This tool shows how heat pump controller settings change the room temperature, the efficiency and the running cost. It works out which setting to change when something is wrong. This guide explains how the controls work, how to tune them and what the tool assumes.
The sections below explain more: read them as you need them.
A heat pump runs most efficiently with the coolest water that keeps the house warm. Weather compensation does this. The controller reads the outdoor temperature and sets the flow temperature: the colder it is outside, the hotter the water. The rule it follows is the heating curve.
Set the curve right and the house holds its temperature in all weather with the coolest possible water. Cooler water gives a higher COP, so the heat pump costs less to run. Comfort and running cost are not a trade-off: the right curve gives both.
A few settings do almost all the work. The rest are set once for the system.
The two ends of the curve. The flow on the coldest day (the cold end) and on a mild day (the warm end). Each manufacturer sets the same line its own way: a curve number, a shift or an offset (section 7). Different settings can draw almost the same line, so judge settings by the line they draw (Charts, Flow temperatures), not by the numbers.
The thermostat setting. The room temperature the controller aims for. On most controllers each 1 °C on the thermostat setting adds approx. 2 °C of flow where radiators are fitted, and approx. 1.5 °C with underfloor heating. This is not the curve number. A Grant curve does not move with the thermostat setting; there it acts through load compensation and Decreasing fixed water temperature. Nor does an Ideal HP290 curve: its Halo Lite RF switches the heating on and off.
Load compensation. The thermostat adjusts the flow temperature when the room drifts from its setting: up when the room is cool, down when it is warm. It helps with sun, cooking, visitors, wind and the reheat after a night set-back. On Generic and Grant, start at 2 where radiators are fitted and 1.5 with underfloor heating (the tool’s choice, approx. the same trim as Vaillant’s Room temp. mod. Active). A New install and the Example start there. At 0 it does nothing, and Why these settings? says so. Too much can make a heavy house swing. Each manufacturer has its own name for it (section 7); the Ideal HP290 has none.
The night. A lower thermostat setting overnight (a set-back), or none. Keep any set-back small: a heat pump warms a house back up slowly (section 5).
Write down the settings and the room and outdoor temperatures first. Change one setting at a time, then wait (section 5). The cold end and the warm end are the two ends of the curve (section 1); the second table gives the setting to change on each controller.
| What you notice | What to change |
|---|---|
| Too cold on cold days | Raise the cold end of the curve |
| Too warm on cold days | Lower the cold end of the curve |
| Too cold on mild days | Raise the warm end of the curve |
| Too warm on mild days | Lower the warm end of the curve |
| About right, but drifts with sun, cooking or visitors | Raise load compensation |
| Cold first thing in the morning | A smaller night set-back, or an earlier start to the day |
| One room too warm or too cold, but the thermostat room is right | That room’s emitter size and balancing, not the curve |
| Some rooms go cool when the heating pauses (often lighter-built rooms) | Shorter pauses: a smaller night set-back, short afternoon reheats; on a Grant, a smaller Decreasing fixed water temperature (section 5) |
| Cycles a lot on mild days | Usually normal (section 4) |
One step on each controller. The cold and warm ends in each controller’s own terms, with the change in flow from one step on a −3 °C day and a 10 °C day (curves where radiators are fitted):
| Controller | Cold end, one step | Warm end, one step |
|---|---|---|
| Generic | Flow on the coldest day, 1 °C: +1 °C at the design temperature, approx. +0.3 °C at 10 °C | Flow on a mild day, 1 °C: +1 °C at 15 °C, approx. +0.7 °C at 10 °C, nothing at the design temperature |
| Grant | Heating curve, 0.1: approx. +2.5 °C at −3 °C, approx. +1.3 °C at 10 °C | Shift, 1: +1 °C on every day |
| Vaillant | Heat curve, 0.05: approx. +1.3 °C at −3 °C, approx. +0.7 °C at 10 °C (curve 0.60) | No separate setting: Room temp. mod. Active, or the OT switch-off threshold |
| Ideal Logic Air | Heating curve (720), 0.05: approx. +1.2 °C at −3 °C, approx. +0.6 °C at 10 °C | Heating curve offset (721), 0.5 °C: approx. +1 °C on every day |
| Ideal HP290 | Temperature level, 1: +3 to +8 °C at −3 °C and +1.5 to +3.5 °C at 10 °C, depending on the level | Temperature offset, 1 °C: +1 °C on every day not verified |
Raising only the cold end. A curve number also raises the flow on mild days, by approx. half as much. If mild days are already right:
Either gives approx. +1.5 °C of flow at −3 °C outside and almost no change near 11–12 °C. A Vaillant has no such pair: raise the heat curve, accept the small rise on mild days, and let Room temp. mod. Active trim a warm room. On an Ideal HP290 the levels are far apart, so one level up with the offset 1 °C down still warms mild days a little; Fix a problem shows the result before anything is kept.
Fix a problem works out the setting and the number of steps, and shows the result before anything is kept. If the tool says the settings are right but the house is cold or warm, trust the house: its real heat loss or emitters differ from the figures entered. Fix a problem then suggests one step, and how long to wait.
Other rooms. The thermostat controls its own room. Other rooms settle at the temperatures their emitters were designed for, if the emitters are sized and balanced (MCS MIS 3005-D: 21 °C living rooms, 18 °C bedrooms, 22 °C bathrooms). Fit the thermostat in the main living area, approx. 1.5 m up, clear of radiators, appliances, doors, windows and direct sun.
A heat pump is sized for the coldest days, but it also has a minimum output: the least heat it can give while running. On a mild day the house can need less than that. The heat pump then runs until the water passes its target, stops, and starts again once the water has cooled. This is cycling. When the house needs only a little less than the minimum, the runs are long: the water settles a little above its target, and the room warms slightly before the water reaches the stop point. On mild days cycling is normal, even on a well-designed system. An oversized heat pump cycles far more, which costs efficiency and wears the compressor: every start adds wear, which is why manufacturers set a minimum off time to limit starts. Installer detail shows the outdoor temperature where cycling starts for the house entered.
Each stop and start wastes a little energy. BS EN 14825, the standard behind seasonal efficiency (SCOP), gives a correction for this with a default factor (Cdh 0.9). The tool applies it, so the COP shown on mild days includes the loss.
When it stops and restarts. Installer detail estimates each run and the starts per hour from the open system volume, the minimum output and the controller’s own rule:
Three things make cycling better or worse:
Cycling on the room stat. With room on/off control (Grant pump blockade on, Vaillant Room temp. mod. Expanded, and always on an Ideal HP290 with its Halo Lite RF), the thermostat stops the heating when the room reaches its setting and restarts it as the room cools. If the results show this on most days, the curve gives more heat than the house needs: lower it until the heat pump runs steadily, or cycles only on mild days.
A house warms and cools slowly, because its walls, floors and furniture store heat. The time constant measures this: long for a heavy masonry house, shorter for a light timber-frame one. The tool works it out for the house entered.
Waiting after a change. Wait approx. two time constants before judging a change; the tool shows the days. For the example house that is a little over two days.
Night set-back. Overnight, from Night starts to Day starts, the house cools, and in the morning the walls and floors have to regain that heat. A curve set to efficiently hold the room at its thermostat setting has little spare heat to recover, so a large set-back leaves the room a little cool well into the day. Below the night settings the tool shows the coolest the room gets, when it is back within the comfort band, and how far below the day setting it runs through the day. It flags a room still below the comfort band 2 hours into the day (the tool’s choice). Load compensation, a smaller set-back or an earlier start helps.
For example, the example house on a 10.6 °C day with a 20 °C night from 22:00 to 06:00: coolest approx. 20.0 °C, back within the comfort band by 06:20, then approx. 0.2 °C below the day setting.
Mixed construction. In a house with heavy and light parts, the light rooms cool and recover faster. With the thermostat in a heavy room, keep set-backs and other pauses short, such as hot water reheats and, on a Grant, Decreasing fixed water temperature.
The design heat loss comes from a survey and calculation (BS EN 12831-1). Real houses often lose more or less than calculated, and real emitters can differ from the schedule. The tool can only work from the figures entered. Four checks help:
The radiators are sized room by room at the design flow, for example 45 °C, each for its own worst case: double ventilation for rooms facing the wind (CIBSE DHDG), an intermittent heating allowance where used, and sizes rounded up. Not every room meets its worst case at once, so the house as a whole loses less than the room heat losses add up to. On a design-cold day it loses the whole-house heat loss, the figure the heat pump was sized to. Free heat from people and appliances helps too. So on a design-cold day the house as a whole needs a cooler flow than the design flow. For example, radiators giving a quarter more than the heat loss at 45 °C flow hold the house with approx. 41 °C flow. The design day is also rare: most of the winter is much milder.
A room facing the wind may still need the design flow on a cold, windy day. If a room is cool then, raise the cold end one step, wait approx. two time constants (the tool shows the days for the house), and judge it on the next cold, windy day. Repeat, up to the design flow. One step on each controller: section 3. Manufacturer-specific controllers: section 7.
Choosing the heat pump in the tool sets its controller; it can also be chosen on its own. The settings then appear with that manufacturer’s names and limits, with the plain name used in this guide in small text beneath, and Suggest best settings and Fix a problem answer in the manufacturer’s terms. Grant’s names are as shown on its touchscreen; Vaillant’s and Ideal’s are from their instructions, and a controller’s screen may word them slightly differently. Generic uses the plain names. Switching controller keeps the same curve as closely as the new controller’s settings allow.
The curve is a straight line through two points: the flow on the coldest day (the design outdoor temperature) and the flow on a mild day (15 °C). It is set for the room at its design temperature. A different thermostat setting, day or night, raises or lowers it by more than the change, as section 2 describes.
Every weather-compensation controller can give much the same line, though manufacturers describe it differently. The controller’s manual shows how, and gives its lowest and highest flow temperatures. Real curves bend slightly; a straight line can run a little warm at the ends.
Load compensation adds setting × (thermostat setting − room temperature) to the flow. The flow restart differential is not verified: use the manufacturer’s figure if it is published.
| Plain name (this guide) | Grant’s name (shown in the tool) | Where (Smart Controller manual) |
|---|---|---|
| Heating curve, shift | Heating curve, Shift | Circuit settings → Heating curve (7.13) |
| Day / night thermostat setting | Preset day / night temperature | Circuit 1 on the home screen → circuit details (7.3.1.3) |
| Load compensation | Room temperature correction | Service settings → Installation controller → Circuit 1 (Section 8) |
| (Grant only) | Decreasing fixed water temperature (Section 8 of the manual: Decrease water temperature) | Service settings → Installation controller → Circuit 1 (Section 8) |
| Room on/off control | Thermostat pump blockade | Service settings → Installation controller → Circuit 1 (7.8) |
| Room differential | Hysteresis | Circuit 1 on the home screen → circuit details (7.3.1.3) |
| Minimum flow | Min. Heat Pump setpoint HEATING | Service settings → Installation controller → Main heat source (Section 8) |
| (Grant only) | Circuit temp. correction | Service settings → Installation controller → Main heat source (Section 8) |
| Heating off above | Summer mode activation temperature. Winter mode activation temperature brings the heating back on below it; between the two the controller stays in whichever mode it was last in, and the tool takes the heating as on. | Service settings → Installation controller → Weather sensor (Section 8) |
How Grant’s curve works. Grant sets the line with two numbers instead of two temperatures.
Starting curve. Grant’s manual starts with the curve through the design point (7.13.1). Its example is 45 °C flow at −3 °C outside; use the design flow and design outdoor temperature of the house. That example needs curve approx. 1.0 with shift 0, below the 1.2–1.6 the manual’s table gives for radiators (Table 7-11; 0.2–0.6 for underfloor heating). The table gives approx. 50–60 °C flow at −3 °C outside, so it suits radiators sized for hotter water. Suggest best settings starts lower, at what the house as a whole needs (section 6), and shows Grant’s design-point curve beside it. Tune from either with section 3: curve for cold days, shift for mild days (7.13.2).
Raising the cold end on a Grant. Grant has no separate cold end. Curve +0.1 adds approx. 2.5 °C of flow at −3 °C outside and approx. 1.3 °C at 10 °C, so mild days warm too. If mild days are right, raise the curve 0.1 and lower the shift 1, as Grant advises (7.13.2): approx. +1.5 °C of flow at −3 °C outside and almost no change near 11 °C. After each step, wait approx. two time constants and judge it on the next cold, windy day.
Decreasing fixed water temperature works day and night. With a thermostat fitted and the pump blockade off, the controller takes this value off the flow once the room reaches its setting, by day as well as at night (manual 7.14.2). For the value, Grant points to its setback figures: 3 °C for radiators and 1 °C for underfloor heating (Table 7.4). It comes off again once the room has cooled by the Hysteresis. So the flow steps down when the room is warm enough and back up as it cools, and the room runs between its setting and the setting minus the Hysteresis. A large Hysteresis, such as the 4 °C in Grant’s checklist for radiators, keeps the Decrease on almost all the time; 0.5 °C keeps the room within the comfort band. The tool models this, day and night, and Suggest best settings allows for it: where the Decrease stays on, the shift it suggests is higher to make up for it. It avoids settings where the Decrease keeps coming on and off, because the room then dips to the setting minus the Hysteresis each time. Where your own settings do that, a note under the chart says so: steps across the season, a dip through the day. With the blockade on, the Decrease applies at night only.
Hysteresis (room differential). How far the room must cool before the thermostat calls for heat again. Grant’s commissioning checklist gives 4 °C for radiators and 1 °C for underfloor heating (Appendix F). Smaller holds the room steadier but switches more often.
Restart. The Aerona 290 stops 2 °C above the flow target and restarts 5 °C below it, as confirmed by Grant. There is no setting for this.
System volume. Grant asks for at least 5 L, and recommends 20 L, of open water per kW of minimum output (installation manual 3.8, Table 3-6). Radiators with TRVs and the hot water circuit do not count. The 6.5 kW model’s minimum output is 2.40 kW, so it needs at least 12 L and ideally 48 L.
Silent mode. Limits compressor and fan speed during its set hours, which also limits heat output (manual 7.5.5, 7.6.2). Level 2 limits more than level 1. On a cold night the house may not keep up during silent hours: check the room holds its temperature, or shorten the hours in cold weather.
Oil-return run. After 4 hours at low speed, the compressor runs faster for approx. 7 minutes to return oil to it (installation manual 8.6.1). A short burst of higher output every few hours on mild days is normal.
Documents (need a signal): Grant Aerona Smart Controller instructions, DOC 0203 Rev 2.4 (June 2026) ↗ (PDF); Grant Aerona 290 installation instructions, DOC 0204 Rev 1.9 (August 2026) ↗ (PDF). For any newer revision: Grant’s manuals page ↗.
Modelled on the VRC 720f/3 system control with the VWZ AI /7 appliance interface, the commonest UK pairing for aroTHERM plus. Installer level: MENU → SETTINGS → Installer level. Diagnostics codes on the VWZ AI /7 need code 17.
| Plain name (this guide) | Vaillant’s name (shown in the tool) | Where |
|---|---|---|
| Heat curve | Heat curve, 0.10–4.00 (heat pump factory 0.60) | Installation configuration → Circuit 1 |
| Day thermostat setting | Desired temperature | CONTROL → Zone → Heating |
| Room temp. mod. | Room temp. mod.: Inactive / Active / Expanded | Installation configuration → Circuit 1 |
| Night set-back, set-back mode | Time-controlled mode; Set-back mode Eco / Normal | CONTROL → Zone → Heating; Circuit 1 |
| Night thermostat setting | Set-back temperature | CONTROL → Zone → Heating |
| Minimum / maximum flow | Min. / Max. target flow temperature (factory 15 / 55 °C) | Installation configuration → Circuit 1 |
| Heating off above | OT switch-off threshold (heat pump factory 16 °C) | Installation configuration → Circuit 1 |
| Compressor start (D.230), compressor hysteresis (D.267) | D.230 (−120 to −30 °min, factory −60); D.267 (3–15 °C, factory 7) | VWZ AI /7 diagnostics codes |
The curve. Vaillant publishes its curves as a chart (VRC 720 instructions 2.12), not a formula. The tool uses a formula fitted to that chart: flow = room + 2.56 × (curve × (room − outside))0.78. It matches the chart within 0.4 °C, including the manual’s own example (curve 0.4 gives 40 °C at −15 °C). The step between settings is not stated; the tool uses 0.05, as on the older VRC 700.
The desired temperature (thermostat setting) raises the curve, as the instructions describe (2.12). The tool raises it by 1 °C plus the curve’s steepness for each 1 °C (section 2), following their wording; their worked example shows a bigger rise. Vaillant has not yet explained the difference, so treat the effect of the desired temperature as unconfirmed.
Room temp. mod. (room temperature modulation) is Vaillant’s load compensation. Inactive: the outdoor sensor only. Active: a cool room raises the flow and a warm room lowers it; the controller adds the room’s shortfall to its room target, target = setting + (setting − room). Expanded: the same, and the zone also switches off approx. 0.1 °C above the setting and back on approx. 0.2 °C below (2/16 and 3/16 °C). Vaillant’s heat pump system diagrams set Active or Expanded (4.9.5.2). The adaptive heat curve (needs Expanded) is not modelled.
Set-back mode. Eco, the factory setting, switches the heating off outside the time periods. Below 4 °C outside for more than 4 hours, frost protection heats to the set-back temperature. Normal works the curve to the set-back temperature. In a heavy house Eco can leave mornings cold; Normal with a small set-back is gentler.
Restart. See the energy integral in section 4 (VWZ AI /7 instructions 10.1–10.2).
System volume. Vaillant gives a minimum water volume for de-icing only: 20 L for the VWL 35 and 55, 30 L for the VWL 75, with the back-up heater off and less with it on (aroTHERM plus instructions 3.11). It states that heating runs without extra water, provided the nominal flow rate is kept.
Documents (need a signal): Vaillant sensoCOMFORT VRC 720f/3 instructions, 0020288151_04; Vaillant VWZ AI /7 instructions, 8000042999_02; Vaillant aroTHERM plus VWL 35–75/8.1 instructions, 8000050729_01. Search for them by document number on Vaillant’s literature finder ↗.
Settings are numbered parameters (Logic Air installation and maintenance instructions, UIN 252130 A02, Table 15). Heating Zone 1 is shown; zone 2 uses 1010–1050. Everything the tool uses is unchanged from the earlier A01 issue.
| Plain name (this guide) | Ideal’s name (shown in the tool) | Factory setting |
|---|---|---|
| Heating curve | 720 Heating curve | 1.40 radiators, 0.9 underfloor |
| Heating curve offset | 721 Heating curve offset | 0 °C |
| Day / night thermostat setting | 710 Comfort temp setpoint / 712 Setback setpoint | 21 / 18 °C |
| Load compensation | 750 Room influence | 20 % |
| Minimum / maximum flow | 740 / 741 Flow temp setpoint min / max | 17 / 55 °C |
| Heating off above | 730 Summer/winter heating limit | 21 °C |
| Minimum off time (heat pump details) | 2843 Compressor off time min | 8 min |
The curve. Ideal publishes its curves as a chart (Figure 43). The tool uses a formula fitted to it: flow = room + gradient × 2.13 × (room − outside)0.75, within 0.7 °C of the chart. At 0 °C outside the flow is approx. 20 + 20 × gradient. The step between settings is not stated; the tool uses 0.05.
Offset and thermostat setting. The offset raises or lowers the curve, the same as changing the thermostat setting by that much; Figure 44’s ±4.5 °C curves check out on this basis. The tool steps the offset in 0.5 °C, which is not confirmed.
Load compensation (750 Room influence) needs a Halo Air room unit. Ideal suggests 20 % for radiators and 30–40 % for underfloor heating, with no TRV in the reference room, or one fully open (5.7.4.2). Ideal does not publish how the percentage acts, so the tool assumes the room shortfall moves the curve by % ÷ (100 − %) of the shortfall not verified.
Optimum start (parameter 790, factory 180 minutes) starts the morning warm-up early so the room is at temperature when the comfort period begins. The tool does not model it, so mornings may be better than shown.
System volume. At least 24 L of free volume for the 4 and 5 kW models and 40 L for the 8 and 10 kW, to limit cycling and allow a full defrost (3.2.1). Free volume is pipework without valves and radiators without TRVs.
Minimum output. Ideal does not publish one. Choosing a Logic Air in the heat pump list fills in the tool’s estimate, marked as such (section 10).
Documents (need a signal): Ideal Logic Air installation and maintenance instructions, UIN 252130 A02 (October 2026) ↗ (PDF). For any newer revision: Ideal’s Logic Air guides page ↗.
Modelled on the HP290 control box (or controller) with the Halo Lite RF programmable room thermostat wired to its Zone 1 on/off thermostat terminals (Halo Lite RF installation guide 5.2). Ideal’s USB stick sets several parameters at commissioning (installation manual 5.5).
| Plain name (this guide) | Ideal’s name, on the HP290 or the Halo Lite RF (shown in the tool) | Where; factory setting |
|---|---|---|
| Temperature curve type | Standard or Custom (Eco is not modelled) | Menu → Weather temp. settings → Zone 1 heating mode; Standard |
| Temperature level, temperature offset | Temperature level 1–8, Temperature offset | The same screen; level 6 (radiators) or 4 (underfloor), offset 0 °C |
| Cold end: flow, cold end: outdoor temperature, warm end: flow, warm end: outdoor temperature (Custom) | T1SetH1 at T4H1, T1SetH2 at T4H2 | Custom curve screen; Ideal: 65 °C at −15 °C and 30 °C at 15 °C for radiators, 50 °C at −20 °C and 26 °C at 15 °C for underfloor heating |
| Day thermostat setting, night set-back | Target temperature; Timed heating schedule (the night temperature has no name of its own) | Halo Lite RF |
| Room differential | Halo Lite RF switching differential | Not published |
| Flow restart differential | dT1SH | For serviceman (password 234) → Heating setting; 5 °C |
| Heating off above | T4HMAX | For serviceman → Heating setting; 21 °C from the USB stick (factory 25 °C) |
| Minimum off time (heat pump details) | t_INTERVAL_H, the compressor anti-cycle | 5 min (6.1.3) |
The standard curves. Ideal publishes them as a chart (Figure 54): eight curves, each two straight lines that bend at 0 °C outside and end at 20 °C flow at 20 °C outside. The tool reads them from the chart: at 0 °C outside, levels 1 to 8 give 25, 30, 35, 38, 45, 48, 55 and 60 °C; at −20 °C, approx. 28.5, 37, 45.5, 50.5, 62.5, 67.5, 79.5 and 88 °C (within approx. 0.5 °C). Colder than −20 °C the tool holds the −20 °C figure (T4HMIN: no heating below −20 °C). The flow never goes above 75 °C (Table 1). Ideal calls level 6 the standard radiator curve and level 4 the standard underfloor curve (5.8.3.1). The levels are far apart, so the offset does the fine-tuning.
Temperature offset. Ideal does not publish its range or step, or say exactly how it acts. The tool adds it to the standard curve, from −5 to +5 °C in 1 °C steps not verified.
Standard or Custom. Ideal advises keeping Standard (5.8.3.1), then gives Custom figures for radiators and underfloor heating. The custom curve is a straight line between its two ends and flat beyond both, like the tool’s Generic controller, but set in fixed temperatures.
The thermostat. The HP290 sets the flow from the outdoor temperature only: the thermostat setting does not move its curve, and it has no load compensation. The Halo Lite RF switches the heating off when the room reaches its setting and on again once the room has cooled. Ideal does not publish by how much, so the tool uses the room differential, starting at 0.5 °C. Ideal’s rules for the thermostat: a set-back of no more than 3 °C below the comfort temperature; and any other programmable thermostat used must have a maximum cycle rate of 1 per hour (installation manual 1.2.4). The Halo Lite RF’s optimised start and stop (factory off) are not modelled.
Restart. The HP290 restarts once the flow has fallen by dT1SH below its target, after at least 5 minutes off (6.1.3). Where it stops is not published: the tool assumes at the target.
Summer. The USB stick sets T4HMAX to 21 °C. Ideal says the output is limited to the minimum above it; the tool treats it as heating off.
System volume. At least 25 L of free volume for the 4.5 to 8 kW models and 40 L for the 10 to 14 kW, so the heat pump does not cycle excessively and can defrost fully (3.2.1). Free volume is pipework and radiators without TRVs.
Minimum output. Ideal does not publish one. Choosing an HP290 in the heat pump list fills in the tool’s estimate, marked as such (section 10); enter Ideal’s figure if you have it.
Documents (need a signal): Ideal HP290 installation and maintenance, UIN 240080 A09 (August 2026) ↗ (PDF); Ideal Halo Lite RF installation guide, UIN 237463 A06. The Halo Lite RF guide, and any newer revision: Ideal’s HP290 guides page ↗.
The estimate. Hot water does not depend on the heating settings, so the tool shows one yearly figure for both. It is standard use for the people living here (SAP 10.3 Appendix J: showers, a bath and other uses, heated from the mains temperature to 52 °C), plus 15% for pipework, the cylinder heat loss and pasteurisation. A hot water figure from the EPC replaces the use, pipework and cylinder heat loss, as MCS 031 a) uses it; pasteurisation is still added. Enter only an EPC’s figure: an estimate from heating design software may be hot water used at the taps only. Real use varies widely between households.
Cylinder heat loss. From the datasheet if entered. Otherwise the tool estimates it from SAP 10.3 Table 2 for a factory-insulated cylinder, assuming 50 mm insulation, adjusted from the test condition (65 °C cylinder, 20 °C air) to the cylinder temperature: a tool assumption.
Cylinder temperature. A lower setting saves through a higher efficiency and less heat loss, not through less hot water: the hot water used stays the same. Hot water should reach 50 °C or more at the taps within a minute (BS 8558, quoted in CIBSE DHDG 4.9.2). Where a thermostatic mixing valve (TMV) is fitted, check its manufacturer’s minimum hot supply temperature.
Pasteurisation. The tool adds the heat to raise the whole cylinder to 60 °C, weekly or daily. The heat pump can do it only if it reaches approx. 65 °C flow, for example an R290 unit; otherwise the immersion does it.
Reheat efficiency. Heat out for each unit of electricity when heating the cylinder. The tool uses each choice as a yearly figure, though the sources measure it differently, as below. The set figures are for a 50 °C cylinder; the tool adjusts them for other cylinder temperatures with its COP model (a tool assumption). Your own figure and the immersion are used as entered.
Reheat differential (hysteresis) and timing (advice, not modelled). A wider differential gives fewer, longer reheats, but the cylinder temperature falls further before each reheat, so hot water can run out sooner. A narrower one gives more, shorter reheats and more compressor starts. Reheats in the afternoon, when it is usually warmer outside, run more efficiently. Avoid the morning warm-up: the heating pauses while the cylinder heats (PDHW). A cheap-rate tariff can change the best time.
This tool helps you choose and compare heat pump controller settings. Results are estimates for the thermostat room on an average day, based on the figures entered. It is not a heat loss calculation, a system design, an MCS performance estimate or a commissioning record. It does not model other rooms, sunshine or wind, and hot water is a standard-use estimate, so real temperatures and bills will differ. Keep settings within the manufacturer’s limits, and follow their instructions where they differ. Change one setting at a time and check the room over a few days. The installer remains responsible for the settings chosen and for commissioning.
How the night is worked out. The tool models the room (air, furniture and the heating water) and the building’s fabric as two linked temperatures. This simplifies the ISO 52016-1 hourly method, which splits every wall, floor and roof into five layers; the tool was checked against that method for a heavy and a light house and is within approx. 0.1–0.2 °C after a night. Treat the figures as estimates: they leave out sunshine, which helps on bright days, and Vaillant’s and Ideal’s early-start functions (Max. pre-heating time; parameter 790).
The daytime room with a night set-back. The results cards, the difference under them and the bar at the bottom of the screen show the daytime average. Without a night set-back that is the steady day. With one, it is the daytime room over 24 hours, so a room that runs a little cool all day after the night shows there. The Across the season chart shows the steady day at each outdoor temperature; with a night set-back, its note gives the chosen day’s average. Most of what a set-back saves comes from the room being cooler, by night and through the day.
Rounding. The night temperatures are rounded to the nearest 0.5 °C, because hour-by-hour temperatures from a simplified model are estimates. Differences, such as how far below the day setting the room runs and the What if? lines, stay at 0.1 °C: the method predicts the effect of a change better than the temperature itself.
The yearly heat uses MCS 031 a)’s degree days for the postcode area (Appendix B, Table 1) and its link between yearly heat and heat lost per degree (2.1 (e)), worked from the whole-house design heat loss at the temperatures it was calculated for: heat lost per degree (design heat loss ÷ (room design temperature − outdoor design temperature)) × 24 × degree days ÷ 1000. MCS 031 a) works the other way, from an EPC’s yearly figure, and uses Table 1’s outdoor low temperature only for that. The tool’s figure is a running-cost estimate for comparing settings, not the MCS 031 a) performance estimate. Each setting scales it by the tool’s own model: its yearly heat ÷ the yearly heat with the room held at its design temperature, so a setting that runs the room warm uses more. Electricity uses each setting’s seasonal efficiency, shown on the cards as Seasonal COP: the heat ÷ the electricity from October to May with those settings. It is the tool’s estimate for this house, not the datasheet SCOP, which is fixed at 35 or 55 °C flow in an average climate and so cannot show the difference between settings. The cards’ Heating this day and COP this day are for the chosen day only, so they change with the outdoor temperature chosen; Heating a year and Seasonal COP change only with the settings. The tool’s electricity figure will usually be lower than an MCS 031 a) estimate for the same heat. MCS 031 a) takes a fixed efficiency from its Table 2 for the design flow temperature band; the tool works from the manufacturer’s COPs at the flow the curve sets. It uses one average day per month, so cold spells, when the COP is lowest and the heat needed is highest, are under-counted and its efficiency is likely to be on the optimistic side. Use the yearly figures to compare settings rather than as a bill forecast. A space heating figure from an EPC replaces the degree-day heat, as in MCS 031 a). Enter only an EPC’s figure, not an estimate from heating design software, which may be based on the total of the room heat losses. With no postcode, the tool uses its own October to May figure with SAP 10.3 UK average temperatures. None of these includes the standing charge: it is the same whatever the settings, and it is paid anyway for the rest of the house. The hot water cost shows under the cards, with heating and hot water together.
Create report, at the end of Results or in the ⋮ menu, makes a settings record for the homeowner: the heat pump and controller, the house figures, the controller settings as found and as left, what to expect on a cold, a typical and a mild day, the yearly cost before and after, a chart of the room across the season, and what to check and when. It opens on screen to check; Save as PDF saves or prints it.
As left is Current settings: press Keep as current settings first, as changes not kept are left out. As found is recorded only when the settings as found are confirmed. Every detail on the form is optional. Nothing is sent anywhere: the installer’s details are remembered on this device for the next job, and the job’s details are kept with the job, so an exported job file carries them. The report is not a commissioning record (Limitations of use).
The list holds only the models whose manufacturer’s documents the tool was built from: Grant Aerona 290, 4 to 15.5 kW; Ideal HP290, 4.5 to 14 kW; Ideal Logic Air, 4 to 10 kW; Vaillant aroTHERM plus, 3.5 to 7 kW. For each model it takes the maximum output at the coldest published test point with 55 °C flow; the COPs at 7 °C outside with 35 and 55 °C flow; the minimum off time where published; and the minimum output as published. Grant gives a minimum modulation at 12 °C outside and 30 °C flow; Vaillant gives the lowest output at each test point, and the tool takes it at 7 °C outside and 35 °C flow. Ideal publishes no minimum output, so the tool estimates it at 30% of the output at 7 °C outside and 35 °C flow, and marks it as its estimate. That is the tool’s choice: for comparison, Grant’s Aerona 290 models modulate down to 27–36% of their nominal output (Table 2-1). Each figure shows its source. Change any figure you have a better one for; it then shows as entered by you.
Standards and data: BS EN 12831-1, MCS MIS 3005-D, MCS 031 a) Issue 2.0 (method and degree days), BS 8558, BS EN 442, BS EN 1264, BS EN 14511, BS EN 14825, BS EN 15316-4-2, BS EN 16147, ISO 52016-1 (structure and values from the Home Energy Model papers HEM-TP-04, 05 and 07), SAP 10.3 (BRE, 12 June 2026, including Appendix J for hot water), RdSAP 10 (BRE, 7 June 2026), Approved Document L, Volume 1 (2021 edition incorporating 2023 amendments), CIBSE Domestic Heating Design Guide, CIBSE TM41.
Manufacturers’ documents: Grant Aerona Smart Controller instructions, DOC 0203 Rev 2.4 (June 2026); Grant Aerona 290 installation instructions, DOC 0204 Rev 1.9 (August 2026); Vaillant sensoCOMFORT VRC 720f/3 instructions, 0020288151_04; Vaillant VWZ AI /7 instructions, 8000042999_02; Vaillant aroTHERM plus VWL 35–75/8.1 instructions, 8000050729_01; Ideal Logic Air installation and maintenance instructions, UIN 252130 A02 (October 2026); Ideal HP290 installation and maintenance, UIN 240080 A09 (August 2026); Ideal Halo Lite RF installation guide, UIN 237463 A06.
The example heat pump and house are illustrative. The heat pump’s figures are typical of a 6.5 kW R290 unit, taken from a manufacturer’s datasheet.
Data privacy. This tool runs entirely on your device. Everything you enter, your calculations and your results stay in this device’s browser storage and are not sent to any server or third party. There is no account, no tracking and no analytics. Nothing is shared unless you choose to share it yourself, for example by exporting a job file or sending an email.
Disclaimer. This tool estimates how heat pump controller settings affect room temperature, efficiency and running cost. It is intended to help competent persons commission and tune heating systems, and to help householders understand them. It is a calculation aid for teaching and tuning only. It is not a design, a commissioning record or a substitute for professional judgement, and it does not by itself satisfy any MCS, Building Regulations or other regulatory requirement. Limitations of use sets out what it does and does not do.
Results are estimates. They depend on the information entered, the simplifying assumptions set out in this guide (one room, steady conditions, average weather) and the published data cited (BS EN 12831, BS EN 14511, BS EN 14825, BS EN 15316-4-2, SAP 10.3, RdSAP 10 and manufacturers’ data). Real room temperatures, efficiency and running costs will differ.
Always follow the heat pump and controller manufacturers’ installation and commissioning instructions; where they differ from this tool, follow the manufacturer. Check any setting against the manufacturer’s limits for flow temperature, minimum output and system volume. The user is responsible for checking all inputs and outputs, for working within their competence, and for compliance with current UK legislation, standards and manufacturers’ instructions. To the fullest extent permitted by law, the author accepts no liability for any error, omission, or any direct or indirect loss or damage arising from use of this tool or reliance on its output.
Backups. Your work is stored only on this device. Clearing the browser’s site data, uninstalling the app, or losing or resetting the device will delete it, with no means of recovery. On the standalone app, Export this job in the ⋮ menu saves a copy as a file, and Import a job opens it again.
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If a calculation looks wrong, the tool misbehaves, or you have an idea to improve it, please get in touch. Use the link below, or Email support in the tool’s ⋮ menu. Your build version is added to the email automatically.
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