Heat Pump Thermostat Settings: Getting the Most from Your System

Why Heat Pump Controls Work Differently

If you have moved from a gas boiler to a heat pump, the single biggest adjustment is how you think about the thermostat. A boiler is a fierce, on-off device. It blasts hot water through your radiators at 70C or more, gets the house warm quickly, then switches off and lets the place cool until the thermostat calls for heat again. That cycle suits a high-temperature appliance that can recover lost heat in minutes.

A heat pump behaves nothing like that. It is most efficient when it runs gently and continuously, delivering water at a much lower temperature, often between 35C and 45C, and topping up the heat your home loses at roughly the same rate it leaks away. Treating a heat pump like a boiler, with sharp temperature swings and aggressive overnight switch-offs, is the most common reason owners complain about high bills or a house that never feels quite warm.

Getting your thermostat and controls right is therefore not a minor tweak. It is the difference between a system running at a seasonal coefficient of performance (SCOP) of 3.5 and one limping along at 2.5, which on a typical heating bill is hundreds of pounds a year. This guide walks through the settings that matter, the myths worth ignoring, and how to commission your controls for the lowest sensible running cost.

Flow Temperature: The Setting That Matters Most

Forget the room thermostat for a moment. The control that has the largest single effect on a heat pump's efficiency is the flow temperature, the temperature of the water sent out to your radiators or underfloor heating.

The physics is unforgiving. The harder a heat pump has to work to lift the refrigerant temperature, the less efficient it becomes. As a rough rule of thumb, every 1C reduction in flow temperature improves efficiency by around 2 to 2.5 percent. Drop your flow temperature from 50C to 40C and you can lift your coefficient of performance noticeably, which feeds straight through to a lower bill.

The Energy Saving Trust and most MCS-certified installers design systems around a flow temperature of 45C or lower at the coldest design condition, and many well-insulated homes run comfortably at 35C to 40C for most of the year. The catch is that lower flow temperatures put out less heat per radiator, so the emitters have to be large enough to compensate. If your radiators were sized for a 70C boiler, some may need upgrading. Our guide on running a heat pump with radiators covers how to tell whether your existing emitters are up to the job.

Where to Find and Set Flow Temperature

Flow temperature is usually set on the heat pump's own controller or installer menu rather than the wall thermostat. On Mitsubishi Ecodan, Daikin Altherma, Vaillant aroTHERM and similar systems it sits inside the main heating controller. If you are not confident navigating the installer menus, ask your installer to set it during commissioning, or to walk you through the consumer-accessible settings. Never guess your way through engineer-level menus, because you can disable safety functions or freeze protection.

Weather Compensation: Let the System Think for You

The smartest thing you can do with a heat pump is enable weather compensation, sometimes called the heating curve or compensation curve. Instead of running a fixed flow temperature all winter, weather compensation reads an outdoor sensor and automatically adjusts the flow temperature to match the conditions.

On a mild 12C autumn afternoon the system might send out water at just 30C, sipping electricity. On a bitter minus 3C morning it raises the flow temperature towards its design maximum to keep pace with the heat loss. The result is that the heat pump almost never runs hotter than it needs to, which is exactly where efficiency lives.

A correctly set weather compensation curve is the single biggest lever an installer pulls during commissioning, yet it is frequently left on a crude default or switched off entirely. If your bills feel high and your installer never mentioned the heating curve, that is the first thing to investigate. The curve is defined by two points, typically the flow temperature you want at the coldest outdoor design temperature and the flow temperature at a mild outdoor temperature. Steeper curves suit poorly insulated homes; flatter curves suit well-insulated ones.

When weather compensation is working well, the room thermostat becomes a high-limit backstop rather than the primary control. The house holds a steady temperature because the heat going in matches the heat going out, and the thermostat only intervenes if solar gain or a wood burner pushes the room above target.

Room Thermostat: Set It and Leave It

This is where heat pump owners most often go wrong. With a gas boiler, turning the thermostat down at night and up in the morning saves money because the boiler can recover quickly. With a heat pump, that same habit usually costs you money.

The reason is recovery. If you let your home drop 3C or 4C overnight, the heat pump has to work hard the next morning to claw that heat back, often by ramping its flow temperature up to its least efficient setting and running flat out for hours. The efficiency penalty of that recovery period frequently outweighs the small saving from the cooler overnight house.

For most homes, the best approach is to pick a comfortable target, commonly 19C to 21C, and leave the thermostat there around the clock, letting weather compensation modulate the output. A heat pump running steadily at a low flow temperature uses less energy over 24 hours than one that swings between deep set-backs and hard recovery.

Is a Set-Back Ever Worth It?

There is nuance here. A shallow overnight set-back of around 1C to 2C can save a little energy in a well-insulated home without triggering a punishing recovery, particularly if you set it back early in the evening and let the house drift down gently rather than chopping the heating off. The key word is shallow. Deep set-backs and full overnight switch-offs, the boiler-era habit, are what hurt you.

If your home is poorly insulated, avoid set-backs altogether until you have addressed the fabric, because a leaky house both loses heat fast and is expensive to reheat. Insulation almost always pays back faster than any control tweak, and it makes every other setting work harder.

Hot Water Scheduling and Legionella Protection

Your heat pump heats your hot water cylinder as well as your rooms, and the cylinder is a separate control story. Hot water needs a higher flow temperature than space heating, typically 48C to 55C in the cylinder, which is less efficient for the heat pump. The trick is to schedule cylinder reheats sensibly.

Most owners benefit from one or two scheduled reheats a day, timed to land just before peak demand, for example a reheat finishing around 6am before morning showers. Avoid letting the system reheat the cylinder repeatedly through the day, and avoid scheduling it during expensive peak electricity windows if you are on a time-of-use tariff.

A weekly Legionella protection cycle is also essential. This briefly raises the cylinder above 60C, usually with help from the immersion heater, to kill bacteria. Set it to run once a week inside an off-peak window. Our explainer on how a heat pump hot water cylinder works covers the controls and safety side in more detail.

Smart Tariffs and Time-of-Use Scheduling

If you are on a time-of-use electricity tariff, your controls can do real financial work. The aim is to push as much heating and hot water load as possible into the cheap off-peak windows, using your building fabric and cylinder as thermal stores.

In practice that means a gentle pre-heat of the house during the cheap overnight or off-peak hours so it can coast through the expensive peak, plus scheduling cylinder reheats and the Legionella cycle inside off-peak windows. A well-insulated home that can hold its heat through a three or four hour peak can shave a meaningful slice off its heating bill purely through scheduling. Our guide to heat pump smart tariffs walks through how to model the savings, and bear in mind that a leaky home can struggle to exploit time-of-use pricing at all.

This is also where leaving the room thermostat alone and trusting weather compensation pays off, because a heat pump that is allowed to run steadily and pre-heat intelligently beats one that is constantly being overridden by manual adjustments.

Understanding COP, SCOP and What Your Settings Affect

Every setting discussed here ultimately moves one number: efficiency. A heat pump's coefficient of performance (COP) is the ratio of heat delivered to electricity consumed at a given moment, while the seasonal coefficient of performance (SCOP) averages that across a whole heating season.

A COP of 3.5 means you get 3.5 units of heat for every unit of electricity. Your thermostat and flow temperature settings push that number up or down in real time. Low flow temperatures, weather compensation and steady running all raise COP; high flow temperatures, hard recovery from deep set-backs and frequent peak-rate cylinder reheats all drag it down. If you want to understand the metric properly before you start tuning, our heat pump COP explained guide breaks it down in plain English.

The practical takeaway is that you do not need to memorise the maths. You need to set the controls so the heat pump runs as cool and as steady as your home allows, and the COP looks after itself.

Seasonal Adjustments Through the Year

Heat pump controls are not entirely set-and-forget. A few seasonal habits help.

Autumn: As the weather cools, check that weather compensation is engaged and that your flow temperature is not creeping higher than necessary. A mild autumn should see your system running at its lowest flow temperatures of the year.
Deep winter: During cold snaps the heat pump will run for longer at higher flow temperatures, which is normal and correct. Resist the urge to crank the room thermostat up, because that just forces a hotter, less efficient flow temperature. Trust the curve. For more on cold-weather behaviour see our guide to heat pump winter performance.
Spring: Ease the system back down, lower the schedule, and let weather compensation reduce flow temperatures as outdoor conditions improve.
Summer: Most owners switch off space heating entirely and run hot water only, ideally scheduled into off-peak windows. Keep the weekly Legionella cycle running year round.

Reading Your System: What Good Settings Look Like in Practice

Numbers on a screen mean little without a feel for what healthy operation looks like. A well-tuned heat pump has a few tell-tale signs you can observe without any special equipment.

The first is long, gentle run times. A heat pump that has been set up correctly will often run for hours at a stretch on a cold day, quietly modulating its output rather than firing up and shutting down every few minutes. Long runs at a low flow temperature are exactly what you want. By contrast, a unit that starts and stops every ten minutes, known as short cycling, is usually a sign that the flow temperature is too high, the buffer arrangement is wrong, or the system is oversized for the load it currently faces. Short cycling wears components and wrecks efficiency, so it is worth flagging to your installer.

The second sign is a stable room temperature. With weather compensation doing its job, your living space should hold within roughly half a degree of target through the day, regardless of whether it is mild or freezing outside. If you notice the house overshooting on mild afternoons or struggling to reach temperature on cold mornings, the compensation curve probably needs reshaping rather than the room thermostat nudging.

The third is a modest, predictable flow temperature. Many controllers let you view the current flow temperature on the main display. On a mild day you would expect to see something in the low thirties, climbing into the forties only when the weather turns genuinely cold. If your flow temperature sits in the high forties or fifties even in mild weather, that is your cue to investigate the curve or the design temperature.

If you have access to an energy monitor or the heat pump reports its own consumption, watch the daily kWh figure across a range of outdoor temperatures. A sensible pattern is consumption that rises smoothly as it gets colder, not erratic spikes that suggest the controls are fighting themselves.

A Simple Tuning Routine for the First Winter

If your system is new or you suspect it was never commissioned properly, a methodical approach beats random fiddling. Work through one change at a time and give each at least a few days to settle, because a heavy building takes time to respond.

Confirm weather compensation is on. This is the foundation. If it is off, nothing else you do will deliver the efficiency the hardware is capable of.
Set a sensible room target and leave it. Pick 20C, or whatever feels comfortable, and stop touching the room thermostat. You are testing the system, not your patience.
Lower the flow temperature in small steps. Reduce the maximum design flow temperature by 2C or 3C, wait several days, and check the house still reaches temperature on the coldest mornings. Keep nudging it down until comfort just begins to suffer, then go back up one step.
Tune the curve, not the thermostat. If mild days are too warm or cold days too cool, adjust the relevant end of the compensation curve rather than the room setting.
Sort the hot water schedule. Settle on one or two off-peak reheats and a weekly Legionella cycle, then leave it.

Done patiently, this routine usually finds the sweet spot within a couple of weeks, and the difference in running cost between a guessed setup and a tuned one is often the most valuable saving you will make all year.

Common Mistakes That Waste Energy

A handful of habits account for most disappointing heat pump bills:

Treating it like a boiler. Sharp on-off cycling and deep overnight switch-offs force inefficient recovery and undermine the whole point of a heat pump.
Flow temperature set too high. Often a hangover from boiler-era radiator sizing. Lower it as far as comfort allows, upgrading radiators if needed.
Weather compensation disabled or never commissioned. This leaves the system running a fixed, usually too-high flow temperature all season.
Reheating hot water at peak times. Expensive and avoidable with a sensible schedule.
Constantly overriding the thermostat. Manual fiddling fights against the steady running that makes a heat pump efficient.

Avoiding these alone can move a system from mediocre to genuinely cheap to run, often without spending a penny on hardware.

When to Call Your Installer

Some adjustments are firmly in installer territory. You should ask your MCS-certified installer, rather than experiment yourself, when you need to:

Set or reshape the weather compensation curve for the first time
Lower the design flow temperature, especially if it involves checking radiator sizing
Adjust buffer tank, low loss header or pump settings
Diagnose short cycling, where the heat pump starts and stops too frequently
Investigate why bills are high despite sensible day-to-day settings

A good commissioning visit, with the heating curve properly tuned to your home, is one of the best-value things you can ask for. If your system was never commissioned carefully, a revisit often pays for itself within a season. Keeping the controls dialled in is also part of routine upkeep, which our heat pump maintenance costs guide sets in context alongside annual servicing.

Key Takeaways

Getting the most from your heat pump comes down to a short list of principles that all point the same way: run cool, run steady, and let the system think for you.

Flow temperature is king. Aim as low as comfort allows, ideally 45C or below, and upgrade radiators where needed.
Enable weather compensation. It is the single biggest efficiency lever and should be commissioned, not left on a crude default.
Leave the room thermostat alone. Pick 19C to 21C and hold it; avoid deep set-backs and overnight switch-offs.
Schedule hot water sensibly. One or two off-peak reheats a day, plus a weekly Legionella cycle in a cheap window.
Exploit time-of-use tariffs. Pre-heat during cheap windows and let your fabric and cylinder coast through the peak.
Call your installer for curve and flow temperature changes. These are commissioning tasks, not consumer menu tweaks.

Dial those settings in and a heat pump stops being a source of bill anxiety and becomes exactly what it should be: a quiet, low-temperature system that warms your home for less than the boiler it replaced.