Keeping a heat pump running well

The commissioning settings on a heat pump are a starting point. The installer chose them from a heat loss calculation and a design outdoor temperature, on a day when the house was probably mild, and left. Whether the system is cheap to run over the years that follow depends on a handful of settings, and revisiting them falls to the owner.

The work is small and most of it happens in the first winter. After that it is an annual check, some of which the owner can do and some of which needs a service visit.

What decides the running cost

The running cost depends on how much heat the house needs, on how hot the water has to be to deliver it, and on what each unit of electricity costs. The heat needed is fixed by the building and the thermostat setting. The water temperature is where an owner has most control, because the machine's efficiency rises steadily as the water it makes gets cooler. The unit price is a tariff question and is dealt with below.

A heat pump producing water at 35 °C can deliver around four units of heat for each unit of electricity; at 55 °C it is nearer two and a half. COP and SCOP explains where those figures come from. So each degree taken off the flow temperature, provided no room goes cold, reduces the bill for every hour the heat pump runs.

The first winter: tuning the curve

A properly set up heat pump runs on weather compensation. An outdoor sensor tells the controller the temperature outside, and the controller sets the flow temperature from a line called the heating curve, which asks for hotter water as the outdoor temperature falls. The installer chooses the curve from the design figures. The first winter is when you find out how cautious the design was, and most designs leave some margin.

The method takes a few weeks. Pick the coldest week the weather offers. If every room reaches its set temperature and the radiators are never fully hot, lower the curve one step. Wait several days, since a house with any thermal mass takes that long to settle, and check again. Keep going until a room falls short, then go back up one step, and leave the curve there.

Two settings matter on most controllers.

The end point is the flow temperature at the design outdoor temperature, typically −3 °C in the south of England and colder further north. Installers set this from the radiator sizing, and it is the setting with the most room in it. A house designed for 50 °C at −3 °C will often heat comfortably at 45.

The slope or the mild-weather point decides how far the flow temperature drops as the weather warms. Getting this wrong in the other direction, so the house is cold on a mild damp day in October, is common and is a separate adjustment from the end point.

Write down what you started with before changing anything. The controller's own log, if it has one, or a note on the airing cupboard door, will do.

Room thermostats and what to do with them

Once the curve is right, a room thermostat has very little to do. The heat pump is already matching the house's heat loss, and a thermostat that switches the whole system off and on interrupts that. Why a heat pump runs all the time covers the reasoning.

The practical setting is to put the thermostat a degree above the temperature the house settles at, so that it only ever acts as an upper limit, and to give the heating curve the job of holding the temperature. If the house overheats on sunny afternoons, the curve is too high for mild weather, and the mild-weather point is the setting to lower.

Thermostatic radiator valves in most rooms can be left fully open. Closing several of them reduces the surface the heat pump has to work through and pushes the flow temperature up for the whole house. The exception is a room that genuinely runs warmer than the rest, such as a south-facing bedroom, where a valve set a little below the others does no harm.

Hot water: the setting most often left too high

A cylinder set to 60 °C on commissioning day is a common sight, because it is the safe default, and it is the setting with the largest and easiest saving in it.

Storing at around 50 °C, with a weekly sterilising cycle above 60 that the controller runs automatically, cuts the electricity used for hot water noticeably and is the arrangement the design standard anticipates. Check that the sterilising cycle is enabled, since a cylinder stored at 50 °C without it is a legionella risk, and check when it runs. It belongs overnight, in a cheap tariff period if there is one, and away from the early evening when the heat pump is also trying to heat the house.

Hot water heating works best on a schedule. One heating period in the early hours, on a cheap rate if you have one, and possibly a short top-up in the afternoon for a house with evening baths, is enough for most households. Hot water with a heat pump has the sizing arithmetic if the cylinder seems to run out.

The tariff

The tariff sets the price of every unit the heat pump uses and can be changed without touching the hardware. A heat pump moves a large share of a house's energy onto the electricity bill, so the difference between a flat rate and a tariff written for electric heating is one of the larger sums on this page. Do you need a smart meter for a heat pump? covers what those tariffs need from you.

Two adjustments follow from being on a time-of-day tariff. The hot water schedule moves into the cheap block, as above. And a house that holds heat well can be run a degree warmer through the cheap hours and allowed to drift back during the dear ones. Both are settings on the controller, and both are worth checking again whenever the tariff changes, since the cheap hours move between suppliers.

Put your own rates into the running cost comparison once a year. Tariffs have changed every year since heat pumps became common, so last winter's answer may be out of date.

Watching the figures

Most heat pump controllers report electricity consumed and heat delivered, and the ratio between the two is the system's seasonal performance in practice. It is worth reading it once a month through the first winter and writing it down.

The figure will be lower than the SCOP on the product sheet, and COP and SCOP explains why. What matters is whether the figure moves from one year to the next. A system giving 3.2 in one January and 2.7 the next has changed, and the likely causes are in the section on things that drift. A cylinder set to 60 °C shows up as a poor summer figure, since in summer hot water is all the heat pump does.

If the controller does not report heat delivered, a separate heat meter can be fitted. Without it the only figure available is electricity used, which is still useful when compared month by month against the weather.

Things that drift

The settings above stay put once found. The physical parts of the system need looking at from time to time.

The outdoor unit. It collects leaves, grass cuttings and spiders' webs, and in some gardens a hedge grows across the air intake. Air has to pass freely through the fins and away from the fan without recirculating. A quick look every month, and a clear-out in autumn before the heating season, is enough.

System pressure. A wet heating system loses a little pressure through the seals over time. The gauge should sit where the installer left it, typically around 1 to 1.5 bar cold. A pressure that keeps needing topping up means a leak somewhere, and a system with a leak keeps taking in fresh water with fresh oxygen, which corrodes the radiators from the inside.

The water in the system. Inhibitor protects the radiators and the heat pump's own heat exchanger from corrosion, and it depletes over a few years. The annual service should test for it and top it up, and the strainer or magnetic filter should be cleaned at the same time. A filter full of black sludge is a sign the inhibitor ran out some time ago.

Refrigerant charge. A slow loss of refrigerant shows up as a heat pump that runs longer and delivers less, and it is not visible from outside. This is the one item on the list that only a qualified engineer can check, and it is the main reason to keep up the annual service.

The outdoor sensor. Occasionally one is fitted where the sun reaches it in the afternoon, or where the outdoor unit's own exhaust blows on it. Either makes the controller think it is warmer than it is, and the house goes cold at the same time each day, which is the symptom to mention if it develops.

The annual check

The manufacturer's warranty almost always requires an annual service, and the service is the sensible time to do the things that need an engineer. The list below is what a service should cover and what the owner can do between visits.

Item	Who	When
Clear the outdoor unit of debris and growth	Owner	Monthly glance; autumn clear-out
Check system pressure	Owner	Monthly
Read consumption and heat delivered	Owner	Monthly through the first winter, then quarterly
Confirm hot water schedule and sterilising cycle	Owner	After any tariff change
Re-run the tariff comparison	Owner	Yearly
Test and top up inhibitor; clean filter	Engineer	Yearly
Check refrigerant charge and electrical connections	Engineer	Yearly
Review the heating curve against the winter's figures	Owner with engineer	Yearly, at the service

What to leave alone

Some settings are on the controller because the installer needed them, and they are not for tuning. The pump speed, the compressor frequency limits, the defrost settings and anything under an installer or service menu are set for the pipework and the heat pump's design and should stay as they are. Changing them can invalidate a warranty and rarely improves anything.

The same goes for the temptation to turn the heat pump off in mild weather. A modern unit at a low flow temperature on a mild day is running near its best efficiency and using very little. Switching it off saves almost nothing, and the recovery afterwards runs at a high flow temperature, which is the expensive mode.

When the house changes

A heat pump is sized and its curve set for the house as it was. Insulation added later, a new extension, a loft conversion, or a change in how many people live there all move the heat loss, and the curve should move with it.

The largest of these is insulation. A house that has had its walls or loft insulated after the heat pump went in needs a lower flow temperature than it was set up for, and the first winter's tuning process is worth repeating from the top. Until the curve is lowered, the heat pump keeps making water hotter than the insulated house needs.