Why are TRVs often left off heat pump installations?

The evidence for the caution comes from testing rather than theory. A BEAMA project at the Salford Energy House, designed to assess the impact of analogue and smart TRVs on air source heat pump performance, found that using smart TRVs on all radiators to provide a setback between heating periods significantly increased heat pump cycling³. The same test found TRV use reduced air temperatures in adjoining zones⁴. Cycling is the mechanism by which efficiency is lost: heat pumps operate more efficiently when the source temperature is higher and the sink temperature is lower⁵.

The rule of practice is not a ban. Building Regulations require a TRV on any new radiator installed⁶, and official guidance encourages fitting them to existing radiators as well⁷. What changes with a heat pump is the order of priority: BEAMA's guidance is that households wishing to limit internal temperatures should initially reduce the flow temperature and programme setbacks, with TRVs used as a secondary measure⁴.

What a TRV does in a radiator system

A TRV controls the temperature of an individual radiator by regulating the flow of hot water to it based on the room's temperature¹. It automatically adjusts the heat output of each radiator to maintain a comfortable room temperature and reduces hot water flow once the desired temperature is reached¹⁰. In an official framing, a TRV gives better control over individual room temperatures⁷.

That is a genuinely useful function in a boiler system. Independent guidance recommends using TRVs on individual radiators to adjust the temperature in different rooms based on usage, turning down TRVs in unused rooms¹¹. Programmable versions add timing control, letting each radiator come on at different times and heat each room separately¹². TRVs can also be manually adjusted to different settings to control the flow of hot water through the radiator¹³.

The controls themselves are simple. TRVs do not usually show specific temperatures; they use numbered settings that correspond to a temperature range¹. The common scale runs from 0 to 6, where zero is off and six is fully open⁹. That coarse scale is part of the problem on a heat pump system, because the difference between one number and the next can represent a meaningful change in how much emitter surface is available to the heat pump at any moment.

Two siting rules apply regardless of heat source. TRVs should not be used in the same room as the thermostat, because the two controls compete¹⁴. Approved Document L states the same rule in regulatory language: thermostatic radiator valves should not be used in the same room as the thermostat, and heat emitters in that room should have manual controls⁸. TRVs are also not recommended in bathrooms, as humidity interferes with their perception of the air temperature¹⁴.

Why TRVs and heat pumps often conflict

The conflict is about how a heat pump is asked to deliver heat. Heat pumps send water to pipes and radiators at a lower temperature than traditional gas or oil boilers¹⁵, and the heat they produce is generally at a lower temperature than a traditional heating system². Because they move heat rather than create it, they can produce several units of heat for every unit of electricity used¹⁶, and they require much less energy than traditional heating systems burning gas or oil¹⁷.

That efficiency depends on temperature lift. Heat pumps operate more efficiently when the source temperature is higher and the sink temperature is lower⁵. A TRV that closes down reduces the emitter area available, which tends to push the heat pump towards a higher flow temperature to serve the remaining radiators, working directly against the condition for good efficiency.

The Salford testing put numbers to the behaviour. Using smart TRVs on all radiators to provide a setback between heating periods significantly increased heat pump cycling³. Cycling means the compressor starts and stops more often than it would in steady low-temperature operation, which is the opposite of how a heat pump is meant to run. TRV use was also found to result in a reduction in the air temperatures of adjoining zones⁴, so closing valves in one room has effects beyond that room.

There is a further mismatch in how the two devices think about time. A heat pump system is typically controlled by the end user via weather compensation or internal temperature control, such as a room thermostat, together with a timer or programmer to control space heating². A TRV responds only to its own room, with no knowledge of what the heat pump is doing or what flow temperature it is trying to hold.

How heat pumps control temperature instead

A heat pump moves heat from the air or ground and raises its temperature via a refrigeration cycle before transferring it to the central heating system¹⁸. Because it transfers heat rather than generating it through combustion, it is considerably more efficient when installed correctly¹⁹. The control task is therefore to keep the flow temperature as low as the building allows for as much of the time as possible.

The primary tools are weather compensation and a room thermostat. Heat pump systems are typically controlled by the end user via weather compensation or internal temperature control, such as a room thermostat, together with a timer or programmer to control space heating². Weather compensation adjusts the flow temperature in line with outdoor conditions, so the heat pump modulates rather than cycling on and off against a fixed high temperature.

The installer sets the relationship between outdoor temperature and flow temperature. The installer should set the heat pump's heating curve when the heat pump is first installed²⁰, and should leave an information pack with instructions on how to adjust the more common settings²⁰. Installers also help with heating controls and performance, and help set up the system to run on the household's preferred schedule²¹.

This is why the advice runs in a particular order. BEAMA's guidance for households with air source heat pump systems wishing to limit internal temperatures is to reduce the flow temperature and programme setbacks first, with TRVs used as a secondary measure⁴. In other words, the heat pump's own controls do the primary work, and TRVs trim the result rather than drive it.

When TRVs can still work with a heat pump

TRVs are not excluded from heat pump systems, and in some circumstances they are required. Any new radiator installed will require a thermostatic radiator valve to be installed⁶, a rule repeated in the Welsh Government's building regulations guidance⁷ and in the Planning Portal's energy efficiency pages²². It is also encouraged to place TRVs on existing radiators⁷.

The practical case for keeping them is room-by-room control where the heat pump is not the only consideration. Heat pumps can work with radiators or underfloor heating, but some types can work without either²³, and underfloor heating is often paired with a heat pump because it suits low flow temperatures¹⁵. Where a system serves a mix of emitters, TRVs on the radiators give a degree of local trim that a single room thermostat cannot.

There is also a seasonal case. Solar PV, a heat pump and a diverter can work together, meaning the heat pump is not needed at all over the summer²⁴, though design, installation and commissioning need careful consideration²⁴. In that arrangement the heating circuit is largely idle in summer, and TRV settings matter less. In winter, PV produces least while the heat pump is needed most, so the heat pump is likely mostly powered by imported electricity in winter²⁴.

Where a supplementary heating appliance is present, it can operate alongside a heat pump so long as the heat pump can provide the full space heating demands of the property²⁵. Hybrid heat pumps can work with another heating system in a building, such as a gas boiler²⁶. In both cases the heat pump remains the primary source and the control hierarchy still applies.

What happens if TRVs are closed on most radiators

Closing TRVs across most of a house removes emitter area from a system that depends on it. The Salford test found that TRV use resulted in a reduction in the air temperatures of adjoining zones⁴, which is the intended effect in the rooms concerned but also a signal that the system's heat distribution changes when valves close.

The more significant finding concerns how the heat pump responds. Using smart TRVs on all radiators to provide a setback between heating periods significantly increased heat pump cycling³. A heat pump that cycles is not modulating down to a low, steady flow temperature; it is being asked to stop and start, which is the operating pattern its efficiency depends on avoiding⁵.

There is a comparison worth noting for anyone considering swapping TRVs for lockshield valves. Trimming internal temperatures with traditional TRVs had a similar impact on system behaviour as trimming with lockshield valves⁴. That finding undercuts the idea that lockshields are inherently better for a heat pump: the system responds to the reduction in flow, not to the type of valve making it.

For frost protection, the guidance is different again. TRVs can be adjusted to the frost setting to make sure radiators use the minimum amount of energy needed to protect the home²⁷. That is a protective setting rather than a comfort one, and it applies to rooms that are not in use.

What installers commonly do in practice

The pattern in the field follows the test findings. Installers set the heat pump's heating curve at commissioning²⁰ and leave an information pack covering the common settings²⁰. They help with heating controls and performance and set the system up to run on the household's preferred schedule²¹. TRVs are then left in place, often fully open, with the heat pump's own controls doing the regulation.

Installation quality matters more than valve choice. Historically there have been issues with poor quality installations in UK homes, including where installers do not size systems correctly, upgrade radiators, replace narrow bore pipework, or properly insulate pipework, though these issues are much less common as standards have tightened²⁸. The installation of heat pumps and associated heating systems is work covered by building regulations and should only be undertaken by competent technicians that have undertaken appropriate training and assessment².

Radiator sizing is often the reason a system is revisited. It is probable that radiators will have to be changed for larger ones to produce an equivalent heat output when replacing a boiler, unless a high temperature output heat pump is installed². Where radiators are replaced, the new ones fall under the TRV requirement⁶; where they are retained, the existing valves can usually stay.

For households commissioning work, independent guidance recommends getting quotes from at least three installers²⁹. The controls conversation is part of that: whether TRVs stay, whether they are set open, and how the heating curve is configured are all decisions made at commissioning rather than afterwards.

TRV settings and heat pump efficiency

Where TRVs are kept, settings are a matter of judgement rather than a fixed rule. Independent guidance suggests setting the TRV at 3 to 4 for rooms used most, turning down to 2 to 3 for bedrooms and rooms used less⁹. The same source notes the midway point as a starting position for non-living rooms such as bathrooms, bedrooms and hallways¹. For unused rooms in cold weather, the frost setting protects the home with minimum energy use²⁷.

The efficiency question is separate from the comfort question. Heat pumps operate more efficiently when the source temperature is higher and the sink temperature is lower⁵, so any control strategy that raises the required flow temperature reduces efficiency. BEAMA's finding that smart TRVs on all radiators for setback significantly increased cycling³ is the practical expression of that principle in a UK test house.

TRVs also appear in official scheme rules. TRV is listed as a measure type in the ECO4 measures table³⁰, and TRVs can be manually adjusted to different settings to control the flow of hot water through the radiator¹³. That places them within the standard toolkit of heating controls rather than outside it.

The wider point for a household is about where control sits. A heat pump governed by weather compensation and a room thermostat keeps its decision-making inside the home, on the heat pump's own controller, with no dependency on a valve head's perception of a single room. TRVs add local trim but do not improve the efficiency of the heat pump itself, and used across a whole house for setback they work against it³.