Do TRV (thermostatic radiator valves) waste energy in heat pump retrofits?

Efficiency is the word all over the place in heat pump systems BUT there are some real misunderstandings when we talk about control strategy. Word SMART is often used with thermostats and TRVs (thermostatic radiator valves). In reality, it is not smart.

TRV history

Thermostatic radiator valves were invented by Mads Clausen of Danfoss (in 1943, promoted in 1952 as energy saving) for early boiler systems that were run hot, to avoid cooled return water damaging the heat exchanger. Such systems had the water flow from the boiler at 80 C or higher. With a simple tap/resistance valve on the radiator to control the passage of water through it, rooms could quickly and seriously overheat. TRVs avoided that discomfort by capping room temperature. A useful side-effect was saving energy.

For a good set-up modern heat pump system, setting an appropriate flow temperature (weather compensation) will minimize overheating. And, of course, increase efficiency (lower flow temperature = higher efficiency). There will still be occasional solar and appliance and other heat gains that need to be accommodated, and some rooms will benefit from being kept cooler than living areas. TRVs can help in such cases.

Context and Motivation

There is concern within the industry that TRVs might cause inefficiencies when paired with heat pumps, particularly by increasing electricity consumption if not managed correctly.

Some installers worry that TRVs, by reducing heat output in specific rooms (zoning), could force the heat pump to increase its output to maintain temperatures in other rooms. This could raise the flow temperature, reducing the COP of the heat pump and increasing energy consumption.

Unzoned property, 2 radiators supplying 3 kW each, total 6 kW

Unzoned property

There is a shortfall in the higher temperature room that must be made up to maintain a constant internal temperature. The only way to do this is to increase the flow temperature, if a heat pump is installed will significantly reduce the performance. If a boiler is installed, there may still be a reduction in performance.

As this might sound counterintuitive, imagine driving your car constantly from 0 to 100 km/h. It is better to drive constantly 60 km/h. The same applies to heat pump (compressor):

Open loop, modulation

Individual control, ON/OFF

Methodology

There is excellent study about this effect. Here is full study:

Hart-Davis, D.; Liu, L.; Leach, M. Do Thermostatic Radiator Valves Waste Energy in UK Heat Pump Retrofits?. Preprints 2023, 2023121608.

https://www.preprints.org/manuscript/202312.1608

The study uses a simplified model to simulate the interactions between TRVs and heat pumps in typical UK housing stock, focusing on a basic scenario with four rooms arranged in a grid.

The model is tested under various conditions, including different internal temperature regulation strategies (e.g., stiff vs. flexible regulation) and external temperatures.

The analysis also explores the impact of different building archetypes (e.g., bungalows vs. two-story detached houses) and room layouts on the heat pump's performance.

Key Findings

The model confirms that under specific conditions, particularly when internal temperature regulation is stiff, the "bad setback effect" does occur, leading to higher electricity consumption when rooms are set back using TRVs.

The impact of TRVs is highly sensitive to the layout of rooms and the insulation levels of internal walls. In layouts where heat flows more easily between rooms (e.g., rooms arranged in an ABAB pattern), the bad setback effect is more pronounced.

The bad setback effect is most significant at lower external temperatures (e.g., -3°C), where heat demand is highest. As external temperatures rise, the effect diminishes, and above 10°C, using TRVs actually starts saving energy.

Building Archetype Sensitivity

The effect varies with building type. For example, a two-story detached house shows a more pronounced bad setback effect compared to a bungalow due to different heat loss dynamics.

Temperature Regulation Strategies

Stiff Regulation: When maintaining very tight temperature control in occupied rooms, the bad setback effect is stronger, leading to increased energy consumption.

Flexible Regulation: Using a more flexible temperature regulation strategy, such as weather compensation, mitigates the bad setback effect. In these scenarios, the heat pump adjusts more dynamically to external conditions, and TRVs can be used without increasing energy consumption.

Practical Implications

TRVs with Heat Pumps: The study suggests that TRVs can still be beneficial in heat pump retrofits, especially when used in conjunction with flexible temperature regulation strategies. This allows households to maintain comfort while minimizing energy use.

Installer Guidance: Heat pump installers should be aware of the interaction between TRVs and heat pumps. In homes where tight temperature control is required, installers might consider alternative strategies or educate homeowners on the best use of TRVs to avoid inefficiencies.

Future Considerations: Further research and more detailed models, possibly incorporating real-world data from diverse UK housing types, would be beneficial to refine these findings and offer more specific guidance.

Conclusions

The study concludes that while TRVs can potentially increase energy consumption in heat pump systems under certain conditions, these risks are manageable with proper control strategies. For most UK homes, especially those with weather-compensated systems, TRVs should not be removed but rather optimized for best performance.

The findings emphasize the importance of considering both the building’s thermal characteristics and the heat pump’s control strategies when retrofitting homes with heat pumps.