Researchers from the University of Salford in the United Kingdom have investigated how thermostatic radiator valves (TRVs) could improve the performance of air-source heat pumps (ASHPs) and have found that these devices may reduce ASHP space heating energy consumption by 6-8% without reducing appliance and system efficienc.
A TRV is a device that is usually fitted to radiators that automatically controls the room temperature by adjusting how much hot water flows through the radiator. TRVs could be used with heat pumps, but they need to be applied carefully. Heat pumps work best with a steady, continuous flow of water at lower temperatures, so closing too many TRVs at once can reduce efficiency. To avoid this, at least one radiator should remain open or a bypass valve should be installed.
“Unlike gas central heating systems, TRVs are often omitted from ASHP installations based on concerns that they may reduce system efficiency and increase energy use,” the scientists explained. “However, open-circuit design relies upon accurate sizing calculations and use of an appropriate weather compensation curve, neither of which can be guaranteed. Furthermore, open-circuit design limits occupant control and can lead to overheating in rooms with uncontrolled heat gains.”
The research team carried out a series of tests to evaluate how adjusting room temperatures with TRVs affects indoor conditions, ASHP performance, and overall space-heating energy use.
Testing took place at a life-sized replica of a Victorian end-terrace home built inside a controlled environmental chamber. The setup allows precise simulation of weather conditions and occupant behaviour while using a conventional radiator-based heating system powered by either a gas boiler or an ASHP.
The system was sized using a design temperature of −3 C externally and 21 C internally to ensure comfort and simplify modelling. To avoid oversizing, the ASHP and radiators were based on measured fabric performance rather than standard assumptions. This resulted in selecting a 5 kW Vaillant aroTHERM plus heat pump system, with a total system flow rate of 0.86 m³/h distributed across rooms according to heat loss.
Radiators were designed for a 45 C flow temperature, balancing practicality and performance, with total capacity slightly exceeding calculated demand. Minor oversizing occurred in some rooms due to availability and planning errors, but overall system capacity was only 4% above requirements. Each radiator was fitted with either traditional or smart TRVs, including models from Danfoss and Schneider Electric, supported by room thermostats and a central hub. Additional flow-regulating valves were installed to aid system balancing and ensure proper operation.
A pre-plumbed domestic hot water (DHW) cylinder from Vaillant was installed to reflect a typical ASHP setup, but hot water testing was excluded due to repeatability challenges. The system included a bypass valve and volumizer to maintain flow. System balancing was carried out at a constant chamber temperature of 4.5 C to assess flow control and system performance. Flow Regulating Valves (FRVs) were initially set using design calculations and then refined using flow meter readings until a stable, near-uniform indoor temperature was achieved.
Overall, the testing showed that system-level ASHP flow changed only slightly, but local radiator flows varied significantly depending on control strategy. These flow shifts also influenced downstream behaviour in temperature difference and power output across the system.
In addition, the tests demonstrated that TRV-based temperature trimming reduced ASHP space heating energy use by 6–8% without affecting heat pump efficiency. The savings closely matched the reduction in whole-house temperature difference between indoors and outdoors. Traditional TRVs behaved like analogue controls, while smart TRVs showed on/off digital behaviour, but neither harmed system efficiency.
TRV use was also found to reduce temperatures in trimmed zones and shifted heat demand to other rooms, increasing flow and output elsewhere in the system. This redistribution may offset some local effects but can require careful balancing or system adjustments.
“People with ASHP systems should be made aware that TRVs should be used as a secondary measure to reduce internal temperatures,” the scientists emphasized. “To ensure the most efficient ASHP operation, the initial course of action should be to reduce the flow temperature until the worst performing room reaches a comfortable temperature. TRVs should then be used to trim temperatures in other rooms.”
They concluded that TRVs can be a useful secondary tool for comfort control in ASHP systems, but they should not replace proper system sizing, low flow temperature design, and hydraulic balancing for optimal efficiency.
The test results were presented in the study “University of Salford & BEAMA TRV Energy House Report,” published on the university's website.
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