Radiator sizing in the heat pump era: Why one size does not always fit all
FRANK DONOHOE of Donohoe Heating Services reflects on how the heating market has evolved over the past 15 years. While radiator styles have shifted toward the decorative, the fundamental physics of heat loss remain unchanged. Frank writes that as the industry pivots toward heat pumps, the reliance on ‘rule of thumb’ calculations can lead to inefficient systems and cold homes, particularly in retrofitted homes, making professional design and understanding manufacturer specifications more critical than ever.
Radiator sizing
I met Robbie Cousins recently, and we reflected on how I had seen the domestic heating market change over the past 15 years since setting up Donohoe Heating Services. I looked back at an article I had written for BS News in 2012.
After reading through it, the core observations remain valid. The article focused on radiator selection, including styles and the correct calculation of heat loss and subsequent radiator selection.
The aesthetic shift
Within the selection of radiator styles, there has been a shift towards more aesthetic choices, with decorative radiators featuring more regularly, particularly in one-off houses. Multi-column and vertical radiators are far more common now; towel rails, of varying colours, anthracite and black being the most common, regularly feature in bathrooms.
Understanding Delta T and Standards
I identified the confusion of the Delta T’s (dt’s) impact on the radiator’s performance. At that time, some manufacturers were indicating outputs at dt50C and/or dt60C, the latter giving a perceived increased heat output performance. However, these outputs were based on temperatures of 90C flow/70C return and 20C room, versus the dt50C EN442 Standard of 75C/65C/20C. This gave an incorrect output, resulting in a deficiency in heating performance from the radiator units.
In recent years, I’ve also noticed EN442 and EN14037 Standards being regularly inadvertently mixed up on specifications. EN442 is for radiators and EN14037 is for radiant panels.
I then introduced the impact of condensing boilers, which were becoming more common at the time, and their lower flow/ return temperatures they used for efficiency.
I mentioned the arrival of heat pumps further complicating this, given the lower operating temperatures.
Jump forward 13 years, and it seems the heat pump is now the heat source of choice and deemed to be the solution to all domestic housing applications. If I’m honest, I’m on the fence. I believe they have their part to play for sure, but are probably best suited to newbuild A-rated houses, providing the system is professionally designed and installed.
I have been asked to supply radiators to houses that are 70 to 100 years old, with no insulation, single-glazed windows, etc., and the client has told me they are using a heat pump provided by their plumber.
The Pitfalls of Retrofitting
I have become involved in more projects of late where the client has invested heavily in retrofit insulation, installed a heat pump with a combination of underfloor heating and radiators.
A common complaint I hear is that they are achieving an internal temperature of 15/16C during prolonged lower external temperatures. On examining the heat loss of these houses, I regularly see low heat-loss estimates and radiators that are too small to provide the heat required at the temperatures delivered by the heat pump.
Moving beyond ‘Rule of Thumb’
I’m still seeing rule-of-thumb heat loss calculations being applied, typically watts/M2 or watts M3!!!!!!!!!
These are among the reasons I believe detailed heat-loss calculations and full design are vital to maximise the benefits of heat pump use and fulfil end-user aspirations.
The impact on sizing and efficiency
In sizing radiators, I depend on heat pump suppliers to provide me with the flow/return temperatures applicable to their products. I typically use 45/40C flow/return Temperatures, as I understand the COP is higher at these temperatures, thus maximising the efficiency for the end user. I have, however, seen flow & return temperatures as high as 55/60C being advised, with no warnings/ advice given as to the decrease in efficiencies when these higher temperatures are being achieved.
Typically, heat pumps are efficient down to 10/7C external air temperature. During prolonged cold spells below 3C, the compressor is forced to work harder, which decreases the COP closer to 1, negating the benefit of using a heat pump in the first place (over, say, a direct electric heater).
The technology is evolving rapidly, with new, higher temperatures being achieved, and the industry needs to communicate these and indicate their impact on heat pump performance relevant to emitter selection.
Let’s take the example of the heat loss above and illustrate the impact of lower flow/return temperatures currently applicable on the radiator output and size.
Heat loss 1114 Watts
Typical Compact Type 22, 600 High as a base, nearest to output is 700mm Long rated 1223 watts @ dt50C (75C/65C/20C)
Depending on the output, it is now common to see two radiators in a room or even a triple panel being used. Radiator manufacturers are now stocking triples to meet market demand.
A call for correct application
Given the encouragement and grant support being applied for heat pump use, I feel the industry needs to focus on the correct application of the product, correctly sized and correctly commissioned. To this end, plumbers should engage with manufacturers/suppliers to ensure the correct calculations are being made. In doing so, the end user is best served.
One size does not always fit all.
About the author
Frank Donohoe, Principal of Dublin based Donohoe Heating Services, has over 40 years’ experience operating in the specification sector and specialises in premium heating solutions. He oversees bespoke installations for prestigious civic projects and contemporary residential properties.