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Ireland's only dedicated building services engineering journal

Sizing and Selection of Radiators for Domestic Heating Systems

Frank Donohoe

Towel radiators have become almost standard for bathrooms and there are several styles available, with a chrome finish becoming more fashionable. Stainless steel models are also entering the market and are particularly suitable for applications where the towel radiator is fed from the secondary services circuit. Buy Oak Vanity Units Online.

With so much choice available the designer must interpret the client’s brief accurately. With the advent of on-line searching and purchase, clients can get overcome and confused with the amount of choice available. This is where the designer must advise based upon experience, professionalism, and indeed common sense.

A pre-tender meeting is a good starting point to ascertain the client’s preferred style or budget options/restrictions. This also prevents over ambitious selection and potential time-consuming re-selections when the project commences on site. It further prevents the client from getting enthusiastic over non-suitable or impractical options.

I usually sit down with the clients to see how they will be “living” in their homes. By this I mean how they live as a family – what rooms they principally use day-to-day? Do they want to have some feature radiators in key rooms? Have their childrens’ bedrooms become living spaces, for study, socialising and general recreational activity? All of the aforementioned need to be reflected in the final design and specification.

Since becoming more involved within the domestic sector of the industry I have noticed several “habits” which are not necessarily good practice but seem to be proliferating within this area. There are two key areas of concern I come across regularly – heat loss and radiator output.

When one mentions heat loss, rules of thumb are used in most cases. There is nothing wrong with a rule of thumb estimate as long as it is recognised as that and not a detailed calculation of the heat requirement. It may be fine for establishing a budget but should not replace correct sizing and specification of plant.

A lot of domestic installers simply multiply the volume of a room by a “factor” often between 7 and 10. Some do not even realise this is based on the old Btu’s per cubic foot model. Others use various “calculators”, which in themselves may be fine, but the difficulty is that there are no records of the design criteria utilised nor room dimensions, etc. In the event of an issue arising there is no discussion document available and this makes dispute-resolution more difficult.

Both of these options do not represent a professional response to establishing the correct radiator size for a given room. These can lead to incorrect sizing of radiators with consequent reduction in performance and lack of comfort for the client. There is no real alternative to a detailed calculation and the correct sizing of the radiators.

Where over-sizing is a result, it could be argued that TRV’s would control the output. This is true, but if the radiators are larger than they need to be, they will be more expensive. In the current competitive climate this could see the job going elsewhere.

I have also noticed considerable confusion in the output interpretation of a radiator’s performance. Sometimes when delta t (dt) is mentioned a mist appears. “Sure I always use the dt60ºC because I get more output from the radiator” is a common response. Delta t60ºC is based on the following temperatures – flow 90ºC; return 70ºC; and room 20ºC – and was the basis of the old BS3528 Standard.

This was superseded in 1997 by the new/current standard EN 442. This is based on a dt50ºC – flow 75ºC return 65oC; and room 20ºC. The “perceived” performance difference between dt60ºC and dt50ºC can give rise to a reduction in output of circa 20% (for 1000 watts required you would need to select 1200 watts @ dt50ºC), which would have a significant impact on the heat input and comfort of the room.

This has been further complicated in recent times with the use of condensing boilers. In order to achieve the maximum efficiency of the boiler, it should operate in condensing mode and this is typically – flow 55/60oC and return 35/40ºC, giving dt’s in the range of 25ºC to 30ºC. From the base of dt60ºC this further increases the “reduction” to circa 55/65% (for 1000 watts required you would need to select 2500 to 3300 watts).

The above figures are for guidance and will vary between manufacturers and the specific exponent for the radiator type. The example from dt60ºC is utilised to illustrate the potential performance issues that can arise in domestic applications.

In practice, modern condensing boilers can modulate their performance range from the lower temperatures (55/35ºC), where maximum efficiency is achieved, to more normal operating temperatures which have no impact on the radiator’s performance. The ultimate selection of the radiator must be a balance between common sense and best achieving the design brief of the client.

Heat pumps have similar, albeit a greater, impact of the radiator sizes.

All new technologies are a step into the unknown for most end users and they frequently have an over estimation of the benefits and lack of understanding of the basics involved. This is where we, as the professionals, need to advise our clients on the specifics of our design and explain to them so they understand the principles behind their installation. ■

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