054 HPM 1115

HPM November 2015

Got a story? Ring us on 01732 748041 or e-mail twood@unity-media.com TRAINING/TECHNICAL Should we worship electrical heating? HPM’s technical expert, John Love, looks at electrical heating as an option to keep a church warm... Heating old churches can present quite a problem due to the often very intermittent use, the very thick structure, which takes a long time to heat up (and cool down) and the invariably very high roofs. I am currently looking at a proposal for heating an old church which has no gas or water supply – it would be prohibitively expensive to install them – and oil heating is out of the question due to delivery access problems. This means that they have no option but to use electric heating. The church is currently heated by high-level tubular radiant heaters which are very old and reportedly inefficient. This is because they are mounted far too high up so you cannot feel their effect at floor level. So what type of electric heating would be most appropriate? For any building with a very high ceiling/roof it is important to select a heating source which does not create a high temperature convectional current of air, leading to stratification which keeps the roof nice and warm, but not the people at floor level. PEWS BEING REMOVED This is why (wet) underfloor heating (UFH) is perfect for such situations because it does not give rise to high temperature convectional air currents. However, the installation of UFH is out of the question for most churches (and this one), unless they are being completely re-ordered with the pews being removed, so we need to look at an alternative. Electric radiant heaters have the advantage that they heat up surfaces and people rather than the air, provided they are at the correct height. The air is indirectly heated by the warmed surfaces, but because the surfaces are only at a low temperature, there is very little convectional heating, so virtually no stratification. However, high level radiant heaters are not generally considered aesthetically acceptable in churches, and frequently the only possible mounting position is far too high. One option put forward to the church for consideration, is the use of electric storage radiators of German manufacture, and at first sight they would appear to be a very good idea, with the claims of dramatically cut running costs. So what are the claims and do they make sense? These storage radiators have a heat retaining core, similar to night storage heaters, but of much smaller volume so it heats up a lot quicker and the heat is transferred to the outer finned surface where it is emitted to the surrounding space, partly by radiation and partly by convection. Once the core is heated up the WWW.HPMMAG.COM Old churches are notoriously difficult to heat, so is going electric a viable option? electrical supply to the heating elements will be switched off by an internal thermostat, but the heat within the core will continue to be given out to the space for a period of time. It is claimed that the heater will only use about half of the electricity that the standard heater would use, without any loss in performance, because the elements are switched off while the heater continues to give out heat. Electric resistive heating is 100% efficient regardless of the type of heater – if the electrical input is 1kW then the heat output will be 1kW, regardless of whether it is an expensive decorative type fire, a fan heater, or an old single or multi-bar radiant type fire. Okay, if you are being pedantic, you could say there is an energy loss with a fan heater due to the motor, but this would be a miniscule amount. So, if the heat loss from a room at design inside and outside temperatures is 5kW you require a heat input of 5kW to maintain the required inside temperature, irrespective of the heat source. To suggest otherwise is contrary to the basic laws of thermodynamics. Arguments in favour of these storage radiators are that they are very responsive to sudden changes in temperature and, therefore, heat demand, compared with night storage heaters which can store a lot of heat which may not always be required, Also, you get more stable temperatures than with a standard electric heater that may be switching on and off on its thermostat, because the core is still giving out heat after the elements are switched off. While this is true, it does not acknowledge that night storage heaters operate on a reduced price night-time tariff, while these storage radiators will invariably be operating at maximum daytime rate, which is far more expensive. If you have only very short periods requiring heating then they may be far more economical, but so 54 NOVEMBER 2015 HEATING & PLUMBING MONTHLY would a cheap electric fan heater. Another argument in favour of these storage radiators, which, strangely, I have not seen mentioned in any advertising material, is that the considerably extended finned surface area may result in a slightly higher radiant to convective heat output, and a possibly lower surface temperature may result in less convectional air movement and, therefore, potential stratification, compared to a standard electric radiator, although I would suggest that these benefits are marginal when it comes to running costs. So, when looking at any manufactured product it is advisable to carefully consider the claims made – if it seems to good to be true, then it possibly is. Returning to our church, what sort of electric heating would be most appropriate? I would consider two options. As I have mentioned above, radiant heating will be the most cost effective, and so, provided there is sufficient wall space available, I would suggest using electric radiant panel heaters that are designed for healthcare premises or schools, because they operate at a reduced surface temperature and convectional heating will, therefore, be minimised, so reducing the potential for stratification. An alternative would be to use under pew electric convector heaters, of which there are several suitable types manufactured. While the heat output is almost totally convective from these heaters, the fact that they are beneath the pews does mean that the convectional air movement is reduced by the presence of the pew seats above, so the potential for stratification is considerably less than if the heaters were freely exposed to the air. John K Love CEng, FCIBSE., FIPHE., FIDHE., MInstR., FConsE


HPM November 2015
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