Readers letters
The beginning of Robin Curtis’s letter (PE November) concerning the insulation of buildings is indisputable, although the continuation does merit comment.
The statement that a supply water temperature of 60ºC to the central heating (CH) system of a building would be incapable of keeping the indoor temperature in the building at around 20ºC “regardless of how many kilowatt hours of heat you deliver” indicates confusion between the definition of temperature and heat.
The heat (thermal energy in kilowatt hours) delivered by a water-based CH system is dependent on:
- the CH water temperature
- the radiator surface area
- the CH water flow rate
So if 1 above is 60ºC after change-over to a ground-source heat pump instead of, say, 75ºC that it was previously, this can be compensated by suitable increases in parameters 2 and/or 3. Whatever the insulation standard of a house, the desired indoor temperature can always be maintained by a suitable combination of the above three parameters.
Considering an existing house in the UK, this may or may not already have a water-based CH system.
If a heat pump is to be installed in the former case, radiators already exist and have presumably been rated to suit the insulation standard of the house and a water temperature that is probably higher than 60ºC. Additional radiator area may then have to be provided and/or the insulation standard may have to be improved.
In the latter case, it’s just a matter of designing the CH system from scratch, and the heat pump is then merely the source of heat.
Whatever the situation, a heat pump is generally beneficial from all aspects – national economy, ecology, the house-owner’s economy and reduced waste of the Earth’s finite resources. In the latter aspect, the heat pump is always beneficial since, at a coefficient of performance of 3 which is typical for ground-source systems, it delivers 3 units of energy for every unit of energy supplied to drive the heat pump. So we get 2 units of thermal energy entirely free of charge (after the end of the heat pump pay-off period).
Antony Kaye, Ekerö, Sweden