Winter conditions challenge ASHPs

Bob Long, MD Eco Innovate Ltd
Bob Long, MD Eco Innovate Ltd
Heat pump customers needn’t be left cold or out-of-pocket by underperforming systems during the winter months, argues heat pump specialist Bob Long.

As recent freezing conditions have reminded us, Winter is not quite over yet. And I continue to receive calls from heat pump users, who are finding that their heat pump is falling short on the quantity and quality of heat output, and/or expensive to run during lower winter temperatures.

When complaints of this nature are received, I can’t stress the importance of heat metering too much. I am still amazed that many systems still don’t have a heat meter fitted.

Knowledge is all-powerful and to have the precise knowledge regarding the economic output of the heat pump is as essential as the fuel computer in a hybrid car.

Although most manufacturers continue to show the best aspects of heat pump performance on advertising brochures, I am pleased to see a number of the big names are now making the actual data for low ambient operation available in their sales literature.

Generally, ASHPs are boldly advertised at +7° C ambient temperature and +35° C water temperature, as these are  the parameters where the most boastful COP can be achieved.

This prominently displayed COP figure is likely to indicate a very respectable 4:1, or more. We must therefore forgive the prospective customer for believing this is representative of the economic advantage they are to enjoy.  Electrical energy costs of around 12p per kW/hr would indicate 3p per kW/hr for the energy supplied by the heat pump.

In principal, this assumption would be correct but, in practice, rarely achieved.

The problem arises when you want to actually use this economically derived energy for the purpose of space heating. A maximum economical temperature of 35°C is not very high up the scale when you consider a conventional heating system using conventional fuel would produce useful temperatures somewhere between +55°C and 85°C, effectively delivering the energy to the space via relatively cheap, conventional panel radiators.

To make adequate use of temperatures around 35°C, the installation must employ either a close-pitched underfloor heating matrix, or specially sized fan/coil units.

Whichever method is chosen, the success is dependant on the heat emitter having sufficient surface area to transmit the thermal energy to the space being heated.

Without exception, surface area is never cheap, and therefore price considerations will lead the installer to look at higher operating temperatures, and smaller heat exchangers.

As operating temperature is increased, so the COP is lowered and very quickly the anticipation of heating for as little as 3p per kW/hr becomes a myth.

Outdoor ambient temperature has a similar effect on the operating economics and as winter temperatures fall below the average +7°C, as stated in the sales brochure, the COP also proportionately falls.

It is no surprise that a prospective customer who has listened to the sales patter and installed a heat pump system now expects to enjoy at least a good level of comfort and affordable running costs.

All too often it seems,  they realise the cold facts and are either not warm enough, financially disappointed, or both.

It doesn’t need to be this way, but installing a heat pump system that provides economical results is by no means a low cost exercise.

Once again, honesty is the best policy and essential in ensuring the value of the heat pump can be recognised, and not compromised!