fitz1 said:
dont think running cost is more expensive, but initial setup cost is about 1000 quid per kilowatt for the pump plus heatrods and associated setup.
Nope. Natural gas is cheaper.
It is very difficult to justify them. Although equipment is getting
cheaper:
http://www.navitron.org.uk
Some background:
What is the COP? Coeficient of Perfromance. COP 3 is 1 kW in and 3kW out.
Generally at heating, it is better than using electric elements directly. An immersion heater is COP 1, a gas boiler about COP 0.85, a heat pump about COP 3. Sounds good, but if air to air, then below 7C the COP drops to near COP 2. That is just for the heat pump (compressor). So, when the inside and outside
temperatures drop, the efficiency of the heat pump drops too.
Then there is the SEER ratings for cooling which is about 10 to 17 for air to air.
Then there is the HSPF for heating, which is between 6.8 and 10.0 for air to air.
Both of the above take into account the fans energy too. The higher the figures the better.
Heat pumps do not generate heat; they move heat from one location to another. Heat is moved from the outside air, water or the ground into buildings for heating and DHW. On average heat pumps consume one third of the electricity of electric resistance heaters. Sounds good so far.
Heat is found in what most people would regard as cool or cold points. Even on a very cold day there is heat energy in the outside air. The temperature of air would need to be absolute zero, -273C, for no heat energy to be available. On a freezing -1C day, the air temperature is 272C above absolute zero. 20C is 293C degrees above absolute zero. So, a heat pump only has to raise heat a relatively small amount; the refrigeration cycle does this.
A heat pump is a large refrigerator compressor. The compressor raises the pressure of the refrigeration gas and subsequently the gas temperature. Heat energy is concentrated by the compressor. The absorber has a similar function to an icebox in a fridge, absorbing heat, being either an outside air-to-air radiator, a buried pipe in the ground or a water-to-water heat exchanger in a stream or pond. The emitter, performs a similar function to the warm pipes at the back of a fridge, is the hot water storage vessel, heat distribution pipework or ductwork heater battery inside a house.
Heat pumps may incorporate reversible compressors to provide cooling, typically incorporated within a forced air ventilation system. Generally in temperate climate UK, heat pumps are only capable of providing comfort cooling rather than full cooling, as heating is the prime function. Cooling is unnecessary in the UK if proper insulation, ventilation and shading is fitted in homes. In other countries heat pumps may provide full cooling.
The cooling aspect of heat pumps offends environmentalists, who frown on summer cooling using fossil fuel as the root power source. Although a heat pump can theoretically recover maybe 7 kilowatts from every kilowatt used, the overall efficiency from power station to recovered heat is around 30%. A well designed house in the UK climate does not require forced cooling.
Air Source:
With air sourced heat pumps, heat is extracted from the outside air. Most domestic air sourced heat pumps in the UK have a maximum output of around 5kW (17,000 BTU/h). The problem with air source is that outside air temperatures may fluctuate widely affecting heat pump performance.
Air-source heat pumps work well down to around 7C, below, efficiency depends on the heat loss of the building being heated. If a building is well insulated with a low heat loss, an air-source heat pump can efficiently heat a house with an outside temperature down to -4C, or lower. A poorly insulated house will require a supplemental heat source to assist at an outside temperature around freezing.
Air sourced heat pumps operate at temperatures colder than 7C degrees for much of the winter. When the temperature is -7C degrees and below, efficiencies drop off sharply, with a COP 3 rated heat pump being closer to COP 2 or COP 1 than COP 3. When outside temperatures fall below 5C the heat pump may require periodic defrosting. The heat pump extracts heat from the house to heat the outdoor coils lowing efficiency further.
The Cost:
Gas is about 1/4 to 1/3 cheaper than electricity per kW to buy. Heat pumps on average are 1/2 to a 1/3 cheaper to run than electric resistance heaters, bringing them "near" to the running cost of gas. Electricity from power station to point of burn is about 30% efficient, because of latent heat and line losses. So, may as well burn natural gas at point of use, which is about 90% efficient. Gas overall is cleaner and cheaper to install and run and appliance service and parts found everywhere.
95% of all Heat Pumps in the UK are in commercial installations.
So, you loose 2/3 from gas power station to point of burn, then ramp it up to near what it was in the first place with the heat pump. Costing more than using natural gas. May as well use gas in the first place and save a ton of money on installation, and use this money on insulation and making the house air-tight. Put enough insulation in and pay attention to air-tightness and the heating system can be just a partial system costing little to run.
The above is a guide and in many cases a heat pump can be the ideal option - usually when heat is being extracted from running water and in of-the-grid locations) - however do the sums first and have insulation and air-tightnes as the priority in a house.
one part of job done foc. 
