Cost of Air Con when used for heating?

absolute_idiot

My 1st post so please be gentle!
I've been thinking of getting a 'proper' split climate control AirCon system.
I've noticed in the technical details that a 9000 btu model can also be used for heating. In round figures the electrical input is 800w with a heating output of 2800w!
I.e output heat power = 3.5 x electrical input.
OK, please dont go into how it all works, i do have a brief understanding.
I would like to know whether this form of heating is cheaper than running a modern gas central heating system because i think that electrical energy costs more than gas?
Please give a brief calculation and explanation on the running cost comparison.

economiser

The 3.5:1 ratio is known as the COP (coefficient of performance). You get this because the electricity does not use electricity to produce heat but to drive a compressor which pumps heat from outside to inside the house. The compressor pumps refrigerant around a circuit between inside and outside. At low pressure outside it evaporates at low temperature and absorbs heat, is pressurised and then condensed inside to give off the heat inside at a higher temperature.

It is certainly cheaper than using electricity for heating but will cost more than an efficient gas boiler. Roughly for every pound you spend with direct electric heating you would spend about about 30 pence with your heat pump and about 20 pence with a gas heater.

real1314

Presumably then, the "output" is measured on the temp raise from the external temp to the output temp?

In which case if it's colder outside than inside, it might be just taking external air at 0 degrees and pumping it in at 15 degrees but that is still "heating"

absolute_idiot

real1314 wrote: »

In which case if it's colder outside than inside, it might be just taking external air at 0 degrees and pumping it in at 15 degrees but that is still "heating"

There is no movement of air between outside and inside. The air in the room recirculates via the heat exchanger. The exchanger is cold in cooling mode and hot in heating mode but there is DEFINITELY no exchange of air between the inside and outside units. The only exchange of fluid is the refridgerant and the small amount of condensate that leaks outside. Any exchange of air in the room is still reliant on the leaks/ventilation that are normally in your room.

absolute_idiot

economiser wrote: »

It is certainly cheaper than using electricity for heating but will cost more than an efficient gas boiler. Roughly for every pound you spend with direct electric heating you would spend about about 30 pence with your heat pump and about 20 pence with a gas heater.

Thanks. So just to confirm... Are you saying that to get the same heat energy from an electrical appliances (circa 100% effy) it would cost 5 x more than the same heat energy from a gas appliance (circa 90% effy), so although the heat pump/aircon heating method is far more energy efficient than gas it would have to achieve a COP of 5 before it even matched the price of gas to make up for the cost of electricity being more than 5 x more than gas?

economiser

As you say absolute idiot (I'm sure you are not) electric heaters are 100% efficient at the point of use. But electricity is in the main generated by fossil fuels where there is an efficiency loss in combustion to generate steam and then losses in the turbines that generate the electricity. Then there is a horrific loss in electrical grid distribution resulting in a cost difference per unit of energy to the end user of about 5:1. It could be a little bit more or a little bit less than 5:1 depending on supplier and tariff but I think you will find it's about that.