4 to 30 -Years
| For those interested it operates in a similar way to a refrigerator working in reverse, it works on the principle of the vapour compression cycle within the pump. There is a volatile fluid known as a refrigerant. The refrigerant in the evaporator is heated by the heat source this causes it to turn into a gas. The gas then passes through the compressor; the compressor increases its pressure and causes its temperature to rise. The hot gas then moves to the condenser where it is condensed back into a liquid and in doing so it releases heat into the house via a distribution system. The refrigerant is then allowed to expand back to a low pressure through the expansion valve and pass back to the evaporator where it repeats the cycle in a closed loop. |
What are they?
There are three main types, these are:-
- Ground Source Heat Pump (GSHP),
- Water Source Heat Pump (WSHP), and
- Air Source Heat Pump (ASHP)
They function by extracting heat from the ground the water or the air respectively and releasing that heat energy at a higher temperature within a building. All heat pumps consume energy usually in the form of electricity to operate the pump that is required to transfer the heat. They can supply all the heating you would require, and ground source systems are the most effective. To compare the performance of heat pump the term coefficient of performance is used (COP) and it described the ratio of heat movement to energy input. In the table below the Best Average COP is used. This a seasonal average, since air source and water source vary in performance with the temperature of the environment. For ground source the underground temperature is however remarkably constant all year.
It should also be noted that you will need a reasonable amount of land for an effective GSHP, unless you drill down, and for a water source heat pump a body of water is essential!
Energetically are they worth it?
According to manufacturer information heat pumps provide between 2.5 and 5 times (or they have a COP of between 2.5 to 5) the energy they consume and by energy consumption they usually mean energy in the form of electricity. Electrical power generation and distribution is covered in another article, however when trying to compare like for like and come to conclusions regarding total energy consumption, the inefficiency of power generation must be factored in, and it never is! So here are the main points:-
- 1kWh of electrical energy consumed by the pump should deliver 4kWh of heat energy, and it would have a COP of 4.
- For a gas fired power station for example 1kWh in your house requires about 2.2kWh of gas energy to generate it, so in reality 2.2kWh is required to deliver 4kWh of heat energy, so should the COP value really be 1.8?
You decide!
Financially are they worth it?
The answer to this question is depends. It simply depends on the form of heating you have currently, if you are mains gas heated this technology make no sense on financial grounds, since one unit of gas energy is four times cheaper than one unit of electrical energy; the system will be cash neutral and will never pay for itself. If your only source of alternative heating is electrically powered at the standard rate, then it could be well worth the investment and pay back in about 8 years.
Data table
| Heat Pump | Best Average COP |
Very Rough Equipment and Installation Cost Grant are available to reduce this cost |
Annual Electrical Energy Requirement (16,000kWh/COP) |
Cost of Electrical Energy |
Cost of "Replaced" Energy |
Payback Time | ||||
| Gas | LPG | Electrical | Gas | LPG | Electrical | |||||
| Ground Source | 4 | £ 12000 | 4000 kWh | £ 580 | £ 720 | £ 1232 | £ 2320 | 86 Years | 18 Years | 8 Years |
| Water Source | 2.8 | £ 8000 | 5700 kWh | £ 827 | £ 720 | £ 1232 | £ 2320 | Never | 20 Years | 5 Years |
| Air Source | 2.8 | £ 6000 | 5700 kWh | £ 827 | £ 720 | £ 1232 | £ 2320 | Never | 15 Years | 4 Years |
| This table gives the potential payback times for a various heat source pumps assuming that the total household heat requirement is satisfied. The requirement is assumed at slightly above the average at 16,000kWh each year. | ||||||||||
many thanks
Richard
re. not replacing an old boiler, you are correct of course, the cost should be deducted. So if you get a quote and you intend not to replace an old boiler, then yes use this number will be the total investment cost, I think this is a very useful point. And I will try to make the point clearer on the site, particularly true for GSHPs.
re. relative efficiency gains, there it goes away from the spirit of simple digestible data, for example we could include heat pump life-cycle, relative maintenance costs (boiler, vs pump) and indeed if its better to invest the money in Greek bonds (I jest). How complicated should the comparison be, and at what point does it become over complicated is the problem here. Since every situation will be very different, it is difficult for a general site like this to cover all eventualities.
cheers
Richard
Surely the calculation should also consider the efficiency of and at what point in the replaced appliance lifecycle the change takes place. If you replace an inefficient oil boiler at the end of it's expected life span the the cost of replacing like for like (a new boiler) should be deducted from the cost of the heat pump, and the efficiency gains of a like for like replacement should be factored in to the pay back time.
The air flow forces cold air out of the system.
If you have a small garden and the washing line is anywhere near in line with the fan - then your wet washing will be froozen like a board.
Any plants in line with the air being forced out of the fan (which is through a wide angle) will be killed by frostbite.
We also find the system regularly flushes itself with freezing cold water, thus cooling the hot water and radiators.
And the Mitsubishi fans are noisy, especially when mounted next to the building - as vibration sound comes through the foundation, and where it's bolted to the wall.
Yes the flow temperature from a Heat Pump is lower than from a conventional boiler and this should have been made clear to you and factored in to the sizing calculation by your installer. Sometimes this means that some radiators may have to be increased in size and usually the pump is operated for longer periods than conventional high temperaturel fuel boilers. This increases effiiciency. Lastly, the initial setup of the flow controller is critical and if the installer has just left it at factory settings without taking into account your requirements it is unlikely to be working correctly.