ASHPs appear to have come on recently and have efficiency of around 3-400%. So I may get a better return warming my house rather than exporting electricty to the grid during winter!
|ASHP in tandem with a gas boiler?||Stuart - 2008-05-06 19:57:09|
|Re: ASHP in tandem with a gas boiler?||Colin - 2008-05-07 21:11:28|
I guess it's do-able, but the questions I would ask are:
2) how would you propose to link the systems and ensure they fire in sequence to always prioritise the more renewable solution.
If you went for an ASHP, like the Daikin Altherma (assuming it meets the output you need) then this can be used instead of a gas boiler, with a direct electric element picking up top-up load and a legionella cycle where the heat pump alone cannot meet the required temperature.
If you're running radiators, and don't plan to change them for larger ones (about double the size), then at a higher heat output you'll only get a CoP of perhaps 1.8-2.5 rather than the 3-4 you'd expect running for lower temp u/floor heating.
Note, if you have solar hot water, Daikin don't like using this ASHP with a twin coiled cylinder - preferring a separate heat exchanger.
|Re: ASHP in tandem with a gas boiler?||Stuart - 2008-05-08 14:20:31|
Thanks for the reply.
I am thinking about doing it because it could overall use less CO2.
I envisage the ASHP being connected in the radiator loop just before the return to the boiler. So all I would expect it to do is help maintain the temperature of the system. As it is only a 3-5 or 7kw system it will only be supplementing the boiler, not replacing it.
The system I am looking at is here as you can see it seems reasonable priced.
I am not use that I understand the impact on the units COP being reduced because of going through my existing radiators. Surely the incorrect radiators will limit the heat transfer from the heated water, not how efficiently it heats the water in the first place. Anyway as I am just supporting the boiler I donít believe this will be an issue anyway.
|Re: ASHP in tandem with a gas boiler?||Colin - 2008-05-08 23:10:25|
Jeepers, that is cheap.
With respect to CoP, a heat pump has to work harder to raise the water to a higher temperature, so instead of taking 1kWh input electricity to raise the temperature by 35 degrees (for example) it'll take, say 2 to raise it by 55. Since radiators work at a higher temperature than underfloor heating the heat pump will have to work harder to raise the temperature higher. They work more efficiently running longer for lower outputs.
If you have a condensing boiler, they work most efficiently when the return temperature to the boiler is below a certain level (about 35C if I recall correctly for gas boilers). Using the heatpump to pre-heat the return to the boiler could therefore cause a decrease in efficiency of your existing boiler (assuming it's a condensing type).
Things to consider:
It sounds a laudable plan but I'm not sure it will give significant benefit.
|Re: ASHP in tandem with a gas boiler?||Stuart - 2008-05-09 09:34:58|
Well that has knocked that on the head. Thanks for taking the time to reply so throughly.
I do have a Worcester boiler so I guess the weather controls are out.
I am very dissapointed as that ASHP was so damn cheap. Is there no way I can use one?