Swimming Pool ASHPs

Casper55

Hello All
I have just spent a few hours reading MSE threads on ASHPs and get the idea that they should not be used to replace domestic gas boilers. I have read that the COP of condensing gas boilers can be as high as 2.3 . manufactures of swimming pool type heat pumps are claiming a COP of 3 or even 4. My interest in swimming pool type heat pumps is purely as a replacment of a boiler for heating a koi pond which heats water to a much lower temp of 10 to 25 deg ,much lower than a swimming pool and therefore I understand this would increaqse the efficiency of a ASHP. I am currently payiing 3.4p per kwh for gas as apposed to 10p for electricity. comparing COP for a condensing boilerof 2.3 and a ASHP of 4. the boiler comes out at the cheaper option.People thoughts on my conclusions would be appreciated, especially anyone who has experience of these types of heat pumps. Best Regards casper

rogerblack

Umm - no.
COP of condensing gas boilers would be around 0.8, if they were measured that way, which they're not.

That is - for every 10kWh of gas you put in, you get 8kWh out of heat.
A COP of 2.3 would mean for every 10kWh of gas that went in, 23kWh of heat came out!

So, for each kWh of heat, you are paying around 4p.
With an ASHP with a COP of 3 (regrettably it drops in cold weather), you'd be paying 10/3 = 3.33p, or 2.5 if you assume 4.

Casper55

Roger black
Thanks for the reply. Please point me to your resource where condensing bolier have a COP of 0.8. If it was so low then why arn,t more sites recommending switching which they don,t.including the energy saving trust. Here is a link that states the break even COP is 2.3. I therefore assumed this was the cop for a condensing boiler and therfore anymore than this you are making a saving
http://www.reuk.co.uk/Compare-Heat-Pump-to-Condensing-Boiler.htm
Please explain how a boiler is measured. My boiler has an output of 23kw . How do I measure the input other than how much gas it burns. Sorry if these are stupin questions and I am reading it all wrong. Using your figures and COPs I am paying 3.4p for every kw of heat by gas & 3.33p for kw of elecrticity for heat pump that is assuming that the heat pump has a cop of 4. A tad optimisic I believe.

rogerblack

Casper55 wrote: »

Here is a link that states the break even COP is 2.3. I therefore assumed this was the cop for a condensing boiler and therfore anymore than this you are making a saving

It's not.
That is the COP at which a heatpump and a gas boiler cost the same to run.

The reason why a gas boiler, and a heatpump are comparable, despite the 'COP' of the gas boiler being 0.8, and the heatpump being 2.3, is that the gas is about 3 times cheaper.

COP - coefficient of performance is a simple ratio of power in versus power out.
For a gas boiler, or most normal heaters, this is under, or equal to one. (and is a figure almost never used)
For a heatpump, it's over one.

The input energy of your gas boiler would be found by reading your meter, and looking up the conversion table to go from units to kWh.
The boiler will produce 23kW of heat, but it will use (perhaps) 26kW of gas.

If you have a heatpump with a COP of 4, it produces 4* the heat compared to electricity in.
If you're paying 10p/unit, then you'd pay 10p/4 = 2.5p per kWh.

Casper55

Roger black Thanks for taking the time to explain that. It all makes sense now. So looking at this link the bigger unit the higher the cop for the same operating temperature http://www.cheshire.luxurypools.co.uk/crystalclear_heater_pumps/Heater_pump_Specs_and_RRP.pdf

rogerblack

Read across the table.
For example - the line 'air 8C, pool 27C' - you get for the various sizes
3.8 3.7 3.6 3.7 3.5 3.6 3.6 3.6 3.2 3.2

It bounces around a bit, but it actually gets worse for the large units.

I do note that my Koi - up to 18" - were just fine with up to 6" of ice on the pond. (the pond was 1.5m deep)
(a bubbler under the ice is _important_, or they will die.)

Casper55

Roger Thanks for the reply. So to chose a unit for a heating supplement to heat in summer it would be the 50Hs-A But for the winter you could get away with the smaller one 35Hs-A. However the smaller one is probably not enough heat and would be running continously