Air Source Heat Pump limit of Coefficient Of Performance being less than one?

For heat pumps the input and output temperatures are critical to the COP, given underfloor heating as the output and a warm day as input the COP can be very high, 10 or maybe more. BUT if the cold end temperature is -10C and the output to radiators at 60-70C (less than normal CH BTW), the COP will be less than 2 with practical working fluids. 

That's why nobody with any sense would run a heat pump that hot.  Something like 40C is more sensible.  If it's for space heating, run it at a lower temperature for a longer time.  If it's for hot water, give the hot water tank a boost to 60C with an immersion heater perhaps once a week to kill off any bugs that live at 40C.

But do you realise how big a conventional radiator needs to be to be useful at 40C or less? It is monsterous! In other words it is completely unsuitable unless it is a new build and can have underfloor heating throughout. Now look and see what the Government says, because they haven't even considered any of these things. The downside of underfloor is that it has a high thermal capacity and takes a long time to heat up. Thus it needs to operate for long periods, even 24/7 to work. All this means that the only useful heat pump system is air to air, meaning real aircon.is the only proper option if it must be reasonably silent.

I stayed in a hotel recently with an air to air system in an older building. The room was cold for more than an hour until the miniscule heat input managed to heat the room, probably 1kW, and that was it on a not very cold day and the fan was quite noisy.

We have a local swimming pool and can heat pump a stream that runs 5 metres behind the plantroom.

Now if the stream is 10 degrees cooler than the 27 degree pool will it be economical to install a heat pump or better to run on gas??

The heat pump is probably a bit cheaper but the capital cost is more. With a 17 degree differential the COP could be 7 or 8 and electricity is about 6 times as expensive, so a long payback period. Good luck. Ask the manufacturer for an accurate figure and actual electricity consumption. Assume a gas boiler is 90% efficient,(although probably a bit more) and compare.

You're thinking about it like gas central heating.  You don't say "it's a bit chilly" and turn the heating on.  Because you turned the heating on in the Autumn, and it's staying on until Spring.  It may run 24 hours a day if needed.  The controller decides when it needs to run to keep the house warm.

I haven't got rose tinted spectacles on this one.  My existing heating system is totally unsuited to conversion to a heat pump, and would need to be entirely ripped out and replaced.  The radiators are old, and not the modern high efficiency ones.  And microbore plumbing is no use at all for high flow low temperature operation.  It's only barely adequate as a gas fired heating system.  So all the figures of how much it would cost for me to switch to a heat pump are likely to be far too optimistic.

The problems are solvable.  But it may need a lot of money thrown at it.

Simon Barker said:

The problems are solvable.  But it may need a lot of money thrown at it.

Anything at a price!

Simon Barker said:

The problems are solvable.  But it may need a lot of money thrown at it.

Anything at a price!

To get a meaningful payback period a COP of 3 or 4 is needed.  To get high COP needs the heat source to be warmer than the output temperature like geothermal which is ideal.

Logic tells me that in my case that a compressor that pumps warm refrigerant into a cold stream where it expands may come back to the plantroom cooler than it went out because of the losses on the insulated pipes [which are not perfect] and the mechanical heat losses that make the compressor housing warm are excessive 

These losses combine to make the COP less than 1 which means it is much cheaper to use an immersion heater and obviously better still to use gas heating despite the negative publicity it is receiving at present.

CliveS said:

To get high COP needs the heat source to be warmer than the output temperature like geothermal which is ideal.

That does not sound right to me. The heat source needs to be hotter than the evaporator. Don't forget that a standard domestic freezer uses a heat pump. The advantage of even a cool stream is that it contains a lot more heat than the atmosphere.

If the heat source is warmer than the output, then you don't need a heat pump at all.  If my house sat on top of a thermal spring, I could just pump the hot water round the radiators.  That's a few watts to run a small pump.

A well set-up heat pump system should achieve a COP of 3 to 4 under ideal operating conditions.  That's the magic of heat pumps.  By "ideal", I mean warm (but not hot) radiators or underfloor heating, and moderately chilly weather outside.

It only really goes wrong when the outside temperature goes below freezing all day, and the outside unit ices up.  Then you need to apply heat to defrost it before the heat pump can start again.  Even then, they should still deliver a COP better than 1.

But a COP>1 isn't good enough if you are using an energy source, electricity, that is at least 3 times more expensive than readily available oil or gas.

On another point, I drove through a town today that was severely disrupted by the installation of mains gas pipes . . .

Grumpy said:

But a COP>1 isn't good enough if you are using an energy source, electricity, that is at least 3 times more expensive than readily available oil or gas.

True.  Until someone works out how to get the price of electricity back down to something sensible, nobody (and that includes me) have ant real incentive to scrap our old gas boilers.

I really want to be greener, but not if it comes at a massive cost.

This givernment is great on targets, and mostly clueless on how to achieve them.

Simon Barker said:

Grumpy said:

But a COP>1 isn't good enough if you are using an energy source, electricity, that is at least 3 times more expensive than readily available oil or gas.

True.  Until someone works out how to get the price of electricity back down to something sensible, nobody (and that includes me) have ant real incentive to scrap our old gas boilers.

Leccy is about 3 times the price of gas (or coal) because the conversion process is only about 30% efficient. So nothing gained unless you can get the COP above 3.

and even if it wasn't, because in the UK we are generating electricity by burning gas in a process that is a lot less efficiant than your boiler, it is the wrong thing to do to switch off the boiler, as we burn more gas using electric heating than we do burning the gas where the heat is actually needed.

Once more than 2/3 of our electricity is generated by another means we can come back and re-consider what is best for the planet.

Mike.

Yes, I agree that electricity is the most expensive option for heating water even with an ASHP; unless COP > 2.

Electric hot water in a house is certainly not the greenest option either.  Wood burners, oil fired boilers and even gas are more efficient and cheaper options and depending on power station chimney scrubbers will probably emit less CO2 overall anyway.