Heat Pump Questions

GunJack

Reed_Richards said:

Replacing a gas boiler with another gas boiler is much cheaper and simpler than replacing a gas boiler with a heat pump. And at current prices for gas and electricity the running cost of a gas boiler are less than a heat pump.  But heat pumps are very green because on average you will get about 3 units of output heat for every one unit of input power.  That, I think, is greener than an electric car and lots of people have been persuaded to pay a hefty premium for one of those.    

however, cars are easy - you don't need to do major reworking of your house to have one....

matelodave

I'd argue that a heatpump is no more greener than a gas boiler until ALL our leccy comes from renewable sources.

Around 50% or more is still produced by gas and by the time it's fought it's way through the national grid the efficiency must be well below that of burning gas at around 90-95% in a condensing boiler.

IMO if it make you feel all warm and fuzzy to go green then thats OK but dont' imagine that its going to save you money.

I installed a heatpump to replace manky old storage heaters and because I didn't want a big oil or gas tank in the garden as we dont have mains gas., not because I thought it was green. However, that said, we've been pretty lucky until SYMBIO went bust as we've always managed to get quite low priced leccy.

It's now jumped from 12.25 to 21.25p/kwh which will add around £700 to our annual energy costs. Mind you, the heatpump probably only accounts for about 4000kwh or £360 of the increase, the rest is general use domestic consumption. 

Reed_Richards

matelodave said:

I'd argue that a heatpump is no more greener than a gas boiler until ALL our leccy comes from renewable sources....

No, that's crazy.  A heat pump is greener because it gives you (on a seasonal average) about 3 kW of heat per 1 kW of electricity.  Even if that electricity comes from a gas-fired power station you can't think that it's got down to less than 33% of it's original energy value by the time it reaches your house.  That would mean that 67% of the generated electricity is lost in the wires of the National Grid.  If that were true then our strategy to combat global warming is all wrong!!!  

BUFF

Telegraph_Sam said:

That in essence is the advice I had been seeking when I first posed my question.

Different scenario: Large traditional countryside residential building in central Scotland built in the 1700's when heat conservation and insulation had never been dreamed of.  Limited improvements in the interim but very difficult to get anywhere near the standards of modern construction. Not serviced by the gas network, dependent on oil for central heating. Outside winter temperatures in Perthshire can be very low. In this oil versus heat pump scenario I have conjectured that it would not be worth considering / investigating the latter even though expenditure on oil is high.  Am I right? [a. subsidized, b. not subsidized]

Have you done the sums for a biomass boiler (although how green they are is debateable)?

Obviously if you were going HP a GSHP would be preferable to an ASHP if you have the land for it but you would probably want to be running with a flow temp of ~40-45C which would most likely require significant changes to your radiators (or you run the heating for much longer than currently). Plus you would need something to boost temp. for DHW. 

p.s. If you have not already found it I recommend reading through Historic Environment Scotland's/The Engine Shed's info on upgrading older/traditional buildings.

doodling

Hi,

Reed_Richards said:

matelodave said:

I'd argue that a heatpump is no more greener than a gas boiler until ALL our leccy comes from renewable sources....

No, that's crazy.  A heat pump is greener because it gives you (on a seasonal average) about 3 kW of heat per 1 kW of electricity.  Even if that electricity comes from a gas-fired power station you can't think that it's got down to less than 33% of it's original energy value by the time it reaches your house.  That would mean that 67% of the generated electricity is lost in the wires of the National Grid.  If that were true then our strategy to combat global warming is all wrong!!!  

A gas fired (combined cycle) power station is around 60% efficient at best and you can assume a 3% loss on the grid so overall 58% at best.  1kWh (thermal) of gas therefore gives 1.74kWh at the output of a heatpump with a COP of 3. Or 0.95kWh at the output of a gas boiler.

This assumes that the gas power station is operating at maximum efficiency.  Unfortunately gas is being used to balance wind rather than as baseload generation so it won't be running efficiently and the heatpump will be even closer to the gas boiler.

Also, in winter the efficiency of heat pumps declines so there is a risk that in cold weather the gas boiler will actually win on a pure gas consumption basis.

Of course, the above calculation is done on a marginal basis on the assumption that additional heat pumps will require additional gas power generation. That is also only true part of the time so score one for the heat pumps...

matelodave

according to this, nearly 60% of our leccy came from fossil fuels over the past 24 hours - Energy Dashboard - real time and historical UK energy figures, analysis and mapping

Telegraph_Sam

BUFF said:

Have you done the sums for a biomass boiler (although how green they are is debateable)?

Obviously if you were going HP a GSHP would be preferable to an ASHP if you have the land for it but you would probably want to be running with a flow temp of ~40-45C which would most likely require significant changes to your radiators (or you run the heating for much longer than currently). Plus you would need something to boost temp. for DHW. 

p.s. If you have not already found it I recommend reading through Historic Environment Scotland's/The Engine Shed's info on upgrading older/traditional buildings.

We talked about biomass boilers but did no serious investigations
The upshot of all that I have read in this interesting thread is that the various heat pump calculations are not simple and the results of switching are not clear cut. The benefits of switching are environmental rather than economic, and either way significant investment would be required. The performance of HP's in less than ideal (cool) conditions is questionable. All things considered makes me think that the downsides outweigh the advantages and HP's are best left alone a least until the environment (economic / physical) changes significantly.
I must see if I can track down the HES info that you refer to, tks.

Reed_Richards

I'm not disputing that nearly 60% of our electricity comes from fossil fuels.  I'm not disputing that it is a lot more efficient to use the energy from gas to heat our homes than to generate electricity and use that.  I'm not disputing that the efficiency of a heat pump declines in the middle of winter, although fortunately electricity generated from wind power tends to increase then.  But the main choice is to heat your house at (say) 85% efficiency using entirely fossil fuels (gas/oil) or to heat it at an average 300% efficiency using electricity.  Given that 40% of our energy does not come from fossil fuels and that we can convert fossil fuels to electricity at 40% to 50% efficiency then running a heat pump will use less fossil fuels than any other alternative (apart from wood), will add less carbon dioxide to the atmosphere and contribute less to global warming.

Telegraph_Sam

That seems to agree with my conclusion that the arguments in favour of HP's are environmental rather than economic, and that there would be quite a price to pay for switching which would unlikely be amortised in a reasonable time scale (if ever).  300% efficiency sounds great but unfortunately does not feed through to lower bills in the present economic environment.  In practical terms the investment required would I feel be better spent on better insulation using the present oil fired heating, less than ideal that it is.

Uxb1

Telegraph_Sam said:

We talked about biomass boilers but did no serious investigations

I know someone who has one of those wood pellet boilers
The whole installation is seriously large. Its in a standard wooden shed adjacent to the house and the shed is maybe 3m wide by 5 meter long
I've no idea of any details costs etc etc about it sorry - it just struck me as OK if you live in multiple acres (they do) of land and of course any excess heat is not being retained within the house but is now in an uninsulated shed in the garden.