Air source heat pump - Gas backup?

waqasahmed

Hey

I was wondering if it'd be possible to have an air source heat pump and on the days it doesn't work (well, it does but at a COP of 1 instead of of 5), if it could perhaps use gas as a heating source?

I'm looking at hopefully getting lots of insulation, solar panels and then an air source heat pump given ground source is way too costly. I believe it would mean that the heating would change to electric, which is fine when the COP is 5, but not as great when the COP drops below 5, given that gas typically costs 4-5 times less than electricity

I'd still want gas for cooking / hot water too, unless the energy from the solar panels is enough to provide hot water heating. Gas cooking however is more a preference more than anything else (even if the solar panels produce enough energy for that) 

QrizB

Yes it's possible. You will have all the fixed costs of both systems to bear.

Do note that the COP for an ASHP is rarely 5 (see eg. this table from Mitsubishi) so if that's your cut-off point you're going to be using gas most of the time.

waqasahmed

QrizB said:

Yes it's possible. You will have all the fixed costs of both systems to bear.

Do note that the COP for an ASHP is rarely 5 (see eg. this table from Mitsubishi) so if that's your cut-off point you're going to be using gas most of the time.

Ah thanks. Would that be somewhat offset by having solar panels? I mean the heating requirement of this house I'm buying are around 10,000KwH a year. Assuming the COP is 3 (on a 3Kw system), I guess that covers my heating needs for spade heating at the very least? 

QrizB

There are a few MSE'ers with solar and heat pumps; @Reed_Richards is one that comes to mind. They tend to find that they mostly need heat in the winter months, when the sun rarely shines.

I've got solar and gas. This past winter I generated less than 100kWh of electricity in each of November, December and January, but needed to burn more than 1000kWh of gas in each of those months for heating. You're likely to fit a more powerful solar array than mine (panel efficiencies have risen a lot in the 10 years since mine was installed) but even so there's not much hope of running your heating on solar-generated electricity.

Reed_Richards

Several contributors to the Green and Ethical section of the forum have installed air-to-air heat pumps which they tend to use for heating in the spring and autumn, and potentially for cooling in summer.  These might fit better with gas central heating than an air-to-water heat pump. 

waqasahmed

QrizB said:

There are a few MSE'ers with solar and heat pumps; @Reed_Richards is one that comes to mind. They tend to find that they mostly need heat in the winter months, when the sun rarely shines.

I've got solar and gas. This past winter I generated less than 100kWh of electricity in each of November, December and January, but needed to burn more than 1000kWh of gas in each of those months for heating. You're likely to fit a more powerful solar array than mine (panel efficiencies have risen a lot in the 10 years since mine was installed) but even so there's not much hope of running your heating on solar-generated electricity.

I'm guessing that's the case even if you've got battery power for said solar panels? 

waqasahmed

Reed_Richards said:

Several contributors to the Green and Ethical section of the forum have installed air-to-air heat pumps which they tend to use for heating in the spring and autumn, and potentially for cooling in summer.  These might fit better with gas central heating than an air-to-water heat pump. 

I was wondering if they could perhaps be used for cooling too tbf. 

I guess the main thing is to get decent insulation in first, to hopefully reduce that 10,000KwH figure further for space heating. That's likely going to have the biggest impact first.

It's certainly be nice to be able to be "off grid" so to speak, but if not  at least to reduce use of it 

QrizB

waqasahmed said:

QrizB said:

There are a few MSE'ers with solar and heat pumps; @Reed_Richards is one that comes to mind. They tend to find that they mostly need heat in the winter months, when the sun rarely shines.

I've got solar and gas. This past winter I generated less than 100kWh of electricity in each of November, December and January, but needed to burn more than 1000kWh of gas in each of those months for heating. You're likely to fit a more powerful solar array than mine (panel efficiencies have risen a lot in the 10 years since mine was installed) but even so there's not much hope of running your heating on solar-generated electricity.

I'm guessing that's the case even if you've got battery power for said solar panels? 

If you can't generate it, you can't store it.

Looking at last December, I needed (very roughly) 1000kWh of heat plus 250kWh of power. If I'd had an ASHP with a COP of 3 that would have been roughly 600kWh of electricity total.

My 2.72kWp solar array generated about 80kWh that month so it would have had to have been 7.5x larger - 20kWp - to meet the demand. That's roughly a £20-25k solar PV system, covering 100 square metres - a moderate-sized suburban garden's worth. A mini solar farm, if you prefer.

Reed_Richards

waqasahmed said:

QrizB said:

There are a few MSE'ers with solar and heat pumps; @Reed_Richards is one that comes to mind. They tend to find that they mostly need heat in the winter months, when the sun rarely shines.

I've got solar and gas. This past winter I generated less than 100kWh of electricity in each of November, December and January, but needed to burn more than 1000kWh of gas in each of those months for heating. You're likely to fit a more powerful solar array than mine (panel efficiencies have risen a lot in the 10 years since mine was installed) but even so there's not much hope of running your heating on solar-generated electricity.

I'm guessing that's the case even if you've got battery power for said solar panels? 

I have a battery and absolutely it is still the case. 

Just think about why winter is cold.  It's because we get a lot less solar energy in winter.  Even though the earth can absorb the solar radiation during the day and re-radiate it at night (like a heat battery), winter is still cold!  

lohr500

I'm not sure if the full details of the Clean Heat Grant have been released yet. It replaces the existing RHI grant which ends at the end of March. But I did read somewhere that hybrid heat pump systems would be excluded.

If you were factoring any grant into your decision making process on having a gas boiler as back up to ASHP, then probably worth investigating further.

ProDave

I have a small ASHP in a self built low energy house, very pleased with it, it works well.

But even as a fan of heat pumps, if you presently have gas, then DO NOT switch to a heat pump.  It WILL cost more to run

Why do you want to change?