Heat pumps - how to make the numbers work?

ed110220

How about spilt air conditioners (air to air heat pump)? The best have published SCOPs of 5 or more, but I'm not sure how achievable that is. With gas at 3.416p/kWh and electricity at 12.054 I'd only need a SCOP of 3.2 to break even.

michaels

ed110220 said:

How about spilt air conditioners (air to air heat pump)? The best have published SCOPs of 5 or more, but I'm not sure how achievable that is. With gas at 3.416p/kWh and electricity at 12.054 I'd only need a SCOP of 3.2 to break even.

Hmm - having expensive gas helps the calcs….with air-2-air do you qualify for an RHI and/or green home grant?

(I also have very bad memories of my GFs house when I was younger which had a warm air central heating system which basically felt like you were sitting in a draught whenever the heating was on)

JKenH

michaels said:

ed110220 said:

How about spilt air conditioners (air to air heat pump)? The best have published SCOPs of 5 or more, but I'm not sure how achievable that is. With gas at 3.416p/kWh and electricity at 12.054 I'd only need a SCOP of 3.2 to break even.

Hmm - having expensive gas helps the calcs….with air-2-air do you qualify for an RHI and/or green home grant?

(I also have very bad memories of my GFs house when I was younger which had a warm air central heating system which basically felt like you were sitting in a draught whenever the heating was on)

When you first start one of the split air conditioners up to warm up a cold room you will feel the air being blown out but once you are up to temperature you don’t really notice the modern ones working. They are useful in spring and autumn to heat up just one room very quickly, for instance the kitchen while you have your breakfast, so you don't waste energy heating the whole house. A lot of the modern ones are programmable and some have built in WiFi so you can turn them on remotely. They do seem quite efficient though left running 24/7. (See the various threads on ASHPs - one linked below)

https://forums.moneysavingexpert.com/discussion/4715287/discussion-ashp-air-air-with-solar-pv/p36

Reed_Richards

If you get a heat pump from an MCS accredited installer, which I understand is necessary to qualify for the RHI payments, then the first thing they do is a detailed room-by-room survey.  Surely that, combined with the increase in your electricity bill, should be sufficient to estimate some sort of heat pump performance figure?  

Cardew

ed110220 said:

How about spilt air conditioners (air to air heat pump)? The best have published SCOPs of 5 or more, but I'm not sure how achievable that is. With gas at 3.416p/kWh and electricity at 12.054 I'd only need a SCOP of 3.2 to break even.

Even with gas at a very high 3.416p/kWh(I pay 2.369 incl VAT with BG) it would need a SCOP of 3.53 to match electricity at 12.054p/kWh - or have you taken into account the efficiency of the  gas boiler?

You then have the problem of heating hot water, and no RHI for air to air.

If you dispense with gas completely you do save the daily standing charge but electricity will be needed for hat water.

Martyn1981

ed110220 said:

How about spilt air conditioners (air to air heat pump)? The best have published SCOPs of 5 or more, but I'm not sure how achievable that is. With gas at 3.416p/kWh and electricity at 12.054 I'd only need a SCOP of 3.2 to break even.

The biggest benefit of these, I think, for PV'ers, is that they will be used when the air temp is warmer (shoulder months and daytime v's mid winter) giving a much higher COP, and of course much cheaper fuel source.
My latest thought is to get another to use off cheap rate night leccy as a boost (during the coldest months) before the GCH comes on in the morning.
But of course air to air doesn't get any RHI, but cross benefits such as A/C do add value.

ed110220

Cardew said:

ed110220 said:

How about spilt air conditioners (air to air heat pump)? The best have published SCOPs of 5 or more, but I'm not sure how achievable that is. With gas at 3.416p/kWh and electricity at 12.054 I'd only need a SCOP of 3.2 to break even.

Even with gas at a very high 3.416p/kWh(I pay 2.369 incl VAT with BG) it would need a SCOP of 3.53 to match electricity at 12.054p/kWh - or have you taken into account the efficiency of the  gas boiler?

You then have the problem of heating hot water, and no RHI for air to air.

If you dispense with gas completely you do save the daily standing charge but electricity will be needed for hat water.

I wasn't thinking of changing my heating system in the immediate future, just contributing to the discussion  on heat pumps. As we use a high electricity to gas ratio, that tariff was the cheapest overall when switched last year (barring a few providers with horrible reviews), so I wouldn't use those gas prices if you're using a lot of gas. I hadn't accounted for the efficiency of the gas boiler, which would tend to lower the SCOP needed to break even.

Pile_o_stone

ed110220 said:

How about spilt air conditioners (air to air heat pump)? The best have published SCOPs of 5 or more, but I'm not sure how achievable that is. With gas at 3.416p/kWh and electricity at 12.054 I'd only need a SCOP of 3.2 to break even.

I'm currently retro-fitting ducting for my MHRV system and it's a bit of a nightmare trying to accommodate relatively small ducts in ceiling/floor joists. I'd hate to think of how to install ducts large enough to carry significant heat without blowing a gale.

Cardew

ed110220 said:

Cardew said:

ed110220 said:

How about spilt air conditioners (air to air heat pump)? The best have published SCOPs of 5 or more, but I'm not sure how achievable that is. With gas at 3.416p/kWh and electricity at 12.054 I'd only need a SCOP of 3.2 to break even.

Even with gas at a very high 3.416p/kWh(I pay 2.369 incl VAT with BG) it would need a SCOP of 3.53 to match electricity at 12.054p/kWh - or have you taken into account the efficiency of the  gas boiler?

You then have the problem of heating hot water, and no RHI for air to air.

If you dispense with gas completely you do save the daily standing charge but electricity will be needed for hat water.

  I hadn't accounted for the efficiency of the gas boiler, which would tend to lower the SCOP needed to break even.

Then, being pedantic, isn't your SCOP of 3.2 incorrect? i.e. 3.416p x 3.2 = 10.93p

michaels

So still trying to make the numbers work, I am trying to estimate how much of my 32kwh gas consumption is for hot water and how much for space heating.  In July when there was probably zero space heating we used about 1100kwh for hot water.  Should I just multiply this by 12 or will we use more in winter because the inflow temperature is lower, and if so about how much more?