LPG Combi to Air Source Heat Pump- Calculations Check

philtrick123

Hi all,

Our LPG Combi is getting towards the end of its life. My lpg contract with Flogas comes to an end in October. I'm trying to work out if its worth changing to an Air Source Heat Pump. My gas safe friend says the Combi replacement would be ~£3k fitted for a good combi.
I'm guessing with the government grant an ASHP would be £5k including upgrading insulation and swapping some rads (note we have wet ufh for the whole downstairs, only two rads upstairs)

My calculations (without changing the house insulation) seem to show I'd be paying £2k extra per year on electric. :-O
1 litre lpg = 7kwh. LPG boiler 80% efficient = 14822kwh. I've used a rough figure for ASHP of 220% efficient.

Am I doing something wrong? Onedrive link to the same sheet included.
Note- I've used my peak rate for the Electric. I do get 4 hours discount rate for car charging, 16p/wkh.

Regards Phil 



https://1drv.ms/x/s!AkxeRAYfHqFliJZCQImyZi0mHEL25w?e=OdLhPk

Reed_Richards

The efficiency of a heat pump depends on the output water temperature you ask of it.  If you run your UFH with a water temperature of 40 C and can get radiators that would work at this same temperature then you should be able to achieve a heat pump average efficiency of well over 300%.  Even at a higher water temperature your 220% figure seems pretty pessimistic.

Remember with the ASHP you would also need a new hot water cylinder.   

HHarry

For the LPG if you need 18528 kwh for water heating at a boiler efficemcy of 80% then shouldn’t the kwh total figure be bigger?  I.e. you put in 23,160 kwh and get out 80% of that whoch is your 18528.

So it’s 23,160 times the unit cost (lpg) compared to 6737 times the unit cost (heat pump)

tacpot12

The latest ASHPs should be about 350% efficient on average. Have a look at this video for some real world data: https://youtu.be/gMMyhBku6Ic

philtrick123

HHarry said:

For the LPG if you need 18528 kwh for water heating at a boiler efficemcy of 80% then shouldn’t the kwh total figure be bigger?  I.e. you put in 23,160 kwh and get out 80% of that whoch is your 18528.

So it’s 23,160 times the unit cost (lpg) compared to 6737 times the unit cost (heat pump)

The way I've worked it out 2617 litres of LPG. Roughly 7kwh per litre= 18528 kwh. 80% lpg combi efficiency, so 14822kwh of real heat output (probably the wrong units- sorry)
I've then used that figure in reverse to work out how many kwh I need for ASHP.

philtrick123

OK I've duplicated the figures and tinkered with them. Use the onedrive link above to view the calculations.
I've increased the efficiency of the ASHP. I've guessed at an increase in price for LPG to 47ppl. I've also averaged my electric price and factored in the 4 hours of cheaper electric overnight.
£393 difference per year. Sound right?

HHarry

philtrick123 said:

HHarry said:

For the LPG if you need 18528 kwh for water heating at a boiler efficemcy of 80% then shouldn’t the kwh total figure be bigger?  I.e. you put in 23,160 kwh and get out 80% of that whoch is your 18528.

So it’s 23,160 times the unit cost (lpg) compared to 6737 times the unit cost (heat pump)

The way I've worked it out 2617 litres of LPG. Roughly 7kwh per litre= 18528 kwh. 80% lpg combi efficiency, so 14822kwh of real heat output (probably the wrong units- sorry)
I've then used that figure in reverse to work out how many kwh I need for ASHP.

No, you’re all good - that makes sense, and your numbers add up.  That doesn’t seem to make ASHP worthwhile.

Reed_Richards

Mains gas currently costs about 10.7 p per kWh.  If, as per your figures, a litre of LPG cost 41p and gives you 7 kWh (at 100% efficiency) then that's less than 6p per kWh.  So everyone currently on mains gas should be fitting a conversion kit for their boiler and switching to LPG, shouldn't they?  

matelodave

I reckon that you should err on the safe side and assume a SCOP of nearer to 3 as it will take a fair bit of effort and tweaking to get it up to 3.5.

I'm not saying that you can't but not many people do get it much above three and I guess that those who do are quite dedicated. Doing your calcs at three would give you a more realistic estimate of your costs and if you achieve better then that will be a bonus.

We've had a heatpump for nearly 13 years and although I cant measure its efficiency as we don't have heat meters etc, based on our EPC and my own calcs I reckon we are just about getting three. However that said, a newer heatpump could well be more efficient.

philtrick123

I'm going to get our Worcester Combi repaired by Worcester- Hopefully it will last for another year- it will be 15 years old next year. Replacing the combi will be the trigger to review LPG vs ASHP.
Ideally I'd like a hybrid ASHP/ Combi system, but I understand the government grant wont cover that.

Which then makes me think- can I install an ASHP system (with grant), and then addon an instant hot water LPG system afterwards. Sound complicated. 

KieranSc

I recently went from LPG to ASHP but as part of a wider house refurb. When I did the calcs there was not a great deal of difference in the running costs and I also included oil.  
If your current electric tariff is 47p pkwh then this is not that ASHP friendly as really you need to keep them running 24/7.


It does seem at the minute that LPG running costs are some of cheapest. 

If you do install a boiler later I think it needs to go between the ASHP and hot tank so not sure you will get benefit of instant heating.