Re: ASHP - Investigations and calculations
Posted: Sun Mar 31, 2024 5:33 pm
Having had my heat pump for a few weeks now I thought I'd give a bit of background and do a calculation of costs so far.
Our house is over 100yrs old built of stone but has had insulation added when rooms have been renovated and underfloor heating was installed when I removed the ground floor wooden suspended floor and replaced it with an insulated concrete slab. There are still quite a few draughty bits that still need to be tackled, the bathroom and utility rooms (both North facing) were always the coldest rooms in the house, quick to cool down!
When I did the heat loss calculations it showed a 9.33kW heat loss at -4.8 degrees and I had targeted a 35C flow temperature for the heat pump.
So a 10kW heat pump looked to fit the bill, a 250l HWC was added along with four larger K3 radiators and a large towel rail for the bathroom. There are still 5 radiators on microbore piping but all the new ones and towel rail are now 15mm.
Being a tight Scotsman buying heating oil was always a pain (though not when it was 10p/L) and would only run the heating for an hour in the morning and same in the evening, unless of course SWMBO said otherwise. Most days the WBS would be lit late afternoon to get some extra heat into the house.
As the heat pump needs to run for much longer than the oil boiler it was initially set to run from 6am to midnight then setback a couple of degrees from midnight till 6am, the initial heating curve was set at 0.6. The house was much much warmer than before so much so that SWMBO complained that it was too hot, WTF I wasn't expecting that! Since then the heating curve has been reduced 0.5, 0.45 and now 0.4 and is on weather compensation only. The heat pump has also been set to run without any night temperature setback, average living room temperature is 25C.
As I was too tight to run the oil boiler all the time to keep the house warm I've done the running cost calculation based on how much oil I would have needed to burn to give the same kWh's of heat.
So from the 9th to the 29th of March the heat pump supplied 2258kWh of heat and used 551.1kWh of electric giving a COP of 4.1.
I looked up the average price of electricity in the UK and it gave a price of 24.5p/kWh so multiplying that by 551.1 gives a cost of £135.02. Cost of heating oil (up here) was 67.56p/L (+5% VAT) during that period so if oil boiler was 80% efficient it would have cost 8.87p/kWh giving at total cost of £200.28 for the same 2258kWh and even if the oil boiler was 100% efficient it would have cost £160.09.
If I had been able to run the heat pump on electric at 7.5p/kWh the cost would only have been £41.33.
Over that 20 days my total electric import was 822.5kWh, if it had all been at 7.5p/kWh that would have cost £61.69 less Ripple rebate £13.65 and export credits £37.72 giving a total of £10.32 or 51p/day.
If I was Richbee I would look at a slightly smaller heat pump and some form of back up heating for the very coldest weather.
Our house is over 100yrs old built of stone but has had insulation added when rooms have been renovated and underfloor heating was installed when I removed the ground floor wooden suspended floor and replaced it with an insulated concrete slab. There are still quite a few draughty bits that still need to be tackled, the bathroom and utility rooms (both North facing) were always the coldest rooms in the house, quick to cool down!
When I did the heat loss calculations it showed a 9.33kW heat loss at -4.8 degrees and I had targeted a 35C flow temperature for the heat pump.
So a 10kW heat pump looked to fit the bill, a 250l HWC was added along with four larger K3 radiators and a large towel rail for the bathroom. There are still 5 radiators on microbore piping but all the new ones and towel rail are now 15mm.
Being a tight Scotsman buying heating oil was always a pain (though not when it was 10p/L) and would only run the heating for an hour in the morning and same in the evening, unless of course SWMBO said otherwise. Most days the WBS would be lit late afternoon to get some extra heat into the house.
As the heat pump needs to run for much longer than the oil boiler it was initially set to run from 6am to midnight then setback a couple of degrees from midnight till 6am, the initial heating curve was set at 0.6. The house was much much warmer than before so much so that SWMBO complained that it was too hot, WTF I wasn't expecting that! Since then the heating curve has been reduced 0.5, 0.45 and now 0.4 and is on weather compensation only. The heat pump has also been set to run without any night temperature setback, average living room temperature is 25C.
As I was too tight to run the oil boiler all the time to keep the house warm I've done the running cost calculation based on how much oil I would have needed to burn to give the same kWh's of heat.
So from the 9th to the 29th of March the heat pump supplied 2258kWh of heat and used 551.1kWh of electric giving a COP of 4.1.
I looked up the average price of electricity in the UK and it gave a price of 24.5p/kWh so multiplying that by 551.1 gives a cost of £135.02. Cost of heating oil (up here) was 67.56p/L (+5% VAT) during that period so if oil boiler was 80% efficient it would have cost 8.87p/kWh giving at total cost of £200.28 for the same 2258kWh and even if the oil boiler was 100% efficient it would have cost £160.09.
If I had been able to run the heat pump on electric at 7.5p/kWh the cost would only have been £41.33.
Over that 20 days my total electric import was 822.5kWh, if it had all been at 7.5p/kWh that would have cost £61.69 less Ripple rebate £13.65 and export credits £37.72 giving a total of £10.32 or 51p/day.
If I was Richbee I would look at a slightly smaller heat pump and some form of back up heating for the very coldest weather.