Heat pumps - how to make the numbers work?

michaels

There is talk of new gas central heating boilers being banned.

I need a new boiler and would like to choose an environmental option but I just can't make the maths work for heat pumps even if they are installed for free under the green homes grant.
https://nottenergy.com/resources/energy-cost-comparison/

Energy Cost Comparison – July 2020

Fuel	Fuel price (p per unit)	Unit	Pence per kWh (after boiler efficiency)	Energy content (kWh per unit)	CO2e emissions per kWh*
Electricity Standard Rate1	20.58	kWh	20.58 (100%)	1	0.288
Electricity Online Rate1	20.78	kWh	20.78 (100%)	1	0.288
Mains Gas Standard Rate2	3.99	kWh	4.75 (90%)	1	0.208
Mains Gas Online Rate2	4.28	kWh	4.75 (90%)	1	0.208
Kerosene3	30.07	Litre	3.41 (90%)	9.8	0.298
Gas oil4	47.95	Litre	5.12 (90%)	10.4	0.316
LPG5	42.59	Litre	6.66 (90%)	6.66	0.241
Butane6	167.99	Litre	23.42 (90%)	7.97	0.241
Propane7	74.24	Litre	11.67 (90%)	7.07	0.241
Seasoned Wood8	22.39	Kg	6.27 (85%)	4.20	0.028
Pellets9	26.33	Kg	6.20 (90%)	4.72	0.053
Smokeless fuel10	42.78	Kg	8.58 (75%)	8.51	0.396
Coal10	31.85	Kg	6.18 (75%)	6.20	0.396
GSHP11	20.58	kWh	5.88 (350%)	1	0.082
ASHP11	20.78	kWh	7.70 (270%)	1	0.107

Basically for us electricity is 13.6p per unit and gas even including the standard charge is 2.35p per kwh.  Assume 90% efficiency for the gas and it puts electricity as 5.2 times more per kwh than gas.

Thus the heat pump would need a seasonal COP of at least 5.2 to make running costs the same - but from the above it seems typical actual SCOPs are 2.7 for ASHP or 3.5 for GSHP thus running a heat pump would cost approximately 1.5 to 2 times as much as a gas boiler - for us about an extra £1000 pa.

Can anyone help me with the maths as I do want to choose the green option.

Thanks

Ectophile

They wouldn't need to ban gas boilers if they were the more expensive option.

The gas works out at 2.35/0.9 = 2.61p per kWh of useful heat.  Assuming a COP of 3.0, the ASHP works out at 4.53p per kWh of heat.

You also need to factor in the costs of a new gas boiler, compared with an ASHP with a grant.  But for somebody with mains gas, a replacement gas boiler will most likely end up the cheaper option.

Reed_Richards

No mains gas where I live.  My last purchase of Kerosene cost me 29.66p per litre but over the last two years the average price has been 45.19p per litre so your oil numbers look over-optimistic.

michaels

Reed_Richards said:

No mains gas where I live.  My last purchase of Kerosene cost me 29.66p per litre but over the last two years the average price has been 45.19p per litre so your oil numbers look over-optimistic.

They say they are July 2020 and I ran a kerosene quote and got similar.  The Gas and Electricity however are crazy.  Assuming you can give up gas means you can include the standing charge which is how I get a cost of 2.35 per unit (or 2.61p per useful kwh at 90% efficiency).  Electricity for us is 13.6p per kwh, could move to a variable tariff at 11p except that I am on a v2g trial so locked into the supplier.  Using the efficiency numbers above and electricity at 11p per unit gives gshp a cost of 3.14p per useful kwh and ashp 4.1p per useful kwh or 20% and 57% more expensive than gas.

These numbers though are competitive with kerosene at the current low prices and would become very good if/when oil prices go go back to more normal levels.

Martyn1981

I think the table is interesting, but you did say that you wanted to choose the green option, and that changes things.
First there's the CO2 emissions side (as per the table) to counter the cost argument.
Second, what about E7 (or similar), you could use this to aid your heating at a lower unit cost. It might also help balance costs to COP (for ASHP) as the cheaper leccy would be at work when the COP is lower (nighttime temps) and higher priced leccy during the day when the COP will be a bit higher.
Third, further CO2 gains by choosing a RE leccy supplier.
Fourth, further CO2 gains/cost savings by utilising PV generation, especially during the shoulder months, to take some of the workload. PV to cover DHW during the sunnier months, and at high efficiency/COP levels too, so plenty of spare left for the BEV(s).
Five, is it reasonable to assume/expect FF gas prices to rise over time to take account of externality costs?

You are not going mad, heat pumps have a very hard time competing against gas in a purely economic argument, but if you are willing to skew the argument somewhat, as per your last line in the OP, then the fight is more balanced.

ASavvyBuyer

You might find the Heat Pump Webinar Sessions here useful:

https://carbon.coop/carbon-co-op-webinar-programme/

Cardew

In economic terms Heat Pumps have never been an option if you have mains gas, and now with ever lower prices for gas(and oil for that matter) it is a no contest.

An important factor that tends to be overlooked is the requirement to run a heat pump for long periods - even 24/7. For occupants out of the house during the day for work/school you still need to keep a heat pump running to avoid the house cooling down and taking hours to bring back to the required temperature; and thus heating an empty property. At night you have a similar problem.

With gas/oil CH with Central heating water at a high temperature you can set the heating to come on, say, 30 minutes before you get up, or return to the property, and have the required room temperature.

markin

E7 is off the cards for new customers for most companies is it not? Until that changes the is little hope of people feeling its a safe option.

tacpot12

Generating your own power through solar power can also help the economics. Some ASHPs are designed to operate with a thermal store so you heat the water in the storage tank during the day when the COP is higher and use the water overnight to heat the house. But fundamentally the economics don't work unless the house is very well insulated; the insulation and air tightness needs to be approaching Passivhaus standards rather than just complying with the current Building Regs. There are ASHP's that are designed for such houses where the maximum output is about 4kW, which is more that the house needs, except on days where the outside temperature dips below 0 degrees C. You could, if you needed to, heat the whole house on the 3kW immersion heater that the thermal store has as a backup. 

Actually this backup facility is one advantage of ASHP/Thermal Stores have over Gas boilers. If your boiler breaks, you might have DHW via an immersion heater but you can't heat your house with this. 

Achieving the airtightness necessary to reduce heat losses to the point where a heat pump is economically viable is very difficult in an existing house; you can't easily access all the parts where air leaks need to be sealed. It's a very expensive job to do, and when done you then need to invest in a Mechanical Ventilation and Heat Recovery system. So I would assert that unless your house has been designed and carefully built to be heated by a heat pump, and has a MVHR system as well, it is never going to be economically viable to heat it with a heat pump. 

This requirement to carefully build houses is going to be a big challenge for the building industry to adapt to. Many house builders are looking at building houses in a modular fashion in factories, where tolerances can be more tightly controlled, and then assembling these modules on site so that only the few joins between modules need sealing on site.

Martyn1981

Just to say that whilst I totally agree about the need to build better homes, and that heat pumps are more suitable for well insulated properties, there is a bit more to it.
My sister is having a heat pump installed to replace two diesel boilers that the property has/had. The original property is 140yrs old and makes up 1/3 of the current building, and half of the front and one side. Those walls are thick and solid and will need internal insulation as part of the work, but the property is suitable for ASHP.
Firstly, we already know the heating needs based on the diesel burnt, so can calculate costs, taking into account boiler efficiency and consumption. Secondly, the property has oversized double panel (and double fin) radiators that can heat the house from cold in a matter of hours - if the old boilers struggled or took all day to heat the property, then it would be fair to say heat pumps would struggle.
So whilst heat pumps may struggle in some situations, if they can cope, then the economics still add up even in a higher consumption situation, since a bigger ASHP is simply going up against a larger FF boiler/consumption, hopefully reducing the kWh consumption of 'fuel' by 70%+ after taking COP and efficiency into account.

[Note - she is also getting just over 8kWp of SSE PV installed, and hopefully ~4kWp SW added in the future, which will aid with some of the heating and DHW requirements.]

Pile_o_stone

Cardew said:

 For occupants out of the house during the day for work/school you still need to keep a heat pump running to avoid the house cooling down and taking hours to bring back to the required temperature; and thus heating an empty property. At night you have a similar problem.

With gas/oil CH with Central heating water at a high temperature you can set the heating to come on, say, 30 minutes before you get up, or return to the property, and have the required room temperature.

Is that not more of a comparison of underfloor heating vs radiators, rather than heat pumps vs boiler?
We have UFH in the annex that my parents live in and the gas boiler goes on and off at all hours as it maintains the temperature of the limecrete slab the UFH pipes sit in. It's a condensing boiler but with such short cycles, I doubt that there is much condensing going on.