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Oil vs Gas vs ASHP costs.

 
Air Source Heat Pumps (ASHPs)
Last Post by Bob77 3 years ago
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 Benseb
(@benseb)
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Joined: 4 years ago
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Topic starter 10/12/2022 4:04 pm  

I've seen a few people comment on how ASHP are expensive to run so thought I would run some numbers...

These are using our real-world ASHP consumptions, etc from a 1800s barn conversion with average insulation - so looking at a new build, the efficiency should be better, and therefore more in favour of ASHP. Treat this as a worst-case scenario!

Nov - mild
Consumed 663kWh
Delivered 2498kWh

Dec (so far) - very cold so far...
Consumed 361kWh
Delivered 1076kWh

Cost of Electric: 33.48p /kWh
Cost of Gas: 10.33p /kWh
Cost of Oil: 78p/Litre * 10.35 kWh/L = 8.07p /kWh

*Above using current October rates from Octopus.

Assuming 90% efficiency for condensing gas boiler and modern oil boiler.

Therefore:

Nov
Cost for ASHP: 663 * 0.3348 = £221.97
Cost for Gas: 2498 * (0.1033 / 0.9) = £286.71
Cost for Oil: 2498 * (0.0807 / 0.9) = £223.99

Dec
Cost for ASHP: 361 * 0.3348 = £120.86
Cost for Gas: 1076 * (0.1033 / 0.9) = £123.50
Cost for Oil: 1076 * (0.0807 / 0.9) = £96.48

Remember, these are rough figures, assuming you have a brand new gas/oil boiler (best case) vs my ASHP actual readings (probably worst-middle case as it's a retrofit in an old property).

Also you can see Oil comes out cheaper in December, as ASHP loses efficiency in very cold weather, However this has been based on a week of sub zero temperatures (it's been -5 here), if we looked at efficiency in the other approx 10 months of the year, you'd expect the COP to be higher, and therefore Oil would be more expensive in comparison. So over a 12 month period, it's certainly looking like ASHP comes out on top or not far off gas. Having used Oil in the past, there's also the massive ball ache of having to get deliveries - we had several bad periods a few years back where we couldn't get a delivery as the roads were too icy!

I think many people are looking at their bills and worrying but not realising that most of this is down to the current cost of fuel, and any heat source is going to be super expensive this year!

Feel free to critique my maths, anything ive missed?


250sqm house. 30kWh Sunsynk/Pylontech battery system. 14kWp solar. Ecodan 14kW. BMW iX.


   
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 Benseb
(@benseb)
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Topic starter 10/12/2022 4:07 pm  

Also this is based on a standard electric tariff. We're on Octopus Go, so 5 hours of our usage is at 8p rather than 30p. Our 'real world' average unit rate is therefore 23p/kWH meaning the above prices come down a bit too.


250sqm house. 30kWh Sunsynk/Pylontech battery system. 14kWp solar. Ecodan 14kW. BMW iX.


   
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 HughF
(@hughf)
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Joined: 4 years ago
Posts: 489
10/12/2022 6:35 pm  

I do the same maths weekly when trying to tell people how the maths adds up….

My gas boiler is 72% efficient so my break even cop is something ludicrously low like 2.34…

Tl;dr - I don’t have my ashp installed yet due to building delays.


Off grid on the isle of purbeck
2.4kW solar, 15kWh Seplos Mason, Outback power systems 3kW inverter/charger, solid fuel heating with air/air for shoulder months, 10 acres of heathland/woods.

My wife’s house: 1946 3 bed end of terrace in Somerset, ASHP with rads + UFH, triple glazed, retrofit IWI in troublesome rooms, small rear extension.


   
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 Oswiu
(@oswiu)
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Posts: 121
10/12/2022 10:52 pm  

I don't fault your maths for your use case, but a lot of people complaining don't know how to use their heat pumps properly. You can't go all that wrong with a boiler really, maybe dropping 10% efficiency from too high flow temps, but doing that with a heat pump can literally double your bill, and that's when the questions start. 



   
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Mars
 Mars
(@editor)
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Posts: 4241
11/12/2022 10:08 am  

@benseb, quick question. Are your numbers based on the boilers and ASHP running 24/7?


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 Benseb
(@benseb)
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Posts: 109
Topic starter 11/12/2022 10:17 am  

@editor they are based on the heat outputs shown. So the cost of gas and electric to produce 2498kWh of energy. Which shouldn’t really matter much with gas and oil how long they are on for. 

with ASHP mine is on 24/7 but with setback of 2c overnight. 

I’ve actually just tweaked this as we’re on cheaper electric 9:30-2:30 so I now turn down the heating 6:30-9:30 then let it play catch-up in the cheap period. It’s not normally noticeable especially with a warm kitchen from cooking but it’s a bit more noticeable when -5c outside this week!


250sqm house. 30kWh Sunsynk/Pylontech battery system. 14kWp solar. Ecodan 14kW. BMW iX.


   
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 scrchngwsl
(@scrchngwsl)
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Joined: 3 years ago
Posts: 91
11/12/2022 11:07 am  

Yeah I've worked out the same for me - producing the same amount of heat that our ASHP is producing using our 20-year old gas boiler would be 40-50% more expensive, even in this awful December we're having. A modern condensing gas boiler might be cheaper right now, but not significantly so, and certainly not enough to outweigh the benefits during warmer weeks/months.

One thing this energy crisis has done is made the payback time on all these green investments a complete non-issue. I didn't really care about payback time before, I just did it because this stuff is better technology, but this winter has brought the payback forward by a good few years.


ASHP: Mitsubishi Ecodan 8.5kW
PV: 5.2kWp
Battery: 8.2kWh


   
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 Bob77
(@bob77)
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Joined: 3 years ago
Posts: 36
12/12/2022 11:56 am  

Posted by: @benseb
↑
with ASHP mine is on 24/7 but with setback of 2c overnight. 

I’ve actually just tweaked this as we’re on cheaper electric 9:30-2:30 so I now turn down the heating 6:30-9:30 then let it play catch-up in the cheap period. It’s not normally noticeable especially with a warm kitchen from cooking but it’s a bit more noticeable when -5c outside this week!

I'm trying to work out the best way to run my ASHP with Economy 7. Our cheap tariff runs from 11.30pm to 6.30am. At the moment I have the thermostat set to full temperature from 4am to 7am so it's warm in the morning, and have it a degree lower during the day, then higher in the evening and setback by a couple of degrees from 10pm. The DHW is also set to heat up at 2am. I wonder though whether it would be better to have the heating on full temperature throughout the cheap period? It's underfloor heating in screed so I fell like it might be better to store more heat in the slab when the electricity is cheaper.

 



   
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 mjr
(@mjr)
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Joined: 4 years ago
Posts: 301
12/12/2022 6:27 pm  

Posted by: @bob77
↑

I'm trying to work out the best way to run my ASHP with Economy 7. Our cheap tariff runs from 11.30pm to 6.30am. At the moment I have the thermostat set to full temperature from 4am to 7am so it's warm in the morning, and have it a degree lower during the day, then higher in the evening and setback by a couple of degrees from 10pm. The DHW is also set to heat up at 2am. I wonder though whether it would be better to have the heating on full temperature throughout the cheap period? It's underfloor heating in screed so I fell like it might be better to store more heat in the slab when the electricity is cheaper.

 

Similar here. My tactic is to have the heating come on early enough to hit target at 7am, which of course varies with how cold the house gets overnight, but is usually some time between 0300 and 0600, with DHW heating at 5am ready for morning washes. You will probably want yours to come on earlier because underfloor heating has more inertia (but doesn't require as high temperatures), but there doesn't seem much point putting it on full the whole time and heating the room while no-one's there because everyone is asleep.

I'm also slightly worried by the mention of "thermostat" instead of weather compensation, but that might just be language.

 



   
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 Bob77
(@bob77)
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Joined: 3 years ago
Posts: 36
12/12/2022 9:53 pm  

Posted by: @mjr
↑

I'm also slightly worried by the mention of "thermostat" instead of weather compensation, but that might just be language.

The room thermostat controls when the UFH calls for heat, and the weather compensation controls the flow temperature that the heat pump delivers. Is that wrong?

The room thermostat is at 19.5C during the day, 20.5C in mornings and evenings, and 18C overnight (until the cheap-rate heat-up period which is set at 20.5). M



   
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