Anyone fancy poking holes in my maths?...

I currently have an oil combi-based central heating system and a small wood burner that can pump out a fair heat (practically heats the house with the doors open). As you may have spotted in my other thread I am considering installing an ASHP to reduce my reliance on both - mainly to reduce my 'carbon footprint' and I suppose hopefully reduce long term costs.
I thought I'd work out some rough calculations for each. I estimate my heating need as 14,000kWh/year.
(CO2e estimates from https://www.carbonfootprint.com/calculator.aspx)

Heating Oil
14,000kWh = 1,353L I think I then need to assume 90% efficiency, so about 1,500L
1,500L = 3.8Te CO2e/year
Assuming £0.60/L long term ave = £900/year

Wood Burner
14,000kWh = 3.33Te dry stacked hardwood = roughly 7.9m3
3.33Te = 0.24Te CO2e/year
8m3 dried hardwood logs locally = £950/year

A/A ASHP
14,000kWh output at an average COP of 2.5 (??) = 5,600kWh electricity input
5,600kWh = 1.29Te CO2e/year
Assuming £0.22/kWh long term ave = £1,230/year

So regards CO2e there are massive differences and wood burner is top of the pops.
Here I should note that I have no close neighbours, the nearest village is 3 miles away and the nearest town is 7 miles away so my assumption is that small particle air pollution is not really a concern for my woodburner (correct me if I'm wrong!)

And regards cost...not much in it, but possibly not looking worth the cost of installing an ASHP - if I can be bothered to feed the wood burner like a demon possessed.

What am I missing??

Will, by ASHP do you mean air to water heat pump?

And yes I think field trial results for A/A heat pumps suggest a COP of 3.5 would be fairer (and in fact a bit conservative. Iirc 4 for space heating is commonly achievable).

Would be interesting to do the figures for storage heaters with a cheap overnight tariff. While changing tariff (and leccy usage patterns, which is at least a good thing to do anyway) is necessary, adding a few is -relatively- easy (and very cheap if you don't mind old ones bought on Ebay, perhaps with a radiator cover to improve looks).

In general I like the idea of adding a storage heater or two to the existing heating system (if gas or oil), to lop off a chunk of emissions without the cost and faff of complete system replacement. Stronger argument in Scotland of course where our overnight electricity is particularly green.

Similar argument for adding a single A/A strategically placed (or single multi split) to augment current system.

Posted By: WillInAberdeenA-A CoP might well be much better than that.

I've found it very difficult to pin down estimated COP at different temperatures.

Posted By: WillInAberdeenWood burner efficiency is less in real life because of the cold draft, unless it is room sealed.

Would there not be a similar 'cold draft' effect from A/A? (depending where you place the unit of course)

Posted By: WillInAberdeenA lot depends on the long term average energy prices we guess at.
In the medium term I'm expecting much lower electricity prices and carbon intensity than you quoted, because costs of offshore wind just keep going down.

Yes, there are always going to be variations in price assumptions. My leccy was 15p/kWh daytime (and 5p overnight EV rate!). It is going up to 31p/kWh daytime this month. I suspect cheaper offshore wind will be offset by cost of further nuclear...20-22p sounds reasonable to me long-term.

Posted By: WillInAberdeenYou already have the oil and wood burners, so add the ASHP too, and then you can run whichever one gives the best greeness per £ each year.

From what I've read, the COP is affected mostly by outside temperature and by flow temperature. So I'm wondering if a slightly oversized unit, running slightly cooler and used more when temps are above freezing may be a good compromise. When it's freezing I'll tend to have the wood burner cranked up anyway!

Based on an improved COP of say, 4.0, and £0.20/kWh...

A/A ASHP
14,000kWh output at an average COP of 4 = 3,500kWh electricity input
3,500kWh = 0.81Te CO2e/year (Still 3X wood burner CO2e)
Assuming £0.20/kWh long term ave = £700/year (quids in...once the installation is paid back)

Posted By: neelpeela slightly oversized unit, running slightly cooler

Why would oversizing make it run cooler? The required flow temp is set by the radiator system, regardless of heat pump (or any heat source) capacity.

Fabric insulation looks cheap and is a one off cost.

Had a good dig and I'm still struggling to find any 'Outdoor Temperature vs COP' graphs for current A/A units.
Surely this is a critical piece of info that every OEM should have to supply?

Interesting Canadian article I've found on ASHPs...
https://www.nrcan.gc.ca/sites/nrcan/files/canmetenergy/pdf/ASHP%20Sizing%20and%20Selection%20Guide%20(EN).pdf
Lots of info to help with sizing...if you know the COP...

Posted By: neelpeelCheap indeed on a new build/extension.

Materials have risen by 20% in 6 months a report today says.

Posted By: fostertom

Posted By: borpin

The required flow temp is set by the radiator system

No, you can define it and adjust it so the HP is not over stressed initially. Lots of good threads on this on the OEM forum

Could you re-state that? Incl which 'it' do you mean, and what's the OEM forum?

OEM - Open Energy Monitor forum - lots of interesting discussions about maximising efficiency. https://community.openenergymonitor.org/c/hardware/heatpump/47

Rather than setting the flow temp by the radiator system (i.e. need house heat - give me lots) the perceived wisdom seems to be, if you need heat, to set the flow temp based on the return temp and gradually step up the demanded temperature.

The knock on of that is that, like UFH, you need to forward look as to when you want the house at a certain temperature.

Usually, the heat pump is not arbitrarily large (unlike a combi gas boiler which is typically much bigger than is needed for heating).

-The heat pump can often produce more heat at a lower flow temperature
-The radiators/emitters can emit more heat at a higher flow temperature
-There is some happy medium flow temperature which gives the best overall heat output, varying also with outdoors temperature
-If spare heating capacity is available then it is usually more energy efficient to run at a lower flow temperature.

The scheme Borpin mentioned ('load compensation') worked well for our previous heat pump - it's a structured way to find the best operating point as the load from the house varies. It worked better than 'weather compensation' because the heat load depended on things like sunshine, wind, and whether the house had been left unheated while everyone was out. Both the flow temperature and the CH circ pump speed were constantly tweaked by the ASHP controller to give best results.

The same principle applies to A-A heatpumps (where the 'flow' is of refrigerant rather than CH water) but probably you get no chance to tweak it, the software has complete control, unless someone has different experience?