Air source heat pump or alternatives to Gas

Also worth bearing in mind that the 90+% stated for gas boilers efficiency are manufactures figures and will often be a stretch from installed reality.

We installed a couple of A2A heat pump units in our downstairs to make use of the tail end of our cheap electricity tariff and new solar. I’ve been quite impressed by how little electricity they consume compared to gas. It’s difficult to compare directly as we still use gas for upstairs but it peaks at about 10kwh a day and seems to have knocked up to 40kwh off the daily gas bill, although that’s comparing with years when it hasn’t been as cold as recently so may have done more. They still worked fine when it went down to -7; the power consumption went up obviously but there was still plenty of heat pouring out. Interestingly it did defrost cycles less often when it got really cold, presumably because there’s less moisture in the air to condense and freeze…? You can tell when it does a defrost; the fans stop and there’s lots of creaking of plastic presumably from the temperature changes.

Whether we’ll keep them long term or eventually go to a complete A2W system I don’t know. These units do the job fine and seem very efficient but the controls are crap and they seem to get confused when the room is up to temperature and they need to produce no or little heat. They’re useful downstairs as the radiators are all in stupid places on microbore pipes; upstairs is more conventional so A2W might be simpler. Aircon in summer will be a plus though, A2W can’t really do that. Can you have both? No idea…

On average even with current power supplies in the UK an ASHP will produce much less CO2 than a gas boiler.
Average UK electricity is 214g of CO2 equivalent per kWh delivered and gas is 204g per kWh delivered.
Even using an electric fire will produce slightly less CO2 on average than a gas boiler (although maybe not at peak times). The targets for completely removing fossil fuels from the power industry means electricity will become less and less CO2 intensive so is often the most effective was to decarbonise.
Heat pump heating is definitely the future so if you can afford to and it will work in your house then go for it.

Average UK electricity is 214g of CO2 equivalent per kWh delivered and gas is 204g per kWh delivered.

Ah thanks, that’s a really useful comparison. I felt like some of the napkin-calculations earlier up the thread didn’t ring true. Can I ask where you got the value for gas from? I’m interested in looking at how they calculate that.

I think it’s from this government spreadsheet:
https://www.gov.uk/government/publications/greenhouse-gas-reporting-conversion-factors-2022
I’m currently using the phone and am off work today but will try to remember to cross check that when I’m in the office.

I may have remembered out by a few grams but those are the figures that the government supplies. It’s usual to use the CO2e value which includes the equivalent emmisions for delivery (pumping or tankers depending on fuel) and from leaks. Natural gas is a strong greenhouse gas and you get small leaks.
If looking at fuel only figures it varies a bit with gas composition but natural gas is about 190g per kWh.

It will be better than 1:1 otherwise they wouldn’t be able to produce a greater output than input.

True but some have a large drop off in actual energy they can produce in low temperatures (probably worse on the bigger end of the domestic models up to 16kw). Still not hitting 3:1 in mid winter.

Hiya,

My house is a very old stone house with reasonable insulation but obviously not enough with the solid walls. We looked in ASHP’s but the advice seems that with the house as it is, that it probably wouldn’t work.
For us a wood burner back boiler seems to be a better idea. I may revisit the ASHP in the future, when I can figure out a better way to insulate the house, but for the moment the wood burner seems to be the better way to heat this house. The Gas boiler which is a modern condensing one we will keep obviously.

Maybe we will then switch the gas CH to ASHP, with the wood burner in the depths of winter providing a top up.

BR
JeZ

I’m probably in a position where we could probably dig up a big chunk of garden. So a ground source heat pump is an option. Doees that generate water at a higher temperature? Are we less likely to need to replace the pipes?

We’re very well insulated and airtight in a passivhaus-style house. We’ve only got electric underfloor heating and it’s been ruinously expensive to the extent that i’ve turned it right down and we’re all wearing 5 jumpers.

We’ve got no radiators but I had a look at fitting wet ASHP system and got quotes between £40 and 75K.

Instead, we looked at air to air heat pumps – basically air conditioning. They have the advantage of being able to heat or cool and because they don’t have to heat the air as much as a water system, the efficiency is better – I think 4-500% is realistic.

It’s about 2.5K per unit with something like 5kw heating power. If the government hadn’t moderated the cost of electricity I would have got one, but as it was we decided to leave it and see if we could manage the house better (it’s only our second winter).

You can get hot water tanks with built-in heat pumps for hot water needs. Presumably you can still use your excess PV with those systems, but its cheaper to run in winter.

I’m probably in a position where we could probably dig up a big chunk of garden. So a ground source heat pump is an option. Doees that generate water at a higher temperature? Are we less likely to need to replace the pipes?

Nope.
Lower flow temperatures than ASHP and similar temperature differential.
Bigger rads and bigger pipes.
Can be fudged to a degree with a buffer running a higher temperature differential but rads will still be undersized.

Instead, we looked at air to air heat pumps – basically air conditioning. They have the advantage of being able to heat or cool and because they don’t have to heat the air as much as a water system, the efficiency is better – I think 4-500% is realistic.

You could basically just stick AC in. It’s a heat pump. Obvs as soon as you use it to cool you are throwing your green credentials and carbon savings out of the window.
I assume the high ASHP prices are due to difficulty routing pipework around the house and therefore costly refit. AC could be a similar issue but pipes are smaller.

You could basically just stick AC in. It’s a heat pump. Obvs as soon as you use it to cool you are throwing your green credentials and carbon savings out of the window.
We haven’t had ours over a summer yet, but as we have solar PV and a battery I’m hoping to justify it to myself that we’ll only need it on days when the PV has generated way more power than we could use anyway so some of it may as well be used to cool the place down…

As for phiiiil, if it’s hot enough for AC, then we have plenty of excess power. But even this summer it wasnt too bad and it probably would get much use.

The installation is very easy if you don’t have it ducted – just a hole in the wall between the compressor outside and the fan unit inside. We have mvhr, but the air volume isn’t enough to distribute the heat effectively.

One thing to keep in mind with just reducing gas boiler temperature is that you still need bigger pipework for the ASHP as it runs at a lower temperature differential to achieve the same output.

Thanks for that, it’s something I didn’t know but I don’t understand the physics; can you expand a bit for me, please? If I’m sending enough 45ºC water round my radiators to maintain 18ºC room temperature, why does the pipe diameter matter? Is it to do with the temperature of the return flow, and if so, how does pipe size affect that?

Just turning down flow temperature isn’t enough, you’ve got to lower the temp difference across the flow and return to mimic a heat pump.

You could do that using the lockshield valve on the radiator though. Also I would suggest a 50 deg flow for rads unless you are in a modern well insulated house.

And most heat pumps will be weather compensated so you would not be getting the maximum flow temp with the mild conditions that are above the south east at the moment.

No chance it’ll be running much closer to 1:1 as it needs some warmth in the air to do it’s black magic.

Well unless this ASHP supplier is lying an ASHP ha smuch better COP than 1:1 in the temps we’ve just had

Urban Plumbers have just posted a YouTube vid covering an ASHP installation. Its worth watching this guy’s other stuff as he does some impressive work.

If you were running a heat pump last night it was definitely lower CO2 than a gas boiler:

While you were sleeping, #windEnergy reached 57% of the National Grid's electricity demand. https://t.co/i6kBudVd44 pic.twitter.com/QEM6bOKmwC

— Winderful (@winderful_uk) December 20, 2022

No chance it’ll be running much closer to 1:1 as it needs some warmth in the air to do it’s black magic.

Well unless this ASHP supplier is lying an ASHP ha smuch better COP than 1:1 in the temps we’ve just had

Point conceded earlier but I’d not be believing the manufacturers either the figure is going to be massaged up and then different manufacturers will have different performances.

Don’t know the exact split but we’ve used something like 3500 kWh for an entire year of heat and hot water, with a permanently warm 4 bed house. That would suggest we have had a very decent percentage of “free” energy from the ashp.

Even if it was only 1:1 for one week (which it wasn’t) then it is still better (cheaper, less energy and arguably less carbon) than bottled gas or oil which are my alternatives.

I like the way people are very quick to contest ashp cop figures but no one contests manufacturers claims about boiler efficiency. Same deal with ice vs electric cars. We have long since given up believing mpg figures for petrol and diesel from the manufacturers but people are all over claims of mileage on an ev.

https://grid.iamkate.com/