High temp heat pumps??

[fixitflav]

Not convinced zoning and set backs are any cheaper to run long term. Even with the gas boiler and nest thermostat geo fencing to switch off when away from 0700 until 1700 (when at work) the difference in our gas usage versus weekends was negligible. I then tried smart TRVs and zoning areas of the house, and that definitely wasn't cheaper and made the house far less comfortable. In fact coming home from work wasn't fun, waiting for the house to warm up again.

I'll second that! When we moved into 1st house (in 1972) we ran the CH on the timer, on in the evening about 5pm ready for getting home at 6. In coldish weather the boiler ran till about 9.30 before going off on the roomstat , set at 20°, off for 1/2 hour, then back on before stopping on the timer at 11. And we weren't warm then, walls and furniture etc being cold means the air has to be warmer to feel comfortable. When we had kids started leaving it on round the clock, much more comfort and didn't see any big increase in bills. Done the same in subsequent houses.

 

[omph]

To achieve the higher SCOPs stay away from the energy suppliers installers, they only have to achieve a SCOP of 2.8 under the terms of the grant. An experienced independant installer would be the way to go but they'll charge for their expertise whereas an energy company will be selling you power.

Have you had any experiences with energy suppliers ? I've heard this too but not seen it in action and the fall out. It's doing a lot of damage to the industry.

 

[ericmark]

The TRV and room thermostat does show how the room has be heated over the day this will clearly vary home to home, as to geofencing did not find it worked too well, as only options for setting with Nest Gen 3 is the Eco and Comfort setting, but I am using a non modulating oil boiler, modulating gas would be better.

I see your point about if worth switching off at night and while at work, my house 21ºC to 17.5ºC in 6 hours

The big problem is the reheating looking as you can see at 4 hours, but not easy to work out how much energy used when I have two room thermostats and 9 programmable TRV heads, the Nest records how long it has asked for heat, but not how long the boiler has run for, and dipping the oil tank does not really help. I would guess we spend around £700 a year on heating a detached three story house, but unless we fitted a heat pump can't tell how much more expensive it would be.

However I want heating to still work during a power cut, and can't see how that would work with electric only. My battery back up and solar panels may run an oil fired central heating boiler, but no chance with a heat pump, so for me heat pump is a non starter.

 

[vulcancontinental]

The TRV and room thermostat does show how the room has be heated over the day View attachment 365853View attachment 365854 this will clearly vary home to home, as to geofencing did not find it worked too well, as only options for setting with Nest Gen 3 is the Eco and Comfort setting, but I am using a non modulating oil boiler, modulating gas would be better.

I see your point about if worth switching off at night and while at work, my house 21ºC to 17.5ºC in 6 hours View attachment 365855 The big problem is the reheating looking as you can see at 4 hours, but not easy to work out how much energy used when I have two room thermostats and 9 programmable TRV heads, the Nest records how long it has asked for heat, but not how long the boiler has run for, and dipping the oil tank does not really help. I would guess we spend around £700 a year on heating a detached three story house, but unless we fitted a heat pump can't tell how much more expensive it would be.

However I want heating to still work during a power cut, and can't see how that would work with electric only. My battery back up and solar panels may run an oil fired central heating boiler, but no chance with a heat pump, so for me heat pump is a non starter.

Your point about the battery pack running the boiler is sound and if enough to tip the decision for oil is fair comment. Especially if you experience frequent outages.
I have no idea of size and duration batteries will operate a burner and circulator.

£700 sounds very reasonable.

 

[robinbanks]

but at that temp it'll have a COP OAT 7C of 2-ish

I'd calculated a similar ballpark, around 1.8, making assumptions as to the Carnot efficiency

Reason I asked is because the OP seems to be specifically eyeing HPs that will drop in replace the way they use their boiler. Aiming to dissuade that approach on running cost price initially..

but no chance with a heat pump, so for me heat pump is a non starter

What size array, inverter and battery capacity do you have?

 

[drives]

The TRV and room thermostat does show how the room has be heated over the day View attachment 365853View attachment 365854 this will clearly vary home to home, as to geofencing did not find it worked too well, as only options for setting with Nest Gen 3 is the Eco and Comfort setting, but I am using a non modulating oil boiler, modulating gas would be better.

I see your point about if worth switching off at night and while at work, my house 21ºC to 17.5ºC in 6 hours View attachment 365855 The big problem is the reheating looking as you can see at 4 hours, but not easy to work out how much energy used when I have two room thermostats and 9 programmable TRV heads, the Nest records how long it has asked for heat, but not how long the boiler has run for, and dipping the oil tank does not really help. I would guess we spend around £700 a year on heating a detached three story house, but unless we fitted a heat pump can't tell how much more expensive it would be.

However I want heating to still work during a power cut, and can't see how that would work with electric only. My battery back up and solar panels may run an oil fired central heating boiler, but no chance with a heat pump, so for me heat pump is a non starter.

You won’t be saving much if anything. I monitored exactly the same thing- heating duration via the nest, versus our actual usage on Hugo. On weekends the duration was more obviously, but that would equate to something like 5kW gas. So for a saving of 20-30p we’d come back from work to a really cold house. The set backs for us just weren’t worth it.

Typical draw from our house (including our 12kW ashp) is around 2.5 to 3kW when heating is on/reheating cylinder. We’ve just had a power wall installed so that will keep us ticking over for a few hours. That said I can’t remember the last power cut we had…..we’re relatively rural as well.

If you already have solar and battery, I wouldn’t write it off. I’d personally never go back to on/off and zoning etc. as our house is so much more comfortable just coasting along and all of the rooms staying warm, for no extra cost. Now we have the power wall taking advantage of the octopus tariffs we should be in fact saving a significant amount.

 

[martygturner]

I would point out that the 75 degree number is for hot water not CH... its just a bog standard heat pump.

 

[drives]

I would point out that the 75 degree number is for hot water not CH... its just a bog standard heat pump.

It will be for both- maximum flow temp it can produce either for your CH or cylinder. What you obviously do is schedule the hot water so it will reheat cylinder (which the ASHP will always prioritise over heating and divert the flow to the heating coil) at certain times. This is typically set at around 50-55 hence why you'll always see a slightly lower COP for hot water versus heating. Our COP for heating currently for last week or so is around 4.2, and for hot water it is 3.5- because it will heat the hot water to a higher temp whereas for heating it is averaging around 36-40 at the moment.

 

[fixitflav]

It will be for both- maximum flow temp it can produce either for your CH or cylinder. What you obviously do is schedule the hot water so it will reheat cylinder (which the ASHP will always prioritise over heating and divert the flow to the heating coil) at certain times. This is typically set at around 50-55 hence why you'll always see a slightly lower COP for hot water versus heating. Our COP for heating currently for last week or so is around 4.2, and for hot water it is 3.5- because it will heat the hot water to a higher temp whereas for heating it is averaging around 36-40 at the moment.

I wonder if there's a case to be made for using the ASHP for CH only, and the immersion heater for HW. Due to the lower efficiency on HW with the ASHP, probably not much in it costwise. Avoids Y or S-plan motorised valves, and simpler controls and wiring.

 

[drives]

I wonder if there's a case to be made for using the ASHP for CH only, and the immersion heater for HW. Due to the lower efficiency on HW with the ASHP, probably not much in it costwise. Avoids Y or S-plan motorised valves, and simpler controls and wiring.

I would doubt it. We have one motorised valve and obviously the immersion will be much less efficient. ASHP HW is currently running at a cop of 3.5 so will be significantly higher efficiency in summer. Immersion would be 1kW in and 1kW out. Unless you had a solar diverter perhaps but then that won’t generate much in this weather.

 

[robinbanks]

Due to the lower efficiency on HW with the ASHP

Conditions have to be fairly unfavourable externally for you to get down to COP1; anything above that you're saving, unless you're really looking for excuses to favour the immersion like "wear and tear on a 4 grand ASHP vs a 50 quid immersion element"

 

[fixitflav]

Conditions have to be fairly unfavourable externally for you to get down to COP1; anything above that you're saving, unless you're really looking for excuses to favour the immersion like "wear and tear on a 4 grand ASHP vs a 50 quid immersion element"

I was just raising a question, not implying anything, but

Conditions have to be fairly unfavourable externally for you to get down to COP1

COP wouldn't need to get down to 1, as there are always some losses between the ASHP and the HW cylinder. . You may have a point about "wear and tear on a 4 grand ASHP vs a 50 quid immersion element".
Also problems with motorised valves come up regularly on here, so eliminating them would be good, in maintenance as well as first cost.

 

[robinbanks]

COP wouldn't need to get down to 1, as there are always some losses between the ASHP and the HW cylinder.

You're splitting hairs somewhat; even at -20, producing 45+ water, all of Mitsubishi's monobloc range have a COP sufficient to counter such losses and not hit 1..

When I said "unfavourable" I meant "the likes of which are seen a couple of times in a lifetime in the UK"

Generally I think it highly unlikely it would be favourable to use an immersion to produce HW when an ASHP is ready and able