air source heat pump help!

trevjen

We moved into a 4 bed new build in April that is all electric with piped underfloor heating and radiator upstairs all supplied by an Air source heat pump.
I am having great difficulty trying to set the heating and hot water to run efficiently and economically. Some sources say leave the ashp running 24/7 and control with room stats and some say only run all day and not at night they also say having water heating on during the night uses more electricity as air is colder ?
I cant seem to get a happy medium where the house is warm and water is always hot. it is costing us around £300 per month in electricity far more than the same size new build we moved from with gas and electric with gas boiler.
Can anyone advise the correct way to set this thing to run as economically as possible please?

QrizB

@Reed_Richards and @shinytop both have ASHPs and have a long thread here where they have been discussing how best to set up and operate heat pumps:

https://forums.moneysavingexpert.com/discussion/6240076/i-bought-a-heat-pump

Hopefully one (or both) of those posters will notice that I've tagged them and will be able to give you some advice.

However, with current energy prices an ASHP will still be around twice as expensive to run as an equivalent mains gas system.

Reed_Richards

I think Underfloor Heating needs to be on 24/7 because it is very slow to respond to changes in temperature so if you allow a room with UFH to cool it takes ages to get it back to temperature.  For my heat pump it makes economic sense to turn the radiators off at night (or set back the temperature as I do) but with more sophisticated controls than I have you can make a case for 24/7 operation.  If you have just UFH running your heat pump needs to know this and to reduce its Leaving Water Temperature accordingly or you won't derive any economic benefit from the UFH.  Most heat pumps have the facility to use Weather Compensation and using that and getting the settings right can help you to reduce running costs.

What time of day you heat your hot water should be determined more by need than anything else.  If you have a big enough tank you can make a case for doing it in the middle of the day when the outside temperature is likely to be highest but such considerations will never make a huge difference.  If your hot water tank has an immersion heater, try not to invoke its use beyond the once a week needed to guard against Legionella.

Sadly a heat pump will be more expensive to run than a gas boiler at current price levels.  Maybe not twice as expensive but very certainly more.   

ccbrowning

I don’t leave my UFH on 24/7, although it does need turning on 2 hours ahead of when you need the heat. Not tested if I can save money having it always on tho. Gas boiler now, but can’t test with heat pump (GS) in a month or so. 

Bulb variable rate. Was £160 for a 4 bed last month. Gas+electricity so a heat pump does sound more £££. 

Reed_Richards

ccbrowning said:

I don’t leave my UFH on 24/7, although it does need turning on 2 hours ahead of when you need the heat. Not tested if I can save money having it always on tho. 

The theory is that if your heat pump knows it has to work harder to heat up you house/zone then it will make your water temperature hotter and so run less efficiently.  If this happens then 24/7 operation could be economic.  But my heat pump has no way of knowing how hard it has to work; it just heats the water to whatever temperature the Weather Compensation algorithm determines then leaves it there until the thermostat is satisfied.  So in my own case it's better to turn the heating off or down when I don't need it.    

shinytop

@trevjen, what make of ASHP is it?  How many m2 is your house?  Do you have heat loss calculations?

It's hard to give advice without knowing more but the things that make an ASHP run inefficiently are low outside temperatures, high flow temperatures and working hard.  You can't change the first so concentrate on the second two.

For hot water, set the temperature 45-50C and run it at the warmest time during the day if you can but it's more important to have HW when you need it.  It won't make a huge difference unless you use a lot anyway.  

For heating it's harder to say and combined radiator/ufh systems are the hardest to get right. I don't know to what extent you can control the radiators and ufh separately; that depends on the set up. But weather compensation set up properly should be most efficient because it uses the lowest flow temperature needed to heat the house.  Start with a weather compensation curve that goes from something like 50C flow at -5C outside to 30C at 15C.  Or try a constant flow of 40-45C.  Try not to let the house cool down too much because the ASHP will crank up the power (and lower the COP) when it has to heat the flow up from cold.

I have a 1990 4 bed house and it cost me about £150 to heat it to 21C or so in December at Ofgem cap rates.  Leaving the heating on 24 hrs doesn't make a big difference for us. It's a different mindset to a gas boiler; slow and steady, running for longer and warm radiators instead of hot ones.   If you try and run it like a gas boiler (and a decent ASHP will happily do this) with 55C flow and lots of on/off cycles it will get very expensive. 

Good luck and let us know how you get on.

Reed_Richards

shinytop said:

For heating it's harder to say and combined radiator/ufh systems are the hardest to get right. I don't know to what extent you can control the radiators and ufh separately; that depends on the set up. But weather compensation set up properly should be most efficient because it uses the lowest flow temperature needed to heat the house.  Start with a weather compensation curve that goes from something like 50C flow at -5C outside to 30C at 15C.  Or try a constant flow of 40-45C.  Try not to let the house cool down too much because the ASHP will crank up the power (and lower the COP) when it has to heat the flow up from cold.

I would go with the latter option first, try a constant flow temperature and see if it is enough to keep your house warm.  If it's, say, near zero  one day and 45 C will keep your house warm then @shinytop's weather compensation suggestion (which would give you 45 C flow at 0 C outside if it's a straight line) might be about right at that sort of temperature.  Then you can see if 40 C flow works on a day when its 5 C outside.

I don't agree with the comment about COP for the reason I explained above.

trevjen

shinytop said:

@trevjen, what make of ASHP is it?  How many m2 is your house?  Do you have heat loss calculations?

It's hard to give advice without knowing more but the things that make an ASHP run inefficiently are low outside temperatures, high flow temperatures and working hard.  You can't change the first so concentrate on the second two.

For hot water, set the temperature 45-50C and run it at the warmest time during the day if you can but it's more important to have HW when you need it.  It won't make a huge difference unless you use a lot anyway.  

For heating it's harder to say and combined radiator/ufh systems are the hardest to get right. I don't know to what extent you can control the radiators and ufh separately; that depends on the set up. But weather compensation set up properly should be most efficient because it uses the lowest flow temperature needed to heat the house.  Start with a weather compensation curve that goes from something like 50C flow at -5C outside to 30C at 15C.  Or try a constant flow of 40-45C.  Try not to let the house cool down too much because the ASHP will crank up the power (and lower the COP) when it has to heat the flow up from cold.

I have a 1990 4 bed house and it cost me about £150 to heat it to 21C or so in December at Ofgem cap rates.  Leaving the heating on 24 hrs doesn't make a big difference for us. It's a different mindset to a gas boiler; slow and steady, running for longer and warm radiators instead of hot ones.   If you try and run it like a gas boiler (and a decent ASHP will happily do this) with 55C flow and lots of on/off cycles it will get very expensive. 

Good luck and let us know how you get on.

Its a Grant Aerona 3

shinytop

Reed_Richards said:

shinytop said:

For heating it's harder to say and combined radiator/ufh systems are the hardest to get right. I don't know to what extent you can control the radiators and ufh separately; that depends on the set up. But weather compensation set up properly should be most efficient because it uses the lowest flow temperature needed to heat the house.  Start with a weather compensation curve that goes from something like 50C flow at -5C outside to 30C at 15C.  Or try a constant flow of 40-45C.  Try not to let the house cool down too much because the ASHP will crank up the power (and lower the COP) when it has to heat the flow up from cold.

I would go with the latter option first, try a constant flow temperature and see if it is enough to keep your house warm.  If it's, say, near zero  one day and 45 C will keep your house warm then @shinytop's weather compensation suggestion (which would give you 45 C flow at 0 C outside if it's a straight line) might be about right at that sort of temperature.  Then you can see if 40 C flow works on a day when its 5 C outside.

I don't agree with the comment about COP for the reason I explained above.

I don't agree with the comment I think you think I'm making either !!  I was very careful to talk about heating 'the flow' and not the house.  When my ASHP detects a big delta between the actual and target flow (e.g after the heating has been off for a while) it definitely works harder until the target it reached. I can see this from my monitoring data. It then drops back to maintain that flow.  According to Mitsubishi's data, working harder reduces the COP.  This is nothing to do with the room temperature. 

shinytop

trevjen said:

Its a Grant Aerona 3

@trevgen,

If you don't know how to change things you could try this; it was a reply by Grant on another forum I'm on.  

"Our Technical Team would be more than happy to help. You can reach them on 01380 736920 from 7:30am to 6.00pm weekdays and 8:30am to 1:00pm on Saturday.

We also have a customer handover guide and a series of videos which explain the Aerona3 controller, its settings and the display buttons. You may find these resources useful:

Handover Guide -

GRANTUK.COM "https://www.grantuk.com/media/4106/aerona3-ashp-handover-guide_july-21.pdf"

Aerona3 Videos - https://www.youtube.com/user/MyGrantUK/playlists

Or post a question on the forum yourself; it's renewableheatinghub.co.uk

Reed_Richards

Shinytop said:
I don't agree with the comment I think you think I'm making either !! 

This is a very technical discussion so I have responded here: https://forums.moneysavingexpert.com/discussion/comment/78902994/#Comment_78902994  so as not to take this thread too far off-topic