Air to air versus air to water heat pumps

EVandPV

Do air to air heat pumps produce as much heat as air to water and does the same efficiency apply ?
Also, does the currently available funding apply to both or just air to water systems ?

Verdigris

You can't get RHI on air to air because it can be used as air-conditioning. I presume the same will apply under the new grant scheme as the installation still has to be MCS certified.

Similar rules of efficiency apply. COP depends on temperature difference between inside and outside. Air to air is relatively cheap but beware. A friend of mine used a couple of units to get extra heat in his all electric house when he was looking after his elderly mother. One unit broke down and he hasn't been able to get it repaired for an economic price.

arty688

Air to Air frowned as they can cool as well as heat so no grants available. Nothing wrong with them though and they are efficient. 

Reed_Richards

An air to air heat pump should be more efficient because it works on the difference between the air outside and the air in the room whereas an air to water heat pump has to make the water hotter than the room in order to heat the room.

EVandPV

Reed_Richards said:

An air to air heat pump should be more efficient because it works on the difference between the air outside and the air in the room whereas an to water heat pump has to make the water hotter than the room in order to heat the room.

Yeah I was thinking the same.
It also strikes me that air to air systems are significantly cheaper so why aren't they being pushed more over air to water ?

Verdigris

You'd have to put the cost of heating water by immersion heater into the financial equation, although if you used a TOU cheap tariff that might not be too significant.

EVandPV

Verdigris said:

You'd have to put the cost of heating water by immersion heater into the financial equation, although if you used a TOU cheap tariff that might not be too significant.

We're on a TOU tariff plus have a solar diverter which sends surplus pv to our immersion heater. We really only use the gas boiler to heat water in the autumn/winter but given the increase in gas prices, using the immersion on TOU would make sense  as you say.

So as we have fairly small rooms with good insulation in the walls and loft, I'm currently thinking we could get away with an air to air system for space heating and do away with the gas boiler and GCH.

Coastalwatch

EVandPV said:

Verdigris said:

You'd have to put the cost of heating water by immersion heater into the financial equation, although if you used a TOU cheap tariff that might not be too significant.

We're on a TOU tariff plus have a solar diverter which sends surplus pv to our immersion heater. We really only use the gas boiler to heat water in the autumn/winter but given the increase in gas prices, using the immersion on TOU would make sense  as you say.

So as we have fairly small rooms with good insulation in the walls and loft, I'm currently thinking we could get away with an air to air system for space heating and do away with the gas boiler and GCH.

The beauty of installing an air to air unit, say in the lounge, is that you can leave GCH intact while assessing it's performance. Comfort vs cost etc.

I put an energy monitor on the socket so then able to log it's precise consumption and running costs.

Then put in a second ashp unit before reducing and finally removing the GCH entirely.

While we got away with two 2kW pumps larger versions are also available.

EVandPV

Coastalwatch said:

EVandPV said:

Verdigris said:

You'd have to put the cost of heating water by immersion heater into the financial equation, although if you used a TOU cheap tariff that might not be too significant.

We're on a TOU tariff plus have a solar diverter which sends surplus pv to our immersion heater. We really only use the gas boiler to heat water in the autumn/winter but given the increase in gas prices, using the immersion on TOU would make sense  as you say.

So as we have fairly small rooms with good insulation in the walls and loft, I'm currently thinking we could get away with an air to air system for space heating and do away with the gas boiler and GCH.

The beauty of installing an air to air unit, say in the lounge, is that you can leave GCH intact while assessing it's performance. Comfort vs cost etc.

I put an energy monitor on the socket so then able to log it's precise consumption and running costs.

Then put in a second ashp unit before reducing and finally removing the GCH entirely.

While we got away with two 2kW pumps larger versions are also available.

Thanks for that. That's something I hadn't considered but makes perfect sense.
So do you find the air to air system copes during extremes of cold weather ?
Or do you have another heating source as backup, ie log burner, leccy heaters, etc
One last thing, are the ac units in the rooms quiet ?

shinytop

I would definitely have considered A2H had the RHI scheme not been so generous.  I think it may eventually become part of the solution to removing/reducing GCH. 

Coastalwatch

No other form of heating and after using just ASHP for the winter of 2019/20 we decided in the April to have the gas supply terminated so losing it's accompanying standing charge. Unfortunately the Utility we were with at the time wanted £120 to do this. I contacted Octopus and asked that if we should transfer to them, had a smart meter installed would they terminate gas at the same time?  They agreed to do this at no charge, timing all dependent upon covid restrictions. The winter of 20/21 was certainly colder with many days below freezing but ASHP coped with this in spite of many posts on here stating that they do not work in subzero temperatures!

Like you we are on TOU tariif, Go Faster 1.30-6.30, so the pumps set to come on around 4am and on extremely cold days left on all day till 9.30 pm. 

Internal units barely audible on the lower settings. The external units are both mounted directly on the gable end wall with one directly above the head of our bed. In the quiet of night it is audble but doesn't wake us and is certainly no noisier than the original GCH circulating pump mounted in the loft.

Think we got off lightly when compared to wet system costs, not to mention doing away with all the  radiators and pipes.