Posted By: mike7Further on the heating mode only suggestion - I wonder if these aircon units are capable of improvement if they are for heating only. I can imagine there are compromises made when they are to be able to function in either mode. Anyone here know enough to say?

The only difference between a heat-only unit and one that does A/C as well as heating is a simple reversing value, to change the flow of refrigerant. Otherwise, the evaporator and condenser heat exchangers are essentially the same - though there's no defrost unit on the indoor coil.

Posted By: mike7And another thing ... if it's a high COP we're after then what we want is great big heat exchangers to waste as little as possible of the delta T , not 'compact' and 'unobtrusive' ones as manufacturers seem often to aim for. Don't we ?

It's not quite that simple - but a big evaporator usually works better at lower outdoor temperatures than a smaller one - but may require more refrigerant and then there's all the other things to consider such as gas flow resistance and so on.

Paul in Montreal.

There's also the way they are driven or regulated. My Hitachi has to be set to try for 32C before it will run at full pelt. Don't seem right.(Edit: This turns out to be because of where the temperature sensor is located - it is where it is able to receive partially heated air due to the low mounting position. It would be better sited more immediately in the unheated air entering the unit. Not really a problem, though).

The more recent one, a Toshiba/Midea, wont go low enough. If I set it to 16C - the lowest it will go - it continues to deliver heat up to 19C. ..Or it would do if I hadn't tweaked it by adding a resistor across the thermistor doing the measuring. I've a feeling that in both cases more sensitive thermistors would be a good thing to try.

Whatever, my limited knowledge means I'm fiddling with something I don't properly understand, and I don't know how to get more aquainted with the technicalities, short of getting a job with one of the manufacturers, and that would be a bit over the top. I guess there's quite a few people around in the UK feeling the same. Might be different in the US and Canada where aircon has been common for a long time.

Sorry Mike you can't put yourself in the 'they' or 'them' bracket as you post on here
and you tinkering with the units to maximise the efficiency seems to prove to me that 'they' (most) would use them with considerably less care, thus potentially losing the desired benefits .

Posted By: davidfreeboroughit is typical of what happens in the UK when reversible A2A heat pumps are added to an existing building.

I bame the parents.

Hello all, hope you don’t mind me bringing this one to life again.

SteamyTea “Yes, if we swapped to HPs of all sorts then out CO2 emissions would go up until we have 60% nuclear generation.”

I thought we were thinking that HPs might cut CO2 emmissions so long as the -average- mix remained the same? Are you saying that marginal increases in electricity output will almost certainly have worse than average carbon emissions, despite closing coal plants?

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David Freeborough “Any benefit in COP from the A2A heat pumps ability to operate at lower flow temperatures need to be balanced against the fact that many people will use them for cooling, significantly increasing energy consumption & carbon emissions.”

To wade into this one, I can’t understand why people would use it for cooling for two reasons:

1. The climate rarely asks for it, as discussed
2. It costs money. Why use an HP to cool, when you can open the windows for free? The days when it’s worth cooling a UK home AND the heat can’t be alleviated for free by opening windows are very very rare, are they not?

I note your point about your office scenario, but workers don’t pay the office energy bills. In a residential setting, it’s unlikely residents would do the same, so I’m not sure you can describe it as typical?

I’m not saying cooling would never happen, but I doubt it would be more than 2% or 3% of the total amount of time HPs were used. I can’t imagine it being even 5% of the time. So this shouldn’t be a significant worry should it? As mentioned, if the govt said they would support only ‘heat only’ A2A heat pumps, manufacturers would supply them in extremely short order as it should be easy.


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David Freeborough: “In a well insulated airtight house,there is a lot to be said for a properly design A2W system which is directly connected to underfloor heating & delivers DHW via a separate cylinder/thermal store. Heat can be delivered where it is needed when it is needed at temperatures just above room air temperature without the horrible sickly smell of warm air from an air conditioning unit.”

Yes, but the vast majority of houses are not new and millions are difficult to insulate well (solid walls) and/or make appreciably airtight. Even if you can stomach the disruption and cost of insulating your solid wall property, surely retrofitting underfloor heating or large radiators and installing A2W is just too much additional expense and hassle to be a mass market solution.

i.e. A2W in new, well insulated airtight houses are a great, but niche, solution. Whereas I think what we’re talking about is something that works for the mass market. The much much lower installation cost and disruption, and potentially lower running cost and carbon footprint even compared to natural gas, of A2A could make them a good solution for heating existing, even relatively poorly insulated, properties?

"There are 10 types of people, those that understand binary and those that don't"

Posted By: GarethCYes, but the vast majority of houses are not new and millions are difficult to insulate well (solid walls) and/or make appreciably airtight.

Solid walls are not especially difficult to insulate. It's just that some (most) people think that the existing aesthetic is more important than saving energy. I happen to disagree with that.

Even if you can stomach the disruption and cost of insulating your solid wall property, surely retrofitting underfloor heating or large radiators and installing A2W is just too much additional expense and hassle to be a mass market solution.

I'd say the easiest by far would be unfastening radiators and fitting some larger ones (or doubling single panels, or using fan-assisted ones, or skirting ones)

Posted By: daserra The consensus is that the hot air they provide is uncomfortable and stifling. Lots of convection and no radiation does not a comfortable house make.

Might that be because they have been used for cooling? - odours may have collected on the heat exchanger. Also the indoor units will have been mounted high. Better at ankle height for heating I find.

Have just read the users notes for my Hitachi aircon - it confirms that odours can collect when the unit is used in cooling mode, and then be released when heating mode is used.

So - another good reason to use them for heating only.

True, Jim, but the 'more attention' of this thread could include introduction of incentives such as FITs or VAT concessions for heat-only hps. Manufacturers would be able to respond to that in a twinkling.

I suggest any such incentive might only apply to smallish units - up to 4 kW output say. In the average UK home that would lop the bottom off the consumption curve in the heating season without adding greatly to the peak winter electric demand. The winter peaks would be met by the pre-existing system.

Posted By: mike7

Posted By: daserraThe consensus is that the hot air they provide is uncomfortable and stifling. Lots of convection and no radiation does not a comfortable house make.

Might that be because they have been used for cooling? - odours may have collected on the heat exchanger. Also the indoor units will have been mounted high. Better at ankle height for heating I find.

The Mitsi's we fit have a mode where they go into heat mode for 1min or so after a cooling cycle to dry off the internal unit. Part of the maintenance we do is to spray disinfectant into the blowers.
I agree low units are better for cooling; they do exist but are quite rare here.

Posted By: Triassic:- " I bet the man fitting it in Malmo knows what he's doing and the man in Edinburgh, well I'm not so sure."

That's one of the advantages of smallish A2As - there's not nearly so much that the installer has to get right as compared to a full stand-alone HP system. I regard it as the low hanging fruit that can be plucked on a large scale immediately to give lower carbon and fuel cost. Against this is the argument that if you can reduce fuel cost by such means, you reduce the potential savings (and thus weaken the justification for investing in the real long-term ideal) of --- ta- da --- Better Insulation.

An A2A would be a lot less hassle to fit I would have thought. Though airtighness may be more important than insulation when you are heating the air first (which is the way it should be).

Posted By: SteamyTeaThough airtighness may be more important than insulation when you are heating the air first (which is the way it should be).

I think you're just splitting airs.

Posted By: SteamyTeaThough airtighness may be more important than insulation when you are heating the air first (which is the way it should be).

It doesn't make a difference. The air is being recirculated anyway - just as it is with radiators (through convection). Air tightness is important to reduce heat loss, irrespective of the heating method. Arguably the only time it doesn't make a difference is with high-temperature radiant heating. In frigid Montreal, there are places with quartz-halogen radiant heaters outside where people have to queue for things like taxis at the airport - even when it's cold, they work well - and there's no air tightness at all then!

As for the earlier comment about A2A heating being draughty and uncomfortable, that's just a sign of a poorly designed system. I live in a house with a forced air heating system heated by a GSHP and everyone always comments on how comfortable the house is - the temperature is very even as there's next to no stratification - despite the vents at floor level (on one floor) there are really no areas that feel draughty. Even candles don't flicker (much to my surprise, to be honest).

Paul in Montreal.