Portable heat pump operation

[arthur jackson]

I decided to stop using the wood-fired Rayburn last year and bought a portable (single-hose air-to-air) heat pump. There’s only me in the house and I spend evenings upstairs and just need to keep the room at 18º. I’m hoping someone here knows about heat pumps because I have a few questions.

I read from https://www.theecoexperts.co.uk/heat-pumps/portable-heat-pumps: "Single hose portable heat pumps have one exhaust hose, which you stick out the window. The exhaust hose releases excess heat and moisture, while the heat pump absorbs air from inside the room, heats it, and releases it back into the room."

I’m not sure what it means by “the exhaust hose releases excess heat” because it’s cold air that’s being expelled.

My first question though is whether it’s also working as a sort of displacement ventilation system because some air is being expelled through the exhaust hose (although I think by far most of it comes back into the room). It feels like it should be better than displacement ventilation because (a) it’s also heating up the air and (b) it’s pulling out a good 1.5 litres of water a night (I’m in Cornwall, UK ).

It also seems strange that it’s presumably drawing in some cold air through the fabric of the building, although the rest of the house doesn’t seem noticeably colder when I’m using it.

 

[TheVat]

1 hour ago, arthur jackson said:

Single hose portable heat pumps have one exhaust hose, which you stick out the window. The exhaust hose releases excess heat and moisture, while the heat pump absorbs air from inside the room, heats it, and releases it back into the room."l

This sounds not quite correct.  Heat pumps extract heat from the outside air and then transfer heat inside, where it adds heat to the room.  If it were to exhaust "excess heat" outside that would seem to defeat the whole purpose of the system.  The heat that comes from use of compression (gases get hotter when you compress them) is supposed to go INSIDE.  

Say it's 40 F outside.  Outside air blows across coolant tubes bringing the (colder) coolant to 40 F.  The air passing over the tube loses heat, passing out of the unit at, say, 30 F.  Then you send the coolant through the compressor and it rises to 65 F and goes inside.  The energy that forced the coolant into a smaller volume goes into the coolant, which then is sent inside to radiate its heat into an airflow that blows into the room.

With the HP you are describing, it relies entirely on the compression of drawn-in interior air, so it would seem rather ineffective, drawing heat from the air it is supposed to warm up and then returning it, with little net gain.  I feel your website writer was a bit confused.

4 minutes ago, TheVat said:

🙂

 

Edited 20 hours ago by TheVat
massive edit issue, how to erase 2nd quote box?

[arthur jackson]

Thank you for the reply. Yes to first para, I think the writer has made a mistake.

Second and third paras: that's how a normal heat pump works. I'm assuming that the single hose portable heat pump takes heat from the internal air as you say but exhausts some of it at colder temperature out of the window to heat the returned internal air.

The COP is supposed to be around 2 (rather than 4 for a proper installed heat pump) and it does seem to give about twice the heat of the 1 kW fan heater I used to use - plus it pulls out a fair amount of water vapour which adds to the effectiveness.

I've been thinking that it wasn't really worth the expenditure but it looks like it has a 2.5 year repayment time with energy prices at around 20p/kWh, shorter if electricity prices go up. Which I'm pleased with since I (think) I've made all the short-term so 2-year repayment time savings like basic insulation etc.

There are dual-hose portable heat pumps which are more efficient - so working as you describe - but I've not seen any for sale in the UK, only in the US.

[exchemist]

10 minutes ago, arthur jackson said:

Thank you for the reply. Yes to first para, I think the writer has made a mistake.

Second and third paras: that's how a normal heat pump works. I'm assuming that the single hose portable heat pump takes heat from the internal air as you say but exhausts some of it at colder temperature out of the window to heat the returned internal air.

The COP is supposed to be around 2 (rather than 4 for a proper installed heat pump) and it does seem to give about twice the heat of the 1 kW fan heater I used to use - plus it pulls out a fair amount of water vapour which adds to the effectiveness.

I've been thinking that it wasn't really worth the expenditure but it looks like it has a 2.5 year repayment time with energy prices at around 20p/kWh, shorter if electricity prices go up. Which I'm pleased with since I (think) I've made all the short-term so 2-year repayment time savings like basic insulation etc.

There are dual-hose portable heat pumps which are more efficient - so working as you describe - but I've not seen any for sale in the UK, only in the US.

Yes that explanation makes no sense. I did find this, which does seem to make sense:  https://www.canopyclimate.com/post/the-best-portable-heat-pump-and-one-type-to-avoid

 According to this, the mode of operation is to draw ambient air from the room, rejecting the captured heat back into the room and exhausting the cold air thereby created to the exterior.  As the writer says, the problem is the expelled air has to be replaced by air from within the house, so you must draw outside air in somewhere, which will be cold, if not as cold as the air the pump exhausts. And in the process it will create a cold draught, which subjectively may make the occupants of the room feel cold, too.
 

Sounds pretty crappy to me, if not actually a scam. 2 hose job must be far better, I feel sure. 
 

But no doubt it will dehumidify the air, which may make it feel less chilly, I suppose.

[studiot]

Just now, exchemist said:

But no doubt it will dehumidify the air, which may make it feel less chilly, I suppose.

Any system that condenses water out of the air will recover the latent heat of that water and if that water is emptied outside there will be some effect.

You can also do this in a conventional freezer / freezer compartment by puting flexible plastic bowls of tapwater into it and throwing the ice outside when the water has frozen.

[exchemist]

1 minute ago, studiot said:

Any system that condenses water out of the air will recover the latent heat of that water and if that water is emptied outside there will be some effect.

You can also do this in a conventional freezer / freezer compartment by puting flexible plastic bowls of tapwater into it and throwing the ice outside when the water has frozen.

Have you successfully heated your kitchen that way, then?

[studiot]

Just now, exchemist said:

Have you successfully heated your kitchen that way, then?

Only the one in my dolls house.

😀

[arthur jackson]

5 minutes ago, studiot said:

Any system that condenses water out of the air will recover the latent heat of that water and if that water is emptied outside there will be some effect.

You can also do this in a conventional freezer / freezer compartment by puting flexible plastic bowls of tapwater into it and throwing the ice outside when the water has frozen.

Interesting yes. Or in this case filling the heat pump's water tank by removing 1.5 litres of water/evening into the heat pump's water tank.

The heat pump also acts as a dehumidifier in summer. I've been reluctant to use it because it runs at around 1 kW and doesn't seem to extract much water into the water tank. Presumably then it's expelling that water outside and I should be checking my room's humidity meter to see how effective it is.

[studiot]

Here is a UK/European datasheet for a 3KVA  single pipe model.

This is the max you can get from plug in UK mains.

Note the risk of explosion/fire

and other caveats.

https://www.appliancesdirect.co.uk/files/pdf/P12HPW 20240311.pdf

 

+1 also to exchemist for finding that link to a sensible (US) evaluation.

 

Interestingly we stayed in the YHA in Penzance in November this year and they had something like this in the room.

Edited 17 hours ago by studiot

[arthur jackson]

2 hours ago, exchemist said:

Yes that explanation makes no sense. I did find this, which does seem to make sense:  https://www.canopyclimate.com/post/the-best-portable-heat-pump-and-one-type-to-avoid

 According to this, the mode of operation is to draw ambient air from the room, rejecting the captured heat back into the room and exhausting the cold air thereby created to the exterior.  As the writer says, the problem is the expelled air has to be replaced by air from within the house, so you must draw outside air in somewhere, which will be cold, if not as cold as the air the pump exhausts. And in the process it will create a cold draught, which subjectively may make the occupants of the room feel cold, too.
 

Sounds pretty crappy to me, if not actually a scam. 2 hose job must be far better, I feel sure. 
 

But no doubt it will dehumidify the air, which may make it feel less chilly, I suppose.

yes but as I say I've not seen dual-hose versions in the UK. I don't think it's a scam - I kept figures last year which I need to check but as I recall I was having the 1 kW fan heater on pretty well continuously while this year the 1 kW heat pump seems to be on for only half the time.

And yes it must be pulling in cold air through the building fabric, but I don't think it pulls in too much because the heat pump is pushing out a lot of air into the room without seeming to pull in much more.

Hence my question as to whether it's acting as a sort of displacement ventilation system, though. I nearly installed one of those last year but that would again be drawing cold air into the house.

The main problem is that the house has suffered badly from damp and mould which don't agree with my chest much, so anything that helps solve that is going to be worthwhile.

1 hour ago, studiot said:

Here is a UK/European datasheet for a 3KVA  single pipe model.

This is the max you can get from plug in UK mains.

Note the risk of explosion/fire

and other caveats.

https://www.appliancesdirect.co.uk/files/pdf/P12HPW 20240311.pdf

 

+1 also to exchemist for finding that link to a sensible (US) evaluation.

 

Interestingly we stayed in the YHA in Penzance in November this year and they had something like this in the room.

That's similar to mine with 12,000 BTU but that's only 1.2 kW input power and 5 A so well below the 13 A plug fuse. Point taken about risk of explosion but I'm not having the rayburn going downstairs and there's no real risk of ignition in the room.

I did see that US article a while ago but it's not my experience (and a fan heater won't extract water), and as I say I can't see a dual-hose version in the UK. I'd love to know how much air is actually being expelled to the outside but again it doesn't feel like it's that much, and I wonder whether there's a benefit from positive displacement ventilation.

Edited 16 hours ago by arthur jackson

[studiot]

Just now, arthur jackson said:

That's similar to mine with 12,000 BTU but that's only 1.2 kW input power and 5 A so well below the 13 A plug fuse. Point taken about risk of explosion but I'm not having the rayburn going downstairs and there's no real risk of ignition in the room.

I did see that US article but it's not my experience, and as I say I can't see a dual-hose version in the UK. I'd love to know how much air is actually being expelled to the outside but again it doesn't feel like it's that much, and I wonder whether there's a benefit from positive displacement ventilation.

My brother lives on the second floor of a block of flats in London and has a properly fitted two pipe system running the radiators and domestic hot water in his flat.

The  200 mm air inlet and outlet pipes for the heat pump were diamond cored through the walls.

I understand from the company that installed them that they have Lincolnshire Council have fitted a number of this type.

[arthur jackson]

I did look at a  multi-drop system but I was a bit concerned about the noise - it would be next to the road and I live in the country so I think it would be disturbing to people going past. The one I have is 56 dB which is just about tolerable in the room - all the other portable systems were 65 dB which wouldn't do my tinnitus any good and also too much for the dog in the room because of his sensitive hearing.

An air-water system might have worked for me since I have a back boiler to the rayburn but the radiators just aren't big enough I think particularly with the lower water temperatures. I like the air-air version because it warms the room up really quickly. Many years ago I had a volkswagen beetle and think I agree with that nice Mr. Porsche who said that all cars are air cooled, it's just that car manufacturers will insist in putting water in the way.

Also my system cost £350 as I recall and just needed a conversion kit for the sash window rather than a few thousand for equipment and installation - even with the improved COP I think it would have made the repayment time too long.

A multi-drop air-air system would have warmed both upstairs and downstairs but I've also got a conservatory which does warm the rest of the house somewhat during the day even in winter.

 

 

 

Eta: I hadn't thought before but I don't think I could have 200-mm holes drilled through the wall. It's traditional Cornish rab construction which means stone-clay-stone. Try to drill a 5-cm hole on one side through that and I've been told you get a 50-cm hole on the other side.