Posted By: GarethC

All interesting points, but this one, and other comments in the thread, struck me. Aren't A/A systems a bit simpler than A/W, even on a 'like for like' (i.e. space heating only) basis? It's been a while since I compared systems, but, for example, iirc A/W needs a heat exchanger box to transfer heat from refrigerant to water (not to mention, this step will inevitably result in some efficiency loss vs A/A).

And then, while in principle existing pipework could be used extensively by an A/W system, in practise, most examples I've heard of involved installing extremely disruptive and expensive under floor heating, or at very least needed extra radiators and seemed to involve a lot of new pipework. Not to say installing A/A wall units would be trivial.

I wasn't aware of the one unit limit too. Another kicking for my plans. Best scenario for me would be an outdoor unit on each gable end. Most pipework would be external, with only short runs into the house at appropriate places. Easier, I think, than long runs from one side of the house to the other.

When comparing complexity of A2A vs A2W its probably prudent to include a DHW system as there cant be many houses that dont need DHW and need space heating only. I think the refrigerant to water exchanger is built into monoblock A2W HPs so no separate exchanger unit. AFAIK, a monoblock unit pumps out hot water direct to UFH/rads/TS/DHW tank.

I dont think its necessary to couple a A2W HP to UFH and that radiators running lower temps will work. It may be that a shuffle of radiator sizes within the house and the addition of a few larger rads is adequate to get heat output. One thing that rarely gets mentioned is that if you are upgrading insulation/draught proofing then the building heat demand goes down and the demand may well be met by the existing rads running the lower temps A2W HPs offer. That would need calculating on a house by house basis but I dont beleive everyone should assume they will have to change rads when a HP is installed.

Posted By: GarethCAnother kicking for my plans.

Not a major kicking though. Just need to make a planning application.

Posted By: philedgeWhen comparing complexity of A2A vs A2W its probably prudent to include a DHW system as there cant be many houses that dont need DHW

Except that the DHW requirement can be met in other ways, such as an immersion. I'll grant that for a specific house it's important to work out exactly what solution will be used. i.e. yes you always need to do a full system plan.

Posted By: LFIs a moderately sorted draft proof house with double glazing, cavity wall and well insulated loft going to be fine. Say 8kW load when heating on. Hot water for combi boiler is about 24kW by comparison on our boiler.

Combi implies no cylinder so my immediate answer would be 'needs further thought'. You'd need point of use heaters everywhere or centrally, or install a cylinder.

Posted By: LFHot water for combi boiler is about 24kW by comparison on our boiler.

Posted By: djhCombi implies no cylinder so my immediate answer would be 'needs further thought'. You'd need point of use heaters everywhere or centrally, or install a cylinder.

Combi boiler size is usually driven be the need to supply a shower and how many bathrooms in the house but there isn't much difference in the price of a system boiler or a combi boiler and given that modern gas boilers modulate power I don§t see the need to install a DHW cylinder for gas boiler usage. For any sort of heat pump however a cylinder makes sense. Point of use water heaters need a lot of power, especially for a shower, and you have to factor in the possibility concurrent usage so I can see some houses having power circuit issues.

The more you dip into this you realise that there is no one size fits all - far from it!

Posted By: Peter_in_HungaryPoint of use water heaters need a lot of power, especially for a shower

Fortunately, electric showers are widely available.

Posted By: rhamduImagine a split air-to-air heat pump installation. But in each room, instead of transferring the heat to air, the heat is transferred direct to a large radiant surface.

the heat is transferred direct to a large radiant surface = radiator ??
So it sounds like an air to water heat pump would (partially) fill this objective although transferring the heat needs a transfer to water first to conveniently transport the heat which is why it is only a partial fix for directly transferring the heat.

Using an A2W heat pump usually needs an increase in radiator size when converting from conventional central heating and this is some of the engineering and economic issues, which can be overcome by reducing the demand e.g.insulation and/or draught proofing

Posted By: WillInAberdeenI'm looking for it too! A refrigerant-to-air radiator, like the one on the back of a fridge, only much bigger, UFH-size. Possibly embedded into the surface of a floor or wall or ceiling.

I think the problem is the volume of refrigerant that would be needed. If the refrigerant needs F-gas certification to install then you want to minimise the quantity of it in the system. Whether it's 'mildly inflammable' propane or one of the 'reduced GWP potential' ones, you don't want the possibility of large leaks. Maybe if you used a CO2 heatpump, you could do it?

Ooh, a quick google gave the first hit https://www.gdhv.co.uk/why-pick-chilled-water-fan-coil-system-over-refrigerant-based-system which says much the same.

Posted By: WillInAberdeenProblem is probably that heating is a niche market, most products were designed for cooling, which requires condensation to be caught.

Good point about the condensation. Also, air movement always increases heat loss from human bodies unless the air is very warm indeed. So a fan is great for cooling.
I raised the idea of refrigerant-filled radiators because a fan is less good for heating. It may transfer heat efficiently to the room air. But if the room air is moving, people may still be shivering.

Posted By: Ed DaviesHow about a panel with heat pipes inside it, maybe using the same fluid as used in solar thermal tubes, so only the bottom strip needs to be heated by the refrigerant?

Now that sounds like a genius idea, Ed. Maybe use low-pressure water as the working fluid? Or propylene? Or something fancy https://www.hindawi.com/journals/ijp/2021/5546508/

But then again, why not just use a regular water-filled radiator heated by a pipe filled with refrigerant?

Yep, that's similar to how the refrigerant radiator on the back of the fridge works. The refrigerant pipes only have to be very skinny and spaced far apart, compared to a water-filled radiator, because the transfer of latent heat from the condensing refrigerant is much more intensive than in a water radiator, which deals with sensible heat. The surface where heat gets transferred to air needs to be many times bigger than the refrigerant pipes' area, this surface area is provided by a fine metal mesh on the fridge, but any conductive material would do (cement fibreboard with an aluminium backing perhaps?). Ceramic or aluminium would be good as they'd protect the refrigerant pipes against getting bashed (which is one reason they go on the back of the fridge).

It's not visible but we have to remember that the radiator on a fridge or an aircon indoor unit, is mostly filled with gas - the condensed liquid just gets in the way so is drained out, asap.

So an indoor heat pump radiator would contain vastly less volume of refrigerant than a conventional CH radiator, and it would be mostly contain low-density gas. So whereas a CH radiator contains kilogrammes of water, the heatpump radiator would contain grammes of refrigerant. Less mass than the refrigerant contained in the supply/return pipework, and not significantly more than the refrigerant in a conventional fan-forced aircon indoor unit, given the same surface area of refrigerant tubing to give the same heat transfer.

The hot side of a heatpump is at high pressure - eg if propane or similar would be at ~15bar (twice a really thin racing bike tyre). That probably means a mechanically strong shape (eg copper pipes), or it will be expensive, or it might deform. Whatever the magic gas is, the consequences of leakages are much greater than if water leaks from a radiator - for this reason generally interfaces are usually minimised, and often soldered or brazed (hotter and stronger but otherwise similar to soldering). Propane is flammable, so there is a maximum mass allowable indoors depending on the room size, and most other gasses used have high global warming CO2 equivalencies in the order of 1000x CO2 - so can only be used by somebody certified.
As it's a phase change thing, I think you want the flow to be gas in at the top, liquid out at the bottom.

Personally, I think A2W/W2W heatpumps are fine things that work well with well insulated homes and (big enough)radiators/ufh in the uk - they're just higher price due to low supply right now. The phase change material needs a heatexchanger somewhere - BPHE (brazed plate heat exchangers) are only Ă‚ÂŁ50 or so, and can couple to a conventional water circuit (likely that's what's in an A2W heatpump instead of the fan & indoor unit of a cheaper A2A unit).

It does sound great, I like that a University is working with a significant manufacturer, and improvements are always welcome!

Having said that - I found the article, and while it says 6.94kW out (1kW elec in) and 75C, these are not in the same sentence. It also says 4.43kW into DHW with 1kW elec, which is probably still below 75C.

To me this reads more like advertising fluff, not backed up by consistent numbers - at what ambient are they measuring? Are pumps and fans included in the figures? Any other zombie power loss ? Are they manufacturing a new more efficient compressor, or optimising existing?

So having done a little searching I found what seems to be a predecessor project: https://cordis.europa.eu/project/id/307169

Its Final Report includes some interesting statements:

"Natural fluids present 4 very important advantages as refrigerants:
• 0 Ozone depletion potential
• Almost negligible Global Warming Potential
• They are not synthetic so that we are 100% sure they are not harmful for the environment.
• Some natural fluids possess very good thermodynamic properties to be employed as refrigerants. Development of equipment specifically designed for them will lead to higher efficiencies than the ones obtained with synthetic refrigerants, contributing to the reduction of the global environmental impact of the future heat pumps and refrigeration equipment."

and:

"the development of improved components and optimization of the unit design and its control strategy cannot be made in “general”, and doing it for each specific application is of course not affordable. Consequently, manufacturers optimize the design of components and equipment only for the applications with the largest market. This means that the components and equipment available for heat pumps are obliged to employ components and designs which have been optimized for Air Conditioning, since the A/C market is much larger than the heat pump market. This constitutes a very important barrier for the future improvement of the heat pumps."

I'm sure there are other points .... and there's a near-infinite amount of reading at https://cordis.europa.eu/project/id/307169/results

The project itself doesn't seem to have published anything yet, or at least I haven't found anything.