Air-to-air heat pump into under-floor void?

Something I've been pondering (which falls into the Crazy Idea But It Might Just Work department)...
Much of the cost of heat pumps systems is getting the heat into the building. Conventional rads require high temperatures which HPs can't provide efficiently. UFH is lovely but expensive. Replacing conventional rads with fan-assisted ones is also pretty expensive.
I also note the fashion for stripped floorboards in houses with conventional suspended floors with ventilated under-floor spaces. Presto! under-floor cooling in winter!
So how about an air-to-air heat pump pushing warm air into the under- floor void, with the air-bricks sealed up? The warm dry air should discourage damp problems (the reason for the air bricks and thus the cold air under the floor). Assuming non-tongue-and-grooved floorboards you've got a ready distribution of warmed air through the floor into the ground floor rooms. You may have to duct some warm air upstairs and into solid- floored back extension kitchens and (more importantly) bathrooms in houses with that layout, and you'd need a source of water heating (electric or solar). But as a cheap renewable-friendly system, what am I missing? (Except anyone actually having tried it, natch!)
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John Stumbles

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2 problems a) you've got no insulation on the ground, so much of the heat will be lost. Perhaps you could lay some sort of liquid foam down, I dont know. b) You'll have the damp problem from hell. Whether you could deal with that with the foam layer plus a dehumidifier I dont know.

clever
Warm air into a ceiling void would wipe out the losses due to no insulation. It would be horribly dusty until all the blowable dust had been blown out.
NT
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Split units cost about £600 each. They would be the easiest way to use ASHP for space heating. Its also the easiest way to get cooling in summer. You might want one with an additional electric heating element just to boost it if it is very cold outside or use a fan heater.
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Lack of insulation, as mentioned.
The humidity problem would be one of excessive dryness, rather than damp. Cold air can only hold a small amount of moisture, even at 100% Rh. Warming up cold outside air will give supply air at an unpleasantly low Rh and usually causes problems with contact lenses, static, thin timber shrinking and cracking, etc..
The other problem would be entrained dirt, causing streaking at any point where the air enters the living space and dust settling on every surface. I can't see it working unless the void was accessible and spotlessly clean, i.e., quite unlike traditional floor voids.
It did work for the Romans, I'm sure it could be made workable.
Ventilation is most effective, IMHO, if supplied filtered at at a controlled positive pressure. Clean air is supplied and air leaks out of the building, keeping airborne dust at bay.
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On Wed, 09 Nov 2011 01:43:06 -0800, Onetap wrote:

Heat loss downwards through the sub-floor: - standard U value for solid-on-earth floor is 0.36 - temperature of the ground beneath the house is, I think, something like 10C. Let's be pessimistic and say 5C - suppose heat pump is pushing in air at 30C (you wouldn't want higher) - then for an 8m * 4m floor (32m^2) you'd lose (32 * 0.36 * (30-5)) = 288W

One presumably has the same problem with air-to-air heat pumps with the outlet fixed high on the wall (as illustrated in manufacturers' literature).

Point. But you find a lot of houses with bare stripped boards on the ground floor; depending on the relative air pressure across the house due to wind there must be times that one has draughts blowing up through the cracks, so same problem. And an A-A HP wouldn't generate a howling gale.
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John Stumbles

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John Stumbles wrote:

none of which means a damned thing if the subfloor is vented and full of a howling -5C gale.

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On Thu, 10 Nov 2011 11:32:54 +0000, The Natural Philosopher wrote:

which I why I said in my original post:
"So how about an air-to-air heat pump pushing warm air into the under- floor void, with the air-bricks sealed up?"
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John Stumbles

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John Stumbles wrote:

well bad juju in terms of heatloss down through the ground and around the walls.
putting 45C air on top of cold ground......
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harry wrote:

Underfloor hot air heating has been used, the Roman hypocaust and related systems. But that was using a floor designed and built for it, not a retrofit to wooden floors (and using fires, not a heat pump).

By saving the cost of installing proper ducting designed for warm air heating.

I suspect to be worthwhile you would have to take up the floors and insulate the ground and walls of the void, by which time there's not much void. And if you are doing that work, you could probably install underfloor heating while you had the floor up....

That's equally true of any air source heat pump using outside air.
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On 09/11/2011 00:06, John Stumbles wrote:

I suspect to overcome the problems one would need to install some form of underfloor ducting... Using the void space directly, while attractive, might have all the practical problems others have listed as well as that of filling the house with the smell of a musty under floor void, laced with the fragrance of decaying rat etc. The mustyness may dissipate with use, but I can't see all the warmth making the space any less attractive to small critters of all sorts.
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Cheers,

John.

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