In a typical home heating system using a combustion to provide heat, and forced hot air to convey that heat throughout the house, the heat exchanger (between combustion products to household air) is made of a material (usually metal) which is permeable to heat, but impermeable to material substances.

What if, instead, it were an entropy exchanger, permeable to both heat and water (but not gasses), and coated with a hygroscopic substance?

Since combustion of hydrocarbon fuel creates H2O as a product, this entropy exchanger would transfer humidity from the flue gasses into the home's air.

This "free" moisture could completely eliminate the need for a conventional humidifier.
-- goldbb, Oct 26 2009

Condensing Boiler http://en.wikipedia...i/Condensing_boiler
More efficient [8th of 7, Oct 26 2009]

PassivHaus article http://www.greenspe...ion/passivhaus.html
a standard for insulation and minimal air leakage and hence heating requirements [Loris, Oct 27 2009]

I keep seeing "Humilifying France", which is a much better idea.
-- MaxwellBuchanan, Oct 26 2009


I wholeheartedly agree.
-- kaz, Oct 26 2009


Do it. Do it now.
-- 8th of 7, Oct 26 2009


Don't most homes have standard central heating, where the boiler heats water that circulates through radiators?

But, in any event, it's going to be very difficult to find a material which is permeable to water vapour but impermeable to combustion products such as CO, smellone etc.
-- MaxwellBuchanan, Oct 26 2009


Even the cleanest exhaust from a forced air heater is not clean enough to run through an expensive filter. The water recovered could simply be collected at the condensate pipe which is already present in modern efficient forced air furnaces and pumped across into the airstream. That is if the water is clean enough. I'm not sure that it is.
-- WcW, Oct 26 2009


[+] you'd have water, maybe some dissolved sulphur compounds... should be good enough to put into the air with filtering. Wouldn't hope too much for membrane life though.
-- FlyingToaster, Oct 26 2009


Usually the problem with houses which need heating is too much damp, rather than not enough.

That's certainly been the case in every house I've ever lived in: Inside, the air is warmed up. Water in it then condenses onto cold surfaces like walls and windows. Air enters and leaves the house through leaks and when doors are opened etc, so there's always more moisture coming in.
The better insulated your house the smaller this effect will be, but still, there's no real need for extra humidity because if nothing else the occupants are producing some.
-- Loris, Oct 27 2009


[Loris] Humidifiers are not uncommon on forced air furnaces, as heaating the air greatly reduces the relative humidity, so that part of this is not bad. Operating properly, in a properly insulated house you shouldn't have to much moisture, I think the problem occurs more with steam/hot water heat, where the air doesn't move, so moist air collects in particular areas.

I agree with [MB] finding a material that will pass water vapor but not CO and CO2 is going to be difficult at best. One that excludes incompletely combusted hydrocarbons is possible, but you're probably looking at a multi-stage filter to get any service life.
-- MechE, Oct 27 2009


I've no personal experience of forced air furnaces. They sound like just the sort of thing to use if you like your possessions saturated with diesel. Are they typical in America?
They're certainly not typical here (uk) since we've got a natural gas pipeline infrastructure.
Presumably forced-air furnaces don't create a moisture problem because they blast said moisture outside through the leaks.

I suggest that in his own understated way MaxwellBuchanan was saying the water permeable/ gas impermeable membrane idea would not be an option.
I think it would probably be possible to separate water from the bulk of the gas mechanically, i.e. condense the water out and pass round a u-bend. It would still have a load of shite you don't want dissolved in it though.

One source of moisture with sealed water-circulating systems is from the drying of clothes - when people dry them on radiators that puts quite a bit into the air. Combining the two problems, could you add an airing cupboard-style enclosure on your hot-air route? That way you'd get the moisture you crave and dry your clothes for free - when you needed space-heating, anyway.
-- Loris, Oct 27 2009


Forced air furnaces are natural gas, oil, and rarely coal or wood. They are more common where central air conditioning is more common, as they use the same ductwork and circulating fan. They use indirect heating same as a boiler would, but the heat exchanger is air-air rather than air-water. Thus there is no more tendency for the exhaust to migrate into the rest of the house than there is from a water system. They are not particularly leaky, and have no outside air requirement for the ducted air (though in large, well sealed buildings outside air often is brought in to maintain freshness).

With respect to humidity, there are two different problems with humidity in the winter. If you simply heat the air without adding any moisture, it will be very dry. This is responsible for high static and dehydration of occupants. It will occur in poorly to moderately well sealed buildings where some outside air is brought up to temperature on a regular basis. Correcting this can lead to the other problem: If the building is well sealed, but not well insulated, moisture will build up inside from occupants and possibly from an active humidifier and you can get condensation on poorly insulated surfaces. The correct solution to this is almost always better insulation, not dehumidification, as an outside temperature just above freezing will leave a RH in conditioned air at an uncomfortably low 20-30%
-- MechE, Oct 27 2009


To clarify - the leaks I was referring to are those from the living space inside the house to the environment. For old houses these can be reduced but are practically impossible to eliminate. Even new houses where it is specifically minimised must have some air exchange - as otherwise the oxygen will be used up and the inhabitants will die.

If your system forces air around the house then you must increase the pressure in some places and reduce it elsewhere. This will drive air through any leaks which may exist, even if you're just moving it around internally.

Without air exchange to the environment there is logically no need to *continually* add water to the system. Think about it. Okay, you'll need some initially - I'd already taken the point. But what about after the place is up to temperature? Add more and it must be building up somewhere!
Therefore we can be certain that either (a) water is only added while warming up, or (b) the water is escaping the house.
-- Loris, Oct 27 2009


If your system has no air exchange then you either have a lot of plants or die rather quickly.

A perfectly sealed house is more efficient with fresh forced air moving through a heat exchanger (and possible enthalpy wheel) than any other option. A leak (the admittedly more common method of air exchange) is probably about neutral depending on where it is. In a water system in a tall building you probably have more natural stack pressure than a forced air system generates (especially in passive solar housing, where the stack effect is used to generate air movement for uniform heating).
-- MechE, Oct 28 2009



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