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I have to agree with Scott that an heat exchanger could be little more than “a very expensive vent” in your application. You should also consider that it might be a very inefficient vent since the temperature differences you mention between inside and outside are small. I believe the efficiency of these machines (In Canada, they are usually called HRV’s – heat recovery ventilators) increases with the temperature differential and, if they work at 100% efficiency, the temperature of the air coming out of the HRV will be the average of indoor and outdoor temperatures. With a small temp diff, you don’t stand to gain much. And, of course, it won’t work at anywhere near 100%, especially if installed in a location like a crawl space where it’s unlikely it would get cleaned.
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I'd like some suggestions on an air-to-air heat exchanger to ventilate under my house - I believe I've seen such items in super-insulated home discussions.
Basically, I want to insulate the perimeter of the crawl space (PIRF) rather than under-floor in my Radon-prone area (Atlanta)? The 1886 house is 50 X 60 on the main floor, on a 2'-4' crawl space that is currently vented, bricked (foamed and light-tight), and un-insulated. I'd like to save the hassle of 3000 sq ft of floor insulation by insulating the 650 sq ft of perimeter, using the savings for a heat exchanger, and using the 100+ year old dry ground as a moderating thermal mass.
Any thoughts?
*If I were you, I'd skip the air exchanger all together. What do you expect it to do? It's not a cheap way to rid the crawl space of radon. I'd insulate the perimeter as you plan but first install 6mil poly on the 'floor' and seal all the current air vents to exterior that let warm moist air under house.Seal any air leaks between house and crawl space to stop radon migration into house.Mike
*Mike - I guess I need to be more clear. The way I understand it, radon gas from the ground would easily come through the gaps in the plastic (80%-90% is maximum recommended coverage for vapor barrier). Really high radon areas have to have fan-forced ventilation through perf-pipe UNDER a slab, so it'll sure get through laid plastic on a crawl-space floor. The reason for the heat exchanger is to be able to circulate fresh outside air into the crawl space without wasting the heating or cooling energy you've already (through your uninsulated floor) put into the outgoing air - i.e. in the winter, I want to push out the radon-laden air from under the house, but use the outgoing 50-60 degree air to warm the incoming 20-30 degree air; the reverse in summer.As far as sealing leaks between the house and crawl space, the house has all heart pine floors over plain diagonal 1 X 12 subfloor, with no tar paper (115 years ago, you know), and 16 rooms/closets with walls on the subfloor. It would really be impossible to seal effectively enough for a gas that can go through minute cracks in a slab. I guess I'm just technically interested in doing the job the best possible way on my personal house - it's not really a problem. With three condensing gas furnaces/air conditioners, 6 damper and thermostatically controlled zones, the HVAC works well now - I'm an engineer before a GC, so I want to able to demonstrate "the best way" to old house clients.Long post, but I love this stuff!
*Not an expert by any means but....It sounds like a very old, very large and very over heated house and I'd take it in a minute! As I recall, any house fitting this description is generally poorly insulated and sealed. I believe it was even the guiding construction rule of the time to keep things as loose as possible. I don't think Radon is considered a problem in a house like this (even if you're built on the nuclear plant itself) because of the constant drafty air change. The use of the word "air-exchanger" and "Radon" in the same conversation sounds more like the scare tactics of a sales rep. But assuming you really do have a problem and you really do have a well sealed and insulated home then here's my penny.To ensure you don't have Radon leaking into the house you would have to maintain the crawl space at a negative air pressure (or the house at a positive pressure). I don't think circulating fresh air through an exchanger will be enough, because your bound to get some cross flow through that leaky floor. So depending on the wind, an open window and who knows what else, radon laden air will travel from the crawl space to the house and back. Needless to say, if the crawl space is negatively pressured, it's ultimately going to suck air through that leaky floor (the direction you WANT it to go if it's going to go anywhere) and send it right on out to the world, taking your heat (or cold, depending on the season) along for the ride. An Air exchanger is designed for balanced flow. Incoming air = outgoing air. If fresh air to the space is coming from the house, that amount of air won't be coming through the exchanger from outside. In effect, your air exchanger will simply become an exhaust fan sucking it up from the crawl space and sending it out doors without returning new fresh air (which is coming from that leaky floor). Or it might just become another source of fresh air without returning the old back outside again depending on how things are setup. Sure, it will capture the heat (or cold, depending on the season) but it won't be returning it back to the space. A very expensive vent.Using the ground mass under the house sounds like a good idea, but it will only ever work well if you can separate it from the rest of the ground. The earth from below will be forever trying to keep that ground at some temperature that doesn't work for you (you can try, but it costs alot to heat the whole world!). Even if you can live with the final earth temperature near the center of the house, without any kind of below ground insulation around the perimeter, the edges of the crawlspace will continue be icy cool in winter and too warm in summer.Working in a crawlspace is never fun, but if I had an unlimited budget, a 100+ year old house, a crawlspace heating and radon problem, I would hire somebody to spray Icynene insulation under the floor and make sure the crawl space perimeter is cross ventilated well enough to make use of natural breezes for keeping it fresh. If I'm to believe the literature, Icynene (spray foam type product) seals every knook and cranny to create a vapour barrier all its own. It would probably be serious over kill for its insulation value (should make a big difference on the comfort of the floor), but it would create the air barrier you need to keep Radon from flowing into the house. Even if the floor "sealing" is not 100% it should be more than enough for natural wind conditions to keep it clear of gas. If you've got a long history of people suffering from Radon poisoning in the house (remember, it has had this same problem for 100+ years), you could go one step further by adding a venilation fan on the prevaling down wind side of the house to supply to create a slight negative pressure in the space.(I love this stuff too!)
*I have to agree with Scott that an heat exchanger could be little more than "a very expensive vent" in your application. You should also consider that it might be a very inefficient vent since the temperature differences you mention between inside and outside are small. I believe the efficiency of these machines (In Canada, they are usually called HRV's - heat recovery ventilators) increases with the temperature differential and, if they work at 100% efficiency, the temperature of the air coming out of the HRV will be the average of indoor and outdoor temperatures. With a small temp diff, you don't stand to gain much. And, of course, it won't work at anywhere near 100%, especially if installed in a location like a crawl space where it's unlikely it would get cleaned.
*Scott and Ron - thanks tons for your input. I guess I forgot my thermodynamics, that thing where heat transfer efficiency is tied directly to delta T. I'm still thinking what to do!
*As I understand Radon Gas, it is present in the soil until negative pressure draws it towards somewhere else. So if you want to put any kind of a fan under your house to reduce the Radon level, make sure that it is drawing away from your house (a fan drawing air out from under the crawl space will increase the negative pressure there and may INCREASE the amount of Radon present). Opening windows and blowing the air with fans can increase Radon levels in structures for this reason. Think of a house as a big chimney, and the Radon levels in it can be a function of how your house draws the stuff up from the earth.Maybe one way to treat your situation, would be to lay perforated drainage tile just under the surface of the earth in your crawl space, hook it up to a Radon Abatement Fan and use the fan to draw that air out. Then you want to provide some barrier between the soil and your living space: plastic layed over the soil, and taped (should really be over the soil in any crawl space anyway for moisture control). This system provides you with a negative pressure drawing the Radon truly away from your house.Then, if you add the icynene insulation to your floors you also get warmer floors.I am not a certified Radon whatchamacallit, but this is the sum of all my research on the topic when my house tested high.The other thing learned was that "Radon Poisining", as you called it, is things like lung cancer that take a lifetime to show up. And they don't always happen. And cigarette smoking probably causes more cancer than Radon does but we still do that. So Radon Abatement, while noble and undoubtably good for us, is maybe not totaly clear cut a science: the "safe" Radon levels in homes are different all over the world, from 4 picocurries (sp?) in the US to 12 in Europe to, I think 20 somewhere. This is all off the top of my head (the data is all in the other office right now), but gives the general idea.