To any HVAC specialists: I’m working on a design for an allergy sufferer and recently came across a mention in an architecture textbook of a hybrid hot water/cold water heating/AC system that would use the same pipes, either in the floor or in wall or ceiling panels. Can’t find anything on this on the web. Anyone heard of such a thing? Maybe in Europe?
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Easy to do. Hard to do well. The details:
Anything through which you circulate hot water in the winter could help cool your house in summer if you circulate cold water. Broadly, the two problems to overcome are:
1) Cold water is only going to be about 30-40F below your desired room temp of 70F. Whereas hot water can be 100F above. So there is less temperature gradient and therefore less cooling (in BTU/hour) than heating for most systems and climates. Maybe okay in extreme northern climates (where -40F is a design temp and you only need to take the edge off of 90F days). But in Phoenix, for instance, the heating system isn't very large and yet the cooling load is. Designs like radiant floor heating (with its large surface area) can do a lot with a small temp difference (heating many houses with only 100F water, a delta of 30F). Baseboards or "radiators", designed for high temp water, wouldn't do as well.
2) Condensation. Less of a problem with dry, hot days. But the Midwest and New England get hot, humid days. If you have any cold surfaces (walls, ceiling panels, etc), there'll be condensation and you need to collect those drips and drain them off. Air conditioners and dehumidifiers have that function designed into them. A large, cool surface area (like a radiant floor) is less prone to that than a very cold small area (baseboards or radiators).
I've done cold water cooling systems but carefully considered what the realative and absolute humidities would be and/or addressed the condensation drainage. It can be especially attractive if you have unmetered very cold water like I do in my own house.
A quick test: Do your towels mildew on the towel rack during the summer? Then don't even go down this road. If your summers are hot AND dry, then it may work. Report with, as the realtors say, your location, location, and location.
With forced air systems, this can be easier to do because all the air goes to one point to be heated up. In cooling mode, all the condensation would occur there and that makes it easier to deal with.
The site is in northern Virginia, and I suspected condensation would be a problem. Forced air heat/AC is the way everyone goes here and I know there are good reasons for that, but forced air can really increase allergies.
We may have to go with the added expense of both a hot water radiant heating system and a separate AC system (perhaps with smaller ductwork). That leaves half the year without forced air, at least.
Thanks for replying.
Pure curiosity question:
What cools the water in these systems? Is it refrigerated, or is it just sucking up water from the main line to the house?
For the record, we have HW heat in our house and are currently getting ready to add forced air cooling with a Unico mini-duct system. Seems like an acceptable compromise.
Couldn't one use a high-end/electrostatic filter and actually reduce allergy problems with forced air?
MKHansen: With summers the way they are in the Mid-Atlantic region, I'd suggest you go with some mass-produced, well-designed-to-drain-water option. If you are willing to retreat to a few rooms in the house on those hot days, you can use single-room air conditioners that mount in the window. Not as upscale as whole-house duct work, but cheaper to install and cheaper to operate.
Darrel: Typically a refrigeration unit would chill the water (or gycol mixture). The unit's condensor (the heat exchanger that sits outside) would ideally be in the shade, on the north side of the house, with good air circulation around it. Personally I would use the unmetered, 39F city water that comes to my house, but most people don't have that option.
Yes, a electrostatic fiter can greatly reduce the dust/pollen/allergy problems of forced air. That, and/or an HRV are not utilized as much as they should be to increase indoor air quality, especially in tight homes.
David Thomas Overlooking Cook Inlet in Kenai, Alaska
"...but forced air can really increase allergies."
I have to take issue with this statement, because it is not true. Forced air systems do not increase allergies (or create dust, as many believe). Forced air system only circulate the air that is already enclosed within your house. They can, if setup properly clean/filter, dehumify or humidify, and add needed fresh air to your house, all of which a hydronic system cannot.
If there is dust in your house, a forced air system will circulate it. If there are allergens in your house, a forced air system will circulate them, too. Niether are created or "really increased".
Also, "We may have to go with the added expense of both a hot water radiant heating system and a separate AC system (perhaps with smaller ductwork). " Ducts for a forced air system are sized for cooling air flow. If you go with "smaller ductwork" (by that I'm guessing you mean smaller than would normally be reuired for the system in mind?) as a way to save money (or for some other reason?), it will not work well and will be noisy.
Don't get me wrong, I think RFH is great. But as Dave said, radiant floor cooling is difficult to get to work, and in all but the dryest climates will not create a comfortable house. You have to move air over a cooling coil to dehumidify it. If you cool the floor in VA, it will get wet.
Tim