Adding solar hot water to a house remodel, we wonder how practical it would be to install a heat exchanger in the gas forced air system ducts to circulate solar heated water, using the “free” heat to supplement the furnace heat. We wonder if, in addition to heating domestic hot water, the solar system could contribute to heating the house? All thoughts, suggestions are welcome. We’re in a Denver suburb.
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I'm not an HVAC guy, but a heat exchanger alone won't do the trick. You need a large storage capacity for the "free hot water".
FHB did an article on a home that used that technology, and if memory serves me, they had a couple of large storage tanks.
Might have been in one of the last two issues.
Dave
You're only going to get "so much" heat from the solar system- if you choose to use some of it to heat the air in the FHA system, just remember that you're going to get less capacity at the domestic hot water system.
Bob
It depends on the capacity of your solar collectors and storage capacity.
I've got a vacation house with solar thermal collectors that preheat the water before it goes into the boilermate. On a sunny day the temperature of the water in the tank(I think it's a 120 gallon tank) will easily get to 120 F so when there is demand for hot water, my boiler doesn't have to come on.
Since it's a vacation place there are long periods when it's not occupied in the fall winter and spring, hence no demand for hot water - but the boiler still comes on to heat the place. My idea is to put a baseboard radiator in that runs off the glycol in the solar collector loop, thereby saving a bit of heating oil and maybe sacrificing a bit of the heat from the preheated boilermate supply. I'd have valves to isolate that radiator in the summer.
If you have enough capacity, I can't imagine why something like that wouldn't work for you. You'd have to get it engineered but, hey, why not?
Good luck and let us know how you make out.
It'll work, obviously depending on capacities. I dismantled several such systems here that had one common, major, flaw. The exchanger was upstream of the furnace filter. Did a great job of filtering, of course was never cleaned and greatly reduced the air flow.
When I pulled the exchangers, the elec bills decreased dramatically, both winter and summer. Make sure your filter is upstream, keeping the exchanger clean. Not vice versa.
Our next house will have one of those recycled exchangers in our air system.
PAHS Designer/Builder- Bury it!
Hi,
These guys carry such coils, and provide some design information.
http://www.magicaire.com/productd.htm
How much collector area do you have?
Gary
http://www.BuildItSolar.com
There was a thread some time back that mentioned the possibility of the legionnaire's disease bacteria developing in tepid water. Don't know the actual risk, but the bacteria can develop in stagnant water between 20 degrees and 45 degrees Celsius. Need to do some more research on it before I set up my own heat exchanger system. Any thoughts about this are appreciated.
Some thoughts... I'm a mere homeowner like yourself, but I have a solar system in an area that is much more adverse to having them installed than you are.
One great aspect of your location is that the insolation year-round only varies 20% or so. We have a 80% variation due to clouds, winter, etc. Thus, your location is perfect for DHW and, to a lesser extent, space heating. The usual lack of clouds (you lucky guys!) only helps.
One thing you have to consider for space heating is how you will handle the summer conditions when there is a bit more sun, yet no heating load. That heat has to be dumped somewhere, whether it's a hot tub outside, etc. you get my drift.
Naturally, if you angle the collectors such that they get little summer and lots of winter sun, (usually lat + 15deg, though you can go steeper) then that'll help in part. Passive solar uptake in the home design is also a good thing to aim for (assuming you design the place to keep the sun out during the summer).
There are lots of hydro-air coils available, and if you're going to go through the trouble of retrofitting all this stuff, I would urge you to consider installing a modulating, condensing boiler at the same time. They do not have to be very expensive, yet they can save a lot of energy doing space heat and DHW preparation. In such a scenario, I would consider buying two indirects for DHW, one heated by the boiler, another pre-heater tank for the solar. Butler solar also sells inexpensive dual-coil tanks that you could use exclusively with the solar until the furnace conks out, then make a switch to a boiler.
Given how expensive a good solar heating system is, I urge you to do everything you can to minimize the infiltration and heat loss. Every BTU saved in that department will repay itself quickly when it comes to making your home net-zero, energy-wise.
Hi,...
"Naturally, if you angle the collectors such that they get little summer and lots of winter sun, (usually lat + 15deg, though you can go steeper) then that'll help in part. Passive solar uptake in the home design is also a good thing to aim for (assuming you design the place to keep the sun out during the summer)."I think vertical mounting can be a good choice for collectors that are only used for space heating. Vertical collectors that have a reflective snow field in front of them collector more energy than a panel tilted at Latitude +15, and they don't overheat in the summer.
My air collectors are mounted on a south facing wall -- I shut them off at the start of summer, and they run at stagnation all summer -- the temperatures don't go above 180F in the collector.
A vertical collector at 45 latitude sees 1600 BTU/sqft-day in Dec, but only 770 BTU/sqft-day in mid summer.
You can also build a small overhang to shade them from summer sun, but still let the low winter sun get to them.One caution is that some evacuated tube collectors don't like to be vertical.Gary
http://www.BuildItSolar.com
Good points. I have seen several solar installs with vertical collectors in the FRG and your reasoning makes sense. Plus, it ought to make drain-back designs a snap.
My air ducts have 2 surplus automobile air conditioner condensor coils in the ducting after the filters that I circulate water (from fireplace coils vs. solar) thru. Have worked without any failure or leaks for over 25 years.
One is out of a '64 Imperial, the other beleive was from a '70 Chrysler. Not sure how the cheaper all aluminum condensors found in most cars after '90 would hold up, but the many I've scrapped out seemed sturdy enough for the lower temp solar heated water unless you have highly PH negative water.