in-slab heat using hot water heater
oldcheese
| Posted in Energy, Heating & Insulation on
I am interested in trying to heat 650 sq. ft. in northern Wisconsin with in slab tubing and using a water heater as the heat source. I am hearing talk about the water heater not lasting very long due to some sort of condensation problems. Does anyone have experience with this? We can get as cold as -30. Thanks in advance for any insight .
Replies
If you're talking about a combination water heater with a built-in coil heat exchanger, yes, they have problems. Some other models out there have their own issues of course.
Use an external plate heat exchanger and make sure the space is fairly consistently heated and you should be ok. If you average 30 BTUs/sq ft under design, you're under 20kBTUs/hr at design and I doubt a more expensive heat source will save you enough in the long run to justify the expense. Of course that's a blind back of the envelope figuring with a list of assumptions a mile long.
The second part of the question is, however, what are you heating? If it's a home, you may want to put some thought to comfort and smooth operation. If it's a shop, perhaps less critical.
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it is very doable, in my experience, to use a conventional water heater as a heater for a radiant slab system. I've put quite few of them in here in northern WI over the past 15 years.
condensation is actually more of an issue using conventional boilers as they, unless they are condensing units, are not good at handling the cooler return water temps that slabs typically produce. water heaters, on the other hand, are designed to handle very cool incoming water.
the primary down side, in my estimation, is an appliance that is lower efficiency than most boilers, though there are those who argue that point.
it is important to make sure the recovery rate of the water heater is large enough to handle your load. this rules out electric water heaters for many (larger) applications but probably wouldn't for your small place. my own slab is heated by a single 5.5kw electric water heater element - or by a wood boiler. for me, off-peak electricity from my utility (Bayfield Electric Coop) is significantly cheaper than the other primary option, propane. not all utilities offer this kind of deal though. if you go with propane, I'd recommend a sealed combustion unit. american water heater co makes a unit that I have used ofen which is called "direct vent" and is sealed combustion without involving a fan. it also has an extra tapping on the side which is very handy for plumbing a space heating loop.
the idea of using a water heater in a dual-use mode where it does both space heating and water heating is controversial. the issue is that the space heating loop will be stagnant for significant periods and could potentially start growing bacteria and other stuff that you wouldn't want to consume. I take the conservative approach and separate the space heating loop off with an external flat-plate heat exchanger. there is at least one on-line radiant outfit that promotes dual-purpose systems without heat exchangers.
if you are wondering about on-demand water heaters as heat sources, I have heard that condensation may well be an issue with gas fired units used for radiant heat. this is because the actual heat demand is much lower for space heating than on-demand water heating and the unit ends up operating throttled way down and can't boil off the condensation the way it does when in water heating mode. thus the heat exchangers are prone to premature failure. I don't have first hand experience with this (yet) but have been told this by a distributor of the Takagi units, which I believe is the only make that promotes use as a space heating appliance.
bottom line - very doable to use a water heater as a heat source - especially for a small place like you have.
Roy
Edited 3/28/2006 9:00 am by royboy