Does radiant heat recycle water
greencomox
| Posted in Green Building Techniques on
With regular radiant heating systems that use warm/hot water to run through tubes in the floor to heat the home, what happens to this water – does it get recycled through the system, heated up again, or does it go down a drain?
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The water travels in a closed loop.... it goes round and round and round....
Some ground source heat pump systems "pump and dump" water in wells, but that's not what you're asking about.
Better yet, once it has equalized temperature in the storage tank the water doesn't need to be heated up as much as it continues around again through the floor tubing. When done right, it is under some slight pressure and only transfers heat from the storage tank. It is not the same water that is in the storage tank. Can be done with on demand units, solar or heat pumps. Lots of options.
A warm floor is way better than forced air heating. When the feet are warm the body is happy. The air temp can even be lower than normal. As in Europe, going up the wall and even in the ceiling has a great comfort when lounging on the couch. No dust flying around the house as well. Easily done on open floors from underneath when the floor joist in the basement are open.
PK
Paul Kuenn repeats one of the traditional chestnuts about warmed floor room heating, that the set point of the thermostat can be lower for a given comfort condition. There is no evidence that this is true. There is plenty of evidence that point source high temperature radiant equipment can create comfort at lower air temperatures - think wood stoves, fireplaces, and those propane towers on restaurant patios - but in all but the leakiest, most poorly insulated homes a heated floor runs at too low a temperature for radiant effects to kick in. Even conduction effects are limited - heated floors typically run twenty degrees or more below body temperature. The heat flow goes from foot to floor, not vice versa. Want warm feet? Wear a pair of slippers.
I have radiant floor heat in my home. There is no storage tank normally in these systems as PK describes unless you're supplementing it through some kind of solar heating through a separate closed loop system. The water in the radiant floor heat system flows through a closed loop, through the floors, back into the boiler, and out to the manifold to the floors again. The only tank in the system is a small 1-2 gallon bladder style tank used to help with the thermal expansion and contraction. The volume is not enough to ever really contribute to the heat as PK describes.
In my situation, radiant floor heat has been a nightmare as the person I bought the home from lied and said it was insulated beneath the slab. After my first winter this was very apparent it wasn't so I demo'd a corner in a bathroom to find no insulation. Not only is this very inefficient, it is also against IRC code which requires R-10 beneath the slab and R-5 minimum around the sides of the slab at the foundation wall.
Michael, That's an awful situation, and I've found less extreme variants of that experience are quite common around here. The standard detailing sent to me by a contractor installing radiant floors in Washington State showed an monolithic slab with two inches of foam underneath and none at the perimeter.
There is also the high cost, which Martin dealt with in a recent blog (and of course received the expected abuse from industry insiders)
Malcolm, to my knowledge, if the jurisdiction follows IRC, R-5 is required along the vertical sections at least 2 feet deep. I'm guessing though that since most inspectors drive-by or don't fully understand the code, the miss this. It's too bad people are idiots and decide to save a a dime during construction as they trip over $100.
The majority of the cost in hydronic, from what I can tell, is in the boiler (almost $2k for 20kw), design ($950 is normal starting price), and installation (use your imagination on this cost). PEX is relatively cheap, along with most standard plumbing fittings. Zoned systems will definitely be a bit more expensive.
With John Siegenthaler's (SP?) book, Modern Hyrdonic Floor heating and basic engineering knowledge, designing and installing the system can be pretty cheap if done yourself.
In my case, I will re-use the boiler and some of hard pipe, regulator, expansion tank, etc. and have to buy a few thousand feet of OFC PEX to lay into the slab. The alternative is a 20KW boiler running 24 hrs a day to maintain a 70 degree home. This would cost me $1500+ a month if I didn't use my woodstove instead at 13 cents a KW.
" If you think using a professional to do something is too expensive wait till you use an amateur " . Just got back from a road trip to tear out completely and re install properly one of these systems . Be vary careful whom you hire , I make a living out of repairing or replacing systems installed by the "very best " in many areas . Problem is that most of these experts got and or get their education from salesmen whom work for manufacturers and have no idea about what the really important things are in any hydronic system . there is no magic equipment and quite possibly more than 90% of experts are not experts at all . I suppose I should not care because this is my bread and butter but I'd much rather see it done right the first time .
Unfortunately, there are very few "professional" installers that do hydronic flooring that know what they are doing. The licensed HVAC contractor, separate builder, a separate design firm, and building inspector all failed to recognize insulation was needed in my case. Even if a professional is doing the work it would be wise to ask questions, do your homework, and supervise when you can.
You are all correct. Yes, I include solar in everything. It paid for itself in 14 years up here in NE WI where they say it is cloudy but somehow the solar fills in 70% of the winter heating needs. 5F degrees this week (way too early) and we had FREE hot floors running at 110F. Toasty especially because we both ride our bikes to work every day of the year. Usually the floor is running at 95 and I don't know anyone who's feet are near core temp. Even folks with fevers usually are at 95 at extremities.
Active solar thermal systems to provide space heating are almost never cost-effective. The cost of the active solar equipment would be better invested in envelope improvements (thicker insulation, air sealing measures, or better windows).
The cost premium for in-floor radiant heating systems compared to simpler heating systems is also hard to justify.
For more information on these issues, see All About Radiant Floors.
Martin,
Still waiting for the plans and details for a PH and PGH from you to show the err of your thoughts on Radiant . When will you send them ?
The challenge has been in place for awhile now , let's put it to bed . Your readers deserve to know and see the truth about radiant .
Richard,
You wrote, "Still waiting for the plans and details for a PH and PGH from you."
I'm not sure why you think that I should provide a plan for a Passivhaus or a plan for a pretty good house. There are lots of ways to build houses, and Green Building Advisor tries to provide advice to lots of builders and homeowners in lots of different climate zones. We do not provide or sell house plans; other sites do that.
If you are interested in construction details, we have those at GBA -- hundreds of them. Here is a link to our details: Strategies and Details.
When it comes to providing "the truth about radiant," I have tried to gather together the best experts' advice on that topic. I presented that information here: All About Radiant Floors.