We are building a ski cabin with a basement (800 square feet basement). While it will have FAU heat, basements always seem to be cold. Also we want a hot tub. Our visits are spaced so that we will probably drain the hot tub between visits. Can we run the hot water from an instantaneous water heater through the floor of the basement to fill the hot tub? Will the concrete crack? Will the hot tub fill with cold water? Will any heat in the floor dissipate too quickly to be of any use? Can we recirculate potable water through the floor?
I’m assuming that the tubing (PEX?) is cheap enough to lay in the slab before we pour and a couple of valves to manually control the flow might be an easy way to make a basement more comfortable.
Eric
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Eric -
If you are heating the floor from cold, it will take a long time - the water will run cold - maybe for a day or so.
Instantaneous heaters aren't the best for this application. Heating a thermal mass (like your floor) for short visits can be problematic, unless you have a way to turn up the heat a day or so in advance.
The concrete will not crack, unless there are other problems.
You are right - the pex is pretty cheap - its probably a good idea to put it in for future plans. A simple radiant system with a gas or oil fired tank based system might fit your application, and something with a high btu rating would be best.
There are disagreements over using potable water in radiant systems - the main issue is stagnancy. Heat exchangers that isolate the water from potable aren't too expensive, but potable systems are simpler.
Radiant basements are very nice - worth the trouble imho.
I can't think of even one advantage of doing this. They're very different uses. One is a loop that runs continuously, while the other is a one-time fill (per visit). Why not just have a valve that directs the output of the heater to one or the other, such as mentioned here: http://www.digelair.com/faq.asp#202 : "Combo-boilers with separate heat exchangers to keep hydronic loops isolated from domestic hot water supply, in gas & oil-fired systems. Or boilers can be installed with indirect water heaters for DHW needs."
Remember, the water in the loop will be sitting for long time periods. You don't want to use that water to fill tube or for other domestic uses. The temperature is not a problem for the slab, but you should look at the whole system. What temp will be used in the slab, 105 or 125? A hot tub gets water in the neighborhood of 102-104. You don't want to run 125 degree water from the slab into the hot tub.
Reading your post again, I wonder if you thinking of not having a floor loop, per se, but just routing the pex through the floor on its way to the hot tub, and thinking the slab will heat as a result of filling the tub. If so, then no, it won't work. The tub will be filled long before the floor is warm. And you'd need to heat the floor long after that to heat the basement. The slab will take 4+ hours just to get to the point where you feel the heat, and then need constant looping hot water to maintain the heat. They are separate applications and need separate solutions.
If you are going to insulate the walls of the basement, then why not insulate the concrete floor (below the slab). The reason why slabs are cold, is because they are contacting an infinite heat sink (Earth). Everybody insulates themselves from the cold slab by putting in a thick carpet and thick carpet, and you still want to wear slippers.
I live in a cold climate and every slab that I pour now will have Rigid Foam Board Insulation (2") under it. If you do run PEX for RFH, then you will for sure insulate under the slab. Why not do it anyways.
I would put the PEX in for future use of RFH, but not to fill the Hot Tub.
Good Luck, Woodnuck.
I highly recommend building your basement from ICFs and insulating under the slab with 2" of rigid foam. These will significantly increase comfort and reduce heat up time.
Thanks for all the help. Of course, now everyone wants to use radiant for the primary heating system. I'll consider that. Maybe I'll add the pipes now and the heat later when I can afford it.
Sorry about my dyslexic spllng. Merry won't let me post any more under her login if I spell that badly.
Eric