Replies

1. 
   Cloud_Hidden | Jul 21, 2001 09:27pm | #1

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   No expert, but have done it here. Air doesn't have the heat transfer rate of water, so that's one disadvantage. Maybe you can post this over in the Energy forum--it's less likely to get lost over there.

   Installing the tube is really easy, with very few "rules". The important part is getting the specifications right so that the tube size and length, manifolds, pumps, heat source, etc are all sympatico. Of course, that's for a hydronic system.

1. 
   David_Thomas | Jul 22, 2001 04:12am | #2

   *Short answer: Use cross-linked polyethylene (PEX) 1/2" or, for long runs, 3/4" tubing in a hydronic system.Longer discussion: The Romans used pneumatic radiant floors in the houses of the rich. A fire was lit in a small basement and then the the exhaust gasses flow under the floor. The "ducts" were all fashioned from masonary and it is unclear (at least to me) if a negative draft was maintained throughout the length of the duct at all times. So maybe the fall of Roman Empire was caused by chronic carbon monoxide posioning rather than the more commonly suggested lead posioning from their plumbing.Of course you would use some kind of air-tight ductwork but that still leaves the very significant problem that Cloud mentions, e.g. the very low heat capacity of air. 3 gpm flows through my 1/2" hydronic PEX tubing. If it goes out at 90F and comes back at 80F, it will deliver 15,000 BTU/hour. So 3 such loops give me more than enough heat delivery for my well-insulated house, even at -40F. To deliver as much with air, you have to move 1365 cubic feet per minute (cfm) through each loop. Or allow for a higher temperature drop, which you'd certainly do. Maybe into the floor at 120F and out at 80F. (120F might discolor vinyl flooring, shrink your wood flooring and sorch your feet, but it is concievable). That would get the air flow down to 340 cfm but that is best moved in 6" pipe. 4" pipe could be done, but the higher pressure drop would require more expensive electricity to move the air. 4" or 6" pipe is NOT cheap to install and, if installed within a slab, raises structural questions about not having continuous concrete (unless it is a monsterously thick slab).I can imagine a set-up where a gravel bed under the slab is used to move the air from one manifold to another. That could be cheap to install and allow for very efficient heat transfer, but so many issues are raised that either a plumber who can design and build a furnace himself or an engineer who can plumb an entire hospital should be involved.Stick with hydronic tubing in the slab. It works well and is well understood by the more competent installers. If you start in the phone book, CHECK THOSE REFERENCES! Better yet, ask around for people who are happy with their radiant flooring systems, after a full year in the house. The controls are (should be) different than the thermostats for a standard furnace. An installer who understands that can do a good job for you. -David

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   Rick_Wainman | Jul 22, 2001 04:17am | #3

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   I am building my first home with a radiant slab. I have qualms about putting water pipes into a cement floor for long-term. I heard that you can use forced air through a slab. Has anyone out there done this? What tips, techniques, etc. do you have? What type of ducting through the slab did you use? Can you come to my place and show me how? (bring your friends along)