The 3/8" Pex is 1/2" o.d. and the 1/2" is 5/8", when not embedding in concrete, the pipe is just fit into a matching size groove with a min 6" radius at the ends. there are companies that sell pre grooved plywood. They make a 1-1/8" thick plywood that can be nailed directly to the joists and you just drop in the pipe (perfect for new construction) there are also some that are thinner and are designed to go over an existing sub-floor. Most of these panels have an aluminum coating on the bottom to reflect the heat upward. A third option is to install the piping from below the floor between the joists, then push foil face insulation up against it. This option is good for homes with existing flooring, but requires a higher source temperature in the boiler than if above floor. If possible it is always best to embed in lightweight concrete as the mass of the concrete will absorb the heat and distribute it more evenly (like a cast iron skillet vs a stamped metal pan). A side effect is that the more mass that has to be heated the more slowly the system will react to a change in the thermostat. While there are some new concrete based materials that are very flexible, it is still too much weight for practical use in an RV. with everything except for hardwood you will want a cover board over the pipe. (just remember to mark where the pipes are so you do not come back later and put a nail or screw through the pipe. (This can be a real challange to find the leak ...and yes I found out the hard way..lol) Nice thing with hydronic is that it is completely silent, and there is no cold drafts that you get when a forced air system starts up. plus on a RV the boiler can be fueled with diesel, and provides almost unlimited hot water for showers/laundry etc.
Dave
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