Here is what I would do if I was you. You don't need that many btu's to heat up them two rooms. Plus you want to get hot water also. haved a certified takagi tankless water rep. come to your home. ( this will be free ) tell him you want a tk-3 pro. ( better heat exchange in the unit itself) it can fire up to 199,000 btu's and down to 11,000 btus. then you want to use a bronze bump with a heat exchanger on one side, plus boiler trim. (1/6 horse power. with a mixing valve for your domestic water. you want to then put two zones on other sided of a heat exchanger with a zone board for 2 zones with 2 pumps one for the baseboard. Ask for high trim baseboard so you can set the water temp down and get max btu's per lininer foot. set the temp for 150 degress durring the winter cold month. Now for the radiant floor get a viega dealer in ur neighborhood. get climate panels that you can put cement nails in and the put a flooring on top. If you wanted to know more about this rout just leave a post. you can have all 3 comming off one unit. The best thing about this type of set up is that you dont need 180 degree water like a boiler. and in summer time you can set the water temp down to 120 or 115. and when your away on vacation you use no gas. The down side is hat the climate panels can be expensive, the tankless water heater has to have a nice size gas line. the equipment to do this job can get pricy. I mean really pricey. with nice equipment comes a good price. Takagi & viega are my brand of choice. but you could use navian and wirlsboro if you wish. all 4 manufactors will be happy to come to your home and sit dowwn and talk to you. ( I sell all four)

OK, so if it's a garage, I assume that you don't want to heat that space so you want to insulate the top (under the slab). Again, typically I think extruded polystyrene is the right material for that although I'd be concerned about pressure from the vehicles on the garage floor. Never heard of using an earth box as a heat exchanger. Seems like there's risk of waste unless you insulate the box really well. Also, I don't really like the "one-way" engineering of an earth box (though I've got one of my own, not yet in service, and to be used for space heating, not for heat exchanging - my box is 12 in to 17 in deep and filled with compacted soil, though the pex tubing is embedded in a sand layer at mid-height of the box). If the pex ever gets damaged in that box, there's not way to fix it unless everything is torn out and you start over. I guess this is less of an issue for you since it's a garage. For me it's a finished family room. I didn't use any aluminum wrap - I don't see how that would help much - the heat still needs to get from the sand to the tubing and I don't see that anything will facilitate the process much. The reason why the book you have doesn't give specifics is because there's too much variability in the concepts. Again, a heat transfer calc needs to be done. Why don't you head over to the UC mechanical engineering department and see if there are any ace heat transfer students around who might run a quick calculation for a little extra cash. I think you could find a grad student in search of some extra beer money. I'd get serious with the idea before constructing.

"Also have mixing valves at each zone with out door reset. So you can control the temp of the water by the outside temp." This. The boiler needs to be sized for the design limits, and the rest of the time (actually almost all the time) is oversized. Using a boiler wit an indirect tank and a reset thermostat allows the tank to serve as the buffer between system size for the coldest hours and the actual needs of the house when temperatures are milder. The boiler heats the tank and runs hot enough to not be damaged. The temperature in the tank is controlled by a reset thermostat based on the outdoor temperature. The zone valve allow final tempering for each zone based on their actual requirements.

Chisue, take a look at your gas consumption for typical summer months. That's likely a good indication of how much you've been spending to heat water (although going forward you'll probably have some savings simply by having one unit instead of two). And it's a starting point for thinking about how much opportunity you have to save money by using a more efficient water heater. edited to add - your summer gas usage may also include cooking and clothes drying too. I'm in an area with high gas prices, but for me heating water costs less than $25 per month. With that, a 25 percent savings (if it were achieveable) is hardly worth any effort and certainly isn't worth incurring any cost for. I've just replaced what I have with the same thing when needed so I have two mid-range grade conventional water heaters. They work fine and I consider the service to be very adequate. Like you, I have two heaters and I've replaced each once in 25 years. azbound, wow, a 5 minute wait for hot water? How long is the wait if you just turn it on to run? This post was edited by snidely on Sat, Dec 21, 13 at 20:48