Radiant hydronic for an existing slab?

[tenthousandfeet]

I'm looking at all the possibilities for heat in my 20x22 this winter, and I keep coming back to solar hydronic. The biggest problem, of course, is that the garage is already built!

I've searched and searched, but I can't find any reference to somebody adding infloor radiant to an existing building. Probably the only way to do so is to cut up the floor and pour a new one, but that would be both costly and time consuming. I've also toyed with the idea of pouring a couple of inches extra on top of the slab, but the heat likely wouldn't permeate the slab underneath (not to mention the fragility of such a thin layer, and the lack of insulation under the slab).

Another thought for taking advantage of the sun as a heat source would be a wall-mounted water radiator of some sort, but without the slab to hold the heat during hours of darkness this would only really work during the day.

Any thoughts? Am I stuck cutting out the slab if I really want free(ish) heat?

 

[tcianci]

I think you have realized the drawbacks to retro-fitting an existing slab. The biggest plus of radiant heat is that the radiating device is the floor so it is not in the way in the garage and the floor is nice and warm in the winter. This holds true only if you keep the garage heated constantly, not necessarilly always hot, but radiant in floor heat is too slow to just flip a thermostat and warm the place up just when you want to use it. If you think you will be able to harness enough solar energy to heat the place, consider a very large insulated tank as part of your hydronic system so that you have a large thermal mass to hold heat to be released into the building over a period of time. Your comment about the slab holding the heat is not quite what is going on...The slab is a heat diffuser, it spreads the available energy out over a large area to provide a very uniform heat, and the heat is introduced at the lowest level of the space making it feel very comfortable but the slab doesn't "hold" the heat. You need a large quantity of heated water to hold the heat and then release it into the room through the radiant floor or other radiating device.
Consider a traditional hydronic system. You control when the system uses fuel through the thermostat. The system is designed with enough input capacity to raise the temperature of the medium (water) so that it can be released into the space and warm the air. With solar, you have no control over when the sun shines or for how long so you have to take advantage of the energy when it is available.. you need a storage device that can be your heat reserve.
Check with solar contractors in your area, if there aren't any maybe there's a good reason. A solid design will help you to set your expectations of performance and budget.

 

[Racecarl]

I intend to use solar hydronic heat with backup or assist from a conventional radiant heat source. Since I work during the day, I don't care how hot it gets in the shop during the day, just as long as it is warm in there in the evenings when I feel like working out there. I realize that radiant hydronic heat does not heat quickly, so temperature swings should be minimal. I hope to capture the heat during the day and enjoy it during the evening.

 

[tdkkart]

One of the issues you're missing out on is insulation both under and around the outside the slab. Typically there is 2"+ of R10 under the slab, and another 2"+ buried 24" or so deep around the outside.
Your current slab is in direct contact with the dirt now, you don't want to loose all your heat into the ground.