I have underfloor heating from a boiler with water supplied at 40C. To protect the floor pipes from frost is absolutely vital [ when I'm on vacation for example ], but using heat from the boiler to do this poses several problems. These floors have huge thermal inertia, a 6" concrete pad for even a small single-car garage will weigh over 4 tons and take several hours to warm up. It will absorb and emit a lot of heat from 40C water before triggering any stat, even one which is fine tuned to 4 or 5 C, below this one risks ice formation before the boiler lag can supply the necessary warm water. If the garage is not well insulated and draft proofed, that adds up to a lot of money. Even if you manage to switch the zone-heat on at 5C air-temperature, your customer will still 'feel' the heat [and the dollars!] coming off the floor- we humans are quite sensitive to infra red radiation. A single-bay garage could cost 20c an hour to frostproof with boiler calls. It begs the question; why did they fit UFH to the garage floor in the first place, if the heating is only turned on the protect the pipes!!?
#1 I eventually went for antifreeze plus a corrosion inhibitor. Use ONLY polypropylene glycol, [food grade], up to 30% depending on frost severity - it's about $10 a gallon. Do NOT use automotive ethyl-glycol, which can eat rubber seals and corrode some metals and cause valves to 'stick'. Ethyl-glycol is also very toxic. Imagine a leak contaminating the whole house, the kids and Muttley paddling about in it!
#2. Insulate the garage floor surface to reduce it's emission of heat, since virtually all the energy leaves the floor as IR. This could be done by laying cheap polypropylene carpet, or by laying a masonite or plywood floor layer, or possibly with paint. Aluminum or white gloss floor-paint has a much lower emissivity than concrete.
#3. Abandon and mothball the garage UFH zone at the manifold, then blow out the water with air or refill with antifreeze.
As a postscript, another refinement might be to fit a remote bulb type thermostat onto the floor water return line in the garage, in addition to the existing 50F air-stat. The air-stat initiates boiler call, but as soon as the garage return flow hits, say, 10C, the water-stat closes the garage zone valve and closes boiler call. You'd need to run a trial to decide on what return water temperature is acceptable, but I would err on the side of caution and try to guarantee at least 5C. Now you are protecting the pipes without necessarily heating the full slab. Normally you'd expect to see about 30C return water temperature, so its possible to safely reduce the floor emmissive power by at least 70% by this method.
[This message has been edited by Alan Belson (edited 03-04-2007).]