I installed below the slab a 1/2 Pex oxygen barrier pex system with four loops at 250 per loop. The loops were placed under the concrete slab on top of a radiant bubble foil white next to the underside of the concrete. I have a pex 4 zone system but wanted to know if I could use a small water heater to keep the slab a little warmer than normal during the winter months as we have a lot of floor tile on the first floor. Wanted to run a closed loop system and used perhaps a water - antifreeze mixture as some of the articles I have read. Wanted to know if I hooked up an electric 30 gallon water heater to heat the water and have it all run by a timer where buy it would come on at several preset times and run for a certain period of time. Was going to use a water circulator but need to know what specifics I need. Just wanted the system to keep the concrete a little neutral in temperature as to the cold it is now to touch. Any suggestions would be greatly appreciated and feel free to contact me. Thanks in advance. Lardog [email protected] House in located in North Texas and has spray foam insulation 2 x 6 exterior walls and roof. Approx 1600 sq ft of tile.

First, radiant foil bubblepack needs an air gap between the foil and the heated surface to go any good at all. Under a slab it does NOTHING! (Well, OK, it works as a vapor barrier, but has an R value of zero.)

If the soil under the slab is dry, it has some R-value though. If your goal is make a slab that coasts along at 58F a somewhat more comfortable 65F the heat loss won't be astronomical. If this is slab-on-grade, insulate the slab-edge with a 2-4 inches of EPS or XPS rigid board around the perimeter of the foundation down to at least the frost line, or 2' deep, whichever is deeper.

In most of TX the freeze hazard is minimal (or nonexistent) for a slab inside a building. Average daily temps in Lubbock in January are ~39F. Inside an insulated not too leaky building it'll coast at temps well above the average. Adding the antifreeze only impedes the heat transfer- use water, isolated from the potable water with a heat exchanger.

The size of the tank is irrelevant- only the power rating of the heating elements, but if the house is otherwise heated by a separate system, almost any standard water heater would be sufficient. If this is your sole heating system we'd need more info as to the actual peak heat loads to know whether it would work It might be good for the whole load- a single 4.5kw heating element is ~15KBTU/H- more than half my heating load at 0F outdoor temps. The great unknown is the heat loss factor out the bottom of the slab through that R0 bubble pack. (It should be a crime to market that stuff as under-slab insulation, which it clearly isn't. Heat is always CONDUCTED out of the slab to the ground, never radiated!)

If it's the slab temp that you want to control, use a floor thermostat, not a timer. (Google "floor thermostat" they're out there.)

As a distributor of reflective foil insulation, I am always dismayed to see it used underslab. It is not the proper useage of the material. I always recommend Crete-Heat or the Barrier for under slab/radiant insulation.

Thanks for the info a little late about the radiat barrier under the slab but what I really need to know is exactly what items I need to purchase such as air gap etc. to tie the the pex manifold into the water heater to complete the close loop system. ALso where would be the best place to puchase these items. Is there a single source?

Dana Thanks for you response. I do have a central hvac system (Propane) for the whole house. Just wanted the radiant floor heat system to warm up the floor tiles in the down stairs area.

Air gap isn' t item, it's a construction constraint: There has to be at least 1/2" of air space between the slab and the radiant barrier material for the radiant barrier to have an appreciable effect (an inch or two is even better, but it falls off fairly rapidly beyond that.)   I'm not sure that's ever easily doable or desirable under a grade slab.  You're neither the first nor the last to get conned into thinking RB would work under a slab. (A friend of mine in MA did the very same thing, but with more $evere consequences due to the cooler climate.)

If you have propane on-site, in most places it'll be cheaper to run the radiant off a propane fired HW heater than electricity.

If you're going to run it off electricity anyway, if you have the headroom you might consider laying down 3/4" XPS insulation on top of the slab between pressure-treated sleepers and put a finish floor above the slab. That by itself will make it warmer, but if you wanted an electric radiant floor there are vendors of low-voltage under floor radiant out there (Z-mesh, etc.).  That's a bigger project than just hooking up a hot water heater to a floor though.

Since it isn't your main source of heat it's likely your current hot water heater could likely run the floor with a heat exchanger, zone relay, and a pair of pumps.  The additional load to the HW heater of heating the slab is quite small compared to say, taking a shower.  A slab thermostat, zone relay, heat exchanger and a couple of low volume pumps (one for the potable side of the heat exchanger, the other for you radiant loops) should pretty much do you.  Keep the slab at 65-67F and it'll stay pretty comfortable in there all the time (just not cruisin' in yer socks warm-floor cushy.)  If your HW heater is fired by propane you could run it warmer and supply a substantial amount of heat to the room at reasonable cost & efficiency (you might want to use a PID-algorithm room thermostat, or one that senses both slab & room temps if you go that route.) 

A standard propane tank water heater has an average combustion efficiency between 78-80% (and will deliver ~76-78% AFUE if it's your sole source of heat.)  Since you're already paying for the standby loss (which is pretty much a fixed # of BTUs per hour x idling hours) increasing it's duty cycle by using it for space heater improves it's average efficiency. If your furnace is a high efficiency condensing version it'll still be a somewhat more efficient way to heat though.