In floor heating retrofit

[andyvh1959]

Not in my garage, but thought I'd pitch this to the experienced here. My house is a early 70's trilevel here in Green Bay. My office and den area is on the slab.ground level. The basement of the house is only under the central living room area, 16 x 24, the rest of the first floor and attached garage are on the slab/ground. The house is near 3,000 sq ft so the gas forced air furnace is good sized. In the winter though the floor in the office/den can be quite, um, cool.

I plan to install a new wood floor over the concrete slab, using the new spacer panels that allow for tubing to be routed under the floor. The panels create an air gap between the concrete slab and the new floor, which itself may improve the floor temperature. My idea is to use a heat exchanger in the ducting in the basement, to heat glycol flowed through the tubing under the wood spacer floor. I can get access to the ducting in the basement and run PEX through the wall at the floor level of the office/den. The floor level of the office/den is slightly higher than the level of the hot air ducting in the basement. So, like a passive hot water heat system in my 1st house, the thermal delta from bottom to top of the system is enough to create fluid flow without a pump. A heat exchanger in the ducting should warm the glycol easily above the current floor temperature, at least enough to provide a much warmer floor than is current in the winter. The heat exchanger could even be a larger heater coil from a large car.

However, since the house was built in the 70's, I'm certain there is no insulation under the slab. The soil here is very sandy, so I'd bet the soil was graded to suit the slab, and the concrete simply poured over the sand. So unless I can get a significant heat gain into the floor I wonder if it'd even be worth the effort. The area to heat the floor is only about 520 sq ft. Thoughts?
Ideas? Worth it?

 

[yeldogt]

I don't think you will get the results trying to heat the PEX with the flowing air.

Your office/ Den is on the main level? ...you are not trying to put this on the basement slab ..correct?.. the basement slab would be bellow grade.

The way I am reading you are doing this over the current slab at grade. What is on the floor now? How much height do you have and what is the step change from the other room.

How much money do you have?

 

[510ebl]

For that small of an area, just to take the edge off the cold floor, would electric pads work for you?

 

[TurnipTruck]

I would just insulate the floor and concrete walls instead of trying to harvest and pump a few btu from a duct.

 

[tdkkart]

I don't think you're gonna get very far trying to beat the mass of that slab with lukewarm pex system. There's just not enough temperature in the forced air duct to get enough heat out of.

 

[johnnyradiant]

The area is small enough and in a conditioned (poorly but still conditioned) space you could easily get by with a small DHW tank to be your boiler for the one room, unlike a stand alone shop 4x or more larger.

Moving heat out of a heat exchange to an air duct does not work all that well going the other way I can't imagine getting close to desired target. Maybe if it was the main duct before any branches or in the flue of an older low efficient furnace you might get the heat transfer you desire, but then you still have to rely on the cycling of the burner and either it won't be on long enough to heat the system fluid or the rest of the house will be hot and looking for a cooling call.

Even those 2' X 2' panels with the dimpled plastic on the underside will make a noticed difference to your floor comfort over what you have. Those alone won't solve the entire issue but you will notice a difference. If you could add to those subfloor panels an electric baseboard radiator, or maybe more comfortable those built-in electric heaters with a fan, which would circ the air which often times makes the room more comfortable instead just a radiator.

Alternatively you could get a radiant floor using an electric kit in stead of a water kit.

 

[kj_mustang]

Unless you are using some type of radiant board with insulation to prevent the slab from stealing all the heat from the pex, I think you are wasting your money.

 

[andyvh1959]

All good points and as I typed the opener I began to realize the futility if the idea. Just to clarify though, the idea was not to route heated air under the floor. I had thought about a water-glycol system, heat exchanger in the forced hot air duct to warm the water-glycol in the floor heat circuit. But like said above, and I agree, the mass of concrete on ground level would likely **** any heat out of the slab so no real gain in floor heat is likely to be felt. Probably best to use the spacer product to get the new floor off the concrete and perhaps add insulation under the spacer floor to reduce room heat loss into the slab. Thanks for the input, I knew I was asking the right group here.

If I did a floor heat system it probably would be best to be a stand alone electric/boiler system. But again, with no insulation below the slab it would simply **** the heat down more than effectively heat the floor to a good comfort level. So, just use the floor spacer system with whatever insulation I can add under that. Still better than carpeting on the slab as it is currently.

 

[yeldogt]

I have done a couple of one story additions .... basically building "bump outs" where I did conventional foundations/slabs. These were extensions to houses with basements where I did not want a basement or crawl (hate crawl). In those cases I used 2x lumber on the flat and 1.5" foam board between. I then attached Warmboard to the 2x lumber. Additional (to the under slab) plastic VB on the slab under the wood.

I had to plan the elevations -- 1.5" for the 2x .... Warmboart is about 1 1/8 .. plus the 3/4 wood floor. pushing 3 1/2 height .... plus the expense, End up with a great floor.

How cold is your slab ? You need a thermal break ..... you may be able to get away with 1" and use a 3/4 retrofit tubing product ... plus a thinner engineered floor.

Bradford white makes an uprated tank water heater with a coil inside just for what you are trying to do. The unit has an uprated burner -- a circulator sends water through the coils and then out to a supplemental radiant zone .... I have used them to warm a kitchen floor.

Those multi level forced air houses from that time period and before can be problematic -- they had somewhat weak insulation ... none in the slab. With hot water heat available -- even a very thin foam on the floor is enough to isolate the slab -- I did my whole office years ago in a walk out basement using a couple panel radiators. That's also a possible solution ...

what's your electric rate ?