Folks with slab heat check in

I heat 1200sq ft with in slab heat using an on demand water heater (soon to have a outdoor boiler). I've learned you can't try and do temp set backs or programmable thermostats with radiant floor, the thermal mass takes to long to react. I think your assumption about the wood stove keeping the air temp warm and letting the slab cool is correct. Would it be possible to move the thermostat to the room you're trying to heat or add a zone valve and second thermostat for that area.

I've never worked with floor temp sensors but it seems like there should be something that could be retrofitted into a small hole drilled in the floor, or maybe mount a regular thermostat next to the floor in a closet or something. You might have to play with the settings some to get it right.

I use a tekmar sensor and it connects to the tekmar "thermostat", which communicates with the tekmar controller....The slab minimum can be adjusted and maintained easily through all that.

There are low cost sensors/controllers for slab sensors. Essentially a thermostat, also looking at the room air temp, monitors a slab sensor, and if either one needs to be met, it creates a demand for heat. So you can maintain a minimum slab temp this way. This is what I plan to do for the addition slab as well as out in the garage/shop.

This doesn't seem to be just what I had planned, but similar.... http://www.pexsupply.com/Tekmar-509-One-Stage-Heat-Thermostat-508-079-4177000-p

My sensor sits in a "slug" on top of the actual slab, in a closet area. When I poured, I didn't have a way to place the sensor in the slab as I wanted, and hold it and all, so I just poured a minislug on top of the slab in a closet corner. I get a bit of overshoot with the temp sometimes, and I'm sure this adds to it. Being in the middle of the slab would be better....but, it all works fine. I also placed the sensor and "slug" next to an inside wall, where I know there is a gap in the tubing, intentionally, so I could tapcon my wall into the concrete without any fears of striking the tube. So I'm on top of the slab, and "away" from the tube....but when the temp is set, it is pretty stable. Sometimes as it cools off in the evenings you feel a drop from 72 to 71, but then it will be back to 72 or 73, and never will you see 70. It is the most uniform, constant, and quiet heat I've ever come across....and we love it.

I never try and setback the floor...indeed, there's no chance, it is way too slow. For the garage and shop, I can imagine wanting to work over the weekend, and if it was at 40 and I wanted to go to 55, I'd have to turn the heat up on Thursday night and no doubt fire the Garn. I think I poured over 50 yards of concrete in the thing, 2200 sq ft. about 6-7" thick. So a 15 degree swing would be some 600,000 BTUs. If I'm putting in 2 gpm at 20 degree drop, or 20,000 BTU/HR in both zones, that's 60/4 or 15 hours to bring it up....and that's without loss...Sounds like it would be an extra firing of the Garn to do this, or one extra load of wood anyways... Though I'm sure I can't get the heat into the floor as fast as the Garn would create it....so there'd have to be a delay in the second load. Big whoopty...for a 60 degree shop all weekend! Can't wait for that phase of all the projects!

That's why folks put in the water to air HX like Nate's modine for sure! Though I almost bet if I did that for the weekend, I'd not have to dump any heat into the garage/shop during the next week, allowing it to drop back down to 40 or so....

Nate, you can add a sensor just on top of your slab somewhere and then maintain a minimum...it might work better than letting it cool down when the stove air is warming the room...causing that time lag to get the floor back up....

Nothing fast about radiant....hmmmm, anyone ever use warmboard? That could be much quicker....though it is PRICEY!

AS a professional in the design/construction business we have found we need to include some amount of hot air heating with the radiant systems. the hot air system is used in the swing season because if it takes a day or two to heat up the slab the slab might be hot by the time the two day cold cycle is gone. other than that its could be that you have some energy nosebleeds at the slab perimeter..most construction details have a pretty good sized thermal bridge where the slab hits the wall. i had this problem (not the thermal bridge but just slow to react heat) in a corner bedroom and I added a small hot air fan heater that is connected to the return line on my Outdoor boiler..they are easy to set up with pex tubing.

NATE379 said:

Yeah all 3 bedrooms have carpet.

I think the issue is either need to let the stove heat the house like I have been doign the last 2 years or let the slab come up to temp and heat the house that way. I'm sure the carpet doesn't help things either, but it's just a few years old so it will be many years before I decide to remove it.... and quite honestly I would rather have carpet. I might do a short pile and thin pad though.

I think maybe what happens is the room is warm from the stove when the door is open. Close the door and after a bit the temp drops some. It ends up taking the night or longer to get the slab warmed up. The line that runs to those 2 rooms goes from the garage across the master bathroom, laundry closet, hall and then to almost the outside wall to the manifold, then to the floor in the two bedrooms.

I know the first year I lived in the house the power had gone out for 2 days when it was -10* outside. I went from 68* to 63* two days later. The slab took that long to start cooling down.

Click to expand...

Heres a link about carpet and radiant floors http://www.radiantpanelassociation.org/i4a/pages/index.cfm?pageid=143

interesting on the slab temperature thermostats bpirger.... will those monitor outdoor temperature and react to turn heat up?

The full blown Tekmar controllers will certainly apply the outdoor reset and adjust water temps....but I don't think those little "thermostat" type things are that smart....but the mixing controller (372, 373 is it from tekmar), certainly do.

This doesn’t seem to be just what I had planned, but similar…. http://www.pexsupply.com/Tekmar-509-One-Stage-Heat-Thermostat-508-079-4177000-p

Bruce, I am going to order one this week. I know where my tubing is so I will drill to mount in the middle of my 7" slab. I have to order a new zone valve and pump so minimum for free shipping will be met. Sprung a leak in the coil yesterday so the forced air is down. Just what I need the middle of deer hunting! Turned up the lower level in-floor to 75* and will continue to hunt. The lower level plus the oven for turkey should suffice....priorities, eh?