Radiant slab - How do you heat yours?

I like the idea of a slab sensor instead of a room thermostat. Set the slab sensor at seventy degrees (or whatever you need to keep the room where you want but not overheated). Not sure how to run the slab only when no other zone is calling, but I am sure the clever folks here can think of some kind of reverse priority set-up. With the slab being kept up to temp, this may not be as big a concern as you imagine.

The people we have done these for will keep the basement a little warmer for the comfort but realize some added benefit on the next floor - radiant & convection. I would pump a bunch of heat into the floor and see how well you like that, it will keep the return water warmer less split and then see if load falls off. I have seen this before. Also all the ones we have done were all T-sats, just need to keep the differential to amount a degree. Heating the garage, well just got to bite the bullet on that one, no bare feet here. 60 degrees is plenty warm for a garage and you could see water temp returns below a 100F and that doesn't feel warm.

My Dad and I installed electric wires in the sand under his slab in about 1990. That system uses off/peak electric and is still heating that house today. It had a sensor in the floor about 6' from the service panel. The floor in that basement was always toasty warm. It was a pain to adjust if the temps warmed up or cooled off a lot in a short time. I think I would try the floor sensor/thermostat, it will give you the most even temp.
Doug

Most of the radiant manufacturers have dual thermostats that have sensor for the slab and air sensing. tekmar has some nice ones also, the 501 series. But remember any heat you put in the slab will raise the room temperature.

You get about 2 btu/ square foot for every degree difference between the slab surface and room air ambient temperature. So install a mix valve to limit the water temperature going to the radiant.

For a slab to feel warm to bare or stocking feet, it needs to run a surface temperature about skin temperature, around 83F or so. If the slab is 83F and the room is at 69F it will output 28 BTU/ square feet. This could overheat the space, depending on the load. The dual stat will allow it to shut down before the ambient temperature gets too high, but the floors may not always feel warm.

We see this issue in super insulated homes with low, low loads. The room warms just from the appliances, lighting, and occupant load and the radiant floor doesn't run enough to provide "warm" floors even though the room or space is a comfortable temperature. I'm learning radiant slabs are not such a good match for low load buildings IF the owner wants the floors to feel warm all the time.
hr

Have exact same problem. Tarm is in finished basement where radiant is in CC floor. I usually walk into utility room while Tarm is charging storage and run floor ( manually turn on pump ) for a couple of hours. Floors do stay warm well into next day.
I had been thinking along the lines of in-floor sensor but I don't like the idea of the possibility of storage being used up on floor. I had been thinking along the lines of a timer also.
Jimbo do you have storage ? Do you have mixing valve on pex manifolds ?

I have radiant slab in my sunroom and what I do is about every 3-4 days I run that zone for about 6 hours at about 120F water. We don't use the sunroom that much in winter except when we use the hot tub (which is in the sunroom). By running every few days we get the floor nice and warm and it stays relatively warm for a few days, then its time to do it over again. If its really cold out I just leave the thermostat set at about 50F and set the water at about 100F. That zone will run a lot more then, but that way there is no chance of anything freezing. About the only thing you can do is bite the bullet and pour the heat to it for a while to get the slab temp up. Just running it a little here and there probably won't do much to warm it up.

ISeeDeadBTUs said:

My basement ~1,200[] and garage 576[] are both radiant slabs. As anyone that heats CC knows, these things can be a bi+ch to heat, even with insulation.

Standby losses from the oil boiler and DHW keep the basement tubes from coming on. The thermostat stays a pretty consistent 66-67 *. But the slab is not as warm to the bare feet. And I am going to have someone down there soon. Plus it seems to me that it would be good to exercise the pump regularly and not take a chance on water out near an edge freezing up.

I am thinking that instead of a thermostat - or even a slab sensor - that what would be best is a timer. Like every hour, run the pump for 7 minutes or something. But since the slab can really drop the inside boiler temp, I'd like it to run only when no other zones are on.

Any ideas?

Jimbo

Click to expand...

I have about 1500sq' of floor with 1700' of pex tubing nailed down on it and then 2" of concrete poured over it. The floor only really feels warm to the feet if you go outside with your bare feet and get your feet cold first. Then walk onto the floor and it feels very warm. You have to keep in mind that your body temp is roughly 98 degrees so your floor would have to be warmer than your body temp. for you to feel it being warm to the touch.

Huff

in hot water said:

Most of the radiant manufacturers have dual thermostats that have sensor for the slab and air sensing. tekmar has some nice ones also, the 501 series. But remember any heat you put in the slab will raise the room temperature.

You get about 2 btu/ square foot for every degree difference between the slab surface and room air ambient temperature. So install a mix valve to limit the water temperature going to the radiant.

For a slab to feel warm to bare or stocking feet, it needs to run a surface temperature about skin temperature, around 83F or so. If the slab is 83F and the room is at 69F it will output 28 BTU/ square feet. This could overheat the space, depending on the load. The dual stat will allow it to shut down before the ambient temperature gets too high, but the floors may not always feel warm.

We see this issue in super insulated homes with low, low loads. The room warms just from the appliances, lighting, and occupant load and the radiant floor doesn't run enough to provide "warm" floors even though the room or space is a comfortable temperature. I'm learning radiant slabs are not such a good match for low load buildings IF the owner wants the floors to feel warm all the time.
hr

Click to expand...

I know what you mean if my floor is warm to the touch the air temp in the room is so hot its unbearable. It also takes a long time to cool down when the temp . is freezing outside it loses about 1/2 degree an hour.

Well here is what I'm trying and so far so good. The house Temp goes up from 71 to 74 @ 4:00AM them goes back down to 71 @ 9:00AM. Then goes back up to 74 @ 3:00 PM and goes back down to 71 @ 9:00 PM. (now I know some have there own idea about that hole thing) But When I hooked in my 1200 sq foot shop of slab heat I was worried about the same thing. I didn't want to starve the house to heat the shop. So what I did is went to Menards and bought a indoor 120V timer. The slab is wired as follows: power comes from the breaker box to an outlet where I have the timer plugged into, the 120V thermostat is them plugged into the timer and them wired to the pump. I have the timer (which has up to 20 set times on it) set to turn on at 8:00AM to 12:00AM then again to turn on at 5:00PM to 9:00 PM. How this all ties into each other is that I fill the boiler now 3 times a day by adding the shop to the system. I fill in the morning and after I get home from work and usually have to toss in a couple pc's before bed. So the house is going to be just off a cycle by the time the shop is allowed to come on. And it is right after I just filled with wood. Seems to be working well so far and didn't cost a hell of allot. I'm sure there is other ways but I'm not an electrical goo roo. Oh and I decided not to run the loop constantly circulating, seemed like a big waste to me both on the added pump needed and the heat loss.

I heat with variable speed mixing controller. You can buy one now on ebay for about $70 or so. You will also need a thermostat...As hr said, the Tekmar 501 series has both the slab sensor and air sensor. Very sweet. There's a 583 on ebay for less than $60.

The controller runs the injection pump at the speed necessary to keep the floor loop at temp. This is all outdoor reset controlled. You can have the slab sensor hold a minimum temp, or run off the air temp, or both.

The controller on ebay is a Weil Mclain, but it is made by Tekmar. I bought 4. I currently am running a Tekmar 363 to do my floor and DHW. I love it. Installed it in 2003 and it has been flawless....

Trzebs13,

I'm not trying to change the subject or turn this thread in another direction but I thought if I posted this here, you would be more likely to see it. I don't know a fraction of what other contributors on this forum know about hydronic heating but I couldn't help noticing the orientation of your circulator. Experience (bad) has taught me that mounting a circulator with the motor down (or up) severely limits the life of the unit. I don't know if it's bearing thrust or lubrication but mounting the circulator with the motor horizontal will give it a longer life and it will run more quiet.

For a home you want a fairly tight temperature control. Most radiant t-stats have a 2 degree differential. Some let you select 1 or 2 degree on/off! A wider temperature swing in a shop would not be as noticeable if you are doing ligh to medium work.

My shop also warms by the heatloss from the EKO and the radiant only kicks in when I run off the LP backup boiler. This is only when I am out of town and the wood boiler is not used.

I miss the warm floors in the shop, but not the LP bills, and I would rather that jacket loss from the EKO end up in the conditioned space, not in the back yard, ala the OWF

hr

Fred61 said:

Trzebs13,

I'm not trying to change the subject or turn this thread in another direction but I thought if I posted this here, you would be more likely to see it. I don't know a fraction of what other contributors on this forum know about hydronic heating but I couldn't help noticing the orientation of your circulator. Experience (bad) has taught me that mounting a circulator with the motor down (or up) severely limits the life of the unit. I don't know if it's bearing thrust or lubrication but mounting the circulator with the motor horizontal will give it a longer life and it will run more quiet.

Click to expand...

You can always PM some one and that is a great way to get some ones attention, with out changing direction of a thread. Just a FYI.

Thanks for the info on the pump. I did not know that. I don't have much choice in the matter due to the space I had to work in. The other thing is why the heck would they put the sticker on the pump that way if your not suppose to mount it with the lettering so you can read it. LOL

Trzebs13 said:

Fred61 said:

Trzebs13,

I'm not trying to change the subject or turn this thread in another direction but I thought if I posted this here, you would be more likely to see it. I don't know a fraction of what other contributors on this forum know about hydronic heating but I couldn't help noticing the orientation of your circulator. Experience (bad) has taught me that mounting a circulator with the motor down (or up) severely limits the life of the unit. I don't know if it's bearing thrust or lubrication but mounting the circulator with the motor horizontal will give it a longer life and it will run more quiet.

Click to expand...

You can always PM some one and that is a great way to get some ones attention, with out changing direction of a thread. Just a FYI.

Thanks for the info on the pump. I did not know that. I don't have much choice in the matter due to the space I had to work in. The other thing is why the heck would they put the sticker on the pump that way if your not suppose to mount it with the lettering so you can read it. LOL

Click to expand...

This way, others may benefit! Looking at the photo, it looks to me like you have enough room to swing it 90 degrees so that the sticker is facing up.

[/quote]

This way, others may benefit! Looking at the photo, it looks to me like you have enough room to swing it 90 degrees so that the sticker is facing up.[/quote]


Good point!

This is what happens to me with the kennel that has floor heat. When the wall thermostat calls for heat it takes a lot of wood (most of, if not all of what's in the boiler) and then the house gets cold. Worries me at night that the kennel calls for heat and the house freezes. The slab does hold a good amount of heat so I have been running it up a bit when I get home from work and then turning it off or way down at night. My problem is this morning when I left home the kennel slab was calling for heat and I know when I get back home this afternoon there will be nothing but coals and a water temp of 80. I am running an Aquatherm smoke dragon as you all say. I am really thinking of getting a propane boiler or tankless heater for out there and just burn the wood insert in the house and selling the smoke dragon. The amount of wood I burn is crazy at times and it is to a point I need to start buying some logs so the payoff isn't there anymore.

ISeeDeadBTUs said:

Thank you to everyone! Lots of good ideas

Just a bit of clarification . . .

1)I do NOT have storage.
2)I am NOT worried about 'over-heating' the basement. (though, know who's moving in that space, I may need to lock the windows from the outside)
3)For a number of years, the TruckChick and I lived in the basement. It's ~1/3 walk-out, plenty of natural light. At the time we heated the tubes with a 40 gal propane DHW tank. Worked until the outside fahrenheight went upside down. During heating season, the floors were ALWAYS warm. After mopping, the CC would be dry in less than 10 minutes.
4)I do have a tekmar mixing valve and outside control, tied to a Vitotronic control.

My reasons I think I need to change from the existing wall-mount thermostat . . .

1)I want the floors warm, the pump exercised
2)I don't want the occupant to be able to play with the stat.

People tend to bump up stats when they feel cold. Not an effective strategy with radiant heat. And if the occupant turns that much CC up, it will kill all the heat in the inside boiler, then all the heat in the GW, then all the zones would go cold.

Click to expand...

You can buy wall thermostats that allow you to put a limit on the temperature that can be set. That might be the best option.

Or the the tekmar mentioned allows a min. and max. setting and has a slab sensor option. So you could limit the floor temperature, and / or have the air temperature over-ride the floor sensor. It's designed to do exactly what I think you are asking for.

hr