Thermostat for Radiant Floor Heat

gerard

I have a small radiant system for my basement family room. Works great except by the time the thermostat reaches temp the room will get warmer for the next few hours, cool down and repeat cycle. I would like to merely turn it on for 45 mins -1 hr every 3-4 hours, experience tells me that will keep it at 72-ish or I would adjust this cycle based on how it reacts. Any ideas on how to accomplish this? I can only find programmable thermostats that have 4 periods. Even a programmable seems overkill but I don't know of an on/off timer. Any help is appreciated.

Ironman

Is it a slab? If so, you need to control the SWT to it using outdoor reset.

As far as a stat: Tekmar 518 or 519.

Jamie Hall

As you have discovered, radiant floors take a long time to react. A very long time... you would probably be best off not using any setback at all on it -- just set it at one temperature and forget it. There are thermostats which actually measure the floor temperature.

May I ask how the flow in the floor and the temperature of the flowing water is controlled? There are some rather sophisticated controls which will vary that flow in accordance with demand, or with outdoor temperature, with the boiler coming on and off to maintain the proper water temperature.

Zman

More info on your system would help.
Do you have a piping diagram or pictures of the boiler piping.
My guess is that the boiler is supplying very hot water to the slab.
For radiant to work well, the supply water temp should modulate down as the outdoor temp rises. It is a simple control strategy known as "Outdoor Reset".

gerard

Thanks for the responses. Very basic system controlled by thermostat. Temp goes down, pump goes on, temp is reached, pump goes off. temp of water is 50 C (about 122F). Because of the small size of room and location I used a direct vent hot water heater, worked well for 15 years. After much research and consultation with the people who built the system (pumps, gauges, etc) I switched to a on-demand water heater. I see it says in some other chats on here that this is wrong but it works well as is, I am trying to even it out and perfect it. I may try the Tekmar recommended above or a standard programmable. I just think 6 times in a 24 hour period for 30-45 minutes will do what I need without over taxing the heater. I can't fit a boiler in the space I have and even if I replace this thing every 5 years, I can work with it.

gerard

oh, and yes it is a slab.

Zman

I would use one of these t-stats. https://www.supplyhouse.com/Uponor-Wirsbo-A3100101-Heat-only-Thermostat-with-Touchscreen?gclid=Cj0KCQjwjvXeBRDDARIsAC38TP7bk0kodv4u0VFvMtsKYBU6QGdRhjTde1AeAP15N4vBj-1X6XTby5caAjlAEALw_wcB

They have a nice feature where they pulse the heat as you approach target temp. If you are using a setback at night, that could be part of the problem. It would also help to experiment with lower water temps. Try 110 and see how it works.

GroundUp

If you're overshooting, the water is too hot. Turn it down to 100 once before investing in ODR and leave the rest as is. If that solves the overshoot issue, then you can look into ODR. I'm no genius or anything but I do specialize in radiant slabs and I have yet to ever see a slab with proper loop field that requires more than 100* SWT to maintain a 70 degree ambient air temperature at -30F outside

gerard

great advice, I give it a shot

Jean-David Beyer

I'm no genius or anything but I do specialize in radiant slabs and I have yet to ever see a slab with proper loop field that requires more than 100* SWT to maintain a 70 degree ambient air temperature at -30F outside

I am not a professional, but I need hotter system water temperature very often. My downstairs is radiant slab at grade. When the outdoor temperature is >= 50F, a supply temperature of 80F is sufficient, but by the time it gets to 0F outside, I need 130F in there. Around here in New Jersey, design temperature is 14F. At 14F, the supply temperature is 116F. These temperatures to maintain 69F in the house.

hot_rod

SWT to a radiant slab would depend somewhat on how much heat energy comes out the top, and how much goes out the bottom of the slab:)

Properly insulated, bare concrete with reasonable tube spacing, and a building load below 25 btu/ sq. ft those SWT may be adequate. The depth of the tube in the slab can make a difference also in the SWT required.

GroundUp

I should have said properly designed slab, not necessarily proper loop field. Insulation is key, both under and around the perimeter. A fairly tight building goes a long way with this as well. I do a lot of sheds/shops out here in farm country and almost all have an excess of overhead doors which are a huge heat loss, and a lot of outdoor wood boilers so ODR is usually out of the question in terms of price so the temp just gets set and forgotten on a thermostatic mixing valve. I set all of them between 90 and 100 degrees depending on the building and slab thickness, and never once have they failed to maintain temperature at -30F.

If it takes 130+ to keep the place 69 degrees, either you're short of insulation or emitter.

hot_rod

Yeah, working shops are more radiant forgiving, generally they are not looking for constant 70F, and if they drop 5 or 10° in extreme cold it's not usually an emergency.

You can run higher surface temperatures in a shop as occupants usually have shoes or boots on, so more output is possible.

100°SWT may provide a 85° surface on your jobs?

85 surface - 65 ambient gives you close to 40 BTU/ sq ft output.

In a residential 80° surface- 70 ambient = 20btu/ sq.ft output.

You have more ∆ to leverage in a shop radiant.

GroundUp

One up the road I did last fall is 7200 sq ft with 20 ft ceilings and 1100 sq ft of R8 overhead door, all 4 zones set to 100* SWT, the building is able to maintain 75* ambient above 0* outside. We had a 3 week stretch of -30* in February and during that spell he did have trouble getting it above 70, but that was due to the boiler falling behind as we could only squeeze 91-92* SWT from the mixing valves. His wood was 40%+ MC with a natural draft stove so I brought a face cord of my dry 15% oak from home to rule out the stove and it cured the issue. I didn't ever check surface temps, but the last few days of that cold spell he was able to maintain 75* inside with -30* outside. This is an 8" slab (semi truck shop) with 200ft loops of 1/2" suspended at 3" and 2" EPS under and around the slab, delta T at full tilt is only about 13 degrees, I would guess surface temps are around 85 on long draws

Zman

GroundUp said:

One up the road I did last fall is 7200 sq ft with 20 ft ceilings and 1100 sq ft of R8 overhead door, all 4 zones set to 100* SWT, the building is able to maintain 75* ambient above 0* outside. We had a 3 week stretch of -30* in February and during that spell he did have trouble getting it above 70, but that was due to the boiler falling behind as we could only squeeze 91-92* SWT from the mixing valves. His wood was 40%+ MC with a natural draft stove so I brought a face cord of my dry 15% oak from home to rule out the stove and it cured the issue. I didn't ever check surface temps, but the last few days of that cold spell he was able to maintain 75* inside with -30* outside. This is an 8" slab (semi truck shop) with 200ft loops of 1/2" suspended at 3" and 2" EPS under and around the slab, delta T at full tilt is only about 13 degrees, I would guess surface temps are around 85 on long draws

It all comes down to the details. Insulation (especially edges), tubing elevation, tubing length and spacing. I believe your claim regarding well detailed slabs. I have also seen slabs that barely make 15 btu/ft with 130 degree water.

Jean-David Beyer

If it takes 130+ to keep the place 69 degrees, either you're short of insulation or emitter.

It takes 130F only when it is 0F outside. Today, if it were heating at all, it would put 80F water into the slab.

Well, it is radiant slab at grade with copper tubing in the slab. Installed in 1950. I have no idea if there is any insulation under the slab and I am pretty sure there is none around the edge of the house. I also do not know the spacing of the tubes. I do know that each room is on a separate circuit, and I can adjust the flow with ball valves. My two largest rooms are full open, and the others, less so. There were "builders batts" insulation in the walls, but I had the empty spaces filled with injected urea-formaldehyde foam insulaton, and the second floor (baseboard) has had an extra 6 inches of blown-in fiberglass installed. The window are now double pane argon-filled Marvin windows, so the insulation is pretty good. When storm Sandy came along, I lost power for 6 1/2 days and it got down to about 36F outside. During that interval, the inside temperature dropped about 11F. So I would say the insulation is pretty good.

I just cannot imagine 70F water into a radiant slab when it is -30F outside keeping the inside at 69F. That must be an amazing custom built house with phenomenally good insulation and windows. My house was just one of a development of houses built in 1950, and heated by an oil-fired GE boiler. One of these; made near here in Bloomfield, NJ.

https://www.historicnewengland.org/explore/collections-access/gusn/316394/

GroundUp

70 degree supply water will never keep a space at 69 degrees without a ton of gain from elsewhere. I'm not sure where you saw that claim, but I'd go ahead and wave my BS flag at that one. 100 degrees like I said on the other hand, entirely possible and plausible