jrfiero
Active member
Thought I'd share my experience the my radiant slab, just fyi.
Warning: Long Post!
Building:
1 ½ story garage, R15 walls, no insulation between first and second floor, R30 roof, no windows first floor, double pane windows and skylights second floor, two insulated 9’ garage doors & one man-door first floor, one man-door second floor.
Footprint 559.7 sq feet (zoning limit 560). 24’3” x 21’2” slab, average ~4.5” thick. ~25,600 lbs mass
2” insulation under and perimeter.
One garage door is opened in the morning and evening for a vehicle entry and exit.
System:
Single Zone, Two loops ½” PEX (~250’ each). Radiantec system, they approved my tubing layout design and provided the balance of the system with everything properly sized ('cept the heater).
Three-speed pump, currently running in the middle setting.
12 psi when pump is running, 17 idle (?). Can be lower, I’ll adjust after AF.
30 gal electric water heater, two 3800-watt elements, 25gph recovery with 90 degree rise. Lower element convertible to 5500 watts. Set to what I think is about 100 degrees, upper element slightly higher.
~36 gallons fluid total.
Filtrete electronic 7-day programmable thermostat. Not specifically for radiant heat, mounted on 2" rigid foam on an outside wall. No slab sensor.
Thirteen gallons of Freeze Free inhibited propylene glycol (36%, Freeze to 0F, Burst to -40(?).
Performance:
Caveat – the first week of system operation was the coldest week we’ve had in six years. Has to have had some effect.
The plan was to leave the garage at 45 during the week while I’m off working, and raise it to 60 starting noon Friday. Rethinking that plan. Warming 25,000 lbs of concrete apparently takes a while.
While I was filling/flushing the system, I had a portable heater in the garage so the air temp was about 54 (outside temp ~30). Slab surface was about 45 according to the handy infrared thermometer. It took way longer than I had planned for the slab to come up to above 60. I had thought it needed to get to 85 for effective heat, but that turned out not to be true (whew!). I prematurely started the pump without letting the tank fully heat – the output initially said 107+ but quickly dropped to 75.
~24 hours after I fired it up the air temp got to 60, slab temps 63-64. Then, for literally days I was not able to catch the pump in the act! It was just sitting there around 60. It was up and down a little so I know if was cycling, just couldn’t catch it, so I don’t have good output – input readings.
The second floor (wood shop) is reading around 10 degrees cooler, but it didn’t have a long time to soak so we’ll see in the future. My $30 supplemental electric oil filled heater should make it comfy.
The weatherman threatened really cold temps (for Northern VA) the first week of operation (was accurate) so I immediately gave up on my 45 plan and raised the lower target to 50.
I dropped the temp to 50 at 6pm one day (will normally happen automatically Sunday), and at 6 am the next day the garage air temp was still 58, 55 by noon. Wish my house would retain heat like that. Must be the concrete.
I have seen the system working to keep the temps at 50, but I haven’t seen it when it starts to check the initial output/input temps. Output as it’s running is 82-83 degrees F, input 71-72. Lower than I expected, but it works fine. The slab is 54 when the garage is up to 50. Doesn’t noticeably overshoot. Upstairs temp is 38, so keeping the first floor down at 45 would be risky for upstairs. But outside has been down to 14 at night and stayed in the 20s during the day, way colder than average for here (Arlington, VA, high 44, low 29).
I need to get a better handle on what the water heater is actually set at, although once it’s running it probably doesn’t matter since the 3800-watt element can apparently supply 83 degree water to a 50 degree slab!
I saw one morning that the air temp was down to 48 (remote thermometer in the house), but I’m almost sure it was the blast of 18 degree air from my wife getting her car out of the garage. So the pump was running when it probably didn’t need to be, the warm slab and surroundings would have reheated the garage. Not sure there’s anything to be done about that.
Future:
Track the time it takes to get from 50 to 60, or whatever my final temp settings end up being. Took a long time the first weekend -~24 hrs.
Play with the heater temp. I’m thinking higher wouldn’t make a lot of improvement in slab temp change (except for the initial hit), so I’ll probably set it as low as it will go to see if that works.
If it takes too long to move from my setback temp to my final temp, I can convert the bottom heater element to 5500 watts. I don’t need that to maintain temps, so I’d rather not do it. But I did, will have to see if next weekend's slab increase is quicker.
Try different pump speeds. The differential is OK now, so I’m in no hurry to change it.
What I’d do differently:
This was the first in-slab radiant for my building contractor. Always a risky idea. Anyway, he didn’t put a thermal break right at the garage doors. When he poured the apron I had him add a thermal break, but I have about 8” of heated slab outside the doors, which I’m unhappy about. No tubes out there, but it’s just wasting heat.
Of course I would like a heat source other than electric. Running natural gas to the garage would have cost $1200, and I can buy a lot of electricity for $1200. Then there would be venting, etc. Solar is an option.
I’d buy a better thermostat (still can, of course). I’d like to know how long the system is calling for heat, etc, and my thermo has none of that. You can get WiFi enabled thermostats now which can sit up and talk. Should have gone that route.
Jury’s still out on whether I should have put underfloor tubing under the second floor. For my purposes, this will work fine, but if someone in the future wants to use upstairs as an office or guest room, might not work as well. I know for sure I’m not retrofitting it!
Anyway, there are some data points for folks who might be considering in-slab radiant. In my case it's really easy to maintain a set temp, and it's very comfortable. I knew going in that setback is not recommended. When I get the electric bill I might not be so impressed, but, oh well.
Jonas
Warning: Long Post!
Building:
1 ½ story garage, R15 walls, no insulation between first and second floor, R30 roof, no windows first floor, double pane windows and skylights second floor, two insulated 9’ garage doors & one man-door first floor, one man-door second floor.
Footprint 559.7 sq feet (zoning limit 560). 24’3” x 21’2” slab, average ~4.5” thick. ~25,600 lbs mass
2” insulation under and perimeter.
One garage door is opened in the morning and evening for a vehicle entry and exit.
System:
Single Zone, Two loops ½” PEX (~250’ each). Radiantec system, they approved my tubing layout design and provided the balance of the system with everything properly sized ('cept the heater).
Three-speed pump, currently running in the middle setting.
12 psi when pump is running, 17 idle (?). Can be lower, I’ll adjust after AF.
30 gal electric water heater, two 3800-watt elements, 25gph recovery with 90 degree rise. Lower element convertible to 5500 watts. Set to what I think is about 100 degrees, upper element slightly higher.
~36 gallons fluid total.
Filtrete electronic 7-day programmable thermostat. Not specifically for radiant heat, mounted on 2" rigid foam on an outside wall. No slab sensor.
Thirteen gallons of Freeze Free inhibited propylene glycol (36%, Freeze to 0F, Burst to -40(?).
Performance:
Caveat – the first week of system operation was the coldest week we’ve had in six years. Has to have had some effect.
The plan was to leave the garage at 45 during the week while I’m off working, and raise it to 60 starting noon Friday. Rethinking that plan. Warming 25,000 lbs of concrete apparently takes a while.
While I was filling/flushing the system, I had a portable heater in the garage so the air temp was about 54 (outside temp ~30). Slab surface was about 45 according to the handy infrared thermometer. It took way longer than I had planned for the slab to come up to above 60. I had thought it needed to get to 85 for effective heat, but that turned out not to be true (whew!). I prematurely started the pump without letting the tank fully heat – the output initially said 107+ but quickly dropped to 75.
~24 hours after I fired it up the air temp got to 60, slab temps 63-64. Then, for literally days I was not able to catch the pump in the act! It was just sitting there around 60. It was up and down a little so I know if was cycling, just couldn’t catch it, so I don’t have good output – input readings.
The second floor (wood shop) is reading around 10 degrees cooler, but it didn’t have a long time to soak so we’ll see in the future. My $30 supplemental electric oil filled heater should make it comfy.
The weatherman threatened really cold temps (for Northern VA) the first week of operation (was accurate) so I immediately gave up on my 45 plan and raised the lower target to 50.
I dropped the temp to 50 at 6pm one day (will normally happen automatically Sunday), and at 6 am the next day the garage air temp was still 58, 55 by noon. Wish my house would retain heat like that. Must be the concrete.
I have seen the system working to keep the temps at 50, but I haven’t seen it when it starts to check the initial output/input temps. Output as it’s running is 82-83 degrees F, input 71-72. Lower than I expected, but it works fine. The slab is 54 when the garage is up to 50. Doesn’t noticeably overshoot. Upstairs temp is 38, so keeping the first floor down at 45 would be risky for upstairs. But outside has been down to 14 at night and stayed in the 20s during the day, way colder than average for here (Arlington, VA, high 44, low 29).
I need to get a better handle on what the water heater is actually set at, although once it’s running it probably doesn’t matter since the 3800-watt element can apparently supply 83 degree water to a 50 degree slab!
I saw one morning that the air temp was down to 48 (remote thermometer in the house), but I’m almost sure it was the blast of 18 degree air from my wife getting her car out of the garage. So the pump was running when it probably didn’t need to be, the warm slab and surroundings would have reheated the garage. Not sure there’s anything to be done about that.
Future:
Track the time it takes to get from 50 to 60, or whatever my final temp settings end up being. Took a long time the first weekend -~24 hrs.
Play with the heater temp. I’m thinking higher wouldn’t make a lot of improvement in slab temp change (except for the initial hit), so I’ll probably set it as low as it will go to see if that works.
If it takes too long to move from my setback temp to my final temp, I can convert the bottom heater element to 5500 watts. I don’t need that to maintain temps, so I’d rather not do it. But I did, will have to see if next weekend's slab increase is quicker.
Try different pump speeds. The differential is OK now, so I’m in no hurry to change it.
What I’d do differently:
This was the first in-slab radiant for my building contractor. Always a risky idea. Anyway, he didn’t put a thermal break right at the garage doors. When he poured the apron I had him add a thermal break, but I have about 8” of heated slab outside the doors, which I’m unhappy about. No tubes out there, but it’s just wasting heat.
Of course I would like a heat source other than electric. Running natural gas to the garage would have cost $1200, and I can buy a lot of electricity for $1200. Then there would be venting, etc. Solar is an option.
I’d buy a better thermostat (still can, of course). I’d like to know how long the system is calling for heat, etc, and my thermo has none of that. You can get WiFi enabled thermostats now which can sit up and talk. Should have gone that route.
Jury’s still out on whether I should have put underfloor tubing under the second floor. For my purposes, this will work fine, but if someone in the future wants to use upstairs as an office or guest room, might not work as well. I know for sure I’m not retrofitting it!
Anyway, there are some data points for folks who might be considering in-slab radiant. In my case it's really easy to maintain a set temp, and it's very comfortable. I knew going in that setback is not recommended. When I get the electric bill I might not be so impressed, but, oh well.
Jonas