Radiant Heat - Boiler Working Overtime?

Folks,
We have a 1200 sq ft full basement that we're heating via radiant heat. There are 4 runs of Pex. We're very happy with it but I'm worried that there is something wrong with the system. When the basement calls for heat the pump turns on and I can see the temperature gauge go up to around 100-110 degrees. There's no gauge on the return but feeling the Pex it's obvious that the water coming back is pretty cool. The problem: It takes at least an hour, sometimes 2, for the basement to warm up enough that the thermostat stops calling for heat. 1200 sq ft is a lot of concrete to heat up but isn't that a long time?
Any thoughts much appreciated.
Thanks, Bill
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Furry wrote:

I don't have a radiant system so I'm writing not from personal experience, but solely from what I've read. Slow response is one of the drawbacks of radiant systems. I'd say that what you are seeing is completely normal. Heating up a 1200 square foot slab of concrete that is probably 4" or so thick is going to take a fair bit of time. This is nearly 15 yards of concrete which is a lot of mass to heat up!
Matt
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Furry wrote:

...
What Matt said...
Would only ask--what was done during the installation for insulation/isolation from the slab? Was this new construction planned ahead for radiant heating or a retrofit? If not, and didn't take special care, not at all surprising to me.
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One of the drawback to radiant heat is the slow response. However, that is not necessarily a cost factor. If the heater is firing for 2 hours at 30,000 Btu, it is using the same amount of fuel as a burner firing at 150,000 Btu for half that time. I don't know your setup so I can't say if it is doing well or not.
You don't want to set back as far when not in use as you would the rest of the house. One benefit, of course, is that once heated, the thermal mass will stay warm longer so if you do want to set back, do it earlier than you would with other heating systems. If you are using a programmable thermostat, that that into consideration when setting the times.
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... 1200 ft^2 of 4" concrete is 400 ft^3 with about 25x400 = 10K Btu/F of capacitance, or more, if there's no insulation beneath. You might warm it from 50 to 70 F in 2 hours with a 10K(70-50)/2h = 100K Btu/h boiler.

It is, if there's an unoccupied setback, compared to a heating system with no time lag, because the heated space needs to be warmer for some time before the setback, and it stays warmer after the setback, so it loses more heat to the outdoors.

No. The former uses 60K Btu. The latter uses 75K Btu.
Nick
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Of course. What I typed is not what I was thinking. Point is, firing time alone is not the determining factor for cost of operation. Longer time at a lower rate can be cheaper than a huge inefficient burner cycling.
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snipped-for-privacy@ece.villanova.edu wrote:

No, the latter uses 150,000 Btus since half of two hours is one hour.
Matt
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The best way to run this type of system might be to have it on all the time but program in a reduced temperature at times when it would normally be off. Normally done with a programmable thermostat that has a "set back" feature.
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Actually, that is much faster than most systems.
Consider yourself lucky.
Get a programable thermostat.
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061230 1341 - Furry posted:

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First of all thanks for your various ideas and feedback. It eventually became clear that the radiant heat was working just fine - the problem was the level of insulation in the basement. There wasn't enough insulation installed on the exterior of the basement foundation when the house was built, which allowed a lot of the warmth to escape through the concrete. There was plenty of insulation installed *below* the radiant heat though. The best solution I've been able to come up with is to use Owens Corning Formular 250 which is Extruded Polystyrene Insulation. It comes in 2" X 2' X 8' sheets and is about $15/sheet. They have an R-10 rating. The stuff is very easy to work with, all you need is a hand saw to cut it to size.
After putting the Formular 250 up [just stood them up against the concrete] for just 30 or 40% of the basement wall the time between reheatings went up from every 4 hours or so to almost 12. Still takes the same amount of time to heat back up, say 3-4 hours.
So I'm going to complete the job, putting the Formular on as much of the basement walls as possible. The thing I'm trying to figure out now is what to use. Is there some sort of caulking or glue that will bind the Formular to the concrete? I'd like to form as tight a connection as possible to make sure air and therefore water vapor doesn't get in behind the insulation and make a great breeding ground for mold. I'm also going to want to tape the seams. Anyone have any ideas on what products to use for gluing and taping?
Thanks, Bill
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On 12 Feb 2007 10:44:06 -0800, someone wrote:

concrete to heat up, so it is no surprise that it takes hours!
Reply to NG only - this e.mail address goes to a kill file.
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Still sounds like the water temp is set too low. Is the burner running almost constantly on the boiler, or is it running only a fraction of the time? I recently had a call from one of my tenants saying the boiler couldn't heat the apartment, and it worked fine once I bumped the water temp up about 20 degrees.
JK
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wrote:

100-110 is a pefect temp for your water, I am running mine at 95 degrees.
--
Bob Pietrangelo
snipped-for-privacy@comcast.net (home)
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Why not just leave the stat alone and let it maintain, rather than trying to heat up the entire slab
--
Bob Pietrangelo
snipped-for-privacy@comcast.net (home)
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I just built a 26' x 26' workshop with radiant heat using PEX tubing etc. I have 6 runs of Pex tubing and it has worked out to be absolutely super! First of all, don't get tied in a knot about nightime setbacks etc., simply set your thermostat for a low but reasonable temperature and forget it. I don't touch my thermostat at all and my heater (gas fired water heater) fires up every 3 - 4 hours for about 5 - 10 minutes and it keep as the shop at a toasty 70 degerees F all the time. Costs are extremely low and I think it's the neatest solution to heating any area - I only wish that I could easily retro-fit my house for the same type of heating!! BTW I have the thermostat on my water heater set to it's lowest setting and I have a reducer valve that controls the flow of water thru the pex tubing - I have that reducer valve set to reduce the flow by about 10%. The combination of the thermostat setting and the reducer valve setting seem to work out just right. I'm in Ontario Canada and this past week it's been -15 Cesius most of the week! Doug
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Bill -
The most effective way to attach the foam board is with liquid nails (the one that is formulated for polystyrene - check the label) and strapping mechanically fastened directly into the concrete. As far as the seams...a lot of things work well, I like to use 1 component foam at each of the seams and the top, but plenty of other things work well.
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: Folks, : : We have a 1200 sq ft full basement that we're heating via radiant heat. : There are 4 runs of Pex. We're very happy with it but I'm worried that : there is something wrong with the system. When the basement calls for : heat the pump turns on and I can see the temperature gauge go up to : around 100-110 degrees. There's no gauge on the return but feeling the : Pex it's obvious that the water coming back is pretty cool. The : problem: It takes at least an hour, sometimes 2, for the basement to : warm up enough that the thermostat stops calling for heat. 1200 sq ft : is a lot of concrete to heat up but isn't that a long time?
Depends? Does it take 2 more hours for the basement to cool off? Is that a long time?
peace dawg.
: : Any thoughts much appreciated. : : Thanks, Bill :
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"Furry" wrote...

As others have stated, the drawback to radiant heat is it's slow warm-up. OTOH, it has a slow cool-down. Depending on how cool it was to start with, 2 hours is not a long time for that area to warm up to temp. However the boiler shouldn't be running continuously. The boiler should cycle and only fire up once the water reaches a certain temp. You really can't have your basement set at 60 then adjust your thermostat for 70 and expect it to heat up quickly. I've found that if you set the thermostat lower (say 65) and leave it, the area will eventually warm up beyond that point and the boiler will only kick in every once and a while. Assuming the radiant heat is installed in the floor, you can also get away with a lower temp setting on your thermostat because since the heat is lower it actually feels warmer.
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hawgeye wrote:

Yes, slow cool down is one of the other disadvantages, but it is really the same phenomenon. I have a friend with a radiant heated home and he often has to open the windows in the spring and fall when the temps cycle dramatically from day to night. When the sun comes up in the morning and the temp outside rises from 20 to 50 in four hours, the house simply can't release the stored heat quickly enough so you end up throwing heat away via open windows.
Matt
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