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.
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!
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.
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.
... 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.
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.
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.
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?
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.
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!
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
: 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
: 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
Depends? Does it take 2 more hours for the basement to
cool off? Is that a long time?
: Any thoughts much appreciated.
: Thanks, Bill
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.
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.
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