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Posted by Furry on December 30, 2006, 1:41 pm
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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Posted by Matt Whiting on December 30, 2006, 1:57 pm
Furry wrote:
> 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.
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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Posted by dpb on December 30, 2006, 2:03 pm
Furry wrote:
> We have a 1200 sq ft full basement that we're heating via radiant heat.
...
> 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?
...
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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Posted by Edwin Pawlowski on December 30, 2006, 2:35 pm
> 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?
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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Posted by on December 31, 2006, 1:43 pm
>> ... 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?
... 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.
>One of the drawback to radiant heat is the slow response. However, that is
>not necessarily a cost factor.
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.
>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.
No. The former uses 60K Btu. The latter uses 75K Btu.
Nick
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