Radiant ceiling water temp question...

jad3675

My house had a CI boiler that struggled to send 120F water through the single zone radiant system - embedded 3/8 copper in the scratch plaster of the ceilings.
The CI boiler is no longer in the picture, and the house has been re-zoned at each floor.
The new mod-con boiler (TT Instinct) has ODR and a dozen or so pre-programmed curves to choose from; I'm wondering what is the best target temp for radiant ceilings?
Is it trial and error? I'm thinking that too high of a water temp would lead to cracked plaster and too low would lead to a cold wife.
FWIW, the 120F water temp of the CI boiler was...ok. It could keep the house at 67 or so, but left a lot to be desired on the gas consumption front.

Jamie Hall

You've pretty well tagged it -- too high and your plaster will crack -- and your wife will be upset. Too low and your wife will be cold -- and be upset.

You are going to have to experiment -- but you've got a good starting point. You know what worked, at least after a fashion, with the old boiler, and I'd be inclined to start right there.

hot_rod

Unless you have a huge heat load, that ceiling should not need to run much above 83-85° surface temperature. 120F supply from the boiler should be plenty to get that ceiling warm.

One nice thing with ceiling radiant is you can run them a bit warmer then floors, as you typically do not stand on ceilings.

90F ceiling in a 70° room would get you around 40 BTU/ sq ft output.

The very best first step would be to do heat load calculation, determine exactly what the system needs to accomplish.

Zman

Are you planning to run outdoor reset?

jad3675

Zman said:

Are you planning to run outdoor reset?

I'm glad HR bumped this thread, I have a little bit of data now!
It is running ODR. I have the current curve set to a max of 140, with the warm weather cutoff set to 68. Boiler is also de-rated to 55%.

On Oct 27th I had it running during the morning (both 1st and 2nd floor zones) - outdoor temp was upper 30's, the boiler setpoint was called at 112F. It ran for a bit over 3 hrs with an avg firing rate of 16%. Avg supply temp was 105F and avg return temp was 94F. A thermal camera on the living room ceiling showed the ceiling @87F in the coil areas.

John

jumper

Depends on ceiling height. Tall bald guys can feel too warm.

hot_rod

Even though it is mostly radiant transfer, radiant ceilings do tend to stratify a bit, It will generally be warmer at the ceiling, which does limit the output some. If 87F covers the load on a 30F day, you may not need SWT over 100-115F at design day

jad3675

hot_rod said:

Even though it is mostly radiant transfer, radiant ceilings do tend to stratify a bit, It will generally be warmer at the ceiling, which does limit the output some. If 87F covers the load on a 30F day, you may not need SWT over 100-115F at design day

Thanks - that helps. The CI boiler never had over 120 SWT (according to the aquastat readout and boiler gauge) and it seemed to do ok...apart from the fact that it condensed because of the low RWT. 

Cincinnati is due for some low temps these week - I may knock the ODR curve down to 120 and see how it does.

hot_rod

If you are running a conventional cast iron boiler it needs to run hot enough to avoid condensing. Typically 150 or so to assure 130 return
most often the boiler runs 150or more and a mix valve supplies the low temperature to the radiant

jad3675

hot_rod said:

If you are running a conventional cast iron boiler it needs to run hot enough to avoid condensing. Typically 150 or so to assure 130 return
most often the boiler runs 150or more and a mix valve supplies the low temperature to the radiant

Oh yeah, the old CI boiler lasted 21 years running 120 SWT. No mixing valves, just poorly plumbed radiant with a chop and drop done. I like to be think the original Honeywell moduflow controlled system was piped properly, but when they replaced the boiler in 2000 and hacked it, they didn't really understand what they were doing.
I replaced all that with as zoned modcon (Caleffi controls, of course) that will hopefully be a bit more efficient. 

Mosherd1

One thing to remember too, if possible, make sure that the ceiling/attic space above is adequately insulated to drive the heat into the conditioned space.