Radiant heat thermostat use
We have an area of radiant heated slab (about 850 SF) that runs off a non-setback type thermostat, at our HVAC guy’s recommendation. He believes that the radiant area will heat best with a constant setting. There is no outdoor reset sensor.
We’ve kept the thermostat at a constant 67-68 degrees, which is very comfortable. Feels more like a room at 70-72 degrees.
The NG bills are pretty steep, though, and we’re trying various things in other places to increase efficiency. Boiler is Weil-McClain Ultra Gold 150 BTUH and does mostly hydronic baseboard (about 4500 SF) and passive domestic HW.
Is anyone using a night setback T-stat on hydronic radiant heat? I’m wondering if a night setback to 60 degrees (say 10:00 PM) and back to 68 (at say, 4 AM to give time for warmup) would save some energy cost.
Thanks in advance –
Jeff
Replies
No, you do NOT SET BACK SLAB AT NIGHT.
Caps for stress. It's important. Do not do it. Your HVAC guy is absolutely correct.
If your bills are high, its in the system setup or the envelope, and that's where the problem needs to be rectified. Is the slab insulated?
Is this a mod/con boiler (Ultra)? I don't hear it called "ultra gold" so I just want to make sure. If it is, that boiler should have an outdoor sensor on it, and should be running outdoor reset on those baseboard units. That could have a pretty significant effect on your fuel bills if it isn't being used currently.
-=Northeast Radiant Technology=-
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Thanks - OK no night setback.
It is the Ultra boiler/condensing type - http://www.weil-mclain.com/products/display/1-Ultra%20gas - I think they've dropped the 'Gold' designation.
Envelope is foamed walls to R-19 +/- Slab is R-12 rigid foam and decoupled from foundation walls.
I'm surprised because with foamed walls the house will hold 70 degrees interior with an exterior of 40 degrees for hours before the T-stat kicks in for the baseboard zone. Maybe $450/month for gas (only) heating/dryer/cooktop in Nov-Dec isn't bad - I don't know. It is heating maybe 5300 SF (4500/basebd 800/radiant) plus keeping a 650 SF garage to 44 degrees +/- on another radiant zone.
Jeff
Edited 12/22/2007 2:35 pm ET by Jeff_Clarke
I would get the outdoor sensor installed, though my next question would be whether you are calling the first month that slab was on a high bill, or whether it's been running for awhile. Just because startup on a fresh slab can draw a whole lot of energy.-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
Just because startup on a fresh slab can draw a whole lot of energy.
Is that because it's having to heat up the slab, as opposed to maintaining a temp once the slab is stabilized? PJ
Everything will be okay in the end. If it's not okay, it's not the end.
Partially yes (it takes a lot of energy to charge up a slab initially), but also partially because on the FIRST startup, you still have a lot of moisture and such in the slab that needs to be "powered out" by the radiant system. This is a highly variable condition depending on when you poured, the mixture of the concrete, heck.. all kinds of variables... but the upshot is when you FIRST start a slab, it is entirely possible that it will take a few days to finally even out and heat up. After that, it shouldn't ever be that bad again. But needless to say, a boiler running full tilt for 3 days or what have you can throw off your fuel bill numbers. And any house that is still under construction is also "all bets off" until construction is final.Maybe that didn't happen to you, but I threw it out there because I get a lot of calls from people on the first month numbers, or on hour twelve of a slab startup with a bit of panic in their voices! "Second month numbers" should be much more accurate, if you have them.-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
OK, minor hijack here. What about in a low-mass installation in an addition where the rest of the house is forced air with setback? I find my boiler is trying to heat more than the room zones when the rest of the house is cooler due to setbacks.
Mike HennessyPittsburgh, PA
If you can do setback with the low mass, fine. But if you have an outdoor reset curve set, you're sacrificing its ability to minimize water temps to do setback. If that's the case, I'd look more seriously at insulating between zones. Or, scrapping that FHA system ;)-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
Thanks for the info. The new rooms are insulated from the old, but certain, er, gremlins, keep forgetting to close the doors. I don't have an outdoor sensor, so I don't think the reset curve will be a problem. I think I'll talk to my supplier/designer and see what he says.
Mike HennessyPittsburgh, PA
I thought the WM Gold had the outdoor reset standard with the unit? I recall the unit being a Tekmar controller only re-labeled.
I very well could be thinking of another WM unit but I thought it was the Gold Ultra.
I'm having an issue with my low mass boiler (System 2000) running ON/OFF every 4-5 minutes but the WM are still cast iron units I was told. I wish I had learned of Rob and this site earlier. :-(