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# In Floor Heating

Member Posts: 30
I have a large commercial room with 20 foot ceiling , infloor temp around 40 deg C , and separate air supply usually at 21 deg C. The room wont warm up in cold weather ( -10 deg. c ) above 16 deg c. Im wondering if the area (cubic feet) is too large. Any suggestions? P.S. Its new construction

• Member Posts: 7,551
Is the tubing in concrete?
Insulation under and at edges?
What spacing?
Have you calc the heat loss for the space?
What is the separate supply air? 21c sounds like your target.
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 30
The tubing is in concrete. Its a new school so I assume the engineers called for insulation under the slab but ill have to look into it-also the spacing. They should also have calculated the heat loss and that the high ceilings would be taken into account when the heat loss calc was done. The air supply is from a mixed air ahu which I have to look at closer to see how low the temp goes. I was wondering if there's a limit on ceiling height when using infloor. thanks for your reply
• Member Posts: 3,952
Twenty foot ceilings are not a problem for radiant heat; even 40 foot ceilings are OK. You just have to do the engineering (heat loss calculations).

Have you spoken to the contractor who did the work?

You said that your infloor temp is 40C. Is that water temperature?
8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

Two btu per sq ft for degree difference for a slab
• Member Posts: 30
No I haven't spoken to the contractor yet . I'm going to look at the building plans first and try to do a heat loss calc and see if their design fits the application. The 40 deg C is the setpoint for the infloor water .
• Member Posts: 7,353
What's controlling the water temp to the slab? It should be a smart valve or variable speed mixing based upon outdoor reset. Also, what type of floor covering and its R value? You may be able to bump the water temp up a liitle more depending upon the surface temp of the floor.
Bob Boan
You can choose to do what you want, but you cannot choose the consequences.
• Member Posts: 30
The water temp is controlled by a digital control system in the building . It varies with the outside temp so its usually at max (40deg C) in cold weather. (a viessman boiler heats the water).
Im going to have to check on the type of floor covering but its ususlly some sort of vinyl . As far as I know the rest of the rooms in the building, on the same system, are ok.
• Member Posts: 3,952
I think 40°C is optimistically too low a setpoint. It should be more like 54°C - 60°C and will depend on tube spacing and the heat loss of the room.
8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

Two btu per sq ft for degree difference for a slab
• Member Posts: 9,546
40 centigrade is 104* water supply that is a bit optimistic, 94 AWT it is quite possible the room in question just needs a higher SWT compared to the other rooms if they are okay.
• Member Posts: 7,551
The problem is more likely in the AHU.
You will have far more heat loss though ventilation than though the building envelope.
A common problem with boilers that supply both AHU water and slab radiant is that the slab drags down the boiler temps and starves the AHU
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 30
I locked the supply air temp at 22 deg c and increased the infloor setpoint to 45 deg c and set the variable speed circ pump to 100.0 % speed. The temp was still down to 17>8 deg today . Valves are set to open continuously. ??
• Member Posts: 30
I meant to say 17.8 deg ( room temp).
• Member Posts: 7,551
How is the AHU heated?
If it is water off the same boiler, what are the supply temps?
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 7,265

Twenty foot ceilings are not a problem for radiant heat; even 40 foot ceilings are OK. You just have to do the engineering (heat loss calculations).

Have you spoken to the contractor who did the work?

You said that your infloor temp is 40C. Is that water temperature?

What is they always said to say about the difference between floor heating and ceiling? One gave you hot feet, the other gave you a hot head.

• Member Posts: 9,546

What is they always said to say about the difference between floor heating and ceiling? One gave you hot feet, the other gave you a hot head.

never experienced either with proper control, and water temps.
• Member Posts: 7,265
That's true. When all of your above is met. No argument.

But often, when people are given the cost of such a system as yours, they go with Polar/Scorched air because it is so much cheaper.

At least that is my experience. Especially when called upon to try to fix the problems. There's no easy fix. Or a cheap one. Especially to the second owner who bought the house in the summer when it was warm and they call you in January.