# optimum slab temp for radiant water



## woodywoodchucker (Jan 15, 2012)

I have radiant heat in my basement floor. Heard somewere that the temp should be below 100*.Is that correct? And why.


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## huffdawg (Jan 15, 2012)

the lower the temp the longer your storage will last.


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## 711mhw (Jan 15, 2012)

My design temp for the water going to the slab is 108 deg. I had my set up designed by a radiant group up in Gardiner ME. because I had no radiant heating experience and all my "stuff" was going into the slab and that's no so forgiving for later corrections, (I have seen a wide renge of suggestions for loop spacing and legnth/layout in my reading) but I think that they try to shoot for the lowest water temps. needed to get the job done for efficency purposes.


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## DaBackBurner (Jan 15, 2012)

711mhw said:
			
		

> My design temp for the water going to the slab is 108 deg. I had my set up designed by a radiant group up in Gardiner ME. because I had no radiant heating experience and all my "stuff" was going into the slab and that's no so forgiving for later corrections, (I have seen a wide renge of suggestions for loop spacing and legnth/layout in my reading) but I think that they try to shoot for the lowest water temps. needed to get the job done for efficency purposes.



+1

Lowest possible supply temperature to satisfy heat loss. A win, win combination.


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## afblue (Jan 16, 2012)

DaBackBurner said:
			
		

> 711mhw said:
> 
> 
> 
> ...



Exactly, if you can run 95 degree water through the floor and keep the thermostat satisfied, thats all you need. The other thing to think about is, Slabs are high mass, very slow reacting, so if you put too hot of water in it, the thermostat is going to shut off and the slab is going to keep giving off heat way past, what is called overshooting. For the "warm feet feel" Maximum surface temp for radiant should be 85F. There are Outdoor reset mixing systems that change the temp of the water based on the temp outside, so you try to match the heat loss of house with a corresponding water temp.


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## woodywoodchucker (Jan 16, 2012)

I checked my water going to the slab and its at 120. I turned it back and will try 90*


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## jebatty (Jan 16, 2012)

A shop but functionally no different than a house. Mixing valve set at 100F, slab sensor set at 61F with 1F differential. Slab sensor shows constant 61-63F. With 1000 gal pressurized storage, burn the Tarm no more often than once every other day for about 6 hours, but this winter burning has mostly been every 3rd or 4th day.


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## kuribo (Jan 16, 2012)

with water supply temps of 100F, what type of flow rates per loop are you seeing?


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## jebatty (Jan 16, 2012)

Based on the flowmeters on the manifold, 4 loops are 0.5 gal and 2 loops are a little less. I estimate 2.75 gpm total. Delta-T average is about 30 for about 40,000 btuh when the radiant is "on."


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## kuribo (Jan 16, 2012)

what is the air temp in the space?


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## jebatty (Jan 16, 2012)

Air temp inside right now is 59F based on a thermometer on an outside wall, outside temp is 13F, wind is calm. Floor sensor in the concrete slab reads 63F, which means that a radiant heat cycle recently completed. The floor sensor temp will fall to 61F when another heat cycle will start. Inside temp may fall as low as 53F at the cool end of a cycle, although 55-56F is more typical. All of this relates also to outside temp and weather conditions. My shop is pretty tight and well insulated, no noticeable drafts. These temps are very comfortable, shirt sleeve comfort, for work in the shop.

As most will say, radiant feels warmer than the air temp would indicate, as radiant warms surfaces and is nearly draft free, except for drafts in the structure.


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## kuribo (Jan 16, 2012)

thanks for the info....

It would seem that one can set the flow rate higher and use a low supply temp or a lower flow rate and higher supply temp. With storage, from what I read here it is better to use as low a supply temp as possible....With that in mind, What is the lowest delta T that is practical in a radiant slab?


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## jebatty (Jan 16, 2012)

I don't know the answer to your question. I seem to remember that for 1/2" pex, the optimum flow rate is 1/2 gpm. Also that concrete should not be supplied at a temp higher than 120F. Others can correct this info. At any rate, 100F works very well for me in heating the slab from storage. Actual temperature from the mix valve varies between 90 and 106F, depending on actual supply temp from 105 to 190F. I have run supply down to the high 80's but don't do that normally.


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## skfire (Jan 16, 2012)

kuribo said:
			
		

> thanks for the info....
> 
> It would seem that one can set the flow rate higher and use a low supply temp or a lower flow rate and higher supply temp. With storage, from what I read here it is better to use as low a supply temp as possible....With that in mind, What is the lowest delta T that is practical in a radiant slab?




I think experimentation on the actual setting is the key, but the Heat loss  vs supply/demand curve is pretty close in general.
The storage temp variance during supply is  one factor you have to consider when dealing with the mixing vlvs and  pump settings.

I have a Delta-T of 30-35 consinstantly, based on my manifold readings, mixing valve settings and  on a 60,000btu/hr on 5f design temp( a bit high end safety factored).

a)Lower zones-4 (1,950 sq ft):All radiant (1/2" pex) in mostly gypkreet under tile/slate, 3way mix set from 110-118(based on variant storage temp), flowmeters set to around 0.5 gpm, wilo eco stratos set to 9ft of head. 
b)Upper zones-3 (1,000 sq ft) : All radiant(3/8" pex) in mostly therma-floor boards under hardwood flrs,  3way mix set from 112-120(based on variant storage temp), flowmeters set to around 0.6 gpm, wilo eco strato set to 7ft of head

So far it is working pretty well, but I am working towards that 40-45 Delta T goal.

I hope this helps, as I am still experimenting myself ....so some very expert members may shed some light.

Scott


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## DaBackBurner (Jan 17, 2012)

Money very well spent from one of the hydronics industry's experts, John Siegenthaler. For $65, a no-brainer, in my estimation.

http://store.hydronicpros.com/Radiant-Precision-SECOND-EDITION/P1010_1002/


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