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# Why can't I calculate my head loss? Garage in floor heat.

#1
12-06-10, 05:52 PM
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Join Date: May 2009
Location: USA
Posts: 19
Why can't I calculate my head loss? Garage in floor heat.

I am totally messing up my calculations.
My new garage has 4 250' 1/2" PEX loops. The manifold supply and return is 3/4" PEX, this loop will be near 100'. I know that is quite a long main loop for 3/4", It is a somewhat temporary setup using a heat exchanger in my house.
The problem: I can't calculate head loss to size a pump.
I have been using Taco's "Selecting Circulators" guide.
I believe the flow through my floor loops should be about 1.5gpm, putting my primary loop at 6gpm. The equivalent length of the Primary loop would be about 118' and the floor loops 257' (or less.) I will be using 40-50% glycol. When I calculate this I get a head loss of 30 feet for the primary, and 30 feet in a floor loop, totaling 60 feet. This is using head loss in one floor loop as it says to use head loss of the highest loop (all being the same.)
I think there are multiple errors in my calculation. I know that head loss is decreased having the parallel loops, but this is not how it says to calculate. I also have a good idea I am doing something wrong because I am way off the charts for pump specs. I know there are similar setups for outdoor boilers, as far as head loss goes. Please help, if there is any for me.
Your assistance is greatly appreciated...the garage is cold here in ND.
Jacob

#2
12-06-10, 06:33 PM
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Join Date: Jul 2009
Location: Toronto
Posts: 1,174
You are not going to be putting 1.5 gpm thru each infloor loop, maybe all 4.
What is the heat loss of the area your heating ?
Do you have a radiant design done ?
In 3/4" PEX 6 GPM is about 25 feet of head at 100.
Gonna be a hard to find / expensive/ power hungry circulator
Maybe a Taco 1400-50. 5.0 Amps Yikes
6 GPM would be about 60,000 BTU, but either a pretty big garage, or you need some more insulation.

Last edited by NJT; 12-07-10 at 06:00 PM. Reason: fixed typo...
#3
12-06-10, 07:21 PM
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Join Date: May 2009
Location: USA
Posts: 19
Wait...1.5gpm in 4 loops is 6gpm total, isn't it? I am looking at somewhere around 45 -50k btu/hr. Just over 1000sq ft in North Dakota.
Not sure what you mean by "1.5 GPM per loop would be about 20 GPM."

#4
12-06-10, 08:27 PM
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Join Date: Jul 2009
Location: Toronto
Posts: 1,174
Should be 20 FOH, my bad sorry

Last edited by NJT; 12-07-10 at 06:00 PM. Reason: fixed post above...
#5
12-06-10, 08:33 PM
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Join Date: Jul 2009
Location: Toronto
Posts: 1,174
You need to get some insulation in that garage.
Can you get second set of 3/4" supply and return lines in there.

Sorry to say but I am not sure your going to get 50 BTU / sq ft out of that radiant slab. I am guessing no one has done a design on it.

Rule of thumb is about 25 to 30 but / sq ft. When you get into situations like this you really need to get a designer in to go over things.

#6
12-06-10, 10:26 PM
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Just for a random dot on a chart ..
My attached garage is about 550 sq ft.. (R22 walls R40 ceiling) and has one loop of about 500 ft 1/2 pex. Its fed from the house manifold (where 5 other loops of 80-200 ft length) all join up.
I use a mixer to the manifold bringing the temps down to about 140 (wood floors in the house).
I keep the garage just above freezing (6c or 43f).. and it seems to hold just fine (it gets to -40f here)
The circulator is a Grundfos UPS-15/42

#7
12-07-10, 06:12 AM
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Join Date: Jul 2009
Location: Toronto
Posts: 1,174
Dave,
You would not have a lot of flow thru that 500 foot loop.
Since you keep the temp low, the heat loss is also low and you can get away with it.
Measure the supply and return temperatures on the loop, you should find they are quite high.

But if it works then it works. No need to reinvent it :-).

#8
12-07-10, 09:04 AM
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Join Date: May 2009
Location: USA
Posts: 19
"Should be 20 FOH"
That makes more sense. Still don't understand why actual head loss for the parallel loops is not used. I don't really want to run another loop from the garage to the house as it will likely not be my primary source for long. It would be nice to have more insulation, but I don't think I will be all that deficient. My heating needs were exaggerated for the sake of my initial calculations. Now going back over things I believe I may have exaggerated more than I thought. I think my demand will be in the neighborhood of half what I was using. Maybe I should have had someone design it, but I didn't do it alone. I went off other people's designs around here, and had some help of a respected company that specializes in outdoor boiler installation. I don't think the design is very far off, I just had a couple numbers a little far off.

#9
12-10-10, 02:20 PM
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Join Date: May 2009
Location: USA
Posts: 19
So I guess I wasn't too far off with the head loss calculations, just a little conservative with my numbers...and quite a ways off with my heating needs.
If I was to shoot for 4.5gpm total flow with near 33 Feet of total head loss, that should put me with a Taco 1400-45, Is that a scary idea? I may be able to get by with a 1400-20 with slightly lower flow.
Now, when I get a boiler in the garage and reduce the primary loop to a fraction of what it will be for now will there be any way to get by with one pump for both options? The IL009 may be enough to get me by in my current situation, and with its curve looks like it could definitely still fit the bill further down the road.
Thanks for the help so far, any opinions on my new info?
Jacob

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