Forum / THE MAIN WALL / GPM in a hydronic slab system

## GPM in a hydronic slab system (10 Posts)

• ### GPM

• Glad I asked. I'll investigate this a little more tonight. Meanwhile: Your btu/hr/sq.ft. output at design (or heat loss and square footage). 27 btu/hr/sq.ft. 20,000 btu/hr/total room 2) The size of the tubing used in the loops. 1/2" 3) The on-center spacing of the tubes in the loops. 16" 4) The desired air temperature at outdoor design temp. Typically 50 degrees F, but may ramp this up to 65 for long weekends. Maybe that's why they supplied me with a three speed pump. It's running at the highest speed now...should I experiment with the lower speeds? I don't quite understand the relationship of flow and btu transfer. Is there a direct relationship between higher flow and higher transfer?
• Thanks, I printed out your reply for reference tonite. I find this whole concept fascinating. Don't have to wear wool sock anymore.
• ### You're welcome

I triple-checked the numbers I gave--based of course on the numbers you supplied. Just wish that I could better explain the relationship between flow, delta-t and output other than to say, "stay within generally established parameters". For your possible enlightenment however, from the Buderus Design Manual for Panel Radiators: Heat loss (Q), water flow rate (GPM) and temperature drop (delta-t) through a hydronic heating system are related to each other as: Q = 500 * GPM * delta-t Said of this formula, "This equation is used extensively for accurate sizing of radiators." Note that the equivalance in the equation is "heat loss", NOT "heat output".
• Thanks again. Last night I double checked the readings in the flow meters. I was in error reporting 2GPM, that IS the highest reading on the scale, but all three circuits are actually running a bit less than 1 GPM, which would be in line with your numbers. Still can't get out of the habit of wearing wool socks, though..my feet were roasting, even though the slab feels cool to the touch. Amazing.
• ### You're welcome

I triple-checked the numbers I gave--based of course on the numbers you supplied. Just wish that I could better explain the relationship between flow, delta-t and output other than to say, "stay within generally established parameters". For your possible enlightenment however, from the Buderus Design Manual for Panel Radiators: Heat loss (Q), water flow rate (GPM) and temperature drop (delta-t) through a hydronic heating system are related to each other as: Q = 500 * GPM * delta-t Said of this formula, "This equation is used extensively for accurate sizing of radiators." Note that the equivalence in the equation is "heat loss", NOT "heat output".
• ### quick cheat

total btu transfer divided by 10,000 = GPM or 1 gpm per loop EIN
• ### GPM for a hydronic slab system

Newcomer here, but expect to be a regular. Plumber by trade 25 years ago, now a DIY, HO, GC doing a 1200 sq ft addition to 480 sq ft cabin. Plan to heat with staple down PEX/plate/OSB sandwich, have framed accordingly for doors etc. Also have a nearly completed garage/workshop/guest bed-bath with a just-installed heated slab. Location is SE MN with -30F typically the coldest day, BUT it's 58 degrees F today. Details of question have to do with the heated slab: question about the GPM that I should be pumping at. This is a single zone, 3 - 250' wirsbo circuits stapled to the 2" rigid under the 24 X 29 foot 4" thick slab. 4" rigid perimeter insulation. Has a Rehau (?) manifold and a Grunfos "medium head" three speed pump controlled via slab sensor and SP 30 Goldline unit. Heat source is a 50 gallon A.O. Smith propane WH. Initial setup has the water temp going in at ~100 degrees and coming out at ~ 80 degrees, running at 14 psi. Hardly needs to go on at all to maintain a slab temp of 55 degrees. Actually, the 10 -150 watt bulbs that provide overhead light warm the air temp up 4- 6 degrees in a 'couple of hours with outside temp at 20 degrees. R40 ceiling, R23 walls, no windows, two insulated garage doors, one man door. Purpose of the system is just to maintain a comfortable baseline temperature to work in, with a side benefit of helping to keep the well pressure tank, BIRM filter, etc, located in an adjacent room, from freezing. The manifold has little red knobs on the return side that screw in and out, controlling little pins, which in turn affect the GPM readings on the little floating meters right above the valves. When wide open, the meters read almost 2 GPM. What should I run them at? Are they intended to provide the right flow so you can maximize the amount of heat passed to the slab, or to control the flow to individual circuits? Or both? Should I run them wide open? Does GPM in a system like this really matter very much?
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