Flow Rate in floor ?

[Bricks]

Our basement slab has 5 loops of 1/2 inch barrier pex total of 940 feet hooked to 1 inch manifolds. The flow rate of the 1/2 inch pex is 1.5 gpm. To figure flow rate do I use 1.5gpm X 5 loops = 7.5 gpm? What would be a recomended flow rate for all five loops?

I haven`t nailed it down completely yet but my main pump will be close to 6.5gpm with a head loss of 7.18. When figuring the main circ do I include the 940 feet of pex in the basement slab?

 

[in hot water]

the flow rate depends on how much heat you are trying to move through a loop.

Some quick numbers from the Radiant Panel "RadPad" calculator.

The RPA guideline is .6 gpm on 1/2", with a loop length not to exceed 300' in length.

At .6 gpm flow (water) in a 300 foot loop the pressure drop would be just under 4 feet of head.

The pressure drop (resistance) (head) is based on the longest loop. if 300 feet is the longest, throw another foot of head for any additional piping, fittings, valves, etc. So you need to move 5 loops at .6 gpm per loop or 3 gpm at 5 feet of head. The circ gets sized to that number.

3 gpm at a 20 degree delta T moves around 30,000 BTU/hr.

1/2" tube, 12" on center in a slab, 300 foot loops, would output 20- 25 BTU/ square foot of floor.

Five 300 foot loops at 12" on center would cover around 1500 square feet of room.

1500 square feet with 20 btu/ ft output= 30,000 BTU/hr

If it's a basement, under heated space 20 BTU/ sq. ft is probably a high number, just depends on windows, insulation, etc.

The best way is to start with the heat load of your room and build the system to that. Not knowing what you are trying to heat, this is some info working from the other direction

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[Bricks]

Thanks..... In Hot Water

My main circ would be running 6.5 gpm can I throttle down the flow through the 1/2 " infloor pex using a valve or am I better staying with a proper sized circ? I will be using a mixing valve for temp control in the slab.

 

[in hot water]

Do you have a circ dedicated to the radiant zone? Or does one circ flow the radiant and boiler?

The pump should size as close as possible to the job (load). Most small wet rotor circs are now 3 speed and offer a wide range of adjust-ability. Final flow adjustments are often made at the radiant manifold. Many radiant manifolds have mini flowmeters right on the ports, along with a balance valve to dial in the required flow rate per loop, regardless of the loop lengths..

Ideally you want to find the circ that meets your spec while operating on the middle 1/3 (knee) of it's curve. That is the most efficient operating area. With your short loops, this Grundfos on speed 1 look to be ideal. IF it is just flowing the radiant portion.

Taco and Grundfos have some good sizing info at their websites.

If you have the budget, the new ECM circs from Laing, Wilo, and the Grundfos Alpha have Auto Adapt functions. They electronically adjust their output based on the load. Ideal for multi zoned system where various zones are turning off and on. With a high efficiency motor they consume 40- 60% less energy. about 3 times the cost of the 3 speed versions.

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