Radiant flooring system not achieving good flow rate

[SolarJoe]

Fresh, new installation in home under construction. Solar thermal panels work GREAT and bring 110 gallon storage tank up to temp in just a couple hours with sunny skies. Problem is on radiant flooring side of system. Main floor is 1600 square feet, broken up into 4 zones of 1/2 inch pex embedded in 1 1/2 inches of gypsum concrete. System was purged of air with garden hose connection and then sat dormant for about a month - and held 40 PSI all that time, so I know the loops are sound and filled with liquid. Hooked up the latest greatest smart Grundfos pump ALPHA II, and using a TECHMAR zone controller, which works as expected. Trouble is, I can't achieve anything more than a fraction of 1GPH, regardless of how I set the pump! I've let air out of the system at the distribution manifold, but just can't get any significant flow!! Picture is attached. System is brand new, and has no leaks. I'm not a HVAC guy, but I am an engineer and understand physics and fluids. With winter on our doorstep, I need to get this working properly!! Any suggestions would be greatly appreciated. I've tried opening ALL loops full open, and nothing changes. The little flow rate site glass windows are only about 1/3 filled with fluid, and no matter how I adjust them (currently they are all open 2 full turns) the little indicators stay at the top all the time. When purging the system with the garden hose, those little adjusters worked just fine on each zone, so I know they work! I'm just moving fluid from the holding tank up about 6 or 7 feet to the main floor directly above. There are no kinks in the lines and I never used less than the minimum radius for 1/2 pex.

 

[Silentrunning]

Joe, I know nothing about these systems other than the principal on which they work. But, like you, I am an mechanical engineer. So let me ask some questions and see if we can’t solve this.

Is it possible a mouse or large bug got into the pex during construction?

Is it possible to disconnect the pump to see what the flow rate is when not restricted?

If it is can you test it with a vertical pipe to see how much this affects flow?

Does this system have a pressure accumulator?

In the meantime I will read up on this type of system to see if I can help you.

 

[SolarJoe]

WHen I initially filled/removed air with the garden hose connected, the system was flowing freely and the flow rate adjustors worked fine - this is how I filled the system, one zone at a time. Then, with the whole system pressurized to 45PSI with the water pressure, I closed the valves, and the system held pressure for over 2 months while I did other things. So nothing could have gotten into the lines. The flow rate was awesome at that point, and the pump was not yet in the system. I just installed the pump and lines to the holding tank, mixing valve and manifolds. Filled them all, and then opened the manifold valves, completing the heat harvesting loop. The system doesn't have an "accumulator", but it does have a pressure compensation tank that is set at 1.5 bar (about 18 psi) by way of an air filled bladder inside the tank. I may call GRUNDFOS and ask their engineers what they think...

 

[Silentrunning]

That would probably be the best move. Please post your findings when you get it operational.

 

[SolarJoe]

The reply I got was incorrect and just not true. I responded, but I'll be damned if I'll spend 24 bucks for a 2nd pass with people that are clueless about my problem... your service is a joke.

 

[rogerius]

Hi SolarJoe,

I'm in process of designing my own radiant heating system and just wondering if after 2 months you successfully started your system. If still have trouble with it, just wish to point a couple items for you to check on your design:
- is enough head on the pump selected? Calculate the head assuming is a call for heat from all zones.
- are all the loops under 300ft in length.
- could you post the schematic of your system so can see the pipe size, connections, etc. Quite difficult to follow on the picture.

Thanks

 

[Silentrunning]

Hi SolarJoe,
- are all the loops under 300ft in length.

The OP has 1,600 square feet and the system is broken down in 4 sections. What would the linear foot requirement for tubing be for this area?

 

[rogerius]

Hi Silentrunning.
Not sure if I understood the question. If you are referring why I asked for each loop to be under 300ft, is because that is the rule of thumb in the industry for 1/2" pex tube which will give reasonable flow rates and Btu deliveries vs. reasonable pressure drops. Over that value the flow will drop, the pressure drop will increase, so the rate transferring the BTu to the slab will decrease.
Now, if you ask me how to calculate the length of the tube required for that area, that will depend on some design parameters: thermal loss in the building, designs temperatures, R-values on the floor, etc.