What I thought would have been a simple system is giving me fits. A 17' x 25' 5" garage slab with 2" of insulation below and on sides has 2 135' loops of 1/2" pex on a single zone. It is a closed system with about six gallons of a 75% glycol solution heated by a small on-demand propane water heater (80k btu, 3gpm Eccotemp). The system uses one circulator, includes an expansion tank, an air scrubber, a Watts manifold, and a strainer. (See photo).
I quickly discovered that my first choice of circulator (Taco 006) was woefully under-powered, registering virtually no flow on either the manifold's or heater's gauges. I was able to make the system work by powering it with the 1/2 hp utility pump that I used to fill the system so I figured a larger circulator would do the trick. I swapped in a Taco 007 and still no cigar, although I did see a reading of .1 gpm flow. And now (problem numero dos) my utility pump trick would not work consistently. It would start fine, showing a flow rate of about 1 gpm but over the course of about three minutes the flow rate would drop below the .5 gpm necessary to keep the heater on. And ... I never saw an air free return of fluid to my transfer bucket, even though any slight leaks in any connections were minuscule weeps.
I temporarily bypassed the heater to check the integrity of the loops and system board and saw flow rates of .4 gpm with the Taco and 1.5 gpm with the utility pump, as well as a nice air free fluid flow.
So, while my initial thought is to add another circulator on the return side of the loop to provide more flow, I'm unsure as to what is causing that drop in flow rate and air intake when the heater is running.
I would be so appreciative of any advice!https://photos.google.com/photo/AF1QipNB43dlP6VdjS4TRdOI8BwuaGBIbLx3rGZQ8lY5