Radiant heat floor puzzle.

I have the spirovent and expansion tank on the suction side of the pump. I'll try tomorrow to move it to the pressure side and see if that clears it up. Sounds very likely to be the problem.

Thanks!

Leave the expansion tank on the low-pressure side off the pump or you risk cavitation issues on the pump impeller. A vent is best places at the highes elevation on the system at a lower water velocity/low turbulence (read "length of fatter pipe") section of plumbing. If yours is one of the Spirovents plumbed designed to hang the expansion tank directly on it, leave it where it is on the suction side of the pump, and put an other vent at a higher pressure higher elevation point:



When using a tankless as a boiler it's important to place the expansion tank & pump in the "acceptable location" point in the left side of in the above diagram, which lowers the risk of macro-boil or "kettling" on the heat exchangers by pressurizing rather than de-pressurizing the tankless, and allows the system to run at a lower pressure. With high-head radiation the vent is still best placed between the pump and the radiation, where it's less likely to ever be negative pressure relative to room pressure.

It's also common in primary/secondary plumbing to place a vent at the hydraulic separator, (even if it's a hydraulic separator built out of fittings & a lenght of fatter pipe rather than a commercial unit) since it's a low velocity/low turbulence point on the system.

This is a good podcast to listen to about "Pumping Away" by Dan Holohan about how circulators move water in closed hydronic systems.

http://flopro.taco-hvac.com/podcasts/index.html?id=32