Bought my house 5 years ago, and converted to gas from oil. At the time I had a lot of renovations and $$ was tight so we used a non-condensing boiler (HB Smith) 100,000 btu. The boiler is set up with 4 zones, the 1st floor has 2 zones, both zones use cast iron radiators (sunrad type).

My original intentions for the basement was to also use cast iron radiators on 2 zones, 1 zone for the basement room and 1 zone for the garage. 5 years later and a new slab of concrete in the basement room zone which now consists of 2 loops of radiant in the concrete floor. The garage zone will still be cast iron radiation.

The boiler set-up is as follows, all zones are run off a Argo ARM-4P relay with a honeywell aquastat set at 180 degrees. All zones have there own circulator and thermostat. The piping comes out of the hot side into my spirovent and expansion tank and then into my header where each zone separates. At the header each zone has a flo-check and circulator pushing the hot side (top of the Boiler) to my rads and returning back from the loop into the cold side (bottom of the boiler). Thats the way the 3 cast iron rad zones are set up. The new radiant zone is as follows, from my header into hot side of a taco 5003 mixing valve, mixed side of mixing valve into radiant zone circulator , then into my supply side of radiant manifold at 120 degrees, return side of radiant manifold tee's off, one side to cold side of mixing valve, other side of tee goes into return on boiler. All returns enter a header seperatly, which then enters the return on the boiler.

The zones are all working great my radiant in the concrete works wonderful as does my cast iron rads, where my problem is in the return on my radiant side, my temp out of the radiant is 80 degrees. I noticed since adding the radiant zone my boiler is firing much longer and normally it would have shut off at the aquastat's setting, now it seems the boiler runs past 200 degrees before the burner shuts down. When running only my cast iron rads it hovered at 185-190 before the burner went out. I added a bypass valve with a gate valve from a tee that come off the hot side and added an additional tee on the radiant return to get the temp to the proper degree the boiler likes (140). After adding the valve I really am stumped, when the radiant is running on it's own with the valve opened fully i can get a max of 130 degrees going into the radiant return, below the tee that goes up to the cold side of the mixing valve. As soon as the other Zones come on, the temp in the return from the radiant below the tee goes cold down to 80-90. And the pipe with the gate valve gets hot.

Not sure where to go with this but I do know I want 140 going back into the boiler on the radiant return. Any help would be appreciated.