Water Heaters - Indirect Water Heater - Too Hot

I have a IHW heater attached to an oil fired boiler. I installed it last spring when I shut off the boiler for zone heating. Brought the low limit to minimum and hi limit to 140 deg. Works fine for the duration of summer. Started up zone heating yesterday. Hi limit 180, low limit 160, diff 10 deg. Turned on first floor zone up to 65 deg. Shortly thereafter house smoke detectors kicked in. Discovered small puddle around HWH and the pressure relief valve opened dispensing hot water onto floor. The steam travelled up to the smoke detector right above. Connection out of boiler is as follows.
Tee pipe out of boiler to zones and to circulator connected to inbound HWH. Return out of HWH to flow chk valve and back into boiler, before boiler circulator. I am assuming that the output flow to the calling zone is also pushing through hot water past the HWH circulator to overheat water in HWH. There is no type of chk valve directly between boiler and HWH circulator. Please advise. http://i253.photobucket.com/albums/hh44/Tigerthetony/IDWH_9-1.jpg