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New Radiant Zone Overtemping Boiler - HELP!!!

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Posted by Dave Tzeutschler on April 13, 2005 at 19:31:31:

Hello! Finally a place to talk to people about my boiler in plain english!! Thanks.

Just finished a bathroom remodel which went great until I tried attaching my new infloor radiant heat to the boiler: I have a gas-fired cast iron Burnham boiler which is currently heating the rest of my house with tradtitional radiators. Used several raadiant companies' websites and diagrams for a retofit in order to use existing boiler for my new radiant heat zone, including a thermostatic mixing valve to decrease the boiler's water temperature for my new radiatn zone.
With the whole house heat running (original zone) in conjunction with the (new) radiant zone, everything works great. But with just the radiant zone, the boiler heats quickly to 180 deg. F, then continues to heat to near boiling (220 deg. F) then I beleive the hi-temp limit shuts it off.
SO, I think one of two things is happening: 1) the mixing valve for some reason is not letting enough water in through the HOT side from boiler and therefore the water in the boiler just heats up--and quickly-- since it has nowhere to go or 2) my radiant zone is too small for this setup (only about 120' of 5/8 " PEX tubing plus copper pipes to 2nd floor) and therefore just overheats quickly.
So what's the solution??? A radiant tech. mentioned an aquastat along the way. Would I tie this in before the mixing valve so it senses water temp and therefore shuts off boiler when radiant zone gets too hot....while continuing to run the circulator pump to circulate that now hot water through my radiant zone? And are they simply a 24-volt switch that could turn the boiler on or off at a hi temp limit (like 180 or 190 deg F)? Could I wire the relay which currently fires the furnace and runs the circulator pump through the aquastat?
I am truly at a loss and I would greatly appreciate ANY help from anyone! Thanks so much in advance.
dave tzeutscheler
St. Paul, MN

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