Post Purge of Low Mass Boiler

BFlynn13579

I have an issue that I think post purge will alleviate but I am unsure. I am a retired plumber but once the heating equipment got so comlicated and every mfr. had a better idea I explored other avenues of work I have a Utica MGB-175K boiler which is replacing on old Buderus Logano G234 that had a Logmax R2107 computer on it. This utica only holds 3.7 gals of water. When the zone t-stat is satisfied and shuts down the boiler if the bolier is in the upper range of its cycle (lets say 170+ degrees) when the circ & boiler shut off together the thermal overshoot brings the boiler temp over 200 degrees. It has even gone to 208+. The Hydrostat 4200a controller does not have a post-purge feature. A preious discussion of this subject referred to the use of a surface aquastat as a proposed means to acquire post purge but the instructions are a little vague. Can someone give a little more detail on how to accomplish this? There is a Taco 4 zone relay controlling the 3 zones of heat and there is a bypass circ loop.

hot_rod

Many of the newer relay boxes include post purge. Replacing the box may be easier than building your own system.

RascalOrnery

I do it with a single aquastat that I had laying around, and a delay off timer. It purges for fifteen minutes after the heat call is satisfied and then single aqua stat prevents it from going too low as to trip the low limit since I have a tankless coil.

I imagine you could use just a delay off timer to purge, but that probably also means utilizing a separate relay to send power to the circulator.

It probably makes more sense to just replace the aqua stat with on that supports post purge.

I am not a professional so dont take my advice too far.