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I'm continuing to try to make incremental small improvements on our two Weil Mclain WGO-5 hot water boiler systems. The next thing I'm looking at is installing some sort of post purge to get the residual heat out of the boilers after each heating cycle. The boilers are cold start and typically run for 30 minutes or so before the Tstat is satisfied. Then they sit idle for several hours until the next call, during which some (most?) of the residual heat trickles away up the chimney.

These are 700+ pound cast iron boilers with 10+ gallons of water inside, so there is a lot of residual heat left inside after a burn, and I'd like to recover some of that with post purge. Recently I read a post from someone who was questioning how the Energy Kinetics boilers can advertise such high cycle efficiency, and Roger from Energy Kinetics explained that their high cycle efficiency was due to (1) low mass of the heat exchanger, (2) small water volume inside the HX, and (3) post purge. I can't reduce the mass of our heat exchangers or the water volume, but I can (hopefully) install a post purge.

Because the boilers are so massive, I would like to get at least 20 minutes of post purge capacity. I have seen some electronic controls that could be wired to give me up to 10 minutes of post purge, but I don't think that's going to be enough for maximum heat extraction, and I don't want to go through the trouble of buying and wiring something up only to find the max purge period is too short.

So I'm wondering if I could hard-wire the circulator directly to this Honeywell aquastat that has an external temperature sensor, which I could mount on the supply pipe coming out of the top of the boiler.

https://www.supplyhouse.com/Resideo-L6006C1018-High-or-Low-Limit-Circulator-Strap-On-Aquastat-65-200-DegreeF-range-5-30-DegreeF-Adj-Differential?utm_source=google_ad&utm_medium=pmax&utm_campaign=pmax_best_sellers&gad_source=1&gclid=CjwKCAiAl4a6BhBqEiwAqvrqun-OunsX_doTrRhlr4d1fho91yeZxAJqRLrSKy71eCVIyvtJvwArThoCxvIQAvD_BwE

I would set the "make" temperature for, say, 100 degrees. Then when the boiler starts cold, the circulator won't start until the water temp in the supply pipe right above the boiler reaches 100 degrees. The circulator will then start, but because there's so much cold water in the system, as soon as the circulator starts, it will flood the boiler with cold water, which will lower the supply temp, causing the circulator to shut off. This process will repeat until all the water in the system has heated sufficiently to allow the circulator to run continuously.

Then after the Tstat satisfies and the boiler shuts down, the circulator will continue running until the supply water temp drops below 100 degrees. At that point, a significant amount of residual thermal energy will have been (hopefully) extracted from the boiler and sent to the radiators.

One possible objection to this setup is that, if the circulator aquastat fails, the boiler will run without water circulation until its own aquastat shuts it down on high limit. I wouldn't want this to happen routinely, but in theory that's the safety mechanism against boiler overheating.

Has anyone seen post purge done this way, and am I missing any obvious alternatives?