Acting like a cold start boiler

Old 04-01-13, 06:04 AM
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Question Acting like a cold start boiler

I have a gas fired hydronic system with a Well McClain CG5-SPDN series 12 boiler. I had a problem with the boiler going over temperature before shutting down and figured out that the limit control was the issue. I went to a plumbing supply house where I gave them the model/serial number and they gave me a new limit switch to install. They did not have the identical replacement and gave me a more or less universal HoneyWell Limit with all the same specs printed on it. On the new one I can set the differential and set it to 20 degrees and also tried it at 30 degrees(original limit was White Rogers and factory set with a fixed 30 degree differential).

After installing it the boiler runs as a cold start boiler. Meaning that during a call for heat the unit will fire and come up to 180 degrees like it is set for and turn off. Then after water circulates and cool the water temp down it turns back on at 150 degrees like it should. BUT it won't regulate it's temperature when there is no call for heat and continues to cool down until there is a call for heat or hot water(indirect storage tank). When the temp is under the limit I turn the themostat up and it fires, I turn the thermostat off and the flame stops immediately(before temp limit reached).

So I went out to a different supplier thinking the limit was the wrong one and got what they told me was the genuine replacement part (came in a well McClain box but was a HoneyWell part - with no instructions or litterature inside.) I installed that limit but without any change, its still acting like a cold start boiler. This limit looks like the original one and is also non adjustable.

Am I crazy and its supposed to be a cold start boiler?

Did Well McClain change something since 1997 when my unit was installed that makes the unit act as a cold start?(maybe to decrease fuel comsumption?)

Or do I need to change the wiring to feed it power differently?
-- I tried to replace the transformer/relay combo piece thinking maybe one side dropped out as its a DPDT and the voltage from the thermostat was filling in but that had no effect
Old 04-01-13, 07:45 AM
lawrosa's Avatar
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Sounds normal to me.... It should have always been a cold start boiler. Unless you had a tankless coil at one time.....

What is the model of the old aquastat?
What is the model of the new aquastat?

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