When to worry about Boiler Return Temperature


Old 11-02-20, 12:58 PM
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When to worry about Boiler Return Temperature

I have a Weil Mclain WTGO-4 boiler heating 4 zones (two low mass fin baseboard, 2 hydroair zones) and a tankless coil for DHW. The previous homeowner converted this boiler to natural gas back in 2018. Given current weather, setting the aquastat to 160Low and 175High is sufficient for heat and DHW.

The plumbing to each zone is very long and runs through unconditioned basement. So, early on during a call for heat, the return water to the boiler can be pretty low (120F or so). Eventually, the return temperature makes it up at 150F or so (I get about the 20F drop across my rads/exchangers).

How bad is it for the boiler that it gets that cool water during the start of a heat call and lasts for probably a few minutes? I know a bypass is ideal in this situation but I dont have the means to install one. Will this low temperature thermally shock the boiler? Will it cause some amount of condensation at my exchanger? The boiler never drops that low (based on FLIR, analog dial and digital temp readouts on boiler), but clearly stagnant "cool" water is still entering the boiler at the start of a call.


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