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Discussion Starter · #1 ·
A question for you boiler pros.

A friend just bought an old house with a Weil Mclain 146,00 btu boiler. I had my plumber doing some service (replace leaky relief valve, new thermocouple, snug up the nut on two dripping ball valves, etc.), Anyway, near the boiler, the supply and returns of the hydronic loop are 1 1/4" copper. It transitions to whatever that old iron pipe for the house. Anyway, in addition to both sides (return and supply) being valved (presumably to isolate the boiler for service), the supply and return side are linked to each other with 1 1/4" copper (and ball valve) Huh? We're trying to understand why this "cross-over" from the supply to the return side. Any ideas? Presumably it stays closed for operation, but wondering why it's even there. Thanks.
 

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Possibly a ByPass loop. In a high mass system its to help prevent shocking the boiler with a lot of cold water which will cause the boiler to run at condensing temps for a long period of time when it cold starts. It really shouldn't be a ball valve, its usually a thermostatic valve than slowly opens up as the temp in the boiler rises.
 

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Discussion Starter · #3 ·
Interesting, thanks. I think it will remain closed.

BTW, what's your take on whether to leave the feed valve open or closed (after everything is bled and the right pressure/altitude is achieved)? He wanted 15-18 psi (for about 24 ft altitude), but it kept going back over 20 with the feed open...so we closed it. Maybe he'll need to replace the pressure reducing valve in the summer months.
 

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Coggs is exactly right. Depending on how the system is designed, how often it runs, and numerous other factors, it could be important to be open or partially open.

This has been a big issue for me, because my homes often have a design load of about 20,000 BTUs per hour, less of course most days, but the smallest oil boiler that works well is 65k. The boiler may run such a short time that the return temps never get above condensing temps (around 130 depending on the boiler). This would cause the heat exchanger to rust out early, as well as other possible problems.
 

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Coggs is exactly right. Depending on how the system is designed, how often it runs, and numerous other factors, it could be important to be open or partially open.

This has been a big issue for me, because my homes often have a design load of about 20,000 BTUs per hour, less of course most days, but the smallest oil boiler that works well is 65k. The boiler may run such a short time that the return temps never get above condensing temps (around 130 depending on the boiler). This would cause the heat exchanger to rust out early, as well as other possible problems.
Why use oil on a 20k BTU design load? Sounds like it could be handled better with mini splits or the smallest modcons out there.
 
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