Bypass Loop for HW Gas Boiler

Old 11-19-04, 11:18 AM
LI Neil
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Bypass Loop for HW Gas Boiler

I have a series 2 150K BTU Burnham HW boiler and everywhere I read that the return line should be above 140 deg to avoid condensation? or something along that line? Can someone explain a bit more to me please?

I don't have this loop, and of course at night with the Stat set back, first fire in the early AM will pump cold water to the return, is this good? I have been to a few new built houses and see no bypass loops as well. Should all cast iron boilers have this loop from the MFG?

Thanks for the insights,
Old 11-19-04, 05:38 PM
KField's Avatar
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A bypass loop is good to have. It is not only to help the boiler but to help the comfort in the living space heated by the boiler. It prevents the thermal shock of a boiler having to deal with a full batch of 70 degree water coming back and completely replacing the hot water that was just there with cold water. If the boiler is starting from cold, all the water heats up at the same time. If the boiler is maintaining temperature and the heat comes on and brings back all that cold water, it will then stop the circulator and some radiators have not yet been supplied with hot water. Most of the importance of the loop has to do with boilers that also make domestic hot water. The loop is piped in during the boiler installation. It is not practical for the manufacturer to do it. It would add a few dollars to the cost also and that would not sit well with the price conscious shopper. If price is the only object, which seems to be the case more than I care to see, then a loop will not be installed. The valve you need in the loop will probably cost $15.

If you have it, all radiators will warm up evenly and the boiler will not be shocked with cold water.

My feeling is that the piping can make or break a boiler job. You can buy all the parts you need at the big box stores, but if you don't put it together just right, it will never operate like one big machine, just a bunch of parts all screwed together.


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