Newly installed boiler - short cycling?

28 Nov 2022
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United States
Just had a 40 year old boiler replaced. Due to a future addition planned in the next few years, we installed a slightly larger boiler. (166k btu instead of 155k btu).

The new boiler is a Weil-McLain CGA Series 3.

The old boiler would turn on when the thermostat called for heat and the burner would not turn off until the thermostat setting was satisfied.

The new boiler turns on when the thermostat calls for heat, then takes a few minutes to reach the 180F setting on the boiler and the burners shut off, within about 1 minute the water temp is back to 160F and the burners turn back on, it take about 1 minute to hit 180F and then the burners shut off again. This continues many times until the thermostat set point is satisfied.

Is this considered short cycling? Or is this a properly installed and functioning system? I assume the former.

A few things to keep in mind is that the outdoor temperatures are still nowhere near the lows that they will be in the winter. So am I worrying too soon? I can't imagine even in the dead of the winter the cycles will lengthen that significantly.

Also the house is 3 zones and at this point it is usually just 1 zone calling for heat at a time (the water will of course take longer to heat if it is multiple zones at once).

So why is this happening:
1) Oversized - perhaps it is slightly, but even if I reduce the gas flow significantly such that I see the flame lower significantly, the water temperature still goes from 160-180 in under 3 minutes and cycles off/on. So I don't think that is the major factor.

2) Should the gap between the maximum water temp (180F) and the temperature which causes the flam to turn back on (160F) be wider? I don't know if I can even change that setting. But if I set the max for 190F and the low for 150F, it will reduce the cycling a little bit.

3) The way it is installed currently the pipe exiting the boiler is about 3 inches in diameter and then reduces to 1/2 or 3/4 for the individual zones. The old system had a 1 inch pipe leaving the boiler. Could this lead to the hot water pooling in the large pipe and not being drawn away from the boiler thus allowing the water to hit 180F much faster in the current setup than the previous setup?

4) Could the pump be moving water too slowly? I have no reason to assume it is, but that would be another reason for the water to heat up too quickly.

As a possible solution (even temporary), can I manually reduce the gas flow (lets say about 20%) by turning the ball valve controlling gas flow? This way the water will heat more slowly and take longer to hit the 180F point? Are there any dangers in doing this, or any risk to damage to the system?

Any other thoughts?
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You might struggle to get answers here as you have posted to a U.K. based forum. Don’t even think the make is even sold here.

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