Low Water Cutoff - Best Placement?

Fixitman

We melted the HX of a hot water boiler last year. Our system had a hot water leak caused by a frozen pipe. Three possible devices in the system all failed to stop the boiler from firing when it shouldn't have. The high limit control, a paddl-style flow switch, and a SafeGuard LWCO failed to protect the boiler.



The boiler is one of 6 AO Smith DuraMax boilers piped on a single header using closely-spaced tees. The individual boiler circulators are pumping away, with the flow switch between the boiler outlet and the pump. The Flow Switch is in the horizontal leaving water pipe. The boiler circulator is a few feet downstream in a vertical pipe, pumping upwards. The piping turns horizontal again just above the pump, and the LWCO is in a short horizontal pipe that is connected to the horizontal header. All the devices, and the tees into the header, are quite closely spaced. Very compact piping. The boiler plant is in the 6th story of a 10-story building.



I've been baffled about why the boiler continued to fire when it must have had little or no water and/or no flow. When we installed the replacement boiler, the LWCO tested fine - though we replaced it anyway. It's a manual reset style control.



I have become suspicious that the circulator may have been keeping the leaving water pipe into the header full enough to fool the LWCO, even though there may have been a fair bit of air mixed into it. There is nothing in the instructions that came with the LWCO that indicates there is anything wrong with the placement. But I'm left wondering whether the entering water piping might not be a better location for the LWCO. The water would presumably be flowing along the bottom of the entering water pipe, not churned up by the pump as the leaving water might have been.



Any wisdom anyone can pass along would be greatly appreciated!

Alan (California Radiant) Forbes

I agree

It's baffling that all three failed.



I'd install two LWCO, one on the supply and one on the return.

Tim McElwain

Put the LWCO

right at the boiler on the supply pipe just as it leaves the boiler so it senses the water just before the boiler, that way when water drops below that it will shut off the boiler. It is more reactive there than on the system piping.