Low-Return Water Corrosion on an Inactive Boiler?

Eric Johnson

I have a late '50s Weil McLain cast iron, gas-fired boiler that I only fire in the early fall and late spring. The rest of the heating season, I'm heating the water in the boiler (through a flat plate heat exchanger) with a wood-fired boiler.

Is low-temp return water corrosion an issue for this boiler when no gas is being burned?

Unknown

If you are not burning gas in the Weil Mclain boiler you will not have a problem with corrosion due to low return temperatures.

Condensation will only occur when the boiler is being fired and the return water temperature is lower than about 130 degrees. The water vapor produced by the burning of the gas will condense on the cast iron passages at lower temperatures, possibly causing corrosion. But as long as you are not burning gas, no water vapor is being produced to condense and cause corrosion.

Joe Brix

Shouldn't be.

The issue with low return water temperature is that it cools the flue gasses to a point below dew-point of the vapor in the gases.

No flame = no flue gas