Water tube boiler corrosion with 70'F water

Hi,
I guess from your description that this is a water tube boiler.

Generally in all well designed boilers, hot gases from the flames circulate around the tubes, and the temperature of the overall enclosed volume within t5he boiler uniformly rises (forced draft will ensure this), to a fairly high value in seconds after ignition.

Even if the water in the tubes is at 16 deg C (60 F), any moisture settled on the tubes (swating) will evaporate in no time into the hot gases swirling around the tubes. This will happen even if the tubes themselves remain cool as water absorbs all the heat for the first few minutes and remains fairly cold.

The reason is that water droplets have their own thermal resistance. The surface of the droplets will ablate away and the part of the droplet adhering to the wall will still remain cool. This will make the frops shrink till enmass the whole body of the drop will boil off. This happens in maximum few seconds.

If there is a runoff of water you observe, it may be because the sweating is locked away in the insulation (porousness) and the insulation will hold it for a long long time till the whole tube itself becomes hot. Which takes a long time.

If the firing is for short period due to less demand, more sweating time leads to really wet soggy insulation.

It is best to switch off circulating pumps in tandem with boiler firing but ensuring circulation remains on till boiler tube temperatures reach normal after halting the firing.

regards
Jayen