Boiler Corrosion

Joe_133

Long story. I have questions about what chemical would be best to stop corrosion and pitting of carbon steel surfaces on inside of my hot water heating boiler.

What I have is an old Kewaunee firetube boiler that I burn wood in. Been in service for the last 15 years heating my house and garage. Maximum 180 degree water.

I don't drain it in the summer. It is not open to the atmosphere- I have a 6 pound radiator cap which doesn't have a vacuum break- in other words this thing can build up 6 psi as it heats up, but as it cools off it holds a vacuum, thus no outside air enters after the initial warm up. I'm using oxy-barrier pex on the plumbing going to the boiler.

Initially the base and the flues were rotted out of this thing- straight from the junkyard. I made an all new base out of new 5/16" carbon steel and welded it on- from the bottom 6 inches up was all new. All new flues 1/8" thick x 4" diameter. The boiler footprint is approx. 33" wide by 48" long and by my water meter the system holds 186 gallons of water.

I've had to replace the flues twice since. The second set were made from 1/4" carbon steel, and this last set I made from 1/4" stainless (not sure exactly which stainless).

This last time the reason I replace them was not leaky flues (they were on the verge) but because the base corroded through.

After I drained the thing and got it out and put it in the shop, I find that every horizontal surface inside this thing was corroded, in most places nearly halfway thru.

The flues were corroded on their tops, and a little around the sides, but from halfway down around to the bottom, they were just fine.

This time I just cut the very bottom of the base off and layed a strip of 5/16" stainless there and welded it on.

What occurs to me is that there was lots of scale and rust in it when I flushed it out, and where that scale was laying that is where the majority of the corrosion was occuring.

I did put a small sacrificial anode in it a few years ago and it is long gone.

I also went to the power house of a local state institution and asked the boiler fireman (they run 125 psi steam water tube boilers) about what I should use to stop corrosion and he did some sort of test on the water and gave me a couple of quarts of a couple of different chemicals- one he called "149" phosphate/sulphite and the other was "202" oxygen scavenger.

At the time he couldn't tell me how much chemical to put in cause back then I didn't know the water capactity of my system- now I do.

But, I've since then wondered if since I heat water and they boil it, maybe their chemicals aren't right for my system.

Can you guys give me a lesson on what to do to keep my system from eating itself up?

P.S. This time, I found some industrial strength sacrifical anodes- discs about 1 inch thick by 4 inch diameter- and have one of them submerged in the soup and hope they are protecting the boiler somewhat til I find some chemical protection.

Mike T., Swampeast MO

Could the fuel be the real problem? Was the boiler designed for wood burning instead of coal, oil or gas? I believe that the combustion gasses from wood are very corrosive, especially if the wood is not very well seasoned.

From your description, it sure sounds as if you're getting significant condensation of the flue gasses. Are you using any sort of "recovery unit" in the flue? If so, I'd get rid of it. Is the boiler perhaps massively oversized and you're essentially underfiring it with a much smaller fire than it was designed to utilize?

Joe_133

Corrosion is on the inside- the water side. No problems on the fire side.

Yes, though, it does get lots of moisture from combustion coming back down the stack. But, I have a nice big cleanout at the base of the boiler breeching and I clean the flues and it out consistently.

Mike T., Swampeast MO

I have zero working experience with firetube boilers, but "lots of moisture from combustion coming back down the stack" sounds like VERY bad news for any boiler that's not designed to recover heat from and handle the corrosive power of condensed flue gas.