Home heating boiler - bacterial sludge

[DOUGLAS LOSTY]

Around 10 years ago I installed a new Slant Fin oil fired boiler (L30) in a converted gravity hot water system. There must be over 50 gallons of standing water in the system due to the 3" feed pipes going to the two pipe cast iron radiation. Due to the crawl spaces involved, it's not feasable to repipe the system. The system has worked well with the only apparent disadvantage being short circulator cycling and very long burn times of around 30 minutes before all the water is heated.

I recently had to pull the domestic coil due to a drastic drop off in hot water output. Much to my surprise the coil wasn't internally "limed" but instead was coated with an inch thick coating of loose black oily sludge, probably bacterial in origin. It washed off easily but I'm worried about the interior of the boiler being coated so badly that its heat transfer is reduced. Am I risking cracking of the cast iron due to poor heat transfer?

I've doubled the size of the expansion tanks and have virtually eliminated popping the relief valve thereby also reducing makeup water and fresh oxygen. Are there any chemicals that would loosen and allow me to flush out the sludge? Would there be any chemicals to stop it reforming? I'd hesitate using anything extremely toxic due to the use of a domestic coil.

tnx,

Doug

[David Efflandt]

Does it smell like swamp muck? Probably hard for anyone unfamiliar with your undisclosed location to guess exactly (is home or city water source well or surface water). Does your water have high manganese content?

[Doug]

It is well water with a high manganese content. I didn't notice a strong odor. During past testing, the well water didn't test as particularly hard but it did show a slightly high manganese and sodium content.

Doug

[David Efflandt]

If it was organic matter, I would expect it to smell gassy or swampy, but would NOT expect that to grow in a heating system.

I don't know if manganese has an odor, but the lack of odor and knowledge that your water contains manganese, could suggest that it may be manganese. I worked for a well driller while in college and heard that manganese could cause black staining of fixtures, but would not know how to remove it.