Anyone ever flush out coils, on a tankless boiler. The only chemical they sell at the supply house is sizzle and that doesn't work that great. There has to be something better on the market?

I would go to the manufacturer's web site, or call them, and ask them for this information.

What brand tankless heater do you own ?

It's a slantfin oil fired boiler with the tankless built inside of boiler.

I am sorry. I am not familiar with this set up. Perhaps, our other Experts can follow up with their own experiences and give you proper advice...

Good Luck... Milo