Best Of

As @Jamie Hall mentioned, a strap on aquastat like the Honeywell L6006C1018 mounted on the return pipe is typical for that application. Eliminate the line voltage thermostat completely.

Have you had a combustion analyzer on it? If it was opened for cleaning you will need to check the combustion. Yearly or every few years it should be cleaned and re-adjusted if needed.

If there’s no regulator at the meter, you’re in a low pressure gas zone where the pressure is regulated somewhere in the street because the piping is very old.

As mentioned by @GGross, more than 1” pressure drop can be too much for some mod/cons. You really need to have a combustion analysis done by someone who’s got a digital analyzer and who knows what they’re doing.

Also, a dirty heat exchanger will throw the combustion off to where you can have the issues that you’re having.

Just FYI, generally speaking with any condensing boiler if you see a 1" pressure drop or greater that would be deemed insufficient even if the number it drops to is within the specified range. Many installs fall in this category and typically seem to be working fine…. until they don't. I would investigate that gas supply personally, no meter or anything at the house? sorry if that's a common install in the city, in my area we always have a meter on the house (natural gas), or an LP tank with a little regulator.

Hello FIshermanPete,

I'd say much more to check.

https://ntiboilers.com/wp-content/uploads/2018/12/TFT-Series-IO-Manual-2018-09.pdf

How old is your igniter? Also, when you cleaned out your condensate did you make sure the line that it connects to was also clean. I just had one like that, a little baffling, I pulled condensate off and after pulling the lid off the top of the HX I started pouring vinegar into the HX and it wasn't coming out the bottom. Shoved a screwdriver up there and sploosh it call came out!

Just a thought. Could be gas pressure. Lots of things to check!