Elusive bonnet "High Temp" trip

RickDelta

Hello HeatingHelp.com community! : )

Re: Small motel with a domestic hot water gas boiler that circulates its hot water to a storage tank next to it.

I was called in for "no hot water" in the motel.

I saw that the high temp exhaust bonnet sensor had tripped-out.

Smoke checked exhaust vent pipe for a draft and down draft back spillage ….. was ok.

Reset sensor and system came back up no problem ….. No fault discovered at this call.

This same call came back two more times after about a five hour span of time in between them : (

In the last call, I was lucky enough to have been looking up into the exhaust bonnet when the fault presented itself ….. I saw something I never witnessed before.

The exhaust bonnet had the most perfect flow display of a four inch high row of FLAMES!!! ….. extending about two inches up into the six inch exhaust pipe to chimney : (

It looked just like the normal (2") gas flame pattern you'd see at the bottom of the boiler!

The total flame length during this failure mode was about 16" (from bottom of boiler flame bars to entry into exhaust pipe).

I checked the gas pressure and was stable at 4.5 w.c. (normal)

The problem was inside the gas valve itself …… it was passing all this gas pressure un-controlled every so often ….. causing the high temp sensor to trip.

Never ran into this before. : (

Note: My inexperience of realizing that the black discoloration of the entire exhaust bonnet (inside) should have alarmed me to this condition at the first call.

hot_rod

https://forum.heatinghelp.com/discussion/198927/elusive-bonnet-high-temp-trip

good that the safeties worked. A classis example of why you never want to bypass a safety.

HVACNUT

What make and model boiler was this? That's completely nuts.