How does this sensor "well" work?

rlaggren

Scenario as follows: AO Smith 225P Series 770 (as far as I can determine from the label - nothing explicitly say "Series 770"). Made 1980 so installed early 80's. 4 years ago I cleaned hx and straighten really messed fins on the bottom of the hx; cleaned exterior of coil. Don't know of any other service (except obviously somebody mashed those fins at some point).

Disassembled, preparing to clean and descale, found various leaks. Trying to review the big picture to decide value of attempting repairs. There's a seepage at the joint at the top of the hx where it's soldered to the threaded fitting w/the well (pics of this fitting) I'm not sure if I can resolder the leak on that joint adequately, but that's not my Q here.

The well in that fitting seaped and it leaked more when I removed the cap nut and wiggled the sensor capillary. The capillary tube moves in/out about 1/2". With the nut off a slotted washer/space came loose and I can see another in there that doesn't want to come out, even though the cap tube move freely.

- What are the chances of that sensor on the cap tube coming out in working condition? I haven't pulled real hard on it yet because I don't know how it's installed in there.

- I don't know the way these wells are made to work so I don't know if that leak means the whole fitting w/the well is bad or just that the sensor on the cap is supposed to seal somehow and that seal is bad but repairable.

- How do I remove that sensor to either replace or reseal properly?

- What's on the other end of that cap tube - it goes into a control box above the gas valve but I can't see if it's removable (like a water heater thermo couple) or whether replacing the sensor means replacing the whole high limit control box.

rlaggren

Here's pics. Can't find an edit button to add above. Huh. Just found it...

Abracadabra

Unscrew the nut and pull the capillary tube out. The thermal paste in there has probably dried up around the tube and is most likely the only thing holding it in. Whether it comes out unscathed is a matter of luck. You can try scraping out as much paste as you can using a bamboo skewer which should reduce the amount of material providing resistance. But be prepared with new paste and a new thermostat if it breaks off. BTW, I've never had any luck attempting a repair on those AOSmith Burkay heat exchangers. If one spot has started leaking it's only a matter of short time before it starts leaking somewhere else.

hot_rod

You might just unscrew the well and replace both the well and control. If there was water in the well, it may have a pinhole leak.

If you want to risk saving the well and control, spray some WD 40 to dissolve the heat transfer grease that may be binding it.

It budget allows I'd certainly advise replacing a 36 year old copper tube boiler. It could haunt you for the rest of it's life

I'd insist on a CO detector in that room.

rlaggren

Thanks for the help w/the small tempest in a teapot. So we proceed w/a new 'stat in mind.

But no purchases until the condition of the copper boiler makes itself clear. I think I can live w/a tiny seep that's barely visible by looking at your finger after feeling it. If that's still the only issue after descaling. TBD.

@abacadabra
Not betting much on fixing any copper leaks; maybe as a last coin flip if it grows too much after descaling. My good torch is in SF and leaks like that where you can't get the joint apart are just playing roulette. But the 1" near boiler pipes look perfect inside so it doesn't seem to be inhaling too much oxygen. The stem leaks at the gate valves can be stopped and that'll reduce the intake some.

@hot rod
Cleaned the chimney, fireside hx, burner, etc 5 yrs ago (? somehow skipped that log entry) after which anything/everything goes up the chimney. The burn looks good and even the pilot maintains good draft. It's just maintenance and planning/decision time. The building does have CO detectors but putting another one in the boiler area sounds like a good idea.

35 years might not be too bad on non/minimally moving parts like that coil-hx. The leak is on a fitting which seems to have received serious weight from the mains overhead - I've replaced 5 hangers so far and it looks like it's missing another two or three. The radiators have been holding up the piping...

Rufus

rlaggren

To follow up: I used Eco-Lyme from Nu-Calgon for three hours to de-scale the inside of the hx-coil. Based on litmus tests there was no visible reduction in chemical strength after 3 hours and since that was the recommended minimum time period, and it flowed was fast and strong I stopped there. Pumped into the drain, out of one side of the purger fitting (purger is a cast block w/two outlets). Bypass was closed. No foaming or other obvious action and no large pieces coming out. Black "silt" type of suspension came out and continued to appear in small amounts even after back flushing w/city water; at a guess, the hx-coil holds about 4 gal and I ran about 50 through it to flush. Came out clear at end but subsequent run-out from the boiler always showed small amounts of black suspension.

The boiler holds 30# pressure w/half# drop after 18 hours. Pleasantly surprised. Nothing at all showed at the joint where I had found small seepage. That's not perfect and water can act a bit differently than air, but it looks worth a try and maybe get another 5-10 years from it.

Thanks for your comments and info. I have other questions which I'll put in a new topic.

Rufus