Runaway Boiler- One Zone Only?

RonKitch

Hello all

I have a 30+ year old crown CXE boiler. Worked like a champ ever since I’ve lived here (5 years). Was awoken Thanksgiving morning to a very loud knocking sound. When I went to investigate I then heard the sound of rushing water- relief valve was blowing off steam and water. Hit the emergency switch.

Boiler has 2 zones: cast iron rads downstairs and baseboard upstairs. One White Rodgers zone valve on supply for downstairs and one on return for upstairs.

In my troubleshooting the last few days I’ve discovered the following:

It was not an airlock, system has been bled thoroughly.
When only the downstairs is on, everything runs as intended. Aquastat hits hi limit (because house is 57 degrees on a cold start). No overheating.
Appears to be hit or miss if both zones are on. Aquastat may trip, may not.
When only upstairs is on, overheating to point of relief valve blowing within 15 minutes. No aquastat trip.
Flow upstairs seems very sluggish. It’s 3/4 as opposed to 1 1/4 for downstairs, shouldn’t it be flowing even faster or at least the same speed?

Am I correct in assuming the upstairs zone valve is at least most of the problem? Not telling the circulator to run?

Intplm.

What was the temperature and or pressure on the boiler when this occurred?

What type of boiler make-up water/feed valve are you using? What kind of expansion tank is installed?

How old are all of these, and is your gauge reading properly?

RonKitch

Temp up to about 260, 35 psi as it blew off, I watched the gauge as I was troubleshooting the second time.

Automatic feeder valve that replenishes to 12psi. , not faulty. I have watched the psi in the downtime, refilled to ~12 automatically after draining, fills fast when lever is raised, doesn’t trickle anything in if not needed. Model fb-38. Don’t see a date, probably original to 90s install.

Expansion tank is Amtrol Extrol model 30, dated 1994. Sounds hollow on bottom and full of water on top 1/2 at and above grooved line.

Gauges reading properly.

As we speak heat downstairs is on, have upstairs off. Has been running as intended for a couple hours. Am home to keep an eye on it.

hot_rod

It could be the expansion tank has failed. As such the relief valve discharges and if no fill water replaces that, the boiler temperature could rise if some of the water has discharged from the boiler?

Easy enough to check the tank and fill valve operation. Push in the schrader, tire stem valve at the tank. Do you get air or water coming out?

If you drain some water from the boiler via a purge valve does fill water enter?

You may need to sit by the boiler and catch it in the act. Usually if the heat call from the zone valve goes off the boiler shuts down.