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Gas Boiler or ? Water Leak - What Might Have Happened?

jdbs3 Member Posts: 32
What might have happened? Read on ...

My mechanicals room (Bosch gas boiler, Tekmar, etc. controllers, DHW) is in the attic.

Early February, 2023 was bitter cold: -9° overnight on February 6 with locals talking about pipes freezing. I've never had a problem with this, so did nothing.

However, the next day, I recalled that I had removed the drywall and insulation on the back of the mechanicals room when tracing the wiring for all system components (see room pictures). So some heating pipes were exposed to the -9° for one forced hot water baseboard zone that was turned off since we do not use the 2 rooms on this zone much.

The other 2 primary heating zones (radiant floor on 1st floor, baseboard on 2nd floor) were on, so the pipes for them should never go below the 67° temperature setting.

Though the temperature in the upstairs bedrooms did drop to 64° for a day, so the boiler was not able to keep up with the call for heat in these rooms. I know how to fix that, and do not think it was the cause of the problem. Or was it related? Read on ...

I went up the next day and reinstalled the insulation and drywall. On finishing this task, I noticed a small wet spot on one outside corner of the mechanicals room, so opened the door to the room. The floor (~90%) was covered in water. Luckily, there is a metal pan throughout the room, and a drain in the floor if the water got too high that (I think) drains to the crawl space under the house. NOTE: I'll have to test this drain this summer.

After cleaning up the water, I searched for the source of the water.

- There were no burst pipes.

- The DHW has a drain valve, and 2 other valves for the hot and cold water (see picture). All were dry. Also, the part of the floor that was not wet was in the rear of the tank, opposite the valves. I would think that if one of these valves leaked, the water would have spread all around the tank bottom circumference..

- That leaves the white hose under the Bosch boiler - the safety relief valve discharge hose. It was also dry.

I then turned on the one zone for the 2 rooms. The water circulated through the zone; there was no gurgling from air in the pipes. I did the same for 2 different radiant heat bathroom floor zones that we never use. They both heated up as expected.

I've checked the mechanicals room a number of times since. It has never been as cold as that day. But still no water on the floor since.

The likely source of the water would seem to be the safety relief valve discharge hose.

Again, what might have happened if this was the source?

I did find this with a google search

"What causes a water heater relief valve to discharge water?
If the temperature's or pressure in your tank exceeds a certain level (often times 210 degrees and 150 psi), the relief valve opens up and allows some water to drain out of the tank. Cold water is then added to the tank in order to lower the temperature and pressure inside."

What might have happened?

And what should I have the gas boiler tech check on the next yearly maintenance?



  • Jamie Hall
    Jamie Hall Member Posts: 22,147
    More likely than not the safety valve released on high pressure. If it's not dripping now, I'd not worry about it much -- but I'd keep an eye on the system pressure itself. One longish shot is that the little line connecting the system to the expansion tank froze, taking it off line for a cycle or part of one. There's no flow in that line, after all.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • jdbs3
    jdbs3 Member Posts: 32
    And the Bosch boiler high limit is set to 3 = approximately 122° F.
  • jdbs3
    jdbs3 Member Posts: 32
    And the PSI for the water boiler pressure has been slightly below the 14.5 minimum filling pressure (when system is cold). Not sure what it was prior to this water issue.
  • jdbs3
    jdbs3 Member Posts: 32

    Thanks for the feedback. As noted, no water issue since. The PSI though is slightly below the 14.5 minimum filling pressure (when system is cold). Is that an issue?

    I plan to have the maintenance tech also look at the PSI when they are out, probably in the April timeframe.

    Thanks again!