How to reduce pressure in hydronic system


Old 04-16-18, 02:32 PM
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How to reduce pressure in hydronic system

Hi all - I think I got myself into a bit of a pickle with my hydronic baseboard heat system and I'm trying to figure out whether it is something I can take care of on my own without calling in a pro.

Background: We had a plumber come out to repair a leaking pipe which required him to turn off the main water line from the well. He turned the main water valve back open before he left. Later that day we had a 3 hour power outage....I'm not sure this has any bearing on the symptom I'm about to describe but I wanted to mention it anyway. We have a "winter-summer" hookup, so the boiler heats the water for the hydronic baseboard heat system as well as our potable/consumable water.

Symptom: The next day, I found that we did not have any hot water when trying to take a shower in the morning. There was plenty of water flow, just no hot water. I went into the basement and found the pressure gauge on the boiler read ~zero and the temperature was down around 105 deg.F. I had also noticed that the boiler did not kick on while I was trying to get hot water out of the shower.

What I did: I noticed that the shutoff valve for the main water supply to the hydronic circuit was closed. I had never taken notice of whether this valve is normally left open or closed, and made the (incorrect) assumption that the plumber had closed this valve prior to doing his work and had forgotten to open it back up when he was done. So, I opened shutoff valve and sure enough to pressure started to creep up on the gauge and then the boiler fired and the temperature started to climb back up to its normal 145 deg.F minimum set point (note- this was on a warm day when the heating system was not running).
Feeling triumphant, I went back upstairs and got on with the rest of my day.

The next day I went back down in the basement and found water dripping out of the pipe coming from the boiler pressure relief valve, and saw that the pressure gauge was reading around 30 psi (the set point on the pressure relief valve) with the temperature at its max set point. Immediately I realized that I should NOT have left the shutoff valve open because it led to the hydronic loop being overpressurized. I understand it should be around 12 psi cold to 15 psi hot.

What I need to do: So, this is where I need some guidance. How do I safely go about relieving the pressure in the hydronic system to get back down to the 12-15 psi range that it should be? I see that there is a valve with a hose connection at the return line of the hydronic loop right before it goes back into the boiler, but my boiler service company hung a tag on there that says DO NOT I'm hesitant to open it! Should I be looking elsewhere to relieve some pressure in the system, or is that the correct place to do it? I was thinking I could turn the system off and let it cool down, then connect a hose to that low point on the return loop into a bucket, crack the valve open and drain out some water until I see the boiler pressure gauge read 12 psi.

On a related note - how long can I let the system operate in this condition before any damage is caused? I'm not concerned about the small amount of water dripping onto the floor, but I am concerned about any internal mechanical damage that could occur. Thanks in advance for any help!

Last edited by SteveYem; 04-16-18 at 02:53 PM.
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Old 04-16-18, 04:02 PM
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Welcome to the forums.

They don't want you to open that valve due to hot water coming out. If the boiler is cool or cold.... it's fine. You actually need to see what the boiler pressure is at low temperature. If it's much higher than 15psi it could signify a leaking fill valve. You may also have a pressure tank that needs checking and possibly recharging.

Shut the water feed off to the boiler. Allow it to cool and then drain water out to reduce the pressure.
Old 04-16-18, 04:59 PM
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Thanks for your reply. It makes perfect sense once this next wave of cold weather passes (hopefully later this week) Iíll shut down the boiler, let it cool down and drain some water out.

Regarding the shutoff valve in the supply line to the hydronic loop, just before the auto fill valve- Should I be leaving that shutoff valve open or closed? I thought the idea was to keep that shutoff valve open and then the auto fill valve would let more supply water in if/when the hydronic loop dropped below 12 psi. Is that not how it works?

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