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Thread: Have some issues with hot water boiler system

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  1. #1

    Default Have some issues with hot water boiler system

    Could use some advice on how to fix or find the problem I'm having with my hot water heater system. I'll give a little background and i'll try not to go on too long. For the last couple year I have had water discharging from the pressure relief valve in the system. I used to just let it fill in a bucket and dump it every couple days. Last summer I decided to try to empty the expansion tank. Turns out the valve to isolate it would not close completely. I had a plumber come in close that tank off and attach a bladder expansion tank. Thought that would solve the problem but still have water discharging. I have a contract with a company to service anything with the boiler. They came by last night and I showed them how the bucket was filling up under the pressure relief valve. They disconnected the new expansion tank and tested the psi. It was 12 so that is working fine. They said it may be the pressure reducing valve letting water in slowly. So he shut off the main water supply and told me to call back if water was coming out. When he left the PSI were at 24 since the boiler was fired up. When I checked it this morning it was at 8 PSI. I turned on the main supply got it up to 12 PSI. Bled the radiators upstairs got all the air out and had to put more water in to get it up to 12psi. Now with the main water supply off at 10am it was at 12psi and when I got back at 12pm it was at 10psi. Do I have a leak? I don't see water anywhere. I've attached some pics below of the system. What would cause it to discharge water if I'm losing pressure with the main water supply off


    Thanks Jay

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  2. #2
    In the trades Dana's Avatar
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    A 2psi difference at different operating points and different time of day isn't much of a concern, since the system volume will vary by temperature, and the pressure will also vary by that much between pump-on and pump-off states. But if it continues to drop even with the fill-valve fully off, you have a leak somewhere.

    If the new expansion tank was sized correctly for your full system volume the t & p valve shouldn't continue to spit with the fill valve fully off. If the expansion tank undersized for the system it could still test fine at 12psi, but could still see overpressure at the system high-temp.

    Bleeding/defective autofill valves that slowly bring the system pressure toward your street water pressure are a common culprit for chronic over-pressure issues. Auto-fill valves aren't really all that useful, and seem to cause as many problems as they solve. Filling/pressurizing the system manually then turning the fill valve fully off and checking it at the beginning of the heating season, or whenever the system complains is generally OK. If the pressure drops low enough you'll hear the boiler complaining with some sizzle rumble & bang due to the micro-boil on the heat exchanger plates becoming more macro than micro, and it'll be pretty loud long before there's any hazard condition or damage.

    If that boiler is 40+ years old (and it looks like it might be), the economics of replacing it with something right-sized and higher efficiency might be favorable. Even if it's in pretty good shape it's probably not getting better than 75% efficiency, and it could easily be under 65%. A high-efficiency boiler would get at least 90%, but you'd not be able to vent it into the old flue, and the water heater would also have to go, since by itself it doesn't put out enough heat to keep the flue drafting, and would have guaranteed flue condensation issue plus potential back-drafting issues.

  3. #3

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    Thanks for the reply Dana. The fill valve has only been completely shut off starting last night. No water has been discharged but it has only been one day. I will keep my eye on that. I will also keep my eye on the psi to see if they keep going down. The 2psi difference was just to illustrate that nothing changed and it went down. The boiler did not cycle and the temperature gauge on it did not change. Nothing really changed. The temperatures on the boiler was the same it just dropped from 12psi to 10psi. Once the system is at 12 psi with the fill valve closed it should never really go below 12psi right? I filled it back up to 12psi at 12pm and now at 2:15 its at 11.

    Also what should the PSI be at when the boiler is running. I've read it should be steady around 20. I remember it always being aroudn 24-25.

    Lastly I'm curious what a new boiler would go for? You are correct about it being 40+ years old. It is the original boiler from when the house was built in 1955. So it is actually 57-58 years old.

  4. #4
    Retired Defense Industry Engineer jadnashua's Avatar
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    A new boiler can be anywhere from a few thousand to over $10K. A nice mod-con would likely be around $10, but NYC with the taxes and cost of living probably more. The pressure will change with the overall system water temp, not just that at the boiler since the water in the pipes to the radiators will cool, lowering the volume, lowering the pressure. Normal operating pressure unless you have a very tall building is typically around 15psi. If the expansion tank is sized properly, the pressure won't change much, maybe a couple of pounds except maybe the first time you fire it up from cold.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  5. #5

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    Thanks for the reply Jadnashua. This is a two family house. I've read online that 12psi is a good number for a two family house. At 12psi all the radiators on the second floor have water to them with no air. The whole day the boiler has not fired on since early this morning. The suns been out and kept the house at 70, the thermostat is set for 68. Throughout the day I have been manually adding water with the fill valve to keep the pressure at 12psi. But throughout the day the temperature of the water on the gauge has been going down. I"m attributing the decline in pressure due to the temp going down. It started at 130 and is now down to 100 degrees. I've filled the system back up to 12psi and will see what happens overnight and when it kicks on in the morning.

    As far as a new boiler I'll have to think about it. I'd like to get this one working 100 percent first. I do like the sound of 90 percent efficient vs 65-70%.

  6. #6
    Retired Defense Industry Engineer jadnashua's Avatar
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    A well sized, properly setup (that's asking a lot, but possible), a mod-con can approach the high 90's in efficiency. Average would be lower than that, but it's possible to get to the high 90's under some conditions. That will NOT happen if the thing is oversized or not setup properly.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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