Residential Hydronic Heat Boiler Odd Temp Readings

Discussion in 'Boiler Forum' started by JohnnyA, Sep 13, 2019 at 7:55 AM.

  1. JohnnyA

    JohnnyA New Member

    New jersey
    If a T&P gauge is too close to expansion tank could this cause the temp READING to fall during flame on and rise during flame off? Scenario...Cold boiler, turn on. Flame on, temp reading rises to 180 then flame off. During flame off temp reading rises to 190, then falls to 180 and boiler relights. Temp reading falls to 170 then flame off. During flame off reading rises to 190, then falls to 180 and boiler relights. So on and so on. Circulator pump runs as normal. (On while thermostat is calling for heat.) Expansion tank is installed off of a tee on vertical supply pipe of boiler approximately 4' from boiler. Pump is installed on vertical boiler return piping, at boiler, with flow going towards expansion tank. (Through boiler and out of supply)
    Thanks much for any information!
  2. JohnnyA

    JohnnyA New Member

    New jersey
    Hey folks. I inherited this boiler set-up. A year ago the circulation pump failed and I replaced it. It was the first time I had worked on a hydronic system so I spent months learning what I could from the world wide web and talking to folks in the industry. I know the system set up is...not right. Theres a chance that the heat exchanger is leaking. Although I have replaced some valves and installed a new expansion tank, Im not going to reconstruct the existing system if I have a bad heat exchanger. So, I'm checking it for an exchanger leak. While doing so, I realized the that according to the T&P gauge, the water temp was rising directly after flame off and then lowering before relight. How does the water temp rise when there is no flame, and fall when when there is? I'm 99% sure the gauge is good so the only thing I can think of is that the temperature fluctuation is only in the pipe coming off of the tee on the supply piping and connecting to the expansion tank, due to the expansion tanks location. But, I don't know. I'm thinking I'm going to at least need to replace the T&P gauge that is connected to the heat echanger. The original one. The broken one. At least I can continue to check the heat echanger for leaks. The system actually seems to run well. Don't know about efficiency yet.
    Anyway, if any of you folks have any ideas about this temperature issue, I'd really appreciate hearing what you think.
    Thank you.
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  4. Dana

    Dana In the trades

    Jan 14, 2009
    Some pictures might be useful for figuring out what's going on with the temperature overshoot. If the circulator pump is turning on/off with the flame it could be completely normal.

    While the placement of the expansion tank relative to the pump is sub-optimal for protecting the pump from cavitation issues it's usually not a show-stopper or disaster. If you can hear the pump cavitating raising the system pressure a bit is usually enough to quell that in most residential systems.

    If the heat exchanger is leaking the system will lose pressure over time unless there is an auto-fill valve. With an auto-fill there is usually a separate valve for isolating it from the potable supply. By isolating it and monitoring the system pressure it should be possible to determine if there is truly a leak.

    Since you're running the boiler when it's not that cold out, can I presume it's because you're heating the domestic hot water with the boiler too? If yes, is it a tankless coil embedded in the boiler, or is it an indirect tank operated as it's own zone? With either, pinhole leaks in those heat exchangers will cause the system to gain pressure over time.

    At what pressure are you running the system? Did you pre-charge the new expansion tank to that pressure (or a tiny bit higher) before installing it?

    What is the high-limit aquastat set to?

    How much radiation is there (zone by zone, if broken up into zones)? Is it all fin tube baseboard, or is it cast iron (rads or baseboard)?

    What boiler model/make /size is this?

    Since you have a heating history on the place, it's useful to run a fuel-use based heat load calculation on it to

    A: determine the oversize factor


    B: determine the needed capacity for any replacement solution, if that becomes necessary or desirable.

    Most cast iron boilers out there are 3-4x oversized for their actual loads, which becomes an efficiency issue. If the boiler looks like it has more than just one or two seasons left in it, installing a heat-purging economizer control can be worthwhile.

    BTW: Paragraph breaks make it easier to read.
  5. JohnnyA

    JohnnyA New Member

    New jersey
    Hi. The circulator pump on my system does not turn off during flame off. Stays running until thermostat reaches user set temperature.

    Expansion tank was pre-filled but I topped it off to 13.5 lbs. before installation.

    Only running boiler at this time for testing purposes. Have separate gas hot water heater.

    Hi limit is set to 180°.

    Fin tube baseboard units only.

    The house is a rancher. One floor about 860 sq. ft.

    Two zones. Approximately 35 linear ft. of copper tube/baseboard units in one zone, and 40 linear ft. in the other.

    Boiler is a 72200 BTU/H Utica boiler model M-100-AGB 2I manufactured in 1983.

    When I replaced the pump, the existing one was installed to flow through the 1" return piping, Y off into the 3/4" pipe and baseboard units, and into the supply side of boiler. I can only imagine it was installed that way due to the placement of the expansion tank. I installed the new pump to flow into return side of boiler, through heat exchanger, out supply side, through 2 zones of 3/4" pipe/baseboard units, converge into the 1" return line via the Y, and into return side of boiler. I assumed that the return line has a larger diameter for a reason. Two zones converging into one pipe?

    The reason I thought it was a leaking heat exchanger is because end of last winter when boiler wasn't running often, I noticed that the flame rod assembly was rusty. Then I realized there was water dripping onto the assembly when boiler was cold, and auto-fill valve supply is open. If I closed the supply to the auto-fill valve, no drip. At all. It took a few hours before the drip began after opening supply to the auto-fill valve.

    The leak appeared to be coming from the heat exchanger.

    Pressure hasn't dropped all summer while boiler was off. Also, no water leaking.
    I decided to remove the boilers outer shell to get a good look at heat exchanger while I run the boiler to cause pressure and expansion in heat exchanger, and look for a leak while operating and after cool down. This is when I noticed the temperature fluctuations.

    At the moment, I have not been able to recreate the leak. Although, when I moved my attention away from the heat exhanger, I noticed a malfunctioning Hy-Vent air bleeder that may have been leaking water down the copper piping, being soaked up by the insulation under heat exchanger, and dripping onto the flame rod/igniter assy. Kind of makes sense why it would take hours for drip to occur. I disassembled and thoroughly cleaned the Hy-Vent and it is not leaking at this time. Waiting Until tonight when it's cooler to run boiler again. I will raise the pressure a bit as you recommended as I believe I can hear pump cavitation and the gauge is only showing operating pressure of 15-17 lbs. although I'm not sure I trust the gauge due to its placement.

    So, my questions at this point are if the Hy-Vent was leaking enough to cause the dripping, does it make sense that it only does so when there is supply to Auto-fill valve?

    Does circulator pump flow direction matter with this set-up?

    If there is pump cavitation would this cause the odd temperature readings?

    And lastly, being the boiler is 36 yrs. old with an outdated/discontinued White Rodgers control, is it worth spending the time and money to reconfigure the piping system, or is it time for a new boiler?

    After I post this I will go to the link you provided to read about running a fuel-use based heat load calculation on it. I will also look into the heat purging economizer control.

    Thanks much for the info Dana AND for the heads up on using paragraphs!

    Hope attached pic helps.

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