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Heating Expansion Tanks

Sounds like you need more expansion volume. I suspect your system is an older, possibly gravity HW system with larger CI radiators? There is more volume there than you realize.

The way to tell if the tank is too small is to watch the gauge upon firing, from cold start to valve weeping or release. If there is a steady rise in pressure as the boiler fires (or a drop as it cools), an undersizing issue would be my first assumption. The pressure should be rising to the 28-29 PSIG range when weeping starts in my experience, (valve being set to release at 30 PSIG). Is this what is happening?

Whatever the static (cold) fill pressure is, this the pressure to which your tank should be pressurized. Your 12 PSIG will accomodate a system height from gauge point to top of the radiator of 18.5 feet and allow 4 PSIG residual pressure at the top of the system. Most two-story homes will be fine with this setting.

If you have a taller house and your tank is charged to 12 PSIG, the system pressure will compress it to a lower available volume to absorb expansion.

You can buy a bigger tank but my recommendation would be to buy another tank such as you have and twin them together. They do not have to be hanging from your piping. In fact, I often simply connect them via a braided flexible washing machine hose to a point upstream of the circulator. The tank rests on the floor. What counts is that point, where it connects.


  • Randall Hill
    Randall Hill Member Posts: 9
    Furnace expansion tank replacement

    I have water coming from my p/r valve of my haot water furnace.I've replaced the p/r valve & recharged the expansion tank to 12psi, but it still leaks when the furnace is heating up.My plans are to replace the 4.5 gal diaphram expansion tank.
    I'd like to know if there are any special tips anyone may have for me.
    I already realize that it's a full tank & caution due to weight is needed. It appears to just screw onto the heating system.
  • Randall Hill
    Randall Hill Member Posts: 9
    Expansion tank

    My furnace was installed in "1995"
    Its a Columbia 100,000 btu Natural Gas, forced hot water.
    The expansion tank presently on it, worked fine until now.
    What ya think ??
  • Brad White_150
    Brad White_150 Member Posts: 29
    I would need to know

    the pressures and the pressure gradient from cold fill to release. Could be a pinhole in the diaphragm or other loss of absorbtion capacity. Any gurgling or air to be vented from radiators? The rate of rise or fall would say "temperature". Given that you replaced the fill valve, the comment below about a leaking immersion coil is worth checking. If there is a coil leak the pressure rise and release would be constant, not just on heat-up. Worth checking still.
    Was the boiler connected to existing piping original to the house or is it all new?
  • A few things to check

    Does your boiler have a coil for hot water ? Or an indirect heater ? When you recharged the expansion tank , was it still connected to the pressurized system ?

    First thing to do is make sure the expansion tank holds pressure on the air charge side - it has to be disconnected from the boiler , or the pressure on the boiler has to be at 0 to check it . If it's OK , I would suspect a bad pressure reducing valve or a leak in a coil or indirect . Since the system was installed in 95 with no problems till lately , I believe the expansion tank is sized right .
  • Randall Hill
    Randall Hill Member Posts: 9
    Old pipes

    The radiators are the original finned type, pipes are original also. No abnormalities are present,such as gurgling (etc). The only change since installed,just started this year. The leaking of water while heating up.
    I'll check the pressure release point,next.
  • Randall Hill
    Randall Hill Member Posts: 9
    Tank Check

    The boiler was at zero when I recharged the expansion tank & I checked it with a guage afterwards.It showed 12-15psi.
    I'm not sure on the coil, or indirect though. It's natural gas. How do I check for the other, where is the pressure reducing valve?
  • Brad White_150
    Brad White_150 Member Posts: 29
    Pressure reducing valve

    is in the CW connection train where the boiler system takes in the fill water. Typically it has a lever valve on top which will fast-fill the system. A common model is the Watts 1156F but it could be of any make, Taco, B&G for example. You recently replace this, right?

    Gas or oil -does not matter what the fuel is, this is on the water side of things.
  • If it's gas

    it probably doesn't have a coil . What makes your hot water for sinks and showers ?

    When you checked the expansion tank pressure the first time , what was it at ? When you checked it the last time , was the boiler pressure still at 0 ?

    The pressure reducing valve should be on the pipe with the valve you shut to work on the boiler . Sometimes it's green , gold or even red colored . Alot of them come with a fst fill lever , make sure that lever is not in the fast fill position .
  • Brad White_150
    Brad White_150 Member Posts: 29
    ??? Ron

    "If it is gas it probably does not have a coil"?

    Short of an easier choice to have a separate DHWH, I am not sure why you would think this. I have seen dozens of systems, gas fired, with an immersion coil either as a back-up, primary or a hold-over from previous installations.

    Good call on the fast fill lever.
  • Randall Hill
    Randall Hill Member Posts: 9
    Pressure reducing valve

    No. I didnt replace the pressure reduction valve. I replaced the releif valve.
    I located the reduction valve with the quik fill on top.
  • Randall Hill
    Randall Hill Member Posts: 9

    I have a separate gas hot water heater,not connected to my heating system
  • Well Brad ........

    Since he did say it was installed in 1995 , and I used to install many atmospheric gas water boilers back then ( not Columbia though ) , I remember NEVER seeing or installing one with a coil . First one we actually did use a coil in was maybe 4 years ago - in a Peerless .

    Those dozens of systems you've seen coils in , were they circa mid 90s ? How many gas boiler have you seen without a coil ?

    I know some gas boilers have a provision for a coil and any boiler can have a heat exchanger added to it . But we both know that's very rare for a fairly new gas boiler . And why I said his boiler " probably " doesn't have a coil .
  • Randall Hill
    Randall Hill Member Posts: 9
    Final Diagnosis

    Should I assume that if it's not my expansion tank, that it could possibly be the reduction valve?
    Thanks for your help,
  • Brad White_150
    Brad White_150 Member Posts: 29
    Laws of probability

    Thanks Ron-

    All the systems I have had or installed (back in my landlord days) had immersion coils. It started as that was what was in stock and made sense from a back-up standpoint. (Perhaps I am a victim of a perception of my own creation?)

    Overall I have seen about 50-50 in my area with and without coils. Some of those were former oil units with Economite conversion burners though, and all apartments had separate DHW regardless.

    Not nit-picking, just curious about your statement of probability. In all probability you are correct :)
  • Brad White_150
    Brad White_150 Member Posts: 29
    That would be

    the next likely suspect, yes. Your statement that it weeps when it heats up though had me going to the tank first. When cooling, does the pressure drop?
  • brucewo1b
    brucewo1b Member Posts: 638
    If the tank is holding

    12PSI shut the valve off before the PRV and see if it works witout pressure increasing to the point of relief valve going off that will tell you if its the PRV.
  • Timco
    Timco Member Posts: 3,040

    It would seem the bottom line is you need to pressureize the x-tank with the tank off the system. See if it holds. If it does, reinstall it, then turn on your fill valve. If the press reducer stops filling at 12, all good. If not, try adjusting it. If still no, you can use the fast fill lever and stop at 12. If the system was at 0, you may need to purge / bleed. When you re-install the x-tank, do yourself & the next owner a favor and install a 1/2" ball valve in line to isolate the x-tank without draining the system.

    Best of luck, do as the other guys said and you will be in great shape!

    Just a guy running some pipes.
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