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Pressure Relief valve, Sizing Expansion Tank

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Greetings, this forum is such a great resource. The information here is priceless.  I hope this question will help others with similar issues. 

The pressure relief valve expels 1-2 gallons per day during the spring/summer/fall. It doesn’t occur so much in winter when the heat is on more often than not. 

The boiler is a SlantFin Liberty L-60P 326,000 btu which burns 2.5gph. 

The boiler supplies 27 cast iron radiators with an estimated 1363 EDR. 

There are 3 heating zones plus a zone for indirect  DHW. 

The first floor is 1.5” to 1” iron pipe the second floor switches to copper from 1” to 3/8”

The pressure relief valve has been replaced twice. 

The Taco pressure regulator is set at 10psi. 

2 plumbers and 3 heating technicians have told me that the Watts E-60 expansion tank (7.6 gallons) is correct and they have no idea why the relief valve is releasing water.

According to the formulas on this site, the boiler requires an expansion tank sized at 18 gallons. This is the formula used to size a diaphragm type tank. 

(EDR x .025) x .55 = tank size

What do you say? Does this system need a larger expansion tank?


Comments

  • JUGHNE
    JUGHNE Member Posts: 11,074
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    Do you have zone valves?

    One possible reason for not over pressuring in the heating season is there might be an air cushion in some radiators, that would provide more air for the system to compress as the water expands.

    In the shoulder seasons that air could be isolated from the DHW heating loop.
    Are you sure the DHW loop has the exp tank included in it?

    There was an old method of leaving a head of air in radiators and sometimes not having an expansion or compression tank.
  • Jon_blaney
    Jon_blaney Member Posts: 317
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    That seem like a lot of water. I would look to see if your auto feed and pressure controls are working properly. I think the boiler is seeing DW pressure. Your operating pressure is more than the 10 pounds the control is set at.
  • Zman
    Zman Member Posts: 7,572
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    Is your expansion tank watterlogged? Is the pressure set correctly?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • neilc
    neilc Member Posts: 2,708
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    how about a picture of the boiler and the tank,
    maybe someone will see something,
    the circ(s) and zone valve(s) also
    known to beat dead horses
  • oldboilerolderhouse
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    JUGHNE said:
    Do you have zone valves? 

    Are you sure the DHW loop has the exp tank included in it? There was an old method of leaving a head of air in radiators and sometimes not having an expansion or compression tank.
    Hi JUGHNE, there are no zone valves. Each zone has its own circulator on the return side and a Taco FLO-Check Valve on the supply side. 

    The DHW loop plumbed exactly like the heating zones. Boiler supply => air scoop /expansion tank => check valve => indirect tank => circulator and back to boiler. 

    The house is very old and your suggestion that the radiators act as a cushion is interesting. All of the radiators work well, they heat up and don’t behave as if they have air. 
  • oldboilerolderhouse
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    The tank doesn’t feel waterlogged. 

    The operating pressure for the boiler goes up to 15psi and reverts to 10psi when not operating.

    Not sure how how to check the auto feed, but I’ll look into it. 

    My pressure gauge doesn’t fit on the Watts tank, gotta find an adapter. The tank says it is set at 12psi and it has been replaced twice. Both times the plumber assured me it was set properly. 

    attached are some photos.  



  • Jamie Hall
    Jamie Hall Member Posts: 23,426
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    Something doesn't compute. If the pressure never goes over 15 psi, the pressure relief valve should never weep. Is that another pressure gauge I see in the last picture? Is it on the heating side? If so, what does it read?

    I very much suspect that a larger expansion tank is in order here.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • neilc
    neilc Member Posts: 2,708
    edited February 2022
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    have you seen your pressure when the safety valve is "leaking"?
    I'm wondering with 3 or 4 circs pumping torwards your tank, and not away from it, if the pressure in the boiler isn't climbing too high,

    but then why would it discharge during the summer?
    known to beat dead horses
  • hot_rod
    hot_rod Member Posts: 22,251
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    Indirect rain direct tank could have a pin hole leak
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    JUGHNE
  • JUGHNE
    JUGHNE Member Posts: 11,074
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    What does the tag on the pressure relief valve say?
    Could someone have put a 15 PSI (steam) valve on your system?
  • oldboilerolderhouse
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    Hi Jamie Hall, the pressure MUST go over 15psi! I just haven’t caught it yet…. Seems to happen early morning or late at night. Especially during shoulder months when very little heat is used and the boiler is mostly supplying indirect DHW. The second gauge was installed to make sure the aquastat is accurate. Both gauges show the same value range. 

    When calculating the expansion tank size based on EDR I get a recommendation of 18 gallons. When using the calculator at supplyhouse using BTU, they recommend a 20 gallon tank. 

    The difference between the 7 gallon tank that is installed and a 20 gallon tank is so great, I decided to post the heatinghelp and get a second opinion. If the current tank is wrong, it’s like someone put a bike tire on a truck. And no one noticed. 
  • Jon_blaney
    Jon_blaney Member Posts: 317
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    Any chance the indirect coil is leaking? The problem exists in the warm seasons so pressure builds up from DW supply. Indirect tank pressure at say 60psi pushing water back to the boiler against boiler pressure of 15/20 psi. Who wins?
    bucksnort
  • oldboilerolderhouse
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    To test if the indirect coil is leaking, I’d need to turn off the boiler and observe if the pressure in the boiler goes up. Is that correct?
  • Jamie Hall
    Jamie Hall Member Posts: 23,426
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    Pretty much...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • oldboilerolderhouse
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    We’ll that sort of test isn’t practical in winter! I’ll give it a try if we get some unseasonably warm days this month. 

    In the meantime can anyone shine some light on this contradiction:

    On one hand numerous licensed professionals have taken my money and have stated the 7 gallon expansion tank is the right size. 

    On the other hand every single manufacturers chart as well as the formulas found on this site indicate I should have a 18-20 gallon tank. 

    My system is 350K BTU with cast iron radiators. 
  • Jamie Hall
    Jamie Hall Member Posts: 23,426
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    Well... the 7 gallon tank is the right size, for most systems. Yours is bigger and they may not think about that.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • bucksnort
    bucksnort Member Posts: 167
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    We’ll that sort of test isn’t practical in winter! I’ll give it a try if we get some unseasonably warm days this month. 


    In the meantime can anyone shine some light on this contradiction:

    On one hand numerous licensed professionals have taken my money and have stated the 7 gallon expansion tank is the right size. 

    On the other hand every single manufacturers chart as well as the formulas found on this site indicate I should have a 18-20 gallon tank. 

    My system is 350K BTU with cast iron radiators. 
    Why not just isolate your indirect and observe your boiler pressure? No need to shut off your heat, just your DHW.
  • Jon_blaney
    Jon_blaney Member Posts: 317
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    "But we always use a 7 gal tank. You need to observe and think, not just react.

    Get a pressure gauge the shows the high pressure. Has two hands, one for the current pressure and one you can reset. Shut off the autofill, see what the pressure does after the next 30 lb relief valve release. It is rises to 30lb and releases again, you have a leak.
  • oldboilerolderhouse
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    Good news, neither the auto fill or the indirect appear to be malfunctioning. Pressure does not go up when the boiler is off. The pressure stays in a normal range when the boiler is on. The high pressure release only occurs in the warm months when the boiler is only being used to provide DHW. 

    The PRV does have a constant slow drip. It’s been replaced twice. I’m going to replace it again. 
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,607
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    @oldboilerolderhouse

    You might check your incoming water pressure if the prvs keep leaking. Possibly your incoming pressure is too high though I doubt it.
  • Jon_blaney
    Jon_blaney Member Posts: 317
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    Do you have two expansion tanks, one for DHW zone and one for the heat zones, or do all the zones come off a manifold?