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Pressure Relief Valves on Expansion Tanks

SpeyFitter Member Posts: 422
So this is a new one for me - on expansion tanks our local Authority Having Jurisdiction is apparently requiring Pressure Relief Valves between the isolation valve and tank. Apparently if there is a fire or high heat source near by and the tank is isolated the expansion tank can blow up. Is this code in your area? I'm interested in what you know or have heard.
Class 'A' Gas Fitter - Certified Hydronic Systems Designer - Journeyman Plumber


  • northernboiler
    northernboiler Member Posts: 55
    That is a new one.

    Never heard of that requirement. Seems to me because the hydronic expansion tank is connected to the system, at the point of no pressure change, any significant increase the system pressure would set the units PRV off.

    What jurisdiction is this?
  • SpeyFitter
    SpeyFitter Member Posts: 422

    I still have to talk to the local Boiler Inspector to hear it from the horses mouth (heard it from a Mechanical Engineer today) but I am in British Columbia, Canada.
    Class 'A' Gas Fitter - Certified Hydronic Systems Designer - Journeyman Plumber
  • Charlie from wmass
    Charlie from wmass Member Posts: 4,314
    So lots of folks will not put in

    isolation valves.
    Cost is what you spend , value is what you get.

    cell # 413-841-6726
  • jumper
    jumper Member Posts: 2,206
    somebody's working overtime

    How often is expansion tank isolated? When will there be enough energy to burst it? This kind of thinking make new nukes so much more expensive than the ones built in sixties.
  • SWEI
    SWEI Member Posts: 7,356
    If an expansion tank ruptures in a fire

    roughly half full of water, what is the likely outcome?
  • Plumdog_2
    Plumdog_2 Member Posts: 873
    Expansion tank weirdness

    I have seen expansion tanks homebuilt of cell core PVC, with a shrader valve and an aqaupex sight glass. This was done numerous times by one of the local yokel Mensa Heat Contractors. Makes ya wonder what would happen if it got pressurized with steam. Maybe just swell up like a balloon or maybe worse.
  • icesailor
    icesailor Member Posts: 7,265
    Local Legends (in their own minds):

    "" local yokel Mensa Heat Contractors "".

    I've met a few of those types. Tough to compete against them because they know so much.

    The homebuilt expansion tank sounds like something to check out. Is it ASME approved?
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    It is within the AHJs ability to dictate this...

    Having just read and reviewed the IAPMOs major mechanical codes, I can tell you that this provision is NOT in the body of the code (can't speak to ICC or others), but the local AHJS can be equal to, or more stringent than the prescribed code.

    Personally, I think it is a moot point and ridiculous provision. Who's going to go to the trouble of isolating their expansion tank when their building is on fire?

    Some times, AHJs get something in their head that they think is dangerous, and come up with provisions to correct it, without really giving it serious consideration or asking for input from those who actually work in the field.

    Sometimes, you can convince them to reverse their decision, but you'd best go in fully prepared.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • rich pickering
    rich pickering Member Posts: 277
    Just had this happen

    AHJ wanted a relief on the tank.  No problem I said.  I'm not paying for it, the customer is, I will make money on this. 

     HOWEVER, because this is a life safety device, I needed the AHJ to provide a stamped spec for the valve, keeping in mind that the tank is also a compressed air tank and a water tank at the same time.  And I'm just a dumb plumber, not an engineer, so I'm not taking the liability.  And  by the way, if it is required at this location, what about every other installation we deal with?  That's a lot of stamped specs because every location is diffferent.

    Apparently, it's not really required.
  • icesailor
    icesailor Member Posts: 7,265
    Mistaken understandings:


    Have I been under the mistaken understanding that one properly sized relief device in a closed system covers the whole entire system to which it is connected to?

    We put 75# pressure relief valves on Well Extrols because the tanks are rated for a maximum of 75#. We put 30# Pressure Relief valves on heating systems because boilers are rated for a maximum of 30# PSI working pressure.

    Maybe I'm wrong. I say "You can't make this stuff up". Maybe I can't, but someone else can.
  • Bob Harper
    Bob Harper Member Posts: 1,026
    clarification on the power of demi-gods

    While ME is partially correct, your local AHJ can NOT just indiscriminately require whatever he feels like. They are code ENFORCERS--not code legislators. Every jurisdiction must have a mechanism for implementing LEGAL code change. This is typically the submission of a proposed change with supporting data and arguments/ logic. At some point, three public hearings are held for stakeholders and citizens to voice their concerns. Then the local governing body votes it into law or rejects it. Only once it has been voted into law as a local ordinance does your AHJ have the power to enforce it.

    Even that process has another level of review: The International Code Council. Where the I-codes have been legally adopted, local ordinances that go above and beyond the current I-codes can be reviewed and in most cases, overturned. This happened in Pa where right after we adopted the ICC suite in 2004, some jurisdictions adopted ordinances such as residential fire sprinklers. The builders went to the ICC and have 14 of 14 cases overturned.

    An AHJ has the power to rule only where there is no applicable code. Even there, he is charged with relying, whenever possible, on reports, listings, or other credible data. In the case of the expansion tanks requiring pressure relief valves, the AHJ would need to show case studies where tanks failed during unfriendly fires that resulted in personal injuries or damage to the structure significantly over and above that which would have occurred anyway. He would have to present test data showing what sort of pressures these tanks could be exposed to and what form of relief would be warranted. For instance, do you install just a pressure relief valve or a temperature/ pressure relief valve and what ratings? TPR valve stems must be fully submersed in the liquid so how are you going to do that in the piping downstream of an isolation valve. Also, what is the incidence rate of such a tank being left isolated as opposed to online? If left isolated, the boiler's pressure relief will likely discharge during heating cycles letting you know something's up and call for qualified service.

    I find this a pretty rare hazard and think its just paranoia. Often, AHJs will read of one isolated incident then make a name for themselves locally by instituting changes based upon imperfect information. Most of the stories about losses are just heresay and not evidence-based from litigated cases. Also, if this was such a hazard, believe me the CPSC, ICC, NFPA, etc. would be all over this.