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Pressure Tank Puzzler

Constantin
Constantin Member Posts: 3,796
... our new house is on a hill in the People's Republic of Cambridge. Being an old town and all that, we get to enjoy about 37psi of pressure at no-flow and 30psi during flow at the hydrant in the street. After the flow test, we installed a new 1.5" main and another 1.5" fire line.

The prospect of having folks on the third-floor take tinkle-showers then made us install a pressurization tank. Per the suggestion of our engineer, we bought the <a href="http://www.amtrol.com/website/content.nsf/0/E29231A3EC244FAE85256A170051F29F?OpenDocument">Amtrol RP25</a> unit, which should give us enough flow to power much of the neighborhood. As you probably know, the Amtrol unit basically consists of a giant expansion tank with a ½ hp motor on top.

Over to the other side of the mechanical room, we have a Vitocell 79 gallon tank into which the cold water is piped. Interestingly, our plumber elected to install a small expansion tank on the cold side of the Vitocell as well. Wouldn't such a tank be redundant? Or is this simply yet another MA regulation gone amuck?

Comments

  • jackchips_2
    jackchips_2 Member Posts: 1,338
    One reason

    for an added expansion tank could be if there is a check valve on your domestic supply after the Amtrol system. Other than that it does appear to be redundant.

    I'd be interested in your plumbers reasoning and what part of our code may require it. Always willing to learn.
  • Ray Landry
    Ray Landry Member Posts: 203


    Potable water expansion tanks are used are when the t&p relief keeps popping. This is generally caused by either a quick closing valve, exsessive water pressure, or when you have a check valve or a pressure reducing valve on your water main. These valves don't allow the water to expand when heated, thus causing the relief valve to blow. It doesn't sound like you have any of these is scenarios, which is kinda strange...
  • Ray Landry
    Ray Landry Member Posts: 203


    Section 2.14 section 8 item E of the mass code says that the only time expansion tanks MAY be required are when there is a backflow prevention device somewhere in either the water main line, or on the water heater inlet line that would constitute an operating hazard.

    So, expansion tanks aren't required, they are just telling you when it's good practice.
  • jim lockard
    jim lockard Member Posts: 1,059
    Maybe in the commonwealth

    your plumber felt it was easier to pull the string then to push it. Too much expan. never hurt. Best Wishes J.Lockard
  • Constantin
    Constantin Member Posts: 3,796
    There is a checkflow valve...

    ... but it is "upstream" from the Amtrol pressurizer. I will go over the system today to see if I can see any additional check valves between the Amtrol and the Vitocell. Thanks to all for your insights!
  • Ray Landry
    Ray Landry Member Posts: 203


    Even if the check valve is upstream of the pressurizer, that still constitutes a situation where the backflow device could cause no room for expansion
  • jim lockard
    jim lockard Member Posts: 1,059
    Constantin

    Think Ray maybe right --Check it out Best Wishes J.Lockard
  • Dave Yates (PAH)
    Dave Yates (PAH) Member Posts: 2,162
    well

    I think I see the logic. He may well have thought the pressurizer's expansion tank is sized for that particular application and given some consideration to how much acceptance the Amtrol system could absorb if it was already at its rated capacity.

    Now, it's entirely possible the Amtrol unit could accept the additional thermal expansion created by the 79-gallon tank, but that could be as mucgh as 2-gallons depending upon the Delta-T of starting/finished temp rise.

    Sounds like he took the more prudent approach, which will also help to minimize pump cycling.

    Remember: V1 x P1 must equal V2 x P2 (Boyle's Law). From that, you can determine the capacity remaining in the Amtrol tank and then go to the Wessels web site to get the exact multiplier for thermal expansion. V2 & P2 can then be used to obtain V3 & P3 once the thermal acceptance volume is considered.

    I'm teaching a plumbing class in two weeks & yours will make for an interesting quiz. Which Amtrol system do you have in place?

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  • A. mitchell
    A. mitchell Member Posts: 8
    code

    mass code states that a pressure blatter tank must be installed when a check valve or back-flow preventer is installed on the main water supply coming in to the dwelling , with the presure tank at the wall with the pump your plumber was probably erring on the side of caution because he is working in the commwealth of cambridge. also in mass inspectors have final say. fellow mass plumber
  • Geo_2
    Geo_2 Member Posts: 76
    amtrol

    I have installed many amtrol pressurisers ( some in cambridge) and have never install an additional expantion tank, never had a problem w/ the systems or the inspctors that checked them out. As you must know constantin the water in cambridge is awful,pull apart a faucet and it's corral like stuff caked in there. May I ask why you called in an engineer?
  • Constantin
    Constantin Member Posts: 3,796
    Had a look last night...

    ... as I suspected, the secondary expansion tank has nothing between it and the rest of the cold water system (see sketch below).

    However, I am perfectly happy accepting that this may be yet another code-related issue that has no bearing on reality, performance, or whatnot. Further analysis with the help of Mr. Yates will determine just what this 2nd tank can do for our system.

    Since I pick my battles with inspectors rather carefully, this is not worth fighting about. Should our analysis show that the 2nd tank is redundant while used in conjunction with a bladder-style pressurizing system (as is the case now), we'll simply replace the 2nd tank with a plug whenever it lets go.
  • Constantin
    Constantin Member Posts: 3,796
    Thanks for the info!

    I believe we have the Amtrol HP25, but will double-check today. I'll also get you the sticker on the smaller expansion tank while I'm over there. I'll post it all this evening.

    Perhaps the fact that the Vitocell may go to 190°F due to the solar water heating system made our plumber add the 2nd expansion tank. Should make for an interesting problem!
  • Constantin
    Constantin Member Posts: 3,796
    Well, it just so happens...

    ...that a very good friend of our structural engineer works for a living on designing water systems for commercial and residential use. We brought him in because of our dodgy water situation in Cambridge WRT to quality, pressure, etc. Also, he had a lot of experience with fire-sprinkler installations and installers.

    All in all, his advice was very good and I'm glad we consulted with him. For example, while he was right that we didn't need a separate fire line if there was a tank + pump in the house, this was not a fight we were going to have with the inspectors. We simply put in a fire line.

    While this engineer did not design our water system (he simply looked over my drawings, specs, etc. and blessed them), the plumber on this job has brought some great insights to the project and changed a number of details for the better. For example, by having two DWV stacks instead of one, our plumber was able to minimize joist penetrations and hence improve structural rigidity and strength a great deal.

    The last thing we still have to deal with is the water treatment system for the home. Our engineer made us aware of one treatment-installer. What have you done in the past to deal with all the Calcium Carbonate and whatnot?
  • hr
    hr Member Posts: 6,106
    I'd check

    to make sure the Amtrol bladder could handel ant temperature that might be pushed back against it. a DHW expansion tank has a bladder that is temperature rated. The Amtrol well tank may, or may not.

    Expansion wise that well tank serves the same purpose for absorbing expansion.

    hot rod

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  • jackchips_2
    jackchips_2 Member Posts: 1,338
    There should be

    a heat trap on any water heater source and wouldn't that prevent hot water migration, Hotrod?

    It seems the only expansion the Amtrol should see would be cold.

    As for Mass. inspectors having the "final say" as am said, that's not true. We verify that the code is followed. There are many plumbers who are just adverse to questioning an inspectors decision for whatever reason. When we do make a decision it has to be backed up by the code, not our personal preference.

    With the information supplied, I see no reason for the added expansion tank and would not have required it.
  • hr
    hr Member Posts: 6,106
    By heat trap

    do you mean those nipples supplied on some water heaters with the little rubber flab to prevent heat migration? Or just a drop leg in the piping for a thermal trap? Are either required or inspected for in MA?

    If you use the heat trapper nipples I would definitely add the thermal expansion tank, as these will act a a simple check valve. At least until the rubber flap disappers :)

    They also used to make some with little floating balls in them. The red nipple had to be on the hot side and the blue one on the cold to work correctly. These were common on electric WHs as "energy savers"

    A thermal expansion tank is under 50 bucks throw one in and don't worry about it :)

    hot rod

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  • jackchips_2
    jackchips_2 Member Posts: 1,338
    Now I'm confused.

    Aren't water heaters fed with cold water into the bottom either by direct connection or a dip tube? It appears

    I did use the term heat trap wrong. I was referencing the inlet.

    How can the hot water migrate back to the expansion tank at the Amtrol?

    In reference to the added expansion tank, my comment was that the second tank is not required by anything in the Ma code but certainly not a problem if someone wants to add it.

    Why do I feel I've made the water muddier? :-)
  • Dave Yates (PAH)
    Dave Yates (PAH) Member Posts: 2,162
    More questions!

    Are there any identifying numbers or model indicated in the Amtrol tank (I can get that from my suppliers if not)?

    What was the factory air charge on the pressurizer tank? That's important to know in order to determine the pressurizer tank's V2 prior to activating the pressure boost.

    What is the final pressure after the pressurizer runs?

    What is the size of the thermal expansion tank? (Make & model.)

    What is the coldest water you see coming in from the street (your water co can give you that info).

    You mentioned 190F as a possible finished temp! Is that correct and, if so, are you mixing that down for bather safety???

    In reference to HR's point, how many feet (aprox) of cold water piping from the pressurizer to the Vitocell tank? Insulated or not? Copper or plastic?

    Thanks, Dave

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  • Dave Yates (PAH)
    Dave Yates (PAH) Member Posts: 2,162
    How it looks on a drawing

    Here's the final test for Thursday night's class. (Not everything I'll be asking is listed!)

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  • Constantin
    Constantin Member Posts: 3,796
    Some Answers...

    Sorry to keep you waiting. Here is the data I can supply thus far. The Vitocell is a 79 Gallon tank with two HX coils (one for the solar system, the other for the heating system).

    The Amtrol unit is the RP25 (link to pdf file).

    Pre-charge: 38psi, 34 Gallon tank, only good for cold water.

    Final pressure: variable, up to 75psi. We'll probably set it to 50-60 psi.

    Coldest water we'll see: about 45°F.

    Naturally, we're mxing down the potable water for safety. The mixing valve is ginormous and has a nice thermometer built-in. Hopefully, that's a thermostatic model so we can set and forget it.

    The second tank is a 2.1 Gallon model. The camera is playing hide-and-go-seek with me right now, so I cannot give you better info on it. White exterior, stainless tank specifically set up for potable water systems. Also, between the 2nd tank and the bottom of the Vitocell is a what looks like a old steam valve handle (i.e. round disk) on a 90° fitting.

    There is about 20' of pipe running from the top of the pressurizer to the bottom of the Vitocell. All copper pipe, currently non-insulated, ranging from 1.5 to 1" in diameter. We'll probably insulate it one day to prevent condensation in the summertime.

    The picture below is a couple of days old. It shows (right to left) the tank, the vitocell, the solar stuff (in the middle), and the mixing valve (sans the Thermometer and mixed output in the middle).

    The maximum height off the boiler-room floor (where the cold comes in to the top shower head is 38' (third floor bath).
  • Dave Yates (PAH)
    Dave Yates (PAH) Member Posts: 2,162
    Wonderful!

    Exactly what I needed to plug in for correct answers!

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  • Constantin
    Constantin Member Posts: 3,796
    And More... but where is the answer?

    ...the second stainless tank is a Flexcon Model WH8 Unit: 2.1Gallons, precharged at 40 psi, max 150psi pressure, 200 °F working temp.

    Once you've administered the test, could you be so kind and tell me the answer? Being a mere homeowner and all that, I don't have the thermal expansion characteristics of water in my head anymore.

    Also, could anyone identify the black knob next to the expansion tank? It's between the Vitocell and the Flexcon tanks and I'd simply like to know what it is (it's been framed in red, upper left-hand corner). Cheers!
  • Dave Yates (PAH)
    Dave Yates (PAH) Member Posts: 2,162
    answer & drawing

    coming soon! You don't need that second tank unless a check valve ends up in the line between the Vitocell and the Pressurizer. If that were the case, a 2.1-gallon TXT would be way too small.

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  • Dave Yates (PAH)
    Dave Yates (PAH) Member Posts: 2,162
    Final answer!

    Here tis. Remember it's Boyle's Law that determines the outcome. V1 x P1 must equal V2 x P2

    We know the street pressure and now the set boost pressure. We lose 1-PSI for every 2.31' of elevation, so that gives us the delivery pressure at the shower head.

    With the combined expansion tank capacity of 34 + 2, you've got an available 36 gallons of air to be compressed. Let's call this V0. P0 will equal the cut-in pressure, which we'll set at 40. P0 x V0 = 36 x 40 = 1440 (that's now "C", the constant)

    We know P1 will be 65 and that the constant equals 1440. Therefore V1 must equal 22.1538

    We know the expansion multiplier will give us 2.47033 gallons, so V3 must be 19.68347.

    Therefore, your final pressure following thermal acceptance will be 73.1578 PSI.

    No way that two-gallon tank was necessary. Go back & re-do the math without that additional 2-gallon TXT to see the slight uptick in final pressure.

    Class was great tonight & they were a great bunch of young men plus one gal.

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This discussion has been closed.