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Two tank solution am I nuts?
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Al Letellier_9
Member Posts: 929
Great advice from the other posts, but here is something that doesn't get mentioned enough and something I see a lot in my insurance work. IF you add a second tank, make sure it is well supported. Two #30 tanks off a single 1/2" tee can lead to disaster when one floods. Those things weigh a lot when full and can put a lot of strain on a piping system. I've seen a quarter million dollars worth of water damage from an unsecured, improperly installed X Tank. When you replace, if you have the room, I always opt for a floor mount tank. A little more money, but a lot more "service room" around the boiler and less chance of a major service problem.
0
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expansion blues
Date: January 29, 2007 11:03 AM
Author: Bob
Subject: same problem
I have the same problem with the pressure relief valve discharging at 30 psi. When cold the boiler water pressure is at ~14psi and gets to 30 psi by the time it hits 175F. I replaced the expansion tank (12 psi) and relief valve but still the same thing. The expansion tank gets hot to the touch near the fill inlet and a few inches down, so I'm guessing there is flow to the tank. And I'm flummoxed. My system has A 7 year old Buderus G234X64 boiler feeding Weil Mclain built in radiators with a two pipe (supply and return) feed to each radiator.The expansion tank feeds off the air scoop downstream from the boiler. I bleed the radiators twice a winter and normally there is air in just one of the 35 radiators. Which makes me wonder how do I calculate the thermal expansion in the system? I replaced the expansion tank with the exact model that was attached. It turned out to still have an air charge of around 12 psi and no water inside. Maybe I need to gang on another tank?
And now another day later here's what I did:
I followed advice given here and figured the thermal expansion from Amtrol's chart. The contractor who installed the system definitely supplied a tank too small. The relief valve has probably been dripping at the upper temperature limit since last year when a different contractor replaced the controller and reset the upper limit. When the relief valve got enough crud it stuck open enough to drip constantly. Since the first tank still appeared to be in working order, I installed a tee and now have two identical tanks and pressure which doesn't go above 24lb all the way up to the upper temperat ure limit of 180F.
Can anyone tell me if this is stupid, and I suppose more importantly, why?
I promise when either of the tanks fails I'll buy the appropriate larger single tank, but please tell me if there might be something wrong with the present arrangement. Thanks0 
If you're stupid, you won't be alone.
My ET30 tank will be slightly undersized if and when I get my 40 gallon buffer tank installed. I plan to just add a second identical xtank.0 
Two Tanks
I often install two tanks regardless, especially on commercial/institutional work. Should one rupture, I am covered and it also allows for future system expansion (such as a plant supporting a future planned addition.
On houses, two smaller tanks (#30's say) cost less than a single #60 in my experience, so I get some redundancy for a few dollars less.
So long as they connect to the same PONPC, you are fine and have the expansion volume you need.
Brad0 
give me
the expansion tank model # or its size & I can give you the answer.
If that's a 4.5 gallon TXT and you bump up from a tank air charge of 12PSI to coldsystem 14PSI, your new air volume in the TXT will be 3.86gallons. That's what remains for accepting thermal expansion.
At 30PSI, the TXT's air volume is squeezed into a 1.8gallon space. That means you generated 2.06gallons of thermal expansion.
If you start at 65F and see the 30PSI upon reaching 175F (and there's no air out in the system to skew the numbers), the expansion factor is .02495. Therefore, your system holds 82.5gallons of water.
it all hinges on the initial tank size.0 
system volume
> the expansion tank model # or its size & I can
> give you the answer.
>
> If that's a 4.5 gallon
> TXT and you bump up from a tank air charge of
> 12PSI to coldsystem 14PSI, your new air volume
> in the TXT will be 3.86gallons. That's what
> remains for accepting thermal expansion.
>
> At
> 30PSI, the TXT's air volume is squeezed into a
> 1.8gallon space. That means you generated
> 2.06gallons of thermal expansion.
>
> If you
> start at 65F and see the 30PSI upon reaching
> 175F (and there's no air out in the system to
> skew the numbers), the expansion factor is
> .02495. Therefore, your system holds 82.5gallons
> of water.
>
> it all hinges on the initial tank
> size.
0 
system volume
Now I'm starting with two Amtrol 60s at 70F and ~18psi and reaching 25psi at 180F. Each 60 will accept up to 2.5 gal in the 7 gal tank. Your guess is undoubtedly better than mine...0 
slight change
That ups your system volume to about 102.5gallons if you're hitting 24PSI.
All thanks to a very dead man  Mr. Boyle.
V1 x P1 must always = V2 x P2
V1 = 7.6 x 2 = 15.2
P1 = 12PSI (tank air charge)
P1 x V1 = 182.4, which now becomes the constant value.
P2 = 18PSI at cold start. 182.4 divided by 18 = 10.13, which is the new dualtank air volume after accepting the pressure change.
24Lbs final PSI at 180F
P3 = 24
182.4 divided by 24 = 7.6, so V3 = 7.6
That's 2.53gallons of thermal expansion from 70 to 180 F & that equals just about 102.5gallons of total water volume.
Who says math is boring!?!0 
I was told there would be no math with this job...
Thanks for the calculations but especially for the computation. The total seems about right for this house.0 
Sage advice indeed
But they were so light when I installed them...
Thanks. Now I won't have to find out how much weight a 1/2" galvanized nipple can hold. And I admit that I didn't think at all about the day when the first one fails. Tausend Dank!0
This discussion has been closed.
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