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Expansion Tank Question

jt3jt3 Posts: 31

Adding a 30 gallon buffer tank to my heating system.  Total capacity of existing system is about 10 gallons so, with the buffer tank it will be about 40 gallons.  Existing expansion tank has a Pure Pro label on it but no specific size or model number that I can see.  Looked up Pure Pro on line and it appears that the installer probably used the Pure Pro 1" Standard Boiler Trim Kit which uses the AMT 30 (Amtrol #30) expansion tank.  Using one of the charts I found on line it says I need about 7% of the system capacity for the expansion tank. (2.8 gals)  The Amtrol is listed as 4.4 gallons capacity which I am guessing is the total volume of the tank.  Is that also the usable capacity or do you calculate usable capacity as a percentage of the total volume, i.e. 50%, 75% etc.?

Guess the question is: Does this expansion tank have sufficient capacity to handle the added 30 gallon buffer tank?




  • jt3jt3 Posts: 31
    Thanks Paul

    Amtrol was one of the first sites I looked at and using the link you posted it came up with what I think is the tank that is already installed.  (Residential System)  All it asked for however was the output of the boiler and the type of radiation.  If I use the Commercial option, it allows me to enter the total system volume along with the minimum and maximum temperatures and pressures.  It then comes up with the AX15 (8 gals), 200L (53 gals), and the 35LBC (10 gals)  Throwing out the 200L which I think is way too large, that leaves something around 8 to 10 gallon capacity. 

    While searching for info on the AX15 and 35LBC I came across this link from Amtrol.  If you go to page 4 there is a table (Sizing Based On Maximum System Temperature) that shows at 190 degrees (Max boiler setting plus 10 degrees for overrun), the Model 30 is good for up to 51 gallon system water content.  That should provide a little margin so the system isn't working at its limit.  (Especially since the system would normally be operating at the 180 degree maximum for only a few days during the heating season.)

    Due to the placement of the existing tank I can't replace it with a larger one.  If I need to increase the capacity of the expansion tanks to 8 to 10 gallons I could hang a second tank on the boiler feed line that goes to the buffer tank.  (Existing tank is on the same line.)  Don't have a problem doing that just don't want to do it if it isn't necessary.  


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