Help With Expansion Tank Sizes - Acceptance Volume

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rwh442

Member
Nov 18, 2008
152
Southeast Indiana
My system has a whopping 150 gallons total and I am using an Extrol 90 as the expansion tank. Maximum temperature is 185 degrees, 12 psi factory precharge on the expansion tank and I am peaking out at 24 - 25 psi system pressure at the maximum boiler temperature I have set. I would like to lower the maximum pressure to around 15 psi as per the Thermo-Control manual.

I looked at Amtrol's website and see they have tanks that hold more water but most of them had the same acceptance volume as the Extrol 90. The SX-30V, 40V and 60V, along with the #90, all have acceptance volumes of 11.3 gallons. But total volume ranges from 14 to 32 gallons between the models. Why would you purchase anything other than the cheapest model if the acceptance volume is the same on all the models? I must be missing something big here - confused. Is the acceptance volume the amount it can expand or the amount it is filled cold?
 
Rob H said:
My system has a whopping 150 gallons total and I am using an Extrol 90 as the expansion tank. Maximum temperature is 185 degrees, 12 psi factory precharge on the expansion tank and I am peaking out at 24 - 25 psi system pressure at the maximum boiler temperature I have set. I would like to lower the maximum pressure to around 15 psi as per the Thermo-Control manual.

I looked at Amtrol's website and see they have tanks that hold more water but most of them had the same acceptance volume as the Extrol 90. The SX-30V, 40V and 60V, along with the #90, all have acceptance volumes of 11.3 gallons. But total volume ranges from 14 to 32 gallons between the models. Why would you purchase anything other than the cheapest model if the acceptance volume is the same on all the models? I must be missing something big here - confused. Is the acceptance volume the amount it can expand or the amount it is filled cold?

The acceptance volume is the most water that the tank can accept without stretching the internal bladder more than it is designed for. Usually if you actually did force the entire acceptance volume into the tank the system pressure would be way too high. (But not always. For instance, the EX-60 and SX-60V both have a relatively small maximum acceptance volume in a relatively large tank.)

Here's a table of many of the popular Amtrol tanks. Assuming an initial system pressure of 12.0 psig, the rightmost column shows what the system pressure would be after the entire acceptance volume had been forced into the tank.
Code:
Pinitial=12.0psig
model   Vtank  Vacc  Pfinal
EX-15     2.0   0.9   33.8 
EX-30     4.4   2.5   47.1 
EX-60     7.6   2.5   25.1 
EX-90    14.0  11.3  123.7 
SX-30V   14.0  11.3  123.7 
SX-40V   20.0  11.3   46.7 
SX-60V   32.0  11.3   26.6 
SX-90V   44.0  34.0  102.8 
SX-110V  62.0  34.0   44.4 
SX-130V  81.0  34.0   31.3 
SX-160V  86.0  46.0   42.7

But your system only requires an actual acceptance volume of about 5 gallons. Starting at an initial tank pressure of 12.0 psig, the final pressure, for each of the tanks, after accepting an actual acceptance volume of 5 gallons, would be:
Code:
EX-90      26.8 
SX-30V     26.8 
SX-40V     20.9 
SX-60V     16.9 
SX-90V     15.4 
SX-110V    14.3 
SX-130V    13.8 
SX-160V    13.6

But are you sure about the max pressure of 15.0 psig? Thermo-Control boilers use a 30 psig relief valve same as most any other boiler. If they really required a max pressure of 15.0 psig, why wouldn't they use a lower pressure relief valve?

Cheers --ewd
 
I don't see why 24-25psi would be a problem? Sometimes a little higher pressure can actually be a good thing. Helps remove air for one thing...
 
Eliot,

Yes - the boiler does have a 30 psi pop off valve. But, according to their boiler web page, they state 15 psi operation. Dunno.

Your set of numbers are close to what I am seeing. So if one was to go from the EX-90 to the SX-40V there would be a roughly 6 psi drop in maximum operating pressure? So I would then probably be running at around 18 psi max.

I'm going to call Thermo-Control Monday and inquire about that 15 psi rating.

Thanks for the info.
 
Stee,

I don't have a problem with the 24-25 psi either. Been running it like that for two years. I just found on their website the other day the 15 psi operating pressure and the questions started to come out.
 
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