# Boiler getting overpressurized -- reaches 30 PSI and PRV releases water

### Users who are viewing this thread

#### Fitter30

##### Well-Known Member
Have to know what size piping is and length of the piping. Look up square inch of the pipe x 12 x how many feet divid by 231 = gallons. That is what in the system. If your close to one size go up one size because didn't figger like what the boiler holds and if theres radiators or fan coils.

#### Dana

##### In the trades
What numbers do you use from the boiler to calculate the expansion tank size?

My boiler is:
Slant fin Sentinel SE-70 DP
C.s.a. input btu/ hr70,000
D.o.e capacity/ hr 58,000
Net i=b=r water btu 0,000

The water volume of the system, and the differential between the high & low limits on the boiler controls are the relevant data for determining the expansion tank sizing, not the BTU/hr ratings.

#### Shellcartels

##### New Member
The water volume of the system, and the differential between the high & low limits on the boiler controls are the relevant data for determining the expansion tank sizing, not the BTU/hr ratings.
Thank you

#### JeffreyArson

##### New Member
There was a similar problem with the boiler, but thanks to this topic, I was able to fix everything.

#### jadnashua

##### Retired Defense Industry Engineer xxx
The specs will likely show how much water your boiler holds...then, the rest of the equation is to estimate the volume in the pipes. What you have for convectors or radiators will vary, but if it’s essentially just a pipe rather than an old style big cast iron radiator, you estimate the total length of the pipes in the heating system, then using the volume for a round column, calculate how much water it can hold (volume of a tube = 1/2 * pi * radius squared * length. A gallon is 231 cu in, so do your length in inches, then, take the result and divide by that 231 to get gallons.

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