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expansion tank size
DanHolohan
Member, Moderator, Administrator Posts: 16,583
on the new tank?
Retired and loving it.
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expansion tank size
I am in Kansas City Missouri and had a new boiler installed on my hot water heating system in my 1908 three story home. When the system is full cold there is 15 lb. of pressure on the boiler pressure gauge. On very cold days the pressure goes up to very close to 30 lb. but does not vent water from the pressure safety valve which is rated at 30 lb. Seems like a big change to me, especially compared to the old boiler and old steel expansion tank where the altitude (the old one had an altitude gauge that read in feet) was 33 feet (15 lb.) full cold and never exceeded 45 feet (20 lb.)on the coldest days. Does this indicate that the new diaphragm type expansion tank is too small, or is this amount of pressure change OK as long as I don't have discharge from the safety valve? Thanks much! Brian Macdonald
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Wondering
Wondering why a pressure increase of 16psi is not acceptable as long as it doesnt exceed the PRV setting & the pressure rating of the equipment online. ??
Terry
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Expansion
Brian,
The expansion tank must be sized according to the amount of water in the entire system. If in fact you have older larger piping in your system that may have originally been used to carry steam, the chances are that the expansion tank that you have now is too small.
This is why different sized tanks are made. You need to measure the piping (diameter and lengths) and account for the gallon capacity of the boiler, and have an expansion tank sized accordingly.
A pressure increase that you are talking about is not acceptable, nor safe.
Good luck,
Joe0 
Joe
If the boiler was on the third floor and the Piping was the same,how would you size the expansion tank.0 
Third floor?
Bill,
I guess that I do not understand what you mean.
Joe0 
15 goes to 28 Lbs?
That's not normal. Either the tank is undersized or it's lost some of the bladder's air charge. You're riding too close to the relief valve's maximum rating.
To Learn More About This Contractor, Click Here to Visit Their Ad in "Find A Contractor"Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 
Amtrol Sizing Calculator
Try this Amtrol sizing for your system and see what you come up with.
Eric0 
Amtrol Sizing Calculator
Try this Amtrol sizing for your system and see what you come up with.
Eric0 
Try this Amtrol sizing program
Try this and see what you come up with.
www.amtrol.com
Sorry I couldn't send a web page link.
Eric0 
Joe
I was thinking of static Pressure were you could get the system to run on a lower psig. You would still need to find out the volume of water in the system to size the expansion tank. Then again I wouldn't want to be hauling tools up three flights of stairs.0 
It is normal..
for the system pressure to increase when the system is heated. It is the job of the expansion tank to control this presure increase within the limit of the system relief valve. It is prudent to allow a 5 psi 'cushion', however, so if the relief valve is 30 psi it makes sense to keep the system pressure from increasing to more than about 25 psi.
Your new expansion tank likely came with a 12 psi air charge from the factory. If your cold fill pressure is 15 psi, the air charge in the tank should be increased to match that, but it must be done with the tank disconnected from the system and no pressure on the water side of the tank. Use oilfree air  a bicycle pump works good.
You may also be able to reduce your cold fill pressure slightly. Measure the vertical height of your system in feet from the fill point to the highest point, divide that by 2.31 to get psi, and add 4 so that the top of your system is pressurized. For example, a vertical height of 20' requires a fill pressure of (20/2.31) = 8.7 + 4 = 12.7 or 13 psi. This will be the cold fill pressure required at the fill point  if the gauge you are reading is someplace else in the system, allow for the elevation difference when reading the gauge.
If the air charge in your new tank is satisfactory and you can't lower the cold fill pressure, you will likely need the next larger size of tank.0 
expansion tank size
Thanks everyone for all the input so far.
To Dan  the expansion tank is a Watts Regulator No.ET60. Also I have calculated using the chart from your web site (excellent!) that I have 546 sq. ft. of radiation. Also per your web site, 546 x .03 = 16 gal for a plain steel tank, 16 gal x .44 = 7 gal. for a diaphragm tank. I don't know the nominal size of the Watts tank ET60. Also it may be pertinent that my system has large pipes, two 3" supply and two 3" returns at the boiler (reduced to 1 1/4" in the near boiler piping). I have also estimated the volume of the old steel tank to be approxamately 27 gal. based on its external dimensions, a cylinder 14" in diameter and 4' in length. So an equivilent diaphragm tank would be 27 x .44 = 12 gal., right?
Jerry, I measured the height from the boiler (pressure guage and fill point) to the top of the system to be 28'. Using your formula, 28 / 2.31 = 12 lb. + 4 = 16 lb. The air charge on the tank is also 16 lb. when removed from the system, so that seems to be correct. So, apparently I need the next size larger tank?0 
expansion tank size
Thanks everyone for all the input so far.
To Dan  the expansion tank is a Watts Regulator No.ET60. Also I have calculated using the chart from your web site (excellent!) that I have 546 sq. ft. of radiation. Also per your web site, 546 x .03 = 16 gal for a plain steel tank, 16 gal x .44 = 7 gal. for a diaphragm tank. I don't know the nominal size of the Watts tank ET60. Also it may be pertinent that my system has large pipes, two 3" supply and two 3" returns at the boiler (reduced to 1 1/4" in the near boiler piping). I have also estimated the volume of the old steel tank to be approxamately 27 gal. based on its external dimensions, a cylinder 14" in diameter and 4' in length. So an equivilent diaphragm tank would be 27 x .44 = 12 gal., right?
Jerry, I measured the height from the boiler (pressure guage and fill point) to the top of the system to be 28'. Using your formula, 28 / 2.31 = 12 lb. + 4 = 16 lb. The air charge on the tank is also 16 lb. when removed from the system, so that seems to be correct. So, apparently I need the next size larger tank?
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This discussion has been closed.
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