Pressure in boiler system

In my system i have an auto fill valve that initially sets the pressure in the system to 12 psi. I have my aquastat set at 190*F. When it is that temp its close to 28-29 psi. I'm thinking that i don't have enough expansion tank volume. I didn't install the system but it has 2 small expansion tanks. My boiler alone holds 135 gallon. Then i have a total of 560 ft of 1" pex and 150 ft of 1/2" pex. The expansion tanks are about 11" wide by 13" long. I'm not sure if that is sufficient. Kevin

That comes out to about 164 gallons total, adding 5 gallons for misc piping also. If it is filled with 60°F water and runs up to 190°F, you need 14.3 gallons of expansion capacity.

Did the pre-charge in the expansions tank get checked when they were installed. They need to be set to the fill pressure.

Some expansion tanks, like the DHW ThermTrol types, come with a 45 psi pre-charge. Drop the system pressure and check the pre-charge with a tire gauge, and determine if the two tanks add up to at least 14 gallons.ds to be adjusted to the fill

Thanks Bob. I think the expansion tanks i have are 4.4gal. So even one more still won't be enough. I guess i will have to find an appropriate sized expansion tank then. I remember checking them when the system was installed 2 years ago. they were charged to 12 psi. Thanks for the help. Kevin

Am i looking for a tank with an acceptance volume of 14.3 or greater or just the tank volume of 14.3 or greater?

The 12 psi set up was designed for a 2 story house ,usually a boiler in the basement feeding the main floor above. if you have 3 stories you need 18psi,since my house is a single story and even though my boiler is in an out building the top of it is actually higher than any of my radiators. I set my system up for 6psi and have had no problems over the last 4 heating seasons.

If you have any air vents or air elimination devices at the uppermost point in the system, you need to have 5 psi at the highest point. Float type air vents need at least 5 psi to operate properly. 5 psi at the top of the system it sort of an industry rule of thumb.

Top of system 15 feet, so 15 X .433= 6.49 + 5 psi + 11.4 psi fill pressure. Most fill valves are factory set at 12 psi, same with the hydronic expansion tanks. But it's always a good idea to check the pre-charge and actual fill pressure once the fill valve is turned on.

I would say my boiler is about level with the main floor of the house and right now i have all the plumbing in the basement. A sidearm on the water heater and an water to air heat exchanger in the furnace duct work. My boiler is an outside pressurized boiler. I have a line run to an uninsulated pole shed with a hanging modine heater. and i would say that the heater in there is higher than the top of the boiler by about 7 or 8 ft. So am i pretty good with leaving it at 12 psi? Thats what my auto fill valve is set for and i'm not sure if it will go as low as 5 psi. Kevin

Sure 12 is fine, really no harm in a bit higher pressure than the elevation calculation.

An old troubleshooter trick is to boost the fill pressure to get rid of air locks in problematic systems. Boost it to 25 psi or so, just to rid the air, then lower it to the correct fill pressure. Higher pressure squeezes the air bubbles smaller so the pump can push them from the high point traps.

An air vent, even a manual screw type is not a bad idea on the Modine. If ever you do have an air bubble it generally rises to the highest point.

I should have clarified that my expansion and fill are both at the top of the system which needs to be 5 psi I gave myself 6 psi just to be on the safe side.I know its not always convenient to plumb this way but in my case it was no problem,being able to do so allowed me to use a smaller expansion tank and when your dealing with large volumes of storage we can use all the help we can get.

An Extrol 30 tank has an acceptance volume of 2.5 gallons and a total capacity (tank volume) of 4.4. You would need 6 of them for your system using Hot Rods calculations, which are correct using the parameters given.

If I were in your shoes I would use something like Extrol's 90 size tank which has an acceptance volume of a little over 11 and give it a precharge of 10psi.