boiler pressure rises over time

spacoli

I have installed a new Slant/fin Bobcat boiler and the pressure rises (up to 20 psi) over a matter of weeks. When looking into this problem I'm finding air in the three 2nd floor radiators and having to bleed them. This drops the pressure down to normal (1 bar or 15 psi). I have had to do this three times since starting the new system.



I have an air eliminator installed (spirovent junior), not sure what the problem is. 



Thanks

EricAune

Is this a retro-fit?

Are you using a diaphragm expansion tank? Is there a make up water line, could that be allowing water to slowly enter the system?



Do you have any pictures?

spacoli

Yes

It is retro fitted into the old supply and return lines.



I do have a  diaphragm expansion tank and the make up water line, pressure reducer and back flow valve.



Is the make up water shut off valve to be left open or shut?

EricAune

Depends

If you have a low water cut-off device then leaving the valve off should be ok.  I'm not sure if the Slantfin has one built in or not. 



If you have been leaving the water feed on and the pressure continues to rise then there is something amiss with your pressure reducing valve.  It is likely to have debris (foreign object like dirt, rust or the like) preventing the rubber seal from holding back all flow when at the desired pressure setting.  I say this because the pressure is raising through time.



The other possibility is that the expansion tank is not sized properly.  If there is not enough room for expansion, as the system fluid heats and expands it has to go somewhere.

spacoli

No low water cut off

I didn't install low water cut off because I didn't have any radiation below the boiler. Maybe I might want to install one and turn the water feed off?



The pressure reduder is new and preset at 12 -15 psi, not sure what's up there?

The expansion tank was juist given to me by the plumbing supplier when I told them what is what for. So it's possible it may be wrong. I beleive it's a number 30?



Thanks

kcopp

The expansion tank.....

is my bet. Looks like a #30 tank. Do you have Cast iron rads? They have a lot more water content... and thermal mass. Upsize it to a #60 tank.

spacoli

Cast iron rads

Yes I do have cast iron rads.