Hot Water Boiler System Over Pressurizing

Contractor_in_distress

I am having a problem with a new boiler system installation where the heating hot water loop is over pressurizing and causing the relief valves on the boilers to open.

Some history: This is a public building that has had public steam for years entering the building and into a heat exchanger to make hot water.  We added (3) Weil McClain hot water boilers and they are working well.  However, sometimes, the system builds up too much pressure and the relief valves open.  They building still uses the public steam sometimes and that doesn't have the same issue.  We replaced the expansion tank and have it pressurized properly.. 

It appears that the overpressurization happens most during the shoulder months when the boilers are locked out during the day on OA temp but then come on cold at night and bring the "cold" temperature up which causing the greatest amount of pressure increase.



Any ideas on why this could be happening or how to address it would be appreciated.

Ironman

Expansion Tank

From what you're describing, the expansion tank is too small. Call you're rep or tank manufacturer and they can properly size it given the right info.



Also, are you pumping away from the tank or towards it?

MikeJ

Tanks

May check to make sure the expansion tank is size correctly, had a similar problem on new 5 story building. Tank was under size.

Contractor_in_distress

Expansion Tank

The engiener re-sized the expansion tank for the whole existing building (something we don't have access to the records). 

One issue we believe it could be is that the pumps discharge into the boiler, then down to the building sytems (boilers, pumps and ET are in penthouse), and then back into expansion tank and suction side of pumps.

Ironman

Penthouse

That's actually the ideal place as it requires less static fill pressure than the reverse.



Is the system piped primary/secondary or low loss header? If so, is the expansion tank connected to the primary loop?



A diagram or pics would be helpful.



Still, it looks like the tank is under-sized based on your info. Everyone makes mistakes. It wouldn't be the first time that an engineer sized something wrong if that's the case. Going to the tank manufacturer is the most reliable way. It's their specialty. And without knowing the water volume in the entire system, there's no way anyone can know how to size the tank.