How to Add Water to a Residential Boiler
Adding water to a residential boiler is an easy task. The pressure gauge at the boiler is how you can tell how much water is in the system. For every 1 psi the water level goes up about 2.31 feet. So if you read 5 psi on your gauge, your water level is 11.55 feet above your gauge. You want your water level to exceed the height of your highest radiator. 15 psi is a good pressure for many residential systems.
How to Add Water
Anytime you will be adding water to a boiler system there are a few things you need to check before you start. Never add water to a hot system. Adding water to a hot dry boiler is like making a bomb. Always wait for boiler to cool down before adding water. Also check for any water leaks that may be coming from the boiler, radiators, or piping. Adding water to a system with a leak will only make things worse.
Find the feed water shutoff valves. Usually they are located near the boiler. The most common setup is to have two shut off valves around a backflow preventer. The backflow preventer prevents any boiler water from getting back into domestic water. If you are having trouble finding the feed water valve look back from the cold water pipe going into the water heater.
The feed water pipe will come off of the cold water line and usually connect to the pipe going to the expansion tank. Slowly open the feed water shutoff valves and watch the pressure gauge. 15 psi is a good pressure to set your boiler to. Once you're at 15 psi you should go around and bleed your radiators. This will cause your pressure to drop again. You may have to repeat adding water and bleeding radiators several times. Remember when you're adding water that the boiler relief valve will pop open at 30 psi.
Why you Need to Add Water
It basically comes down to two things that cause your boiler pressure to drop, either a leak or air. If you see your pressure gauge at zero or considerably low, it's not a bad idea to do a walk around of the boiler and radiators and check for leaks. Radiator valves are common places to find leaks. If you see no evidence of a water leak then most likely you had air in the system. Many boiler systems have an automatic air bleeder at some point in the system. As this device gets rid of the air in the water the pressure will slowly drop.