Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Why is my boiler occasionally overfilling?

Hi there!

Today my wife texted me at work with the distressing news that the boiler was not coming on and the house was getting cold. Of course, this news came in the midst of a huge project I had to get done before the weekend.

But when I got home, I found the problem was that the boiler had overfilled itself, as was evident by checking the liquid level. This happened once last year, too. The pressurestat was showing something like 15 oz/in.

I was able to easily drain several excess gallons from the tank, causing the pressurestat to return to zero and correcting the water level. The boiler is now humming away just fine and heating up the house.

What could be the issue here? My guess is that the water level got low, and the automatic water feeder clicked on and overdid it.

But before running out and replacing that unit, I thought I'd check with the experts here, who have always proved so incredibly helpful.

Here's the side of the boiler with the water feeder, etc.:

Any ideas?


  • Jamie Hall
    Jamie Hall Member Posts: 23,452
    It may be set either to feed too much at a cycle, or not to wait long enough for condensate to return. There are four dip switches inside. What are they set for?

    It could also be that it doesn't shut off properly when activated.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    EBEBRATT-Ed Member Posts: 15,633
    The problem could be in the feeder leaking by or in the low water cutoff that controls the feeder.

    My guess would be the feeder leaking by.

    It could also be a manual fill valve leaking by

    I guessing you do not have a tankless water heater in the boiler because it's gas or an indirect water heater.

    I would feed the boiler manually for a while and keep the feeder shut off, maybe turn it on when you are home.

    If you get any warm days when you don't need heat you could mark the water level in the glass with some tape and leave the feeder on and see if it starts to fill when idle

    The only other thing would be remove the low water cutoff probe and clean it and check the feeder for leaking by.

    Process of elimination