“Could I Have Flooded my Basement by Bleeding my Radiators?”


“Dear PoPville,

I bled my radiators Friday night. Then I was woken up at 6am Saturday morning by my downstairs tenant that the basement had flooded. We shut off the boiler and the water flow stopped. It was coming from the pipe in the attached picture. Could it be that I tightened the radiators too much? Caused pressure on the system? Any advice would be great heat is off until I figure it out!”

6 Comment

  • Per the last HVAC guy that came to look at my furnace/boiler – if water is coming out of that pipe that means the boiler reservoir has overflowed and there is too much water in the system. You should be bleeding your radiators each winter…but it looks like either you’ve introduced too much water or there is too much pressure.

  • gotryit

    At the risk of completely misdiagnosing from afar, look into your expansion tank. There should be a small trash can sized tank near your boiler.
    By bleeding, you may have taken out all (or most) of the air in your system (normally a good thing). But then, when the water heats up, it expands and has to go somewhere. The expansion tank has calibrated volume to allow the water to expand into it without overpressurizing. If your tank is busted or you don’t have one, then the water would just expand until it tripped the overpressure relief and discharges out of that pipe (which prevents your boiler from exploding or bursting a pipe).

    Not sure how accurate that is from looking at just one picture, so take it with a grain of salt.

    • ah

      That is my guess as well. I would hazard that the expansion tank for one reason or another needs replacement. If you know what you’re doing you may be able to fix it by pumping air into it, if the bladder is not broken. Otherwise replace – perhaps $200 with labor.

  • That is the pressure/temp safety valve. It opens when either the pressure or temperature of the boiler gets too high. Several things could cause it to open, but you should have a pro check it out asap.

    You did nothing wrong in bleeding the air from your radiators. The relief valve may just coincidentally have gone bad (they do this) or you may have a problem with your make-up water pressure reducing valve. There are other possibilities but those two are the most common.

  • That’s a blow off valve for when the PSI is too high.
    Bleeding radiators is a two part deal. You let the air out and add water to the system (it is a mostly closed system) most people have an autofill valve which automatically adds water to the system as the air is let out.
    It sounds like your autofill valve may be shot (calcified) and could not close back up once it opened to add more water after you let the air out. As other commenters have mentioned you might want to look at the expansion tank but for it to flow indefinitely there would need to be more water being added, which is why I suspect your fill valve (it kind of looks like a brass bell). Some can be manually closed by loosening (not tightening that adds water) a long machine screw that comes out of the top. Given your question you should just rest assured you didn’t do anything wrong and call a pro, no offense but it doesn’t sound like DIY HVAC is your forte.

  • Bleeding your radiators should not cause the system to overflow. I agree that it may be your water intake valve. I also think that as far as hot water systems go, unless you are very well versed in how they function and are trained in their repair – get a technician in to do the work. This kind of repair often involves sweating copper pipe, calibrating water pressure and draining and filling systems…lots of fun.

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