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Steam Boiler preheating domestic how water heater?

We bought our home in January.  It appears that someone once tried to use the steam boiler to preheat the water going to the hot water heater.  The first photo shows the pipe from the left (the upper pipe)that heats up when the boiler comes on and then heats all of the water going into the cold water intake for the hot water heater (that is the lower pipe coming towards the camera, I was standing next to Hot Water heater).  The whole system is circular as the second photo you can see shows the hot water exit pipe (from the left) connects with the boiler (pipe from the right) at a T that goes up and becomes the pipe on the left of the first photo.  When the hot water only is running through the pipe the upper pipe does NOT heat up and does not heat the cold water intake but when the boiler comes on it does heat the pipe and heat the cold water intake water.

But some mornings we have no hot water in the tank. My theory is that as the boiler runs all night it heats the cold water intake water and the hot water heater does not turn on because it thinks it is full of hot water.  Does that make sense?

Should I turn the valve off that connects the boiler to the cold water intake pipe?

This seems like a bad hack of an idea.  Or is there a valid reason to have connected this way?


  • MDNLansing
    MDNLansing Member Posts: 297
    Not a good idea

    Inside the water heater the supply line drops all the way to the bottom of the tank. The output side starts higher up the tank. Because of this, you can get convection that sends hot water in the bottom of the tank and passes to the top of the tank. This can trick the thermostat into thinking the water is hot and the burner never fires.

    My sisters house had this same thing and the same issue. We tried closing the valves but they were stuck so I ended up just removing it all. Shes never had an issue since. I think your diagnosis is correct. If nothing else, removing it will get rid of the corrosive mixed metal environment and make you pipes last longer. There's really no need to preheat anyway.
  • JStar
    JStar Member Posts: 2,752

    Just be careful not to fully isolate the tankless coil in the boiler. When the water in the coil heats up, it needs a place to expand, or else....KABOOM!
  • JohnNY
    JohnNY Member Posts: 2,624
    We commonly see this in NYC.

    It's common to have the option of running water through the coil to preheat the cold and passively reclaim some of the standby losses inherent in any atmospheric boiler by sending some energy into your domestic water heating system rather then let it disappear up the chimney.

    Diverting the water through the coils in summer when the boiler isn't firing doesn't render any benefit, but during the heating season, we often restore the valves to the preheating condition for our service and maintenance clients.

    Your problem with no hot water in the morning is not caused by this connection to the boiler since the water in the tank is only warmed by the coils while the hot water is actively being used and chances are pretty good the water coming through your coils is far hotter then what your water heater can produce anyway.
    Contact John "JohnNY" Cataneo, Master Plumber
    in New York
    in New Jersey
    for Consulting Work
    or take his class.
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