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Should the water supply flowing into a steam boiler be cold water or hot water?

FHillsFHills Member Posts: 19
Should the water supply flowing into a steam boiler be cold water or hot water (from the hot water heater) or does it matter?  

When I made the oil-to-gas switch, the plumber also installed an A.O. Smith natural gas hot water heater (the upright storage tank variety with anode rods inside).  He had hot water going into the boiler.  After 15 years, that hotwater heater began to leak, and I had to shut off its cold water input.  This deprived the boiler of hot water so I had to (a) either shut off the boiler and live without heat or (b) allow cold water to flow through the leaking hot water heater so the boiler had water input and mop up the leak on the floor.  

To get around this problem, the plumber who replaced the hot water heater (with another AO Smith) redid the piping so that the boiler is fed by cold water.  I wanted him to give me the option of feeding hot water or cold water into the boiler.  He said there's no good reason to feed hot water into the boiler, because the amounts of water added to the boiler is small and would make little difference from a heating efficiency standpoint.   In We Got Steam Heat, Dan writes that tap water contains oxygen and minerals that are corrosive, so it is better to fix leaks and limit the amount of fresh tap water flowing into the steam heating system.  By heating tap water, doesn't the hot water heater remove some of the oxygen from tap water and doesn't its anode rods absorb some of the corrosive elements in the tap water.  In this sense, does it make sense to feed hot water from the hot water heater into the boiler over cold water from the tap?

Thank you!


  • FredFred Member Posts: 5,257
    Cold Water is Fine

    Most boilers are fed cold water. That water should be introduced somewhere along a wet return, if possible so that it mixes with return water and is warmed before it enters the boiler. That reduces the risk of any thermal shock. Feeding from the hot water heater is not doing any harm but certainly is not offering much, if any, advantage given that water really needs to reach its boiling point to effectively burn off excess oxygen. A water heater simply doesn't heat water that hot.
  • FHillsFHills Member Posts: 19
    Thank you.

    I see.  The cold water input does feed into the return, so the set up is fine. 
  • Jamie HallJamie Hall Member Posts: 6,693

    it's not as though you were feeding a whole lot of water -- or at least I hope you're not.  If you are really fanatical about blowing down a float type LWCO, you might -- might -- use as much as half a gallon a week... for that.  And normal operation should use much less (my decent size system has finally managed to get up to 7 gallons; took almost four years to use that much).

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • ChrisJChrisJ Member Posts: 8,627

    The only time I've heard of hot water being an issue is if you have an autofeeder.  I believe the seals in autofeeders aren't designed for hot water and may fail.
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment

    Steam system pictures
    Central air project pictures
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