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Flow/Check Valves on an Indirect

I am installing an indirect water heater as the only zone off of a steam boiler.  I've looked at a bunch of installation diagrams from various manufacturers, and most show one or more flow check valves to prevent gravity circulation.  I am a little confused about the necessity, and since they add significant head loss, I don't want to add them for no reason.  On the boiler supply leg, hot water exits the boiler below the water line, drops close to the floor to the circulator, and then gets pumped up to the boiler supply input on the indirect.  Since both the indirect and the boiler have an immediate downward leg, it would seem that both have effective heat traps, eliminating the need for a flow check (or even an internal flow check on the pump itself).  On the return side, the indirect exit and the boiler return are essentially at the same level.  Since the indirect is going to have some stratification, the water at the bottom of the tank near this exit is likely to be much cooler most of the time, so I wouldn't expect to see significant losses to a cooled off boiler.  If the boiler is hot, it's actually not a bad thing to get some  flow in the return leg to the indirect since I'd rather have the heat in the indirect as opposed to sitting in the idle boiler.  So I am not sure why I need the flow check on the return leg either.  I must be missing something here, but I can't figure out what.  Any help would be much appreciated.


  • lchmblchmb Member Posts: 2,992
    post this

    Your probably better off to post this under the strictly steam section.
  • trompertromper Posts: 4
    just a hot water boiler in this case

    There really isn't anything steam related about the question.  The fact that it's a steam boiler isn't relevant here because when it's heating the indirect, it is operating below steam temperatures.  It's really just functioning as a single zone hot water boiler in this case.
  • Mark EathertonMark Eatherton Member Posts: 5,843
    Heat seeks equalibrium...

    It WILL migrate from hotter to cooler, even if it has to flow downward to do so.

    If you don't have the traps installed, you heat up the tank, and the boiler shuts down, and the tank loses its heat back to the boiler, where there is most probably a chimney draft, which sucks out all the paid for BTUS, and cools down the tank, and the boiler starts back up, and the vicious cycles continues.

    If you are really concerned with parasitic operation, consider using a line voltage zone valve wired in parallel to the pump. Much less pressure drop, but just one more electro/mechanical device to go wrong.

    If you are using the condensate of the boiler,definitely use a 3 piece bronze or stainless steel circulator, or figure on replacing it often,,,

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • trompertromper Posts: 4
    Sort of...

        I am not arguing that the heat will seek an equilibrium over time, but there are a couple of different things happening here.  If I took an isolated hot water tank, the hot water would be at the top because it is lighter, and the cooler water at the bottom.  Conduction will very slowly cause the temperature to equalize, but there isn't any convective flow, the lighter hot water is already at the top where it wants to be, and the cooler at the bottom where it wants to be, so everyone is happy.  The same is true in our bigger system, we'll always have some conductive losses because the cooler boiler is physically connected to the warmer hot water tank through a nice thermally conductive fluid-filled copper pipe (and the flow check makes no difference for the conductive losses).  We really just want to block the convective flows where they can occur.

        In my system, since my boiler supply drops down to the pump, and goes back up to the indirect boiler input, I have an effective heat trap leg on both the boiler and indirect, so no convective flow is possible.  Hotter water can't exit the indirect (or the boiler) because it can't flow down the heat trap leg.  You can (and people do) use this same gravity trap on the domestic hot water exit from the tank to prevent convective flow losses into the rest of the plumbing.  So I am pretty sure that a flow check is not needed on the supply side.  Some indirects have the boiler input on the bottom, so you might need a flow check in that situation, since you couldn't easily have a gravity trap (at least not without lifting the water heater up).  On the return leg, I think I do need a flow-check because I don't have any heat trap legs, so I could get convective flow within the same pipe from the warmer water in my tank to the cooled water in the boiler.  This effect might be minimal since the water at the bottom of the indirect tank is cooler anyway, but I think it makes more sense just to be safe and deal with the head loss from the flow-check up front, rather than having to add it later and then possibly need a different pump to deal with the added head.  I think I am all set now.  Thanks for the suggestion about avoiding the head loss with an actuated flow-check, but I also like the idea of avoiding complexity wherever possible, so I think I just have to accept the head loss and call it a day.
  • lchmblchmb Member Posts: 2,992

    When it's only heating the indirect that is true. But when it's heating for steam it's a different animal altogether. My suggestions was due to the type of materials used on steam system's compared to a standard hydronic boiler. I dont claim to be a steam specialist, I just know where to go ask the questions...;)
  • trompertromper Posts: 4
    Definitely some steam specifics

    The indirect gets set up as a priority zone to keep it below steam temps when it is calling for hot water, but you are correct that there are some steam specifics that have to be accounted for in order to get proper operation (specifically to avoid pump cavitation).  Noel Murdough has a very good write-up on the basic setup
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