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Where to put the fill falve in near-boiler piping?

Venus Member Posts: 4
I apologize in advance for what is probably a very basic question.

Dan's books and almost all other piping diagrams I've seen always show the fill valve connected to a system via the pipe that goes from the air separator to the expansion tank, but I've never seen an explanation for why this is the convention.

For example, in a primary/secondary piping setup where the boiler is on a secondary loop, the air separator and expansion tank are of course on the primary and all zone circs pump away from that point.  Is there a particular reason that the fill should definitely be attached between the air separator and the expansion tank, or could it live on the boiler's loop instead?

Thanks in advance for the help!!



  • There is a very specific reason

    why the fill line enters the tapping of the air eliminator.  It is part of the purge assembly and must be attached to the system downstream of a hose bibb and ball valve in that order.

    If you have Dan's books, you will see that.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour
  • Venus
    Venus Member Posts: 4
    Think I have it, please confirm

    Ok, basically I think the issue is that you want the fill at the point of no pressure change in the system.  To put the fill anywhere else risks a fluctuating pressure, and you get unexpected results from your fill.

    Am I close?

    Thanks again,

  • The fill valve

    has nothing to do with the point-of-no-pressure-change, but has everything to do with where it connects to the supply.  It has to connect to the supply after the hose bibb and ball valve to properly purge the system.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour
  • "Pumping Away"

    If you have Dan's book, "Pumping Away", see page 48 which explains how and why you set up a purge assembly.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour
  • pipemd
    pipemd Member Posts: 3
    Heating basics

    Where the expansion tank is called "the point of no pressure change". The feed should be connected here and the expansion tank must be placed prior to circulators pumping out to the system. This will allow you to add the head pressure of the pump to the fill pressure. This will help to move air out of the system as higher pressure goes to lower pressure.
  • Venus
    Venus Member Posts: 4

    Thanks pipemd, that makes perfect sense!
  • JStar
    JStar Member Posts: 2,752

    The expansion tank defines the point of no pressure change. Piping your water feed to the expansion tank ensures that the boiler cold-fill pressure will never be affected by the circulator pump.

    The air separator can be anywhere in the system, but it works better on the hotter supply side.

    The fill valve position has no affect on purging the system. You can purge the lines from any point if you have an isolation valve and drain in the right spot. You could purge it from the basement or the attic, if set up properly.

    You should always pump away from the point of no pressure change. In a one pipe system, that point is the tank on the supply piping. In a P/S loop system, the common piping (the two closely spaced tees) represent the point of no pressure change to that particular secondary loop.

    If you want to eliminate all doubt about the water feed...pressurize the system and close the feed valve.
  • I like

    to install a bypass around my fill valves for high pressure purging.  It makes air elimination a snap.  And I put all three essential components for purging in the boiler room, close together so that I'm not running around the house.  And when I'm piping the system, I will even bring back a valved 1/2" line from a high point that I feel will trap air even with a power purge so that I can vent it without leaving the boiler room.

    I think we are all talking about the same thing.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour
  • JStar
    JStar Member Posts: 2,752

    The fill valve has very little to do with purging, other than being the source of water for the purge. You could just as easily run a garden hose to a hose bib, and purge the system from there. The reason for piping the feed to the expansion tank is to eliminate the possibility of the circulator pump over-pressurizing the boiler.

    If you install a purge drain, isolation valve, and water feed, in that order, you can purge from anywhere in the system. It is more convenient to have it all right in the near boiler piping.
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