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Three-way mixing issues

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I’ve got a gasification wood boiler install that isnt circulating the hot water well. I think it’s how I have my return supply mixing valve located. I’m not quite sure how to plumb it to fix things. I’ I’m pumping away from the heat, which I thought was best. Should I swap to pulling through valve and back to boiler?

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  • Ironman
    Ironman Member Posts: 7,397
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    You have to have the pump inlet located at the “MIX” outlet of the valve, pulling from the valve into the pump.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • hot_rod
    hot_rod Member Posts: 22,468
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    This is all a pressurized system, the storage tank is in series with the boiler?

    What mix valve do you have at the tank for return protection?

    The valve you show is for the radiant? How big is the radiant zone? That is probably a 3 Cv valve, so 5-6 gpm is about what you could expect with a typical small circ like a 15-58, 007, etc.
    Were there screens on the H&C ports of that valve? They tend to plug and reduce flow.

    For return protection you would want a valve with a 10Cv or more. It depends on the valve you have at the boiler. If it is similar to the radiant one, ideally you want to pump away from the mix M port.
    Send a pic of the boiler valve.

    I think the boiler would warm quicker with the protection valve before the tank? Heat delivery would be the same amount of time, but the boiler will come up faster.

    Having the storage piped separate, a 4 pipe method, would avoid always flowing through the boiler. When the tank is hot and the boiler is not firing, you end up running that hot water through the boiler and loosing some heat up the flue. The boiler becomes a cooling tower :)
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    NealFoley
  • NealFoley
    NealFoley Member Posts: 36
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    Bob, that is the mixing valve for return boiler protection… it has screens, so I’ll remove them. Pressurized system. In this application, the boiler is bigger than needed I think, so running thorough the boiler and losing heat isn’t too much of a worry. That heat loss helps keep the boiler room and space next to it warm. If I pump to the boiler, then I can put the pump just after the mixing valve, to the left of it in the photo…
  • hot_rod
    hot_rod Member Posts: 22,468
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    I think that valve is undersized by quite a bit. What model is that mix valve? Many of those 3/4 and 1" thermostatic valves are around 3 Cv. Meaning a 3 gpm flow rate at 1 psi pressure drop.

    If it is this valve attached below with a 1.37, flowing 7 gpm, a whooping 26' of head (pressure drop) . Two Cv calcs attached, one for a 3 Cv valve, one for a 1.37.

    First it would be good to know how many gpm you are trying to get into the building, bot radiant and high temperature? Then size the valve to that flow rate.

    Bottom line the valve is your bottleneck, the piping if it is 3/4 to some extent. Depending on how much flow, BTU, heat you are trying to move from the boiler to the system?


    Also you really do need to pump away from the mix port. It needs to draw both H and C through the valve to mix properly.

    I'm not sure how deep you want to get into a repipe. I'd get a higher Cv valve, ideally a protection type mixer those are 10- 14 Cv.
    And move it to the boiler, with the circ pulling out of the MIX port.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    NealFoley
  • NealFoley
    NealFoley Member Posts: 36
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    hot_rod said:

    I think that valve is undersized by quite a bit. What model is that mix valve? Many of those 3/4 and 1" thermostatic valves are around 3 Cv. Meaning a 3 gpm flow rate at 1 psi pressure drop.

    If it is this valve attached below with a 1.37, flowing 7 gpm, a whooping 26' of head (pressure drop) . Two Cv calcs attached, one for a 3 Cv valve, one for a 1.37.

    First it would be good to know how many gpm you are trying to get into the building, bot radiant and high temperature? Then size the valve to that flow rate.

    Bottom line the valve is your bottleneck, the piping if it is 3/4 to some extent. Depending on how much flow, BTU, heat you are trying to move from the boiler to the system?


    Also you really do need to pump away from the mix port. It needs to draw both H and C through the valve to mix properly.

    I'm not sure how deep you want to get into a repipe. I'd get a higher Cv valve, ideally a protection type mixer those are 10- 14 Cv.
    And move it to the boiler, with the circ pulling out of the MIX port.

    That’sa taco 5000-3
    It seems like it’s the right valve. I had to fire up the system tonight after making some changes to the radiant slab circuit and bleeding more air out. Going to be cold tonight. Tomorrow I think I can move that pump to the return side and pull through the valve.
  • hot_rod
    hot_rod Member Posts: 22,468
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    drives me crazy when a valve like that indicates a 20 gpm max. flow rate


    Here is a better "look" at the valve from the flow chart included on that sheet. Looks like 115' head at 20 gpm. A 3 Cv valve would be 44 psi drop, 102' head at the indicated 20 gpm.

    Flow rate through that valve at 1 psi drop is a bit tough to see on their flow chart, I suspect 3 Cv or less. The bottom left of the flow curve chart.

    Call up the supplier or manufacturer and ask them to spec a circulator to move 20 gpm through the mix valve as their sheet indicates.

    If you even could move 20 gpm, that would be some high flow velocity in 3/4 pipe

    Now I doubt you need to move 20 gpm, but possible 6, maybe 8 gpm? 8 gpm would be 16' head. The boiler size and load of the building would determine what you are actually needing to circulate.

    Move the pump, throw away any strainers at the valve and you should see some improvement.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    NealFoley