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Wrong Zone Valve & Pump?

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by Sawyer, Nov 21, 2011.

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  1. Sawyer

    Sawyer Minister of Fire

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    It always seemed that our forced air heat was slow to respond and the coil fan ran a lot whether on gas or wood. I thought I would measure all the pipe in the pre-existing system and assign equivalent lengths to the fittings and accessories to give me a total head.

    When I went to apply an equivalent length to the Erie zone valve to the coil I notice that the 1†pipe was necked down to ½†and then up to a ¾†NPT Erie zone valve with a 3.5 Cv rating. Head is 9’ at 7.5gpm. My overall head calculated to 18.85’ @ 7.5gpm. This being the case the installed Taco 007 is highly inadequate and incapable of delivering adequate flow to efficiently satisfy the coil.

    If my assumption is correct I should replace the current configuration at the 1†pipes and replace with a 1†NPT Erie valve which if I understand the information correctly would allow 8gpm @ 2.31’ head. This would lower my system head to about 12’ head at 8gpm which could be handled by a Grundfos 15-58 rather than going to a more expensive and expensive to operate pump. (I’m gambling this because my existing 007 system is incapable of delivering 8gpm but is still able to heat my house so with the new pump I should increase flow about 60%).

    Is my thinking and troubleshooting correct? I guess if this does not help I always have a spare 15-58 when I need one.

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  2. in hot water

    in hot water New Member

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    How many GPM are you trying to flow through the valve? A valve with a Cv of 7.5 would flow 7.5 gpm with a 1 psi drop.

    That is an inverted flare type of valve. The nuts on either end will swivel and allow you to remove the body. Erie may have an inverted flare valve with a higher Cv that will fit into that dimension? Lower cv valves are commonly used with air coils as they can shut off against higher pressure. A 3.5 may have a 30 psi shutoff. a 5 Cv 25 psi and a 7.5 Cv 20 psi. The higher the cv of a valve the larger the hole and the lower the pressure it can shut off against.

    Or remove the valve and inverted flare adapters and sweat in a 3/4 or 1" zone valve with a 5 or 7.5 Cv. The flare connections were to allow the valve to be removed and serviced, but usually the motor fails long before the body or flapper inside.

    Here is the pressure drop thru a 3.5 Cv valve flowing 10 gpm or about 100,000 btu/hr.

    hr

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  3. Sawyer

    Sawyer Minister of Fire

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    I am trying to get 8gpm, 80,000btu/hr. At 15psi system pressure this should shut off properly?

    If I replace the existing valve with the 3/4" NPT valve with a 7.5 Cv, would I have to calculate the 1/2" reducers separately for equivalent length. wouldn't the psi drop be quite high over those 1/2" reducers at 8gpm? Or am I misunderstanding this application?
  4. ewdudley

    ewdudley Minister of Fire

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    You shouldn't have to worry about system pressure affecting the valve's ability to shut off, it's the pressure difference across the valve. Study the valve spec and I expect you'll find that the valve will have no problem with six or eight psi of head from a small circ.
    The 1/2" reducers will add plenty of resistance, but it's not like a weakest-link situation. Maybe look up the Cv for a 1/2" ball valve to get some idea of what their contribution to total resistance might be.
  5. in hot water

    in hot water New Member

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    I don't think those reducers are costing you more than a couple feet of head. If you were to remove that zone valve and look inside it probably has about a 3/8" hole. It's a pretty short distance of reduction in those bell reducer flare adapters.

    8 gpm thru a 3.5 Cv valve is 5.22 psi drop. Not ideal but certainly workable. Look for a flow restriction somewhere else in the piping. The only way to know what actual flow you have would be to install some gauges. Either a flow setter or pressure gauges across the inlet and outlet of the pump. Measure the ∆P (pressure drop), across the pump, pull out the pump curve for that pump and see what the actual flow rate is.

    Depending on the type of coil and the tubing size the coil itself could be causing a large pressure drop. If you can find any data on the coil you could look up that info on the manufacturers site. I've seen folks try to use AC coils for hydronic applications, those 1/4 or 3/8 tubes will not flow enough to transfer much energy.

    hr
  6. nate379

    nate379 Guest

    Before you buy some expensive parts you could always remove the valve, zone valve and fittings and sweat in a "temp" 1" copper. See how that works out.
  7. Sawyer

    Sawyer Minister of Fire

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    Hr, you made me do homework, but thanks, I think I am also learning as usual from this forum ;-) . I calculated 8gpm thru a 3.5Cv valve is 5.49psi (Cryo-tek 100 – specific gravity 1.05) which times 2.31 = 12.7’ head. My coil is rated as at 3.5’ 9gpm.

    If a Nibco ½†brass ball valve (http://www.nibco.com/assets/FIG171_1.pdf pg4) has a Cv of 4.3 (3.6psi@8gpm = 8.3’) those ½†reducers should also be a problem at 8gpm?

    Not much I can do about the coil but a 1â€, 7.5-8 Cv valve would definitely appear to lower the head substantially and allow me to use a smaller pump with lower energy consumption.

    I am also assuming (I hate to use that word) that if I deliver more BTUs to the coil the fan will run less and still satisfy the heat demand.

    All of this definitely makes me comfortable changing out the 007 for a 15-58 which should do the job with the valve and connection opened up to 1â€.

    Nate, after calculating all the head created on the valve and fittings it seems a good solution to change the fittings and the valve body. It should certainly allow more gpm to the coil.
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