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An option to 3 way mix valves

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by Bob Rohr, Dec 19, 2012.

  1. Bob Rohr

    Bob Rohr Minister of Fire

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    I wonder that larger boilers, 60KW or more, might be better served with dual pumps. A couple small, off the shelf pumps like the Grundfos 15-58 could move around 15 gpm with a low pressure drop boiler and piping circuit.

    Once you put a 3 way thermostatic you add that pressure drop, and if it isn't a 100% bypass shutoff valve, you always bypass some flow.

    With dual pumps you could move 100% of the output and use a variable speed function for the return protection function. When the pump speed ramps down, so does the power consumption.

    With a 3 way thermostatic, the pump runs 100% speed and always works against the pressure drop of the valve so the pump output is lower. The higher the floe rate, the higher the pressure drop. It's a parasitic device, really.

    Attached Files:

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  2. Tennman

    Tennman Minister of Fire

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    Always enjoy your input Bob. Isn't that about what the Laddomat does with one pump?
  3. Bob Rohr

    Bob Rohr Minister of Fire

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    Yes it is the same function as a Laddomat. After reading all the complaints and issues with various mixing valves I thought another option would be helpful. Circ pumps are very reliable, easily sourced, inexpensive, and easy to troubleshoot.

    A variable speed control is ideal, but it could be operated with a simple setpoint or strap on aquastat. When the temperature at the boiler drops to 130° F the second pump shuts off. It would cycle that pump until everything reaches steady state. But that is what the onboard EKO pump control does anyways, just use that output to run the system pump. Now you have return protection and good flow provided by a couple $60.00 circ pumps, or less if you shop e-bay :)
  4. 700renegade

    700renegade Member

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    A very forgiving parasite however..... Keep in mind the power consumption decreases as you throttle a centrifugal pump. Counter-intuitive to most, a dead-headed cirulator draws very little current.

    When a centrifugal pump is throttled back, eventually you reach a condition where the liquid contents of the pump are simply spinning around in the volute. No fluid requires pressurizing since the fluid at the outer wall of the volute is already at shut-in head pressure maintained by centrifugal forces. In the absence of friction, conservation of rotational kinetic energy maintains everything spinning like a top with no need for external energy input, no net force on the impeller vanes, no torque, and hence no load on the motor. ( Of course we do have friction, so there is a minor load)

    Regardless, your sketch is certainly a viable option.
  5. heaterman

    heaterman Minister of Fire

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    In my humble experience, pumps have proven far more reliable that any type of mixing device be it motorized or thermostatically controlled.

    We just did a system almost exactly like that with the exception being instead of a storage tank filled with water, the storage medium was a concrete radiant floor. Works great. Keeps maximum load on the boiler without letting temps drop to the danger point. Nearly instant response in contrast to the lag seen with mix valves, especially the thermostatic type.
  6. BoilerMan

    BoilerMan Minister of Fire

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    This is interesting Bob, it's what I did on my slab injection setup for return protection. I posted this thought, but your diagram showes it much better than I could explain it. Basically a variable hydraulic coupleing with 0-100% modulation capability. It has worked well for the past two years.

    TS
  7. chuck172

    chuck172 Minister of Fire

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    Is the boiler loop pump controlled by the boiler temp? If the boiler loop drops below lets say 140*, the storage tank pump stops, the boiler pump continues till the min temp is reached?
    Can you give a more detailed theory of operation?
  8. ewdudley

    ewdudley Minister of Fire

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    Same idea here, plumbing adjusted to incorporate power-fail thermo-siphon. My boiler manufacturer specifies minimum return temperature of 150 degF. There are 160 degF mixing valves available but during design I didn't know if that would be too hot. With adjustable return temperature I thought the boiler loop with injection would guarantee that I could tune for correct operation, not to mention being less expensive.

    At start up boiler loop pump P0 runs according to 'launch temperature'. A 007 or 15-58 on high speed will flow upwards of 15 gpm on a boiler loop.

    PID controller maintains return temperature with pump P1according to return temperature setpoint. PID control uses a constant cycle time of 80 seconds and turns pump on for however long it takes to maintain the correct average return temperature. Longer cycle means less wear and tear on pump P1 relay, shorter cycle means smaller temperature swings around set point. On paper 80 second cycle could wear out P1 relay after a couple years of 120 days a year, six hours a day. I'm using a 15-58 on low speed for PI injector, which should flow something like 7 gpm. Variable speed control could be used, but with the high rate of flow of P0 and the lower rate of flow of P1 on-off control works correctly and efficiently.

    I run with a return temperature setpoint of 160 degF. With 110 degF storage return this means boiler return temperature runs a little above 160 degF for about 50 seconds while injection pump is off, then return temperature dips to 150 degF for 30 seconds while injection pump is on. What's nice is I can do a poor man's outdoor reset and get a higher or lower supply temperature by adjusting return temperature up and down with the return temperature controller, depending on how hot my radiators need to be.

    When supply temperature goes above 180 degF P0 is switched off and 15 gpm pump P2 is switched on to 'top off' storage without idling.

    On power failure thermo-siphon check valve opens.

    boiler_recirc.001.jpg

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