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DHW circulator speed

28W Member Posts: 141
Sorry to beat this drum again . . . but I'm wondering about the circulator speed setting on my DHW. It is on a modcon system with primary/secondary piping and a low loss header. On the boiler side, the Grundfos circulator is set to speed "2". My question, is, what speed should the circulator for the DHW be set at?

If I understand the universal hydronics formula correctly, increasing the circulator speed will give the system a greater capacity to transport BTUs to the DHW in any given time period. . . but how fast is "fast enough"? I'm assuming that trying to get the return water cool enough for condensation is not a goal with the DHW, so therefore there's nothing wrong with setting the circulator to "3" and just letting it rip. On the other hand, if a slower setting will work just as well (with a greater delta T), why not use that setting?


  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    what speed should the circulator for the DHW be set

    I am just a homeowner. I have a mod-con and and indirect attached to it.

    The mod-con is a W-M Ultra 3 and the indirect is one of their tank-within-a-tank Plus Line indirects (Triangle Tube, I think).

    They assume I am supplying 190F water to the indirect, that it is connected across the supply and return of the boiler loop; i.e., before the closely-spaced Ts. Their instruction manual for the Indirect has a table (Table 3, page 29) that shows what circulator to use, depending on what size boiler and what size indirect. It even tells you the gpm and pipe size to use. So for mine, they recommend a Taco 007  and 1-inch pipe. This is determined by two factors:

    1.) They need enough flow to get the heat into the indirect fast enough to deliver what they think is a reasonably fast recovery rate.

    2.) When the indirect runs (priority load), they turn off the boiler circulator so as not to send heat into the secondary loop (system loop), and all the boiler heat must go somewhere fast enough to protect the boiler. I.e., the indirect circulator must be at least the size of the boiler loop circulator so there is enough flow through the boiler.

    I have the 80K BTU/hr size boiler and the Plus 40 (36 gallon) indirect. It turns out my hot water demand is low (only one of me in the house), so I use a supply temperature of 175F instead of 190F. I get a temperature drop of more than 10F but less than 20F under these conditions. So just before the aquastat is satisfied, the water going into the indirect is close to 175F and the return water is about 160F. At that point, I assume there is no condensing. However it takes several minutes (between 5 and 10) for the water temperature to get that high because the indirect is soaking it up almost as fast as the boiler is producing it, so some condensing takes place as it warms up. Sometimes, even in the summer, the condensate pump runs as the boiler warms up supplying the indirect.

    Right now I run the indirect at about 125F until the money tree blooms and I can get a mixing valve installed. Then I propose to run it at 140F to reduce the legionnaire's disease bacteria count.
  • 28W
    28W Member Posts: 141

    I'm a homeowner also, and still learning. My Viessmann Vitodens 100 puts out (I think) 176 degree water to the DHW, and, like your system, it gives priority to the DHW (i.e., shuts down radiator circulator when DHW is running.

    I've also noticed a small amount of condensing during the initial start up of the DHW.

    I think you should run the DHW at 140 now. Since there are no young children in your house, the risk of anybody getting scalded is pretty much zero. I run mine at 140 and it does not have a mixing valve on it. Everybody knows to run the water and adjust the temp before getting into the shower. Since boosting the DHW temp to 140, I've noticed that the dishwasher works much better.
  • CMadatMe
    CMadatMe Member Posts: 3,086
    DHW Head and Flow Rate

    First what is the pipe size for the domestic? Gotta figure the DHW flow rate and the supply pipe size is critical to how many gpm's you can carry to the indirect. Then what is thenpressure drop across the indirect. Whose indirect did you use?
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • 28W
    28W Member Posts: 141
    It is 1" pipe

    1" pipe to the the indirect.  It is a Mega Stor MS40.  I don't know the pressure drop. . . .  Thanks.
  • CMadatMe
    CMadatMe Member Posts: 3,086

    8gpm @ 4' of head. Go to the curve for the pump and select the proper speed based on the chart. MS-40 accounts for 3' of head and that minial pipe isn't much...
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • 28W
    28W Member Posts: 141
    edited September 2012
    Found the curves

    OK I did my homework . . . It looks like for the Grundfos 15-58, at 4' of head, setting 1 will deliver 6 GPM.  Setting 2 delivers about 11 GPM, and setting 3 delivers about 14.5 GPM. 

    So based on this, setting 2 appears to be the winner.  Thanks for the excellent guidance!  Much appreciated.  I am slowly beginning to understand this stuff.
  • Paul48
    Paul48 Member Posts: 4,469
    Mixing Valve

    Let me say this loudly.....INSTALL A THERMOSTATIC MIXING VALVE!!!!! I read a story about a woman that began to have a seizure, just as she turned on the shower.She survived but is horribly disfigured for life. It is a gamble, not worth taking. Bypass the mixing valve for the dishwasher.
  • 28W
    28W Member Posts: 141
    By the way . . .

    Now you've got me curious.  What BTUH and delta-T do you use when calculating GPM for a DHW?  I imagine there must be an industry standard value for BTUH, based on typical DHW demands.
This discussion has been closed.