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Variable speed?

bill_105 Member Posts: 429
So I finally get around to using a Wilo stratos on my own home. Then my neighbor asked me if I was using one on on the indiract. My answer was no. Then I got thinking, (always dangerous) now would a delta T circ as in Taco VDT have any help with an indirect?


  • Paul Pollets
    Paul Pollets Member Posts: 3,656
    Pumping the indirect

    It would be a waste of $$  to use the variable speed pump for indirect DHW recovery. Why would you want the pump to work at anything less than maximum flow?  The tank coil should see maximum flow while recovering to shorten the recovery time. I typically use a 15-58 on high speed for up to an 80g tank. Too much flow (overpumping) can cause errosion of the fittings or even coil damage.
  • Brad White
    Brad White Member Posts: 2,398
    I second Paul on this

    Get the job done and get back to heating the house, or not. DHW production is a short-term task, not one for much finesse. The time it takes compared to any savings or control -there is no control really, just firing to your tank setpoint- has no payback.

    Save the VS circulators for heating. Many more hours and hours are where the savings are.

    My $0.02

    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • Steve Ebels_3
    Steve Ebels_3 Member Posts: 1,291
    Me three

    Not a worthwhile application for a VS circ. When it's making domestic water you want max flow period.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    Me four....

    Let's get 'er done! Fire that mother up, and put the petal to the metal! Take the fire out of the combustion chamber, and stick it in the tank, PRONTO. There are people standing around waiting for a hot shower ;-)

    Don't dally around. Get it done so you can go back to doing a partial space heating load where the VS pump makes sense.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Me five (sort of)

    The nice thing with this app using a Stratos and the optional ext off interface module you have an easy priority DHW demand control (direct 100% of the heat from the boiler to the DHW system by turning off the space heating pump).

    I agree constant speed for indirect for residential but what do you think of ECM pumps for commercial indirect apps?  We are seeing installations where the return temp from the indirect (obviously from the boiler loop of the indirect HX) is modulated to protect non-condensing and helping condensing boilers condense (using variable flow ECM circs).  Or effecting the efficiency of the heat transfer of the indirect.  Something to think about.

    One more thing to think about, how about ECM pumps on commercial DHW recirc apps?
  • Brad White
    Brad White Member Posts: 2,398
    edited September 2010
    Steve, some thoughts on those Apps

    I can see the efficacy of VS for domestic to avoid or lessen boiler condensation but only during a cold start-up and only when you have a return water sensor to govern that set-point, of course.  The principle would apply to commercial or residential the same, as I see it. Once the tank is warm, you may get a first slug of cool water but warmed by passing through the indirect on the way back to the boiler. After this it climbs to near equilibrium and holds it there until the tank temperature is satisfied.

    As for efficiency of the indirect, it is all about temperature. If you have say a 10 gpm flow rate to an indirect and take a 30 degree drop, call that 150 MBH load. If the EWT is 160 and LWT is 130, you may be a little on the edge of condensing if a cast iron boiler.  But your average water temperature (AWT) is 145F and this drives the rush to heat the tank. This is a fairly conventional situation. Varying the flow downward could only hurt the situation.

    Were you to add 50% to your flow rate, say 15 gpm,  you would have a 20 degree F. temperature drop with an AWT of 150F [(160+140)/2], only five degrees higher and with a pressure drop increase double your initial rate and a Watt draw penalty  of 337%, all things being equal. 

    Now, if you STARTED with the 15 gpm situation and dropped to 10 gpm, see the savings, but acknowledge that you would be closer to condensing OR you would have to raise your boiler high limit temperature, which many do.

    Re: Recirculation: Commercial or not, I see that as a simple time/temperature function. A timer says to have DWH available at a the most remote fixture at certain times of the day. The thermometer says that the line is satisfied. There is no P, I, D or PID logic to this because once the line is warm, either by the circulator or by the users drawing DWH to the fixtures, the circulator is off.

    When coupled with the hours this actually runs and that once satisfied the pump shuts off anyway, it need not be a variable speed event. Open to suggestions, but I just do not see it as effective or a good use of excellent technology.

    Pressure boosting, DP or DT control, yes, absolutely!

    My $0.02 anyway-

    Good and thought-provoking Steve. Maybe there is something here, just that I do not see it.

    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    Mini bronze circs

    Miniature DC powered bronze circs at the furthest points of use, variable speed. Now THERE is an idea whose time has come.

    Those little 1 watt pumps could be controlled by motion detectors strategically located through out the dwelling. If motion is detected, pump is enabled, and input voltage to the pump would be a function of differential temperature between the source of supply, and point of use. Big difference = maximum allowable voltage. Little difference = little voltage. When no motion is detected, voltage drops to zero.

    All the little pumps would need to have their own individual check valves. There would be one common return headed back to the source of supply.

    Thinking outside the normal pipe here. :-)

    This could also have application in larger commercial buildings, like high rise apartments, hotels, hospitals etc.

    Remember me in your will Steve ;-)

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • hot_rod
    hot_rod Member Posts: 21,793
    ECM circ on DB

    A drainback solar is another good application to lower energy consumption. On my first DB I has a standard and an ECM together, drop off the standard pump once the siphon is started, and run on about 17W with just the high efficiency circ..

    I raised my DB tank way up in the shop now a single Alpha has enough lift to fill and run the system by itself.

    Mike our engineer has a ECM circ on his DHW recirc, sipping electricity to move tiny flows.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
This discussion has been closed.