Buffer tank, DSH and a hot water recirculator pump

Discussion in 'General Discussions' started by Graham C, Dec 26, 2014.

  1. Graham C

    Graham C New Member

    Hello all,

    Quick question (i hope)

    I have had 2x 7 series (4 ton) units installed recently. Both units have the DSH plumbed to a single unpowered buffer tank, then the buffer tank feeds an A.O.Smith electric heat pump water heater unit.

    I have a recirculator loop and pump in the house for my hot water (which was there prior to the water furnace install). Previously this connected to the 20 year old oil hot water heater. Should the return feed for the hot water be plumbed into the input of the buffer tank? or the input or the A.O. Smith tank? my contractor has given me his view but i'm not sure agree. I would like to see what other poeple think on this situation. Let me know if you need more info about the setup.

    I'm assuming (correctly or incorrectly) that the return water is already "pre-heated" so it doesn't make sense to run this back into the buffer tank (which could be anywhere from cold water to 110F based on whether or not the W.F Series 7 machines are running).

    Thank you in advance for your advise.

  2. Bergy

    Bergy Member Industry Professional Forum Leader

    Recirc lines should come off the finish tank.

  3. Graham C

    Graham C New Member

    Excuse my ignorance.

    "Finish Tank" is which one?

    also -when you say "come off" do you mean they should be fed from or return to?

    Thank you kindly.

  4. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    A proper DSH set up would be an upstream (buffer) tank and a downstream (finish) tank. If one wishes to circulate hot water through the house it should exit and return to the finish tank only.
  5. Graham C

    Graham C New Member

    Hi there,

    I've attached a two page pdf with my crude drawings of how everything is currently plumbed, and how I think I want it to be plumbed. The contractor is against changing it but have agreed to change the layout to whatever I want. Can you take a look at the second page of the PDF and see if that is correct? and also if I am missing any check valves?

    also - I've noticed that if the circulator pump is unplugged I still get hot water in the return loop (even if no hot water has been used in the house for 12+ hours). So i'm assuming that this is due to the pumps on the DSH pushing the water - should this be happening? please let me know your thoughts

    Thank you again.


    Attached Files:

  6. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    Looks right.
  7. Graham C

    Graham C New Member

    Thanks - so no extra check valves?

    Do you think just re-plumbing to the "proposed" layout will stop the water furnace internal pumps from pumping hot water around my circulator loop?
  8. TurboTom

    TurboTom New Member

    Wouldn't it be better to have your hot water circulator pump return go to the electric tank? Otherwise your preheat tank will get hot water from the return and in short time be the same temp as the electric tank.

    OOPS looked at the wrong drawing that is what you are proposing. ;)
    Last edited: Dec 30, 2014
  9. birkie

    birkie Member

    Suppose the circulator were demand controlled with a timer or temperature sensor to stop the circulator once the hot water has fully populated the plumbing. In that case, wouldn't the opposite be true: you would want to plumb the return side to the buffer tank, as the retuning water temperature will almost always be lower than the finishing tank?
  10. Graham C

    Graham C New Member

    The pump is connected to an Aquastat already.

    I think it should still return to the finishing tank. The buffer tank could be cold water if the Geo units are not running. Why would I want to dump return hot water into cold water?

    Appreciate anymore discussion on this before I get them to re-plumb!
  11. ChrisJ

    ChrisJ Active Member Forum Leader

    "The pump is connected to an Aquastat already." So that means the pump stops when the hot water has reached the fixture/aquastat?

    Which should mean that the water returning to the tank is the room temp(or colder) water that was in the pipes.

    Wouldn't that water going into the Finishing tank cause the elements to come on?

  12. birkie

    birkie Member

    Right, so let's assume that the aquastat is set up to assure that the circulator does *not* run if the temperature at the most remote tap is sufficient. Let us also assume that the circulator only starts pumping in response to some external stimulus (like a button or motion sensor), unless prohibited from doing so by virtue of having sufficient temperature in the pipes already (hence the aquastat).

    With that control scheme, the water returned by the circulator is cool/ambient.

    Plumbed to the finishing tank, any cool/ambient water introduced by the circular must be actively heated by the finishing tank.

    Plumbed to the buffer tank, there is a nonzero probability that the cool water may stand a chance of being heated by the (cheap) desuperheater heat, rather than the expensive finishing tank. In the worst case scenario that the desuperheater is inactive, then we simply have cool/ambient water entering the finishing tank indirectly through the buffer rather than directly from the recirc loop.

    So I can imagine a scenario that, with the appropriate control scheme, plumbing to the buffer tank may offer a small theoretical advantage.
  13. Graham C

    Graham C New Member

    The aquastat is on the return leg of the hot water (about 3ft before the pump) and is set to 120F. The hot water from my tank is set to 125F. The aquastat will turn on whenever the return water drops below 120F. As my basement is probably colder than the rest of my house I'd assume the return water going back into my tank to be around 120F.

    Maybe my understanding is wrong? I wouldn't expect the water to get to room temperature if the aquastat is set to 120F. Before anything was changed (pre-Geo) the hot water system (oil) was set to 130F and the aquastat to 125F
  14. birkie

    birkie Member

    Ah, your understanding is correct. The issue is that your system uses almost the exact opposite control scheme than the one I described. So you need to plumb to the finishing tank, just as everybody has suggested.

    Your system's control logic guarantees that the hot water system is always full of hot water, and therefore will return hot water to the tank. It provides comfort and relative simplicity, but costs energy because the hot water tank is continually heating the hot water plumbing as heat is lost into the conditioned space.

    The system I described is almost the opposite. In it, the circulator does not run unless two conditions are met: there is a demand for hot water, and the hot water plumbing is cold. After a hot water usage event is over, the hot water plumbing is allowed to cool, and remain cold until hot water is needed next. A temperature control is used only to minimize circulator usage. (i.e. if there is hot water demand while the plumbing is already hot, circulator usage is unnecessary). I posit that plumbing the circulator return to the buffer tank can be beneficial with this rare (but very efficient) control scheme.
  15. Graham C

    Graham C New Member

    Perfect - thank you. This discussion is just what I needed to re-assure myself that the changes I am asking for are correct.

    I wonder if anyone with WF experience can comment about the fact that my pump has been unplugged for a week and even first thing in the morning (after 12 hours of no hot water running) my loop is still hot! the W.F must be pumping around, which doesn't seem right.
  16. dgbair

    dgbair Just a hobby Forum Leader

    Well if your circulator loop does not have any built it check value it's surly possible that you could be moving hot water through that loop via the pump in the WF.
  17. Graham C

    Graham C New Member

    The circulator pump has a check value inside it.

    So i'm assuming that the W.F pump is pushing hot water through the buffer tank, into my finishing tank, and then out into the loop.

  18. TurboTom

    TurboTom New Member

    Its probably gravity, hot water rises in your loop pushing the cooler water back to the tank through your pump
  19. Graham C

    Graham C New Member

    hmmm interesting, it'd have to rise all the way through the house and back to reach the return leg near the pump, is that possible?. I'm pretty sure this didn't happen prior to having the Geo units.
  20. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Put the recirc pump on a 5 minute demand switch or master bathroom motion sensor. - it is folly to run hot water recirc when no one needs hot water.

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