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Viessmann: DHW pump into Boiler not into the indirect?

tivotivo Member Posts: 16
I have recently installed a Viessmann Vitodens 200 with an indirect.  I already piped the DHW pump to pump into the indirect and the pair are working just fine.  After re-reading the installation manual, I just noticed on EVERY layout diagram, it's labeled "IMPORTANT: DHW circulating pump must pump into Vitodens 200-W WB2B boiler (as illustrated)".

I have always piped to pump into the "indirect" but Viessmann wants to pump into the "boiler" instead and it's a "must"?  Does anyone know why?


  • TomTom Member Posts: 458
    shot in the dark

    I haven't installed a Viessman yet but my guess is flow rate, if you are pumping into an indirect than you are going to be cutting down flow and head. The heat exchanger requires a certain amount of both when firing, especially on DHW call. Viessman is protecting the Hx by doing asking you to pipe this way, many boiler manufacturers ask us to do.

    Just a guess though...
    Montpelier Vt
  • GordanGordan Member Posts: 891
    edited February 2013
    Not flow

    It doesn't matter where on a closed circuit a circulator is located, since the pressure drop is additive it will come out to the same pressure drop at the same flow.

    Putting the circulator so that it pumps into the heat exchanger will make sure that the heat exchanger has the greatest absolute pressure, which raises the boiling point of water inside the heat exchanger. This is not so much of an issue with low pressure drop heat exchangers.
  • Paul PolletsPaul Pollets Member Posts: 3,022
    Vitodens 200

    The reason is flow rate. If you did not install the low loss header, follow the piping instructions exactly, or you will have problems and program faults.
  • GordanGordan Member Posts: 891
    Not trying to be contrarian, but...

    All other things held equal, how could the location of a circulator possibly affect flow?

    And how would the low loss header enter the picture, since the DHW is branched out before the LLH?

    It's entirely possible that my understanding is incomplete, in which case this will be a very welcome lesson. Thanks in advance!
  • Paul PolletsPaul Pollets Member Posts: 3,022

    The boiler's small HX requires a minimum flowrate by pumping towards the HX rather than pumping towards the indirect tank coil and adding the tank's HX head loss as well as the boiler's HX. The low loss header is required when the system flowrate design exceeds 6,5gpm.(or is less than 1.5gpm)

    The DHW tank can be piped in either before or after the LLH, with the code changed in the system setup for piping the tank before the LLH.
  • tivotivo Member Posts: 16
    edited February 2013
    Not so convinced

    I'm also not so convinced it's the flow. If the flow is the culprit, by properly sizing the pump, pumping into the indirect should still work, if maintaining the flow is the key. But it's viessmann saying a "must" to pump into the boiler that's bothering me.

    Also, I already piped it to pump into the indirect and I don't have any problem or fault code. That's why I'd like to find out the reason behind viessmann's thinking.
  • ced48ced48 Member Posts: 383
    Seems To Me

    that as long as the boiler's minimum flow is being maintained, it should not matter. For example, I am planing to do the same thing with a small Lochinvar, the boiler requires 3 gpm, the indirect 8 gpm, seems about impossible not to have enough boiler flow-
  • GordanGordan Member Posts: 891
    If you've got only one DHW circulator...

    ...then it has to overcome the head of both the indirect and the boiler's heat exchanger by definition. It doesn't matter where along the path you put it, as the flow has to be the same anywhere in the circuit. And the same flow through the same set of components means the same head loss.
  • ChrisChris Member Posts: 3,056
    edited February 2013
    Point of No Pressure Change

    If you look at the piping diagram the exp tank and feed are on the return. On a DHW call the boiler pump does not run. The DHW pump will be then pumping away from the point of no pressure change. If you are not providing priority then the DHW tank should be piped down stream of the LLH and then the boiler coded as such.

    Head is head and you will note that the recommended boiler pump is sized for a 40 degree rise. If your using a Viessmann EVI tank pressure drop across the coil is pretty much nothing. Not using a Viessmann tank, make sure you account for the pressure drop across the indirect as well as the boiler when your selecting the DHW circ selection. Which 200 are you installing?
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • tivotivo Member Posts: 16
    Not PONPC

    I wish it was that easy.

    In layout 1 on page 29 (, there is no LLH, the boiler pump is on the supply and Viessmann still emphasizes "IMPORTANT: DHW circulating pump must pump into the Vitodens 200-W boiler (as illustrated)".

    The boiler installed is VItodens 200 WB2B 35 (125k) and the indirect is EVI-300V, I believe it was 89 gals.
  • TomTom Member Posts: 458
    According to the manual

    "The following pumps have been selected based on boiler heat exchanger head loss and boiler piping to a low-loss header. Before using the following pumps for a DHW tank application, find out the proper pressure drop through the tank, the required temperature difference through the coil and system piping head loss of the domestic hot water"

    That's from the tech book from viessman. Always putting it on the return may help ensure that those who didn't size the pump properly for the indirect don't hurt the HX.
    Montpelier Vt
  • Paul PolletsPaul Pollets Member Posts: 3,022

    Is the heating circuit for radiators or high temp? Is the heating circuit flow rate greater than 5.7gpm? If so, you need the LLH. Since the Vitodens 200-35 has a 126K input, it would be presumed your flowrate would be higher than 6gpm. The tanks are either 53g, 79g or 120g. If your high temp circuit has less than a 5.7gpm design, than the LLH is not needed.
  • ChrisChris Member Posts: 3,056
    I'm Very Familar

    With all the diagrams. Reason one is going to be PONPC and reason two if I'm a betting man and know them all to well is going to be because they want to make sure your making the flow switch which is on the supply and not tricking it. I'll get the answer out of them and post it.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • ChrisChris Member Posts: 3,056
    Here is the anwser

    We field this one all the time. Inside the LLH you want to always pressurize the Hx to prevent O2 from coming out of solution and flashing to steam. The boiler will do that in 2 seconds with an air slug so pressurizing the return inside the LLH prevents this situation. Goes against pumping away but it works!
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • Steve WhitbeckSteve Whitbeck Member Posts: 669

    You allways pump towards the point of most resistance.

    And it is not a true closed circuit. There are two loops tied into the boiler. And the indirect pump pumping away from the boiler is also pressurizing the return from the boiler loop going to the system piping.
  • tivotivo Member Posts: 16

    Let me try to summarize all the comments and correct me if I'm wrong, the reason for Viessmann's "DHW pump must pump into boiler" comment is:

    Higher Pressure at HX -> Higher Boiling Point or Less Chance of Dissolved Air Coming out of Solution -> Less Chance of Steaming

    I'm still not convinced the PONPC is the reason simply because whether pumping into the indirect or away from the indirect (i.e. into the boiler), I'm not changing whether it's pumping into or away from the PONPC, the direction of the DHW pump in either case is still the same and the PONPC is not in the picture, so to speak.

    In any case, if less chance of steaming is the reason, then whenever we pipe an indirect, the DHW pump should always pump into the boiler or simply away from the indirect.

    I was flipping my notes from a recent Taco training, all the diagrams show the DHW pumps into the indirect not the boiler.  I guess they are not using a Viessmann boiler.  :)

    But I'm still surprised that other boiler manufacturers are not as concerned as Viessmann to make the same warning on the piping diagrams.

    Anyway, thank you all very much for your comments!!
  • MWMW Member Posts: 25
    Flow Control

    What about flow control? Is it possible to inadvertently pull from the space heating loop when pumping toward the indirect. If so then it is possible to introduce high temp boiler flow into the low temp space heating loop. I think Viessman and most boiler manufacturers recommend this piping arrangement for two reasons.

    1. Proper placement of check valves and flow direction guarantee adequate flow through boiler HX during both DHW and space heating calls.

    2.Above mentioned practices prevent high temp migration into low temp circuits and vice-versa.

  • TomTom Member Posts: 458

    If you google your question it shows other threads from the wall and a few of them show a series of answers. Chris said it also may be for the flow switch that is internal with these Viessman, I didn't realize they had flow switches but that certainly can be a factor. What you stated before is true as well the HX protection and moving water across it at the correct flow.

    Thanks for interesting post.
    Montpelier Vt
  • GordanGordan Member Posts: 891

    Where's my gold star? :-)
  • tivotivo Member Posts: 16

    I googled it everywhere maybe I wasn't using the right keywords. Could you tell me what keywords you used in your search? Also, regarding the flow switch, if the pump is properly sized, it will make the flow switch regardless the location of the pump. So, back to the same question, why is it a "must" to pump into the boiler?

    I actually called viessmann and the tech actually didn't know the answer! I'll see if he'll follow up with me.

    To add to the confusion, I already piped the dhw pump to pump into the indirect and it's working fine for months now. I'm not proud of my "mistake" but I just want to get to the bottom of it, why is it a must to pump into the boiler when by properly sizing the pump, it is a non-issue?

    To me, keeping the pressure at the highest at the HX seems to make the most sense.
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