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Low-Loss Header: am I understanding this correctly?

28W28W Member Posts: 141
The installation instructions for my Viessmann LLH (pg. 49 of main install instructions) appear to indicate that the heating circuit flow rate must be greater than the boiler circuit flow rate (Vprimary < Vsecondary). Am I interpreting this accurately?


  • Jean-David BeyerJean-David Beyer Member Posts: 2,565
    am I understanding this correctly?

    Note: I am a homeowner, not a heating professional.

    I looked at their description and they do say that.

    I do not understand why they say such a thing. What I looked at was the manual for the LLH itself, where they would not know what boiler or what indirect was to be used. Perhaps they assumed one of their own boilers and their own indirect.

    Since the purpose of closely-spaced Ts and of Low Loss headers is exactly to decouple the primary circuit from the secondary one, it should make no difference what the relative flows are from the point of view of one flow affecting the other.

    So why do they want one flow greater than the other? Especially in the case of the secondary being split up over many zones. Imagine that the system is designed to make the primary and secondary flow equal as it passes the Ts or the separator. Then as the zones in the secondary circuit are turned off, eventually you get that only one secondary zone is running. And by Murphy's law, it is the smallest zone in the system. In that case, the flow in the secondary would be much less than the flow in the primary. And that should be just fine if the system is zoned with circulators. If not, a pressure differential bypass valve would be needed to bypass the excess flow when most of the zone valves are shut off.

    The boiler loop, would never know that the flow was reduced. It might realize that the secondary circuit was not sinking as much heat as was expected, but presumably the boiler would reduce (modulate) its output so it would not overheat.
  • Paul48Paul48 Member Posts: 4,492

    I think your thinking regarding flow and zone sizes is backwards. You size for the longest zone (highest head). As you reduce head, you increase flow.
  • Kevin__FlynnKevin__Flynn Member Posts: 74
    Low-Loss Headers

    While higher flow rates on the boiler side than the system side won't ruin the boiler, it may and probably will reduce the boiler's efficiency.

    There are two things to keep in mind. First, condensing boiler efficiency increases as return water temperature decreases. Second, one of the rules of primary/secondary piping is; whatever leaves a Tee must, first, enter that Tee.

    For the sake of easy math, let's assume that the boiler flow rate is 10 GPM's and the system flow rate is 5 GPM's. In this scenario, the boiler pump is pulling 10 GPM's from the Low-Loss Header back to the boiler but, the system pump is only returning 5 GPM's from the system back to the Low-Loss Header. The boiler pump has no choice but to circulate 10 GPM's so it takes the 5 from the system return and it also takes an additional 5 from the boiler supply. This means that 50% of the hottest water from the boiler supply is mixing with the return water back to the boiler, thus, elevating the return water temperature to the boiler.

    If the pump flow rates are reversed, as is recommended in the manual, this mixing takes place at the system supply tapping of the LLH and it insures that the coldest water in the system is being returned directly to the boiler.

    Is this scenario always possible, especially with multiple zones? No, but that's the goal. This, by the way, is not an issue isolated to Low-Loss Headers. The same physics hold true with closely spaced Tee's as well.

    Kevin Flynn
    Kevin Flynn
  • Jean-David BeyerJean-David Beyer Member Posts: 2,565
    I assumed that the system was already built,

    and that there was little to be done about the sizes of the individual zone sizes. Sure, as you reduce the head, you increase the flow. But what has that to do with having more flow in the secondary than in the primary? If you change the head in the secondary, you change the flow in the secondary. Turning zone valves off and on will change the head. So even if the head in the secondary is such that the flow is greater than in the primary loop with all zone valves open, as soon as you start closing them, the head goes up and the flow goes down.

    So the flow in the boiler loop is to be constant, and the flow in the secondary loops is going to vary. Therefore, even if, under some conditions, the flow in the secondary loop is greater than that in the primary, in other conditions, with some of the loops off, the overall flow in the secondary loop will be less even if a constant speed secondary circulator is in there. True, the flow in the last circuit running may be higher than it was when other circuits were running too, but from the boiler's perspective, the amount of heat sunk in the secondary circuit will be less. The boiler loop will not be aware of the different flows in the secondary. It will be aware that the heat loss is less. And in designing the secondary circuits, a pressure differential valve will not really help. The flow may be higher than lacking one (and that is worth doing if you do not use a ECM (delta-P) pump), but it will not sink any more heat unless the bypass circuit goes to an outdoor radiator or something. That might make the gas company happy, but that is about it.
  • gennadygennady Member Posts: 722
    edited August 2012
    this is the reason i stopped doing zoning.

    Between low boiler efficiency, morning boosts, heat loss trough uninsulated partitions, there is no fuel savings in zoning in the same temperature range loads.
    Gennady Tsakh

    Absolute Mechanical Co. Inc.
  • 28W28W Member Posts: 141
    edited August 2012
    This is a great explanation

    Thanks, Kevin.  I'm a newbie (and not a contractor), so forgive the ignorance.  As the pump speed out to the radiators increases relative to the boiler pump speed, don't you get cooler water being supplied to the radiators (more return water mixing into the radiator supply), and also a smaller delta-T on the radiator return? 
  • Kevin__FlynnKevin__Flynn Member Posts: 74
    Low-Loss Header

    We're all ignorant about something. The good news is, ignorance can be cured.

    Once the mixed temp at the system supply is determined, the Delta-T will be determined solely by the btu and flow-rate on the system side.

    Take a look at the link, specifically page 3. You'll see the formula to determine what your mixed temperature will be.
    Kevin Flynn
  • 28W28W Member Posts: 141
    OK . . . now I get it.

    This is the formula that started this whole string. Now that you've explained it, I understand the math. Thanks!
  • YorkietomYorkietom Member Posts: 7
    could this be my problem

    After reading about low loss headers here I am wondering if this is the issue we are having, we have a rinnai boiler, installed last year by our company,with a low loss header and piped to the book, we had 3 zones all with circ, 1x 1"radiant and 2 x3/4'baseboard and we stubbed out for a future basement  zone. All was fine until we added the basement zone and now all the other zones overheat until the basement stops, all the circs have flochecks and we added flow checks to the return and still they flow thru, we were going to try slowing the flow thru that zone but after reading about flow here, I am curious if that would help, any ideas on the issue? I have a good pic of the system.
  • kcoppkcopp Member Posts: 3,230
    is it wired ....


     Looks ok in the piping dept.
  • ZmanZman Member Posts: 4,566

    I assume you have triple checked the direction of the circulator and the orientation of the mixer. Could the mixer be malfunctioning causing the ghosting? I can't really picture it but maybe that is what is going on.

    The circs on the other zones, Are they turning on?

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • YorkietomYorkietom Member Posts: 7
    wiring and direction ok

    the wiring is all set and I actually tested the taco control to make sure there was no backfeed, only the circ on the zone calling is energized, and the direction of all the circs and mixer is good, we just tried closing the problem zone supply and return valves half way and we will see. 1 thing though that comes to mind is that the zone creating the problem is the basement zone and it only has about 10 feet of baseboard, maybe a oo7 is too much ? Thanks for any and all input :]
  • SWEISWEI Member Posts: 7,356
    Just to be clear

    you have a dedicated 007 driving a zone that consists of 10 feet of baseboard and nothing else?
  • sunlight33sunlight33 Member Posts: 124
    Pump setting

    Is it better to keep the Grundfos on both the primary and secondary loop to the same setting such as Med, or do you set the boiler pump to Lo and system pump to Med to further minimize the return temp? By the way, is there a way to monitor the temperature sensor inside the Viessmann LLH?
  • WeezboWeezbo Member Posts: 6,232
    : )

    That must get quite a bit of heat to the baseboard : ) immediatement ....

    Oh O ! there i go speaking Canadian again

    *~//: )

    seems the basement baseboard it almost a bypass.

    rather than a zone.
  • Chris_110Chris_110 Member Posts: 3,056
    Doesn't Matter

    You can run the boiler on a different delta-t then the system and in Viessmann's case you must since the max flow rate across the HX on the Vitodens 100 and 200's -19/26/28/35 is 6.2gpm

    In the boiler control when you scroll the information via "I" button - Common Supply Temp is your low loss header temp.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • sunlight33sunlight33 Member Posts: 124
    LLH temp

    The comm. supply temp is around 108-120F, so the boiler is in condensing mode.

    As to the pump setting, you were saying it didn't whether the two pumps are both in med or one in med the other in low, did I understand it correctly?
  • Chris_110Chris_110 Member Posts: 3,056
    Didn't You

    Just have this boiler installed? Suggest you speak with your installing contractor. To answer your question. The boiler pump speed 2 and don't know what your gpm and head requirements are for the system side so can't tell you what speed to put the system pump at.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • SWEISWEI Member Posts: 7,356
    Zone circs

    haven't used those since ∆P circs arrived on our shores.  What size are they, and what are the loop heads/losses?  If I had to guess, I'd say about 2% of the ones I encounter are properly sized.

    The Q-Seires has a variable speed internal pump which is managed by the onboard controls.  Nothing to change or set.
  • WeezboWeezbo Member Posts: 6,232
    for the 10 base board,

    i would expect to see it on a zone valve as a take off on another zone rather than on aa circ of it's own..

    as a thought ..

    *~//: )
  • Greenzone22Greenzone22 Member Posts: 1
    Basement zone problems

    The problem I see for why you are getting flow in the 3 zones when then basement mixed zone is the only call, has to do with the location of the spring check on the return line of the basement zone. Move the check valve to the other side of the shut off valve closer to the return header, after the cold line to the mixing valve. This will prevent any flow from the other return lines.
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