To get email notification when someone adds to a thread you're following, click on the star in the thread's header and it will turn yellow; click again to turn it off. To edit your profile, click on the gear.
The Wall has a powerful search engine that will go all the way back to 2002. Use "quotation marks" around multiple-word searches. RIGHT-CLICK on the results and choose Open Link In New Window so you'll be able to get back to your results. Happy searching!
In fairness to all, we don't discuss pricing on the Wall. Thanks for your cooperation.
Need to contact us? Visit

Low Loss Headers

NronNron Member Posts: 163
low loss headers dont have to be expensive , look at Caleffi or Sinus headers and you find good prices for great materials , I will post soon with one being installed


  • Bob ForandBob Forand Member Posts: 305
    Can someone explain ?

    I am hoping that someone can fully explain a low loss header and why they should be used in a system over P/S piping ? Why they are now even more important with Mod/Con or low mass boilers. The feeling is that we are headed towards using more and more low loss headers in residential installs. I just don't know how to justify the price. I am sure there is a great reason to use, but I don't know why. I am sure this is a question many wonder about...
  • llh

    I usually use p/s for piping my mod con and have found that by using a llh that i saved alot of room and that it incorparated a air elimator and gives great hydronic seperation and takes up alot less space espically if you are piping your p/s mod con properly by have at least a 10 inch piece of pipe before and after your 2 tees you save alot of space also i have noticed that there is very little migration from the boiler to the system side through the seperator ,i will agree the price espically califee is up there and that the price when added to a install does some times put it over the edge but it definetly saves alot of time and space between being able to just pipe your system manifold into it and your off ansd the fact that it's has air elimation and a place to attach your water feed ,expansion tank and even a system side sensor and as for building your own between time and materal it's kinda a wash but i'm still thinking of building one any way just to see how it compares peace and good luck clammy
  • Brad WhiteBrad White Member Posts: 2,393
    Low Loss Headers

    To build on Clammy's points and to construct things as I might, I can offer this:

    In conventional P/S piping, what supply water flow is not used by the system or a given zone flows immediately back to the boiler (or to lower temperature downstream zones).

    EDIT: This condition of bypass to the return applies to intermittent circulation or variable flow volumes in the secondary circuit without a concurrent drop in the primary/boiler circuit.

    In any system but particularly a ModCon system, this can and will raise the return water temperature, possibly slowing or even stopping condensing, especially if you are running higher temperatures at that time. You really do want your lowest possible return water temperature and P/S fights that to an extent.

    (Conventional P/S is good for cast iron boilers though, for the same reason. Even absent specific control, you do not want your boiler return temperature to get too low by comparison.)

    Now, with a Low Loss Header (LLH) or hydraulic separator by any other name, you have a much larger vessel. This favors more secondary branches and is more simple than multiple sets of P/S tees. Typically and diagrammatically, the hottest boiler water enters the upper port while the system flow leaves on the opposite upper port. System return enters the lower port on that side and returns to the boiler via the lower port on the opposite side.

    A proviso here in any case is that the system side flow is higher than the boiler side flow. Not much, just higher. In that flow configuration, ALL of the entering boiler flow goes out to the system with a small portion of system return flow being recirculated. Because in-ee equals out-ee, what goes back to the boiler is 100% system return water. This has all been passed one hopes through an emitter and represents the coldest system water possible.

    Another feature is in multi-zone systems. On the system side and assuming zones have similar resistance, one, two, three or more zones can operate without interfering with boiler flow. The normal P/S benefit.

    If I have a ModCon with one zone, I would have trouble justifying it, especially if it is properly matched to load (as they all should be). I currently have a Vitodens and LLH installed serving my Susan's house as a single zone but there are taps for future radiant zones so it is "ready". If it were to only be the house forever, I would not have installed the LLH.

    Naturally, one primary reason for using P/S (by any means) in a ModCon system is to assure a minimum if not constant boiler flow rate. Those with low water content and high pressure drops are particularly sensitive. A LLH and dedicated boiler circulator takes that issue away.

    Of course there is judgment to be used here- One has to either balance the boiler and system zones to meet this criteria or one has to have variable flow that listens to the system (Viessmann Vitodens comes to mind).

    A LLH can also be used in a conventional system but if so, I would set it up with higher boiler-side flow to help raise the HWR temperature back to the boiler. I still get the P/S benefit of hydraulic decoupling (although I could also do this with closely space tees).

    Using a LLH is not a "must" in every system. It should not be a "default" device without reason, especially in a competitive market. A LLH is a good tool to use when properly applied to solve or prevent a specific identifiable problem.

    My $0.02

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

    -Ernie White, my Dad
  • seansean Member Posts: 22
    radiant boiler v.s hot water heater

    should i use a boiler for radant heat or a bradford white 75 gallon 80000 btu water heater.the house is 2500 sq ft.water heat is cheaper but is it wiser??
  • Brad WhiteBrad White Member Posts: 2,393
    Sean, you may want to post this as a separate thread

    EDIT: Never mind, I saw that you just did. :)
    "If you do not know the answer, say, "I do not know the answer", and you will be right!"

    -Ernie White, my Dad
  • singhsingh Member Posts: 865

    Siggy at PM live made a good point about one less circulator pump not needed,
    Whereas, traditional P/S piping still needs that primary circulator.

    To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"
  • TimcoTimco Member Posts: 2,802

    Can someone post a diagram for the llh use / install method? Do the zone's pumps move the water through the header? Why no pump on the boiler side???

    Working on steam and hot-water systems isn't rocket's actually much harder.
  • J.C.A.J.C.A. Member Posts: 2,981
    Thanks Brad !

    I was proposing the same theory to Mr. Milne just the other day...but you laid it down in terms that I couldn't come up with on such short notice.

    What a great and eloquent explanation of the reasons for a LLH and P/S method. I particularly agree with your statement that closely spaced tees can accomplish the desired result, in the proper long as the system is piped properly and all the rules of hydronics are followed.

    Condensing boilers CRAVE COLD WATER! Throw all you got at it!!! It's what makes them work more efficiently. Cast iron needs some type of protection to make sure it doesn't condense. Condensation will reek havoc with conventional cast iron boilers, and shorten their lifespan dramatically.

    I need to get a better vocabulary happening! :P~ JCA
  • Brad WhiteBrad White Member Posts: 2,393
    Here is something

    from my sketchbook. I do some thinking while taking the train.

    This sketch is of a way to optimize a set of four Aerco Benchmark 2.0 boilers which were designed by another engineering company as P/S.

    The delta-T's currently are narrow, 3 to 5 degrees and the over-pumping, well, a mess for another topic.

    Anyway, the principles are the same. The sketch does show some controls (Delta-T control to VFD's on pumps) but that is a refinement.

    The bottom part of the sketch is the "hardware", should I have to have a large LLH built; fortunately, Taco is coming out with a line which goes up to some decent sizes.

    In that sketch, T1 is the boiler temperature. T2 is a mixed temperature (on which the emitters are designed and which can vary with OD reset). T3 is the system hence boiler return. Does this help?

    Singh- Maybe I am missing something, but I do not recall Siggy saying that the boiler-side circulator could be eliminated. The LLH by definition would not allow secondary circulation to impose itself on the boiler loop, especially with the predictable flows that lower mass boilers demand.

    Maybe I misunderstood? Not a challenge, just hate to think I missed a key point. Thanks!

    EDIT: I see now as you posted, the in-boiler circulator. Makes perfect sense in light of that. Thanks Singh! Now I know I am not crazy. The little voices tell me so. :)
    "If you do not know the answer, say, "I do not know the answer", and you will be right!"

    -Ernie White, my Dad
  • singhsingh Member Posts: 865

    I should clarify.

    Boiler side pumps are not eliminated with LLH.
    However, If using a mod/con with P/S , you'll have the pumps into boiler, then a primary loop with a primary pump, and then secondaries.

    So, as I understand it, using a LLH and condensing boiler allows one less pump. And less energy consumption as a result.
    Does not matter so much with non condensing boiler.

    To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"
  • John FJohn F Member Posts: 1
    Low Loss Header Article

    There is an article in the Library discussing low loss headers (Hydraulic Separation). Go to Separation and open the "Hydraulic Separation" paper.

  • Brad, I have to be contentious on this again.

    the LLH is not a buffer tank. the amount of effect it has on boiler return water is nearly nonexistant.

    Flow characteristics and flow mixing should be *exactly the same* minus the very small amount of additional fluid mass in an LLH between a LLH and a P/S tee setup. What goes in must come out... LLH doesn't change that. Doesn't change flow rates. Doesn't change the specific heat of water.

    If pure water would go back with an LLH, it would do so with P/S as well. If not, it won't in either case either.

    The only reasons to use LLH's are for looks, time savings, space savings. For any return water benefits, or cycling benefits, you want a buffer tank, not a LLH.
  • Brad WhiteBrad White Member Posts: 2,393
    You are absolutely correct Rob

    Thanks for calling me on that and do I feel dumb :).

    I had to sketch it out to verify the obvious. You are right and I cannot argue that in fact a LLH follows the same principles. Indeed it does make the setup more forgiving but in the end it all gets down to control and balancing. Change the relationship of flow and all bets are off.

    Here is one place I went wrong in my thinking- P/S with intermittent or variable circulation on the secondary zones. When any zone circulator stopped or slowed due to TRV's backing off, partial or even full primary hot water flow would pass through to the return.

    This of course could happen with a LLH just as well. Key is isolation and control, maintaining the flow relationship higher in the secondary circuit. Constant circulation or variable circulation with primary circuit feedback (changing both to maintain the relationships) is essential. This too is something I have been doing on large commercial jobs and it is this which assures the coldest water back to the boilers, not the LLH itself. As you stated, the LLH does make it neater and saves a bunch of piping.

    In actual practice, the LLH, if tall and vertical, does create a slight thermocline, but that is more theoretical than practical because upward flow breaks it.

    Still, in that case it does act like a small (very small) buffer tank. A buffer tank also makes a very good LLH, a tangential point.

    Regardless, I learned something today or more to the point it got me thinking about my thinking. I appreciate being challenged.



    p.s. Sorry about the posting, it is turned 90 degrees clockwise. The original was "landscape".
    "If you do not know the answer, say, "I do not know the answer", and you will be right!"

    -Ernie White, my Dad
  • David107David107 Member Posts: 1,117
    What setup would benefit from both LLH AND Buffer Tank?

    And in that case would the LLH always be incorporated into a Buffer Tank or vice versa?

    Edit: I now realize Rob and Brad have answered these questions and the others I edited out; so obvious that I missed it. But thanks for this thread.


  • Steve EbelsSteve Ebels Member Posts: 1,284
    Rule of thumb

    Anytime you have a required difference in the flow rate of the boiler side vs the system side, you have a good application for a LLH. P/S piping will also accomplish the same thing but many times requires an extra circ and a lot of additional piping if you need the same temp fluid to each zone. Usually you'll see a boiler pump, then a primary loop pump and finally your secondary or zone pumps. With a LLH you need only the boiler pump into one side and the system pump(s) out the other side.

    The picture is a good illustration of this. The boiler side, to the left of the header can flow as high as about 35 GPM with both boilers running. The system side can go as high as 70-80 GPM with all the zone circs on. Way more flow than the boilers need or would like to see.
  • hot rodhot rod Member Posts: 6,486
    You need to run the calculations

    just as you would with P/S. this will determine the mixed temperature output of a hydro separator.

    You may have one of 3 cases.

    Flow in boiler side equal to flow in distribution or flow in distribution greater than boiler side, or flow in boiler side greater thean distribution. there is a mixed temperature formula to determine that temperture.

    It is quite possible to have a boiler side at 180 and a mixed outlet of 130 or less with the flows being un equal.

    And of course that outlet temperature will change as load side flow rates change.
    Find a copy of the Caleffi I-dronics issue 1 for a good clear formula happy explanation.

    hot rod
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • ALHALH Member Posts: 1,790
    My Take

    Low loss headers have some benefits that P/S piping typically does not.

    1. The LLH can be a perfect location for connection of the expansion tank.

    2. The LLH is an excellent location to remove air.

    3. The LLH can act as a sediment trap.

    4. The LLH can provide a good location for the system supply temperature sensor in a well rather than strapped on a pipe.

    These are all benefits, but there are other ways to accomplish these goals.

    I feel that avoiding the need for P/S piping with mod/cons is the way to go. The system then needs only one circulator. Obviously there is a flow threshhold where this is not practical, but for most residential work, P/S piping with mod/cons can be avoided with proper system design. The heat exchanger design of the Triangle Tube Prestige lends itself best, among stainless steel mod/cons currently on the market, to avoiding the need for P/S piping.

  • what makes the prestige special there?
  • ALHALH Member Posts: 1,790

    Lowest head loss through the heat exchanger of any stainless steel mod/con on the market that I know of.
  • Mike T., Swampeast MOMike T., Swampeast MO Member Posts: 6,928

    oops. double-post. NM
  • Mike T., Swampeast MOMike T., Swampeast MO Member Posts: 6,928

    Here's a couple drawings that might help. One of a low-loss header; the other of primary/secondary.

    Both low-loss headers and primary secondary have the same major purpose: to allow multiple circulators to operate independently in the same system without being affected by each others' flow and head losses. This is often called "hydraulic uncoupling".

    Hydraulic uncoupling aside, they are similar but go about their job in slightly different ways. Study the LLH diagram and you'll see how it guarantees return temperature to the boiler is exactly equal to return temp from the emitters as long as secondary (emitter) flow is greater than primary (boiler) flow. You'll also see that the flow paths allowing this are completely logical and straight forward. Compare this to the primary/secondary diagram and you'll see how it requires reverse flow between the closely spaced tees in the primary loop. This does happen, but I still have a hard time believing that no mixing (and no increase in return temp) occurs.

    Another purpose of the low-loss header is to provide some additional volume (dare I say buffering?) in the primary loop. This was especially important with condensing but not modulating boilers. Why? Because they transfer heat so rapidly and efficiently that during periods of low load it is quite possible for even a very short firing at full output to exceed the high (safety) limit temperature.

    It's very important to understand what's happening in a LLH with regards to supply temp and delta-t--both at the boiler and in the emitters. As long as secondary flow is greater than primary (and this should be true as much as possible) some of the return water is flowing right back into the supply to the emitters thus lowering its temperature. Note how T1 (primary) > T3 (secondary) in the LLH diagram.

    The more secondary flow exceeds primary, the greater the amount of mixing and the lower the secondary supply temp relative to primary supply temp. In other words it takes a higher and higher supply temp in the boiler to maintain the necessary supply temp to the secondary. Normally, this isn't too much of a problem but some bad things can occur if you take the secondary flow greater than primary flow to an extreme:

    1) The worst thing that can happen when secondary flow greatly exceeds primary is that the boiler will be unable to maintain the required supply temperature to the secondary. Provided the mod-con is maintaining its target temperature based on the mixed temp in the LLH (e.g. T3) the control system will automatically compensate. In the more common situation however where the mod-con is modulating based on the supply temp in the boiler a higher than normal reset curve may be required. In a truly extreme situation such as can quite easily occur with mod-cons driving hydro-air systems it is quite possible for the required supply temp in the boiler to exceed the high limit--when that happens the mod-con will be forced to reduce its output to avoid tripping a safety.

    2) Another thing that happens when secondary flow greatly exceeds primary flow is that the mod-con is forced to operate with less efficiency. Why? Because it must have a supply temp significantly higher than actually required by the emitters. In any given system, a mod-con is most efficient when operating at the lowest possible temperature with the highest possible delta-t. Poorly conductive radiant systems (e.g. "bare tube") and hydro-air again are the places where you're most likely to see this problem--everything will work (provided you don't hit problem 1), but you will loose some efficiency.

    So how do you guarantee that secondary flow is greater than primary flow? Great question with a somewhat difficult answer. Presuming the secondary circulator operates at fixed flow (e.g. constant speed), the only way you can guarantee this is with careful system design. This is very easy in a single-zone system, but it gets more and more difficult the more on-off controlled zones are added. Careful design and setting of the reset curve will minimize this possibility--the goal is to have the best possible emitter balance among the zones with the lowest possible reset curve. Why? This will keep as many zones as possible calling for as long as possible. Be aware however that the lower the reset curve, the longer it will take to recover from any room temperature setback...

    So what happens when secondary flow is less than primary? With a low-loss header, the flow inside the header reverses, thus bypassing supply water from the boiler directly into return to the boiler. This increases return temp to the boiler and decreases the efficiency of a mod-con. With primary-secondary, there is no longer a flow reversal between the tees and everything is moving in a logical direction. This brings up a major difference between the intended purposes of a LLH and primary-secondary. Primary-secondary is mainly about conventional boilers and keeping the boiler return temperature UP to avoid condensation damage while LLHs are mainly about keeping return temperature DOWN for mod-cons. Again though even when using a LLH with a mod-con you want to design carefully and use the lowest possible reset curve to avoid this condition as much as possible.

    Now comes the most interesting thing (well to me at least) about the LLH diagram. Note how Qprimary (heat supplied by boiler) is stated to be equal to Qsecondary (heat consumed by system). How does this happen?

    There is ONLY one way for this to occur in any real-world system. Primary flow MUST be variable! Just being variable speed alone is not enough. It must be controlled in a way that takes both the burner output level of the mod-con and the difference in temps between the boiler supply and system supply! At the present time, the only boiler I know of that is capable of doing this is the Vitodens 200. This accounts for a good portion of what can almost be seen as its' "magic" ability to maintain space temperature without sensing space temperature.

  • Mike, again I have to be contentious.

    What goes in, must come out, with P/S or a LLH. That is irrefutable fact.

    If secondary flow is greater than primary, with either P/S or LLH, return temp is return temp of the secondary circuit. No reverse flow occurs with P/S.

    If secondary flow is less than primary, then return temp is mixed up a bit from the boiler supply water that returns to the boiler. Again, regardless of whether you use LLH or P/S.

    buffer capacity is negligable in the vast majority of LLHs. You want a buffer tank for that.

    LLH's are easy piping for P/S piping arrangements. They collect several functions into one item.

    But functionally there is nothing an LLH does that is not done by P/S piping and assocated components. Nothing of measure or note, except save time and look pretty. Perhaps it's a better guarantee of proper worksmanship as well. But that's a pretty small concern.
  • Mike T., Swampeast MOMike T., Swampeast MO Member Posts: 6,928

    Great takes Andrew! Especially trying to avoid the use of the LLH in the first place. To my knowledge every mod-con on the market can be used with a single circulator (no LLH or primary/secondary), but the high head loss of the HX in most mod-cons (Triangle Prestige a notable exception) can make this difficult unless you keep the head loss in the rest of the system extremely low.
  • ALHALH Member Posts: 1,790
    Pretty pipes

    I agree that LLH's are nice but not necessary for residential work.
  • hot rodhot rod Member Posts: 6,486
    The Prestige 110

    does have a bit over 8 feet of head at a 10 gpm flow. As such a 15-58 will need to be on speed 3 to maintain a 20 degree delta t. According to the graph I have from Triangle Tube.

    Better than most of the coil type HX design., but not to be ignored. Not nearly what a cast iron boiler pressure drop is :)

    hot rod
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me

  • well, the time savings may be worthwhile, and the cost of emulating a large low drop collector for multiple-separation situations may make the LLH more attractive... as another person noted, if you have multiple circuits that need the same supply temp, you eliminate paralell P/S circuits with an LLH and some short, fat headers (thanks Siggy!). And ditto with Multiple boilers.

    But that's not LLH magic, LLH just makes it easier to do that kind of piping. Perhaps in a cost effective manner.

    But if you have one boiler and one water temp, a LLH is a complete waste of money IMHO.
  • ALHALH Member Posts: 1,790

    When designing a hydronic heating system, any piping added to the boiler adds considerable cost. Avoiding as much external piping as possible increases the ROI of the high efficiency appliance considerably. It also saves on electricity costs and components subject to failure.

    The ultimate in simplicity seems to be a high-efficiency combination water heater. It combines two appliances and makes solar integration with domestic and space heating as simple as a pre-heat tank. The only problem is that I have yet to see a high-quality high-efficiency combination water heater.

    In my opinion, simplicity is key to realizing the benefits of efficient heating. One service call can make a big dent in any efficiency savings.
  • siggysiggy Member Posts: 78
    Low Loss Headers


    Low loss headers are one form of a broader concept called hydraulic separation. There are several ways to achieve this effect including closely space tees, LLH, and even specially designed tanks.

    PM Engineer magazine will be presenting a 90-minute Webinar on the subject of hydraulic sepatation at 2 PM on October 25. If interested you can get more information at this link:

This discussion has been closed.


It looks like you're new here. If you want to get involved, click one of these buttons!