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Hydraulic separator yes or no?

My partner and I have been debating the necessity of  hydraulic separators for a while and frankly are having a hard time seeing the need and justifying the added expense.

Our basic radiant system is built with a mod/con boiler with a dedicated pump injecting into the radiant distribution loop through closely spaced tees. Most of our systems employ constant circulation through non-electric remote thermostatically controlled zone valves (TRV's).

We are told that besides venting air and collecting dirt, the advantage of a hydraulic separator is that at certain points in system operation, the gpm through the boiler will exceed the gpm of the heat distribution loop. The design of the separator then provides a path for the boiler supply water to mix with return water and divert directly back into the boiler return pipe.

I have spent more hours than I would like to admit, in wet-head obsessive fascination, watching boilers run and playing with my clamp-on thermistors. My conclusion is that when gpm's through the boiler exceed gpm's through the heating distribution loop, a portion of the boiler supply water goes back upstream, where it diverts right back into the boiler return pipe. Just like in a hydraulic separator, but without spending half a grand to do it.

I can see separation piping scenarios for buffer tank applications, but on most residential situations I am thinking it is better to put the money into Uncle Hydronoids retirement fund. Am I missing something or is this just more gear being foisted on the unwitting?


  • Jean-David BeyerJean-David Beyer Member Posts: 2,665
    Hydraulic separator.

    I should point out I am not a heating contractor. But I have been interested in primary-secondary near-boiler plumbing because my mod|con requires it according to the manufacturer. My contractor used closely spaced Ts to accomplish this, but an hydraulic separator would accomplish the same function, as well as provide air and dirt separation.

    I do not know if the air separation in an H.S. is as good as in a fancy air separator such as Spirovent or the Taco 4900 series, but the professionals should know this.

     In my system, I do not believe I have space for the hydraulic separator, but it seems pretty clear than in other situations, the H.S. would require less space. Labor might be less than making up the closely spaced Ts; I do not know what the loaded salary of plumber would be for the shorter time to install an H.S. compared to the C.S.Ts.

    The main question, it would seem to me, is not whether to use an H.S. vs CSTs, but whether you need primary|secondary plumbing or not. That, it seems to me, depends on the the required flow rate through the boiler, and whether this needs to be independent of the flow rates on the secondary side of the system.
  • FortunatFortunat Member Posts: 103

    I think you are basically right.

    A properly plumbed and installed pair of closely spaced tee's provides hydraulic seperation, just like a hydraulic separator does.

    So other than ease of plumbing, air and dirt removal, I'm not sure there is a huge argument to be made for that particular piece of hardware.

  • Mark EathertonMark Eatherton Member Posts: 5,837
    I am on record for saying that LLH are for people who don't know how to...

    do PROPER primary/secondary piping. Key word here is PROPER. Just having the two tee's as close together as possible doesn't cut it.

    There is a need for straight piping before and after the two tweener tees, and certain distances before any pumps.

    When you think about the cost of your labor, and if you VALUE your labor where it really should be, then by incorporating the hydraulic disconnect, dirt separator and air separation into one unit that doesn't really require a certain amount of straight piping before or after it, then the use of a LLH will decrease install time, increase performance and reliability.

    Obviously, your milage may vary....

    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.
  • CMadatMeCMadatMe Member Posts: 3,066
    Depends on the Job

    I'm a Viessmann guy and we use them dependent on the delta-t we are running on our system side. The boiler pumps are sized for a 40 degree delta-t across the boiler so if my system side is a 20 I'm using them.

    On the smaller Vitodens 100's they are not needed if the system flow doesn't exceed 6.2gpm or get as low as 1.7gpm. I use Taco's on these boilers and it's 1/4 of a grand. They make 2 nice small sizes, mounted on strut and ready to hang,

    On the Vitodens 200's we always use them. The boiler has a temp sensor that gets immersed in the LLH to provide temp feedback to the boiler that it takes into account during modulation to keep a 40 degree delta-t across it.

    From a design standpoint I prefer them over pri/sec because I don't have to worry about temp drops across multiple zones when mutiple zones call. I know what temps are going into each zone.

    Attached is a little webinar that Caleffi put out that may give you more insight.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • Jean-David BeyerJean-David Beyer Member Posts: 2,665
    Delta T across boiler.

    The instructions for my boiler (W-M Ultra 3) require closely spaced Ts. They provide two strap-on temperature sensors, one to go on the return pipe of the secondary circuit before it goes down the return pipe into the primary loop; the other to go on the supply pipe of the secondary circuit after the primary loop enters the other T. So the boiler and I know the delta-T right there. Depending on the conditions in the secondary loop, these temperatures could be the delta-T across the boiler. I know the boiler uses the output temperature for modulation (to follow the outdoor reset, for example). It may use the return temperature as well, but I do not know that. But if not used, it is just there for my amusement and I do not see why they would bother to do that.

    In actual operation, my boiler runs with much lower delta-Ts than 40F; I do not recall seeing a delta-T over 10 degrees there; it is usually quite a bit less. The indirect hot water heater is plumbed differently, right across the output of the boiler on the primary side. The supply and return temperatures for that are measured at the entry and exit of the heat exchanger, and tend to be about 15F. Both the primary loop of the P-S plumbing and the indirect loops use Taco 007 style circulators, so to a first approximation, the flow rate through the boiler is the same.

    It seems to me that the flow rate must be the higher of two factors:

    1.) fast enough to get the heat generated by the boiler out to the load of the system. This is over a relatively long time compared to item 2, though it may be less than a minute in a system where the boiler contains only 3 quarts of water.

    2.) fast enough to prevent localized heating inside the heat exchanger from getting too high for its design and compromising its lifetime. This can be a very short time. My heat exchanger is aluminum and has lots of pins on the fire side that should not get too hot. Perhaps primary-secondary plumbing is not absolutely necessary in some cases, but in the installation manual they may not wish to confuse the installer with all the detail to decide such things and just go with a safe approach that should work in all cases.
  • Mark EathertonMark Eatherton Member Posts: 5,837
    The problem is...

    Weil McLain does not make a LLH. If they did, they would require it on every installation.

    You could use a LLH on any boiler, regardless of the number of sensors required for operation, and simply place the sensors where they will see what needs to be seen to dictate firing rates.

    A LLH is really nothing more than a fat spot in the piping where water velocity slows down (air elimination and dirt removal) and provides for hydraulic separation to keep one pump from influencing the other. and it doesn't have to be made of steel. It can be made of any material commonly used, like copper or steel. But the factory labor (if you value YOUR labor) will almost always be less expensive than field labor, and the quality of the product will be higher.

    I am not a complete Kool Aid drinking convert, but there are some applications where it makes sense, and reduces our most expensive commodity, that being labor.

    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.
  • NRT_RobNRT_Rob Member Posts: 1,013
    most of the time

    you don't even need primary/secondary. I know, I know, they all show it.

    but if your system pump can push the total system flow through the boiler AND the loops, and you can maintain minimum flow through the boiler (pressure bypass, larger zones), there is no need for extra pump.

    Basically, unless your total system flow is too high to pump through the boiler directly I don't even see a need for extra pumps and separation, never mind a hydraulic separator.

    If hydraulic separators were about 2/3 or half the cost they are now and had super duper air separation, I would see them as useful when p/s is needed. As is, we just use webstone purge tees which are much cheaper and ensure close spacing and fast install. then install a good air separator.

    never really got "dirt separation" in a new system. radiator retros or something, sure. otherwise... purge the system.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
  • Jean-David BeyerJean-David Beyer Member Posts: 2,665
    A LLH is really nothing more than a fat spot ...

    ... in the piping where water velocity slows down"

    That is what my closely-spaced Ts amounted to when the contractor was done. For my size of boiler, W-M said I should use at least 1" pipe for the near-boiler plumbing. My contractor used 1 1/4" becaise he got better results that way. He then used 1" for the relatively large downstairs zone and 3/4" for the upstairs zone. (To make matters worse, there are about 70 feet of 1/2" tubing in the floor up there.) So the primary loop and the common part of the secondary loop are all 1 1/4", and the air separator is even larger inside, for slow flow rates there.

    Since I am not a contractor, I do not know the loaded labor rates for the plumbers employed by heating contractors. I suppose it is like automobile dealers who charge a lot for labor, though the employees get a lot less. In addition to profit, though, the dealer has to recoer the cost of the building, the hydraulic lifts, heating and lighting, insurance, and so on. Similarly, I am sure heating contractors are up against the same issues. So I do not know if the additional cost of a low-loss-header is less than the labor required to build the same thinig.

    In my case, it took about 15 minutes to cut and put together the copper for the closely spaced Ts. And some more time to plumb in the fancy air separator. Ihave no dirt separator. I do not know what a small low loss header would cost in comparison.

    I do not know how my contractor figures out costs. I needed a manifold to split the 1" pipe down to 5 copper tubes for the radiant heating zone. The contractor made it up from copper tubing, a bunch of Ts, and some small ball valves. It worked, but I would have thought they would have used a pre-built one instead.

    Maybe the costs are too close. I would not know. But maybe it is a case of "when all is said and done, more is said than done."
  • Jean-David BeyerJean-David Beyer Member Posts: 2,665
    if your system pump can push the total system flow

    The trouble with my system is that there are two zones in the secondary side, and these are in parallel.  I am pretty sure when the downstairs zone is running, that a single pump could get enough flow through the boiler. I am not positive, because one of the 5 rooms down there has its valve almost totally shut off because there are two computers in there, and one of them produces a lot of heat. Now if both zones are on, if anything, the flow would be even higher.

    The problem would come when only the upstairs wanted heat. In that case, the total load is far less than the minimum boiler output (always less than 6,500 BTU/hr, and the boiler will only modulate down to 16,000 BTU/hr). So I strongly suspect that the flow would be insufficient. W-M seem to think I need the full output of a Taco 007 through a short loop to keep the HX cool. Now maybe if I stuck a bypass valve across the downstairs zone so when it is off, the flow would continue unchanged, I might get away without P-S piping. But W-M do not actually say what the flow rate must be. They just specify the pump type and the minimum pipe size. Not quite enough data.
  • NRT_RobNRT_Rob Member Posts: 1,013

    we find the boiler guys are ok as long as we keep the dt across the HE at 30 or less.

    so on low fire, you don't need a lot of flow to do that. and if you only have a small zone open, you are probably on low fire.

    however we almost always end up using a pressure bypass in that situation unless the smallest zone has a higher flow rate than the minimum for the boiler.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
  • JackJack Member Posts: 1,045
    Those of you

    contractors who "really" know how to design and install systems probably don't...definitely, don't need them. Rinnai ships them with the boiler and as one of their reps, I am thrilled that they do so. From my perspective, the "A" guys never have a problem, The "B" guys hardly ever have a problem. The "C&D" guys, well, you know. I think they provide margin for error and simplify troubleshooting over the phone or on site because they allow some separation of the boiler loop and the system. So, you A&B guys rejoice, as we rejoice in you! ;)
  • CMadatMeCMadatMe Member Posts: 3,066
    I Wish

    More manufacturers would start putting the LLH piping arrangement in their install manuals. It's a much better concept than pri/sec. As HR stated previously you can use the on each and every boiler that requires pri/sec. I think what most contractors miss is the fact that NO TEMP drops across the zones as multiple zones call.  They give you a much better grasp in controlling the system side. The boiler can do it's own thing and you can do whatever you need to on the system side with your flow rates.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
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