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flow control valves on a home-run TRV system

I'm looking at some distribution manifolds that have flow meters/flow limiters built in. I'm wondering if there is any reason to pay the added expense for these in an extensively zoned, home run distribution system with all wall mounted radiators having TRVs.

My thinking is that since the TRVs are controlling the flow, there is really no need to balance the system... If a radiator zone with long piping naturally gets less flow than other shorter piped zones, the TRV would be more open on the long piped zone than the shorter piped zones, balancing the system flows.

But then again, perhaps it is possible that all TRVs would be close to fully open, and the longest PEX runs might not get the flow required to heat the room. This would likely mean the reset curve needs to be adjusted, but even so, if this occurred, the radiators have a valve independent of the TRV that could be used for balancing purposes. I could choke down the shortest runs, giving more flow to the longer ones to achieve balanced flow.

Is this logic sound? Would there be any advantage to having the flow meters/limiters on the distribution manifold in this type of system.



  • Harvey Ramer
    Harvey Ramer Member Posts: 2,217
    While it may not be absolutely critical to use a manifold with flow balancing, it is a good idea. This ensures that all radiators recieve adequate flow during rapid outdoor temperature changes. It also helps prevent excessive system flow and corresponding higher than necessary btu input from the boiler. Having a balanced system before the TRV's results in the most stable conditions for the boiler and the longest runtimes. You should also use a pump that features a proportional pressure curve. Such as "auto adapt" on the Alpha circ, or the "proportional pressure mode" on vr1816 Taco circ. These pump modes have technology built into them that is designed to keep the TRV's modulating somewhere in the middle of their flow rate command spectrum. What this does is allow continous, but moderated input from the boiler, further discouraging an over accelerated firing rate.

    The physical design of most TRV's does not match the flow balancing capabilities of the balancing valves on a manifold. It is important to have a well balanced system prior to the TRV's to obtain optimal results from an overall system standpoint.
  • Big Ed_4
    Big Ed_4 Member Posts: 1,775
    More important ...Reverse return is very inportant on a parallel system which will equalize the pressure drops across each loop . First supply to leave manifold its return will be last on the return manifold.... I am assuming your using panel radiators .. As Harvey mentioned use an Alpha type circulator would be a good choice with TRV's.. I would only look at flow meter manifolds on large radiant systems if one needs to fine tune the system .
    I have enough experience to know , that I dont know it all
  • roundrightfarm
    roundrightfarm Member Posts: 54
    Thanks for the input and affirming that balance is still important with TRVs.

    Yes, we are using panel radiators. The building is 4 levels, with the boiler room and dist. manifold in the basement (1st level). This means that there will be a large difference in loop length... anywhere from 20', total loop length, to 140'. Despite this, flow calculations showed that 3/8" pex is adequate for all loops. However, someone suggest using 1/2" on the longer loops, which made a lot of sense to me. It just so happens that the 4 longest loops have the largest sized radiators of the system, requiring the most flow. A few shorter lengths also have the large size rad as well.

    This 24 rad system will be divided into two manifolds of 12 loops each. I'm assuming for balancing purposes that I should put the longest runs on one and the shortest on the other. Is this correct?

    Any other advice on how to design balance into the system would be appreciated.
  • hot_rod
    hot_rod Member Posts: 15,055
    There is only so much you can do with manual balance valves. If you balance the 12 loops with PRVs full open, as the valves close flow will change to the remaining loops.

    That is why the delta P circ is important, to help eliminate the over pumping that a fixed circ would provide.

    The biggest advantage of manifold distribution is you do not have the head loss across the mains as you would with a parallel or reverse return.

    The TRVs and ∆P should work fine, adding a manifold with flow balance and flow indicators is a nice addition, but will not add a lot of additional control. The TRVs are proportional control valves.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • roundrightfarm
    roundrightfarm Member Posts: 54
    thanks hot rod for introducing the pdf to the discussion. It is advocating the use of pressure-independent balancing valves (PIBV) on the return manifold to control flow rates in each loop of multi-zoned system. It sounds like you can set these valves for a specific flow, across a wide pressure range. So, as the TRVs open and close, and the system pressure changes, each loop should still receive the same amount of flow.

    Is it possible, that without any balancing valves, under design conditions, that the longest radiator loops would not get adequate flow?

    I imagine a situation where the shortest loop's TRV needs to be fully open to provide adequate heat. This means that the longest loop, which would also have to be fully open would not get the required flow, due to the extra head dissipation. In practice, this would mean raising the ODR curve, causing the shortest loop to choke down, sending more flow to the longest loop. Is this right?

    If so, does this mean balancing is important for boiler efficiency? or put another way, does balancing allow for maximum comfort at the lowest ODR temps on a TRV controlled system?
  • hot_rod
    hot_rod Member Posts: 15,055
    Your circ pump should be sized to flow the highest head loop and provide adequate gpm for the zones.

    Under full load, all zones calling you will (should) have adequate flow.

    If you use a fixed speed circulator, as zones close, the remaining zones will be over pumped.

    The ∆P circulator should mitigate this condition, manual balance valves on the manifold would allow you to dial it in closer.

    The PIBV would assure exact flow to every zone under any combination of zones calling. PICV are usually applied on larger commercial applications to dial in dozen, or hundreds of heat emitters, it it probably a bit over kill for your system.
    Correct, you specify a flow and we build them to that requirement at the factory. More expensive versions have field changeable cartridges.

    If you plan on using a ∆P, size it correctly, and set it up properly I think you will be close enough to ideal :)

    We do make TRVs that you can dial in a Cv, balance with the TRV, but we do not offer them in the US, yet. They seem to be more into "exact" balancing of TRVs in the European market. A lot more are applied over there so the market supports a more expensive version of TRVs.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,837
    I think you can do just fine without the extra balancing because the nature of TRV's is that they will balance themselves out. The closest loop will satisfy sooner, which then send more flow to the balance of the system.

    The ONLY time you will actually see that condition, would be during a cold start up. Once up and running, it most probably won't happen again, even at design condition.

    I agree that the use of a delta P pump is important.

    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.