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  1. #1
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    Water Balance Fun

    I'm doing TAB on a job that has been interesting from start to finish. I thought you guys might like to hear about some of the fun.
    The building has a history of not heating or cooling properly. For this reason the redesign is over kill of the highest order. Every space has a fan powered box and every space has a design airflow and heating design that considerably exceeds the need.
    The present problem is that most of the FPB water heating coils were downsized at the last minute but they forgot to down size the circuit setters and control valves. A few coils were up sized. The circuit setters are Nexus Orturi and the design orifice DP'S vary from 0.81 inches of water to 129 inches of water to provide design GPM. The control valves were provided the same size as the circuit setters.
    I'll give my take on this after getting some of your comments but thought you guys would like to add your two cents.
    No man can be both ignorant and free.
    Thomas Jefferson

  2. #2
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    Do you really mean 129 " water?

    "The control valves were provided the same size as the circuit setters." I learned fairly early on to not size by line size; you size by the specs of the flow. Strainers was the area I first learned that in, sizing strainers to the line size often resulted in lack of flow. Especially with a finer mesh screen
    I do a triple evac with nitro to remove non condensables.

  3. #3
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    Thread Starter
    Yeah 129 inches of water. That isn't the drop across the valve but the drop through the circuit setter orifice.
    Control valves are normally one pipe size smaller than the line size but in this case where the flows are small they are normally the same size as the circuit setter.
    No man can be both ignorant and free.
    Thomas Jefferson

  4. #4
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    Thread Starter
    One of the problems is there is a 1" valve that has a design water flow of 0.6 GPM. Another one is a 3/4" valve with 0.6 GPM design.
    No man can be both ignorant and free.
    Thomas Jefferson

  5. #5
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    Mar 2008
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    Honeywell you can buy better but you cant pay more

    I told my wife when i die to sell my fishing stuff for what its worth not what i told her i paid for it

  6. #6
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    Aug 2019
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    Sounds like an interesting job. Are these series or parallel boxes? Will the hwc's produce a useable leaving air temperature ? Are the hot water control valves 2 or 3 way? Sizing the control valves based on the CV is the way it should be done . I will certainly have more thoughts on this one , probably a bit unorthodox though .

  7. #7
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    Thread Starter
    The boxes are parallel. The leaving air temperature is good. The control valves were originally sized by Cv. The control valves are two-way equal percentage.
    The coil sizes were changed but everybody forgot about the circuit setters and control valves.
    No man can be both ignorant and free.
    Thomas Jefferson

  8. #8
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    Aug 2019
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    Is this a multiple story building ? How many boxes and coils are you dealing with? The reason I ask is you maybe better off not balancing each coil and letting the two way valves do their thing . Slowing down the pid loop helps when valves are oversized. Being the fan is your 1stage of heat you could have the valve open 2 degrees lower than the fan comes on .

  9. #9
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    Thread Starter
    It is a two story building with 22 FPB'S. I have advised them that since the control valves are two-way equal percentage that balancing the water is not only a waste of time but the system will work better if not balanced. On the oversized valves the equal percentage aspect is out the window. The valves will hunt and their life will be shortened somewhat but there is only two choices at this point. Replace the valves now or replace them later after they fail with the right valve. The oversized valves are better off not balanced also because the circuit setters will be so near to closed that they will plug. The control valves won't because they will open enough to clear any debris that might plug them.
    No man can be both ignorant and free.
    Thomas Jefferson

  10. #10
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    Aug 2019
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    Slowing the loop speed down will help a bunch with the hunting . Most all loop speeds are way too fast anyway as the control guys want to see stuff move when they change a setpoint . The problem is that the system doesn't really want to work that way . You might want to drive the valves open and set the flow to each floor . The other thing I would do is while they are driven wide open in the heat mode ( and the parallel fans on ) is get a snap shot of the leaving air temperatures of each box. If the control system doesn't have that feature I would just shoot one grille off each box with a Raytek . Very good move by suggesting they not balance the water . As these are two way valves I assume there is a vfd or a bypass for the pump . Getting the right setpoint is much better than trying to balance this turd . You are definitely steering them in the right direction , we will see if they listen .

  11. #11
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    Thread Starter
    I can't proportion the water to each floor because there are only circuit setters at each coil and none for the loop. As oversized as some of the control valves are slowing the loop speed will not help much. The control guy is really good and after I alerted him to the problem has already slowed it down.
    This situation is rare with the folks I'm working with. The engineer is good and values what I do and gives me a free hand. The installing contractor is real smart and particular and also trusts me. The owner's people don't know much about this kind of thing but trusts all of us. The problem is a result of last minute changes and the water got overlooked.
    I ran the calculations for air discharge temperature and all the boxes looked good but one and it was borderline at design airflow. I cut the primary air minimum a little and got the design up to 95 F with 180 F water. Even with outside air reset the box should heat with no problem.
    I also calculated the threshold of cavitation and it is about 24 PSI. Given I only need about 9 PSI max cavitation should not be a problem unless vena contracta recovery is a problem due to the valves throttling so near to closed. What do you think about having the control guy not control flow on the grossly oversized valves? I've thought about having him throw them half open and holding them there as long as heat is needed.
    Looks like we are thinking a lot alike brad on this. Welcome aboard.
    No man can be both ignorant and free.
    Thomas Jefferson

  12. #12
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    Aug 2019
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    On the one that you had to reduce the primary minimum airflow down , why not just let it go to zero ? There are enough boxes that have minimums to provide ventilation so I wouldn't hesitate to do this to achieve a good LAT . Actually , you might take the worst 3 to 5 of them and go to zero minimum. Not really sure about driving the valves half open on a call for heat . I didn't think of that so let me ponder a bit . I think I would question the control guy a bit about how slow the valves will drive . To me I would want at least 6 minutes for them to go from closed to wide open . Every control guy I know thinks 120 seconds is plenty . Thinking further , you could balance the flow ( kinda ) on each floor by taking the return water temp on each one and tweaking the isolation valve for one floor a bit . If you are thinking about cavitation I assume there is no VFD or bypass ? Is there diversity between the coils and pump ? Yes , we do think alike and thanks for making me feel welcome . Cheers , bud .

  13. #13
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    You hit on how I would prefer to handle this when you suggested setting the total flow to each floor. If I had the branch circuit setters I would set each floor to 2/3 design and have the outside reset minimum bumped up about 10 F on the minimum side. That would allow me to have the pressure to deliver the water where ever needed but the max flow would be limited. That would in essence give me a rarely used diversity that would only be used during warm-up on not so cold days. I love hot water because it is so forgiving but at the same time the forgiveness opens so many possibilities in situations like this it gets more complicated than chilled water. Almost any way you do it will work so you have to look at the long run.
    It really gets interesting.
    I forgot to answer about my concern with cavitation. There is a VFD but I am wondering if the vena contracta will extend beyond the valve into the pipe when the valve is throttling close to the closed position. The recovery factor on these valves is 0.7 but that is with the valve full open. What happens when controlling almost closed? In this application I really don't know for sure but suspect some cavitation will occur but will probably not be to the level of damaging anything. I am concerned about cavitation as a result of velocity not pressure.
    Last edited by WAYNE3298; 11-30-2019 at 09:18 PM.
    No man can be both ignorant and free.
    Thomas Jefferson

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