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reverse return question

Brad White_9Brad White_9 Posts: 2,440Member
Your pipe fitter is probably correct and is looking out for your best interests, IMHO.

Here is the cautionary fallacy about reverse-return piping:

It allows the same "potential" pressure differential at each unit served. It works (by "works" meaning intent of being self-balancing) only when the unit's served have the same pressure drop. It is doubtful that your WSHP's have close to the same PD's.

The other is that reverse-return piping as a design concept works the first time you design it. Unless provisions have been made in the form of extended header pipe sizes with tees and valves, any addition to the system is going to "violate" the design precept.

(If the system was important enough to design reverse-return in the first place one has to assume that the concept is essential to it's operation.)

Enough theory; back to reality: Will one heat pump at about 1/60th of the total throw things out of whack to the point where the others will be adversely affected? Probably not if it is remote from the pumps; Probably so if it is close. But how do you know that this is the first one added? Takes a big picture.

Here is what I do for WSHP jobs: I use Griswold constant flow valves. WSHP's are rather fussy about constant water flow and this is the only way I know of to approach this. Your other WSHP's may already have them; that alone is reason to do so in this case.

My first course of action is to exhaust the option of "doing it right", tying to the tail of the return main even if you have to chase it to a larger pipe size. And if Griswold's are installed elsewhere, here you must also.

Here is my second choice of action: I would at least for the one you are installing, put in a Griswold valve. If you do not, or if manually balanced and someone tampers with it, you create a DP bypass where you need it least, close to the pumps. That one path could thwart flow to the others, it is true, and be noisy in the process as it fights total head.

My $0.02,

Brad

Comments

  • Matt_21Matt_21 Posts: 140Member
    we are working

    on the ground floor of an existing 3 story building with a water source heat pump system. we are adding a heat pump to the system. the engineer shows us tying into the 3" supply main and the return tying into the 1 1/2" return. the system is reverse return. our new heat pumps will be the first supplied and last returned. there is no way to tie the new 1 1/4" return into the 1 1/2" return, live so we were looking into tapping into the 3" return main. the heat pump would now be 1st supplied 1st returned. my pipefitter contends this will throw the system out of balance. my feeling is the small amount we are flowing through the heat pump is too small to affect the whole system. he also thinks it will throw off the water temperatures. there are 18 heat pumps on our floor and i'm guessing about 60 in the whole building. will this be okay to do?
  • Matt_21Matt_21 Posts: 140Member
    brad

    thanks for your response. this system doesn't have any balancing valves what so ever. each branch to the whsp have two ball valves and the flexible hoses you buy from mcquay and thats it. i think the pumps are located on the first floor somewhere so i don't think that is an issue. the return main at the first heat pump (last return) is 1 1/4" and increases to 1 1/2" at the next heat pump. after the return ball valve serving the first heat pump, could i tee in my new heat pump which also has a 1 1/4" line? now i would have 2 1 1/4" returns tying into the 1 1/4" return main. i'm afraid the 1 1/4" won't be able to handle the flow. each whsp is 7.6 gpm. if this won't work, i think i'll go back to the 3" return and install the griswald like you said. is the problem with the manual balancing valve once you have it set is the flow is variable in the system or it's more of a concern of someone changing the setting?
  • Brad White_39Brad White_39 Posts: 18Member
    Two 7.6 GPM Heat Pumps

    walk into a bar... Ooops! Wrong forum! :)

    Where was I?

    Two 7.6 GPM heat pumps connected to a 1-1/4" line is pushing things a bit at 15.2 GPM, (that is, how can you tell that really is your flow?). You need to be sure.

    There generally is no flow diversity in a WSHP system as you know. It is close but should work if you have enough head. How far until the pipe size increases to 1-1/2"?

    If you want to try something, how is this: Take a look at heat pump performance at reduced water flow (they have a range). See if the performance still meets the intent of the zones. Then you will know if you can share the line or not. Otherwise I would see about extending that 1-1/2" line back to you as a header. That should handle three WSHP's of that size no problem. But do install balancing valves. Using ball valves as balancers is really guessing at things. You could have a lot more water flow to spare with proper management (balancing).

    My balancing valve concern is if someone changes the setting. Lock that valve if you go that route.
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