Zone Valves : Setting the Differential Pressure Bypass Valve

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lsayre
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Post by lsayre » Thu. Dec. 24, 2015 5:59 am

Once you know your closed loop systems friction head for the singularly specific case wherein all zone valves are open you can properly set your differential pressure bypass valve from this data.

The simple formula to use to determine the pressure setting to dial into the differential pressure bypass valve (DPBV) is as follows:

PSI = (Friction Head x Water density)/144
Where friction head is in feet
And where water density is in pounds per cubic foot

Example for friction head with all zone valves open and circulator running = 5.8', and nominal water temperature = 150 degrees F. (whereby water density = 61.2 lbs per cubic foot):

PSI = (5.8 x 61.2)/144
PSI = 2.47

Dial in 2.47 PSI on the spring tensioner atop the DPBV and you are good to go.

*** Some manuals suggest dialing in a tad more pressure than you have calculated (typically 0.5 PSI), and I presume that this is to allow for the potential of error in the friction head measurement and/or for water density variations due to temperature swings, or to compensate for spring tensioner pressure vs scale accuracy error within the DPBV unit itself. Or perhaps to allow for the spring to weaken with age and use. ***

 
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Rob R.
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Post by Rob R. » Thu. Dec. 24, 2015 6:31 am

Larry, do you get that reading off some sort of gauge on the bypass valve?

If I was doing a fresh system I would probably just use one of the new variable speed circulators and skip the dpbv.


 
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lsayre
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Post by lsayre » Thu. Dec. 24, 2015 6:37 am

Rob R. wrote:Larry, do you get that reading off some sort of gauge on the bypass valve?

If I was doing a fresh system I would probably just use one of the new variable speed circulators and skip the dpbv.
There is an adjustable spring tensioner atop the DPBV which is scaled (for the case of the Taco version at least) in PSI. Others (made in Europe) are likely pressure scaled to the metric system and would need conversion to PSI.

I fully agree that variable speed circulators make this all unnecessary. And as their cost comes down they are approaching the combined cost of a standard circulator and a DPBV.

Bu then again, most people find that they also need a bypass valve in order to temper the returning water so as to not shock their boiler. The DPBV serves this function admirably. When only one zone is open the DPBV is forced to maximally open, and as more zones open up it closes progressively until (if set correctly) it is fully closed when all zones are calling for heat, so perhaps in a zone valve application it serves this purpose even better than a fixed position bypass valve which never closes or adjusts as conditions change.

The DPBV would fight against a pressure sensing variable speed circulator and confuse its programming, so this pairing should not be attempted. It's either a standard circulator and a DPBV or a variable speed circulator with a fixed percentage of open valve bypass (or no bypass valve at all). Tough choice. If doing it again I just might chose the variable speed circulator route also.

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