A building's fixtures total 180 water supply fixture units. Of that total, 120 units belong to flushometer valve water closets and urinals and the remaining 60 units belong to lavatories, sinks and flush tank fixtures. Take these conversions as given data: on the predominantly flush tank curve 60 units equals 26 gallons per minute and 180 units equals 52 gallons per minute; on the predominantly flush valve curve 120 units equals 65 gallons per minute and 180 units equals 82 gallons per minute. The design procedure on this project directs that a system containing flushometer valve fixtures be converted on the flush valve curve using the total of all fixture units in the system. What design flow sizes the service?
- AA design flow of 52 gallons per minute
This reads the full 180 units on the flush tank curve. The flush tank curve assumes no fixture dumps 25 gallons per minute in a ten second burst, which is exactly what a flushometer valve does, so it undersizes the service badly.
- BA design flow of 65 gallons per minute
This converts only the flushometer valve fixtures and forgets the lavatories, sinks and tank fixtures that share the same pipe. Every fixture connected downstream of the section being sized has to be in the total, whatever its type.
- CA design flow of 91 gallons per minute
This converts each group on its own curve and adds the two demands, 65 plus 26. Each curve already contains a diversity allowance, so adding two curve readings applies that allowance twice and produces a flow the building will never see.
- A design flow of 82 gallons per minute
Why D is correct
One curve is entered one time with one number. The flushometer valve fixtures force the flush valve curve, and the total load on the pipe being sized is all 180 fixture units, so the reading is 82 gallons per minute. Converting the two groups on two curves and adding them double counts the diversity that each curve already contains.
What this question is testing
This checks whether you can decide which demand curve a system belongs to and what total belongs in it, which is the first decision in every water sizing problem and the one that costs the most when it goes wrong. It also checks whether you understand that a demand curve is not additive.
On the job
Mixed systems are the normal case in commercial work: a shopping center with tank type closets in the tenant spaces and a flushometer valve restroom in the anchor. The estimator who converts the two halves separately and adds them ends up buying a service one size larger than the job needs, and the plumber who reads the whole thing on the tank curve ends up with flushometer valves that will not complete their cycle when two fire at once.
Memory technique
One pipe, one curve, one total: never add two curve readings together.
Exam tip
If any flushometer valve fixture is on the system, the flush valve curve governs and the whole fixture unit total goes into it once.
Where to look it up
Water sizing procedure and the demand curves live in IPC Appendix E, with the load values in the fixture unit table there. UPC states size water piping from their own fixture unit table and their own sizing appendix, and the numbers are not the same, so confirm against the code your state adopted.