Maximum DFU's for a laundromat. Plumbers please help!

[Sifu]

Is there a diversity of demand (or some similar method) that is or can be used when calculating the maximum DFU's for a branch drain and a building drain? I have a proposal for 28 commercial washers installed in two bulkheads of 14 each. Each machine discharges 45gpm, which is converted to 90 DFU's per IPC 709.3. Each bulkhead is drained by a single branch drain, both of which converge into a single building drain. By my estimation each branch would be required to be 8" and the building drain would be required to be 12".

This seems really high, so maybe my reasoning is flawed. The proposal is for 4" branch drains, and a 4" building drain. This seems really low.

709.3 says "where discharges to a waste receptor or to a drainage system are only known" in GPM, then you calulate it at 2 DFU's per 1 gpm. Per t709.1, each would only have 3 DFU's. The MFR has only provided the chart below, showing discharges in gpm. I must be reading something wrong.



All guidance is appreciated.

 

[north star]

# = # = #

Sifu,

Your calculations are correct ( per the IPC )
requirements.......As long as you can provide
your calculations and the applicable Code
Section(s), then, IMO, you are on the correct
path for compliance.

You can provide your data to the contractor
and let them rebut with their Code Sections &
calculations.

# = # = #

 

[Sifu]

The bulkheads each have 12"x18"x16' drain troughs, with a capacity of about 180g. The simultaneous release of every machine at maximum discharge rates would result in a maximum discharge of 216g. The trough would slowly release the volume of water into the 4" branch drain. The other is slightly less. I am thinking the flow rate of the trough meters the flow into the branch drain so that the even under maximum potential discharge the trough will not overflow, and the branch and building drains will not back up. Any flaws in that theory?

 

[Paul Sweet]

I think that 709.3 applies to continuous flow. The washers just discharge once or twice per cycle.

Table 709.1 shows 6 FU for a shower flowing 25.8 gpm to 55.6 gpm, which is way less than 2 FU per gpm. Table 709.2 shows 5 FU for a 4" drain.

I think it would make sense to size the trough drains for 14 x 6 = 84 FU. According to Table 710.1(1) a 4" drain would be adequate for each trough, and a 4" main drain would be adequate for the total 168 GPM.

If I were designing it I would probably be a little more conservative and increase the trough capacity to hold the maximum possible discharge of 216 gal. each, and make the main drain 6" (Table 710.1(2)).

 

[Sifu]

There is a huge gap between the 2 DFU/gpm and the prescriptive table. In this case, the building drain is existing and they may have worked somewhat backwards from there. If all machines discharge at the same time at maximum volume there is a slight possibility that the trough could not keep up but I think that unlikely event is covered by the requirement for an additional 3" floor drain.

 

[PolYoil]

A 2-inch drain for each washer sounds small. Some codes allow multiple washers on a line, but oversizing the drain helps avoid backups. Venting properly is just as important too.

 

[ICE]

The odds of all of the washers operating simultaneously are slim. If they did all operate simultaneously the odds that they would all discharge at the same time are slim. There is a sweet spot in sizing the pipe that facilitates scouring. Too small or too large defeats scouring.

 

[PolYoil]

A blocked drain in a laundromat can become a real headache fast, especially with high DFUs. If the system isn't sized correctly, lint and detergent buildup will make things worse. I've seen cases where upgrading to larger-diameter piping and adding lint interceptors made a huge difference. Also, regular maintenance helps a lot.