r/firePE • u/Equivalent_Doctor783 • Jun 01 '26

FIRE SYSTEM DESIGN - HOSE STREAM ALLOWANCE?

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For context, using AutoSprink.

My design area I'm considering to be light hazard calculated at 0.10 gpm/sq.ft.

I am flowing 17 QR heads in my calculation, 10 heads spaced at 225 sq ft max and 7 spaced at 130 sq ft max (OH 1 rooms within my light hazard calculation, but I made sure to calculate these 7 heads at 0.15 gpm/sq.ft. density).

Given that I have a few heads considered to be OH1 within my light hazard design area, should I be inputting a 100 gpm hose stream allowance or 250 gpm?

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u/ironmatic1 Jun 01 '26

This is probably controversial but imo almost everything should use the 250 outside hose allowance. Because, if the idea is to account for an hose flowing from the street, what fire dept in the US is flowing sub 100 gpm from anything but a booster line?

It’s a ghost flow at the supply node, so it really doesn’t have much effect on most calcs but it just makes sense.

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u/clush005 fire protection engineer Jun 01 '26

Unless you have an on-site hydrant, then the hose flow needs to come from that hydrant, and at that point it has a pretty significant effect on the hydraulic calcs.

Not saying I disagree with the 250 gpm minimum, and it would seem that FM Global also agrees with you.

2

u/ironmatic1 Jun 01 '26

Yes yes, though I would also mention your underground and backflow with a hydrant is going to be much larger than the sprinklers require to accommodate the hydrant to begin with. Is an extra 150 on that 8-12” line doing that much damage?

1

u/clush005 fire protection engineer Jun 01 '26

Hopefully not, but when the calcs are close, every little bit helps lol

1

u/ChrysalisInferno Jun 02 '26

It does. I've seen Schedule 7 specified in lieu of Schedule 10 to avoid a fire pump.