Water heater size for emergency shower

[Spacemouse]

I am sizing a gas fired tank-type water Hester for an a combo emergency shower eyewash. Looking to size water heater to deliver 140 F and then use mixing valve to temper water to 80 F to fixture.
My question: do I size the water heater for the full 23 GPM required at the combo fixture for the 15 minute duration or do I size it for the ratio of hot water going to the mixing valve (which would be much less than 23 GPM)for 15 minutes?

 

[PEDARRIN2]

The ANSI standard Z358 4.1.2 for emergency fixtures requires 20 gpm for 15 minutes which would require a total of 300 gallons of the tempered water. Depending on what plumbing code you use, tempered water is defined as a certain temperature range. In the IPC, it is 85F to 110F.

So you are required to be able to deliver 300 gallons of the 85-105 F water. How you do that is up to you. There are instantaneous heaters that will do it, but you will need a lot of gas or electricity for the quick heat up. Or you can use a storage heater with enough recovery and storage volume or temperature to get the 300 gallons of water.

Remember recovery is usually denoted for an hour. You will need to use 1/4 of that vaulue since you only have 15 minutes of use.

Sometimes I have designed for a lower tempered water temperature so that I did not have to store as much water.

 

[Spacemouse]

Would you still size the water heater for the 300 gallons, or just for the fraction of hot water that goes to the mixing valve?

Example: if cold water at 40 f, hot water at 140 F, and mixed water temp leaving the mixing valve is at 70 F, the ratio of hot water to cold to the valve is approx = (Tmix- Tcold)/(Thot -Tcold) = 0.3. If the water for case when both emergency shower portion (20 GPM) and face wash (3gpm) are active at the same time, 23 GPM x .3 = 6.9 GPM. Then over 15 minutes would be approx 104 gallons instead of 345 gal.

 

[PEDARRIN2]

You would size for the fraction. But also size for any other hot water demands in the building. An emergency fixture is more of a dump load than other fixtures would be.

You are producing 300 gallons of water at the tempered water temperature. The amount of water from the heater would be the fraction of hot water at hot water temperature required to produce the tempered water temperature.

From ASPE, if Tm is the tempered water temperature and Th and Tc are the hot water and cold water temperature, the portion of the 300 gallons you need to generate is calculated by

P = (Tm-Tc)/(Th-Tc)

300 x P = gallons of hot water in the heater to be generated.