Systems & CodeQuestion 51 of 110
A thermostatic mixing valve blends 140 F stored water with 60 F cold water to deliver 110 F tempered water at a public lavatory. Using a mass balance, what fraction of the delivered flow must be the 140 F hot water?
a.37.5%
b.50.0%
c.62.5%
d.75.0%
Explanation
Set 140f + 60(1 - f) = 110, so 80f = 50 and f = 0.625, or 62.5% hot water. The mixing valve holds storage at 140 F to control Legionella while blending down to a safe 110 F delivery. The cold fraction is the remaining 37.5%.
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