12
Appendix B – Evaporative bypass cost calculations
Energy required to convert water to vapor (BTU/day) = GPD required * latent heat of vaporization * water weight (pounds per gallon)
Humidity required is 6.5 GPD as defined by AHRI for the example above
Latent heat of vaporization is 1000 BTU per lb of water
One gallon of water weighs 8.3 lbs
Energy to convert 6.5 gallons of water to vapor = 6.5 * 1000 * 8.3 = 53,950 BTU
Energy content of natural gas = 1031 BTU per ft3 natural gas
53950 / 1031 = 52.33 ft
3
/day of natural gas
Cost of natural gas = 52.33 ft
3
* ($20.19/1000 ft
3
) = $1.06 per day
Cost of electricity to convert water to vapor = [(53950 BTU/day) / (3414 BTU/kW hr)] * ($0.11/kW hr) = $1.74
Water cost = Total used * (cost of water + cost of sewer)
(43.2 GPD)(.003 + .003) = $.026 per day
Cost to heat water with natural gas (heating 60°F to 120°F) = (43.2 GPD)(8.3 lbs/gallon)(1 BTU/lb-F)(120-60°F)($20.19/1000 ft
3
) /
(1031 BTU/ft
3
) = $0.42 per day
Additional information can be found online at www.forwardthinking.honeywell.com/truesteam
Appendix A - Humidity requirements in GPD based on AHRI Guideline F
500 1000 1500 2000 2500 3000
Average 1.3 6.6 10.8 15 19.3 23.4
Loose 2.6 10.7 17.1 23.5 29.8 36.1
Tight 0.1 2.3 4.4 6.5 8.6 10.7
*Based on people living in the home.
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