Increasing Free Hot-Water Generation
Published Friday March 16, 2018
Increasing Hot-Water Generation
I have a theory that by increasing the tank size for the desuperheater (the component that gives me free culinary hot water whenever the heat pump is running), that the active hot water heater energy usage will be reduced by an additional 66% during winter and summer months when heat pump usage is the highest.
This has been shown to be the case through calculating the shortfalls and surpluses of each day throughout the year.
Adding an additional 40 gallons increase free hot water availability in December by 59% and in January by over 65%.
With an additional 60 gallons, the gains top out. In January, an additional 60 gallons of storage capacity would satisfy all culinary hot water needs and the resistive water heater would not have to turn on at all.
While my calculations are sound, they are still purely speculative. However, I am confident that they are sufficiently accurate to justify spending the money to add an additional 50 gallon passive water tank.
Time will tell if it makes a difference or was a waste of time and money.
By knowing flow-rate and temperature delta, you can calculate the BTU output of the desuperheater.
While the spec sheet claims it can produce up to 18,500 BTU of heat, based on direct temperature measurements of input vs output water temps and an assumption of flow-rate, (0.4 gpm per ton of conditioning), I come up with only around 1800 BTU of heating capacity. This is in line with the time it takes for the heat pump to bring 50 gallons of water up to temperature.
This small BTU heating rate appears to vary depending on the temperature of the incoming water. It heats more quickly when the water is cool and slows down as it approaches the temperature set-point of the active water heater.