Published Sunday October 29, 2017: 

Increasing Hot Water Generation

The Tranquility 30 ground loop heat pump has a cool feature that whenever it is heating or cooling the home, it also heats the culinary water supply to supplement the regular hot water heater. On the coldest winter days and during the hottest summer days, this desuperheater (that's really what is it called) works so well that 100% of our culinary hot-water needs are met. 

I have a theory that by increasing the passive tank size for the desuperheater, 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. 

Currently I have a 50 gallon passive hot water tank that the heat pump uses to create free hot water and a 40 gallon active, (powered) hot water tank to make up the difference in water temperature. 

Adding an additional 40 gallons increase free hot water availability by around 45%.

40 Gallons= 45% increase in free hot water supply. 

50 Gallons = 66% increase in free hot water supply. 

60 Gallons= 70% increase in free hot water supply. 

Above 60 gallons the gains quickly diminish. 

My goal is to get the hot water heating energy demands as close to zero as is practical. Reducing this energy demand, (especially during the winter months) is paramount to being able to go totally off grid. 

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 Tranquility 30 spec sheet claims it can produce up to 18,500 BTU of heat from the hot water generator, 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.