Why is the solar fraction for the DOE Very Small Usage Pattern (10 gallons per day) lower than the DOE Low Usage Pattern (38 gallons per day) in some cases?

There are several factors that could cause the solar fraction for the Very Small Usage Pattern to be lower than the Low Usage Pattern.

1. The DOE Hot Water Usage Patterns withdraw different amounts of water at different times of the day.  The timing and amount of each hot water draw relative to the amount and timing of the solar energy being collected impacts the total annual solar fraction.
2. The relative impact of system inefficiencies is different for the different DOE Water Usage Patterns.  The ICC-SRCC performance models also account for the standby losses (energy lost as the hot water stored in the tank cools) and parasitic losses (energy used to run components like pumps).  For the 10 gpd draw, the energy in the standby losses and the hot water draw is comparable to the standby losses for a basic electric resistance tank.  But for the 84 gpd draw, the energy in the standby losses are 13% of the hot water draw.
3. The impact of enclosed volume in piping between the collector and hot water system varies with draw size. For all OG-300 ratings, the piping between the collector and hot water system is assumed to be 25 feet each way.  For ½” copper piping, that results in 0.33 gallons enclosed in the piping.  For ¾” copper piping that number grows to 0.67 gallons.  For the DOE Very Small Usage Draw Pattern (shown below), the individual draws are very small, leading to relatively small amounts of hot water from some passive rooftop-mounted systems reaching the backup water heater for a given draw.  For example, if a 0.5 gallon draw is started, and ¾” pipe extends from the rooftop system to the backup water heater, none of the water heated by solar reaches the backup heater before the draw ends.  Therefore none of the water heated by solar enters the hot water system.  Instead, it sits in the piping and cools significantly before the next hot water draw.  Even for larger draws, relatively little water heated by solar enters the backup water heater.  Using the same example, a 2.0 gallon draw with 3/4” piping results in only 1.3 gallons of water heated by solar reaching the backup water heater during the draw.  In this situation, the backup water heater must supply a larger percentage of the hot water called for in the hot water draw.  Plus, after a draw, colder water is introduced from the water supply that must then be heated to meet the needs of the next hot water draw. For active systems that use a pump to circulate the hot water between the collector and a separate tank, this effect is far less pronounced.  These pumps usually activate when the water in the collector reaches a pre-set temperature range and deliver it to the tank regardless of whether a draw is occurring.

Performance in real-world conditions will vary based on differing tubing lengths and sizes, controls, system designs and actual hot water draws.  But for OG-300 ratings, variables like the piping length is held constant to allow for comparison between the performance of different systems using the same hot water draws.