EnergyPlus treats the initialization of design day sizing periods the same as any other run period. The way Kiva is handled during warmup is described in the Engineering Reference Documentation:
The traditional “warm-up” period in EnergyPlus (of repeating a single day) presents several challenges for foundation heat transfer calculations:
As the ground can have time constants on the order of years, a single day is simply not long enough to adequately capture the thermal history of the ground.
Any repetition of a single day would erase any pre-calculated thermal history and likely take much longer to converge.
Instead, Kiva instances are initialized independently from the rest of the simulation using the accelerated initialization method developed by Kruis (2015). This method looks back in the weather file and simulates long timestep (on the order of weeks or months) calculations using an implicit numerical scheme. These long timesteps allow Kiva to capture a long term history of the ground without running the entire building model.
The initialization of the ground relies on assumptions of indoor air temperatures (as they are not yet calculated by EnergyPlus). When a thermostat is assigned to a zone with Kiva foundation surfaces, the assumed temperature is equal to the setpoint (or a weighted average of heating and cooling setpoints depending on outdoor temperature). For zones without thermostats, a constant 22 C indoor temperature is assumed.
The calculation of design loads from foundation surfaces is not well established, and is part of the focus of an ongoing ASHRAE Research Project.
To answer your question: No. The initial condition is not a uniform temperature, it is initialized based on long-term dynamics of the ground. The state of the ground is then held constant during the warm-up period so that it is not erased and doesn't impact the time to convergence for the warm-up period.
This is not ideal, but a reasonable approach given the current limitations of EnergyPlus's warm-up approach.
