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I initially thought it could be two things:

• Either this is just due to the Building's Temperature Converge Tolerance Value that has a default of 0.4C
  • Lowering that temperature to 0.2C didn't change anything
• Or this may be a very similar issue as the one I posted here and where it was rightfully pointed out to me that this was because of the AvailabilityManager:NightCycle reporting tolerance of 1C by default. I'm thinking having set the Supply Air Fan schedule to always 0 might be triggering some built in availability manager with a hardcoded tolerance of 1C
  • Setting an Always 1 schedule for the supply air fan didn't help either

Then I realized it was something else entirely. I actually have a DOAS supplying unconditioned air to the zone as well. I've created all my systems (DOAS and ZoneHVAC) by using the OpenStudio API. Turns out I have assigned the ZoneHVAC component to the zone before I assigned the DOAS's AirTerminalUnit (ATU). As OpenStudio will generate the cooling and heating priorities based on this specific order (as pointed out on unmethours and on uservoice), my ZoneHVAC had a priority of 1 and the DOAS 2, the exact opposite as what you need to do.

From there I had two solutions: either assign the equipment to the zone in the right order, or just use setCoolingPriority and setHeatingPriority (see SDK Doc)

Here's a ruby example of the second method:

# I've already added the ZoneHVAC equipment, now I add the DOAS ATU
atu = OpenStudio::Model::AirTerminalSingleDuctUncontrolled.new(model, model.alwaysOnDiscreteSchedule)

# I could just have created this terminal BEFORE creating the other and it would have ended as first priority for cooling and heating
# But I'll set it after the fact as a good reminded that OS will by default use whatever order equipment to create the ZoneHVAC:EquipmentList
# You should always have the least controlled equipment (DOAS, etc) first in line, to let the most controlled one pickup the remaining load
zone.setCoolingPriority(atu, 1)
zone.setHeatingPriority(atu, 1)