Let’s say the design condition is a 10°F outdoor temperature just before sunrise (no sun) with no internal gains in the space, and maintaining the room at say a 70-F setpoint under these conditions results in a quasi-static load of 464 kBtu/h. Multiply this by 1.25 and we get 580 kBtu/h.
If sized according to ASHRAE requirements for baseline building equipment, the ASHP has to have a capacity of 580 kBtu/h, and we assume this should be at the “rated” condition (this is where ASHRAE is more than just a bit unclear). If we then go and look at the rating conditions for an ASHP in a baseline system (EER tables at the end of 90.1 chapter 6), we see that the ARI rating condition is an outdoor temperature of 17°F. Thus, following this logic, our baseline ASHP should be capable of providing 580 kBtu/h at 17°F outdoor temperature. If the load is greater or the outdoor temperature is lower, it may not actually meet the load. However, as the design load was determined at 10°F outdoor temperature and the equipment must be 25% oversized, this probably will never by a limiting factor for a baseline ASHP system. There will, however, be another very significant limiting factor: The ASHP probably will have a lower limit, such as 17°F, below which it will provide no useful heat…
