Location of Preheat coil

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This is a baseline case. It does not have a heat recovery coil.

Thanks for your opinion. This is a requirement for the baseline case. We are happy when the baseline case consumes more energy i.e., happy if the preheat coil can be located in the OA stream.

I was just speaking from a design perspective. I think there'd only be a preheat coil before the mixing box for frost control with exhaust air heat recovery. The Engineering Reference (16.8.1.4) says: "The preheat coil should be controlled to maintain a minimum supply air inlet temperature thereby eliminating frost buildup on the heat exchanger core." My interpretation would be one preheat coil to bring the OA up to 35F, then another preheat coil to bring the MA up to 20F below heating setpoint.

I agree the location(s) should be stated explicitly in 90.1.

Say, OA is 0ºC (32ºF), and the indoor heating setpoint is 20ºC (68ºF). The preheat coil heats the air up to 9ºC (48ºF). If the preheat coil is located in the OA stream, the coil operates. If the coil is located in the mixed air stream, the mixed air temperature is the mixture of OA (0ºC (32ºF)) and RA (20ºC (68ºF)), which can be above 9ºC (48ºF), and the coil would not operate. That happened in my baseline case. The location matters.

Excellent point. I would say in most VAV systems this would happen (the temperature after the mixer would be above 9'C), as the return air flow from the zones is preheating the total air flow going to the AHU coils. The main thing is deciding what the purpose of the preheat coil is. I would always go back to the configuration of the system you are comparing with and put it in the same position, or if you aren't comparing it look at its purpose (is it to ensure the components in the air handling units don't freeze?) and put it in the correct position.

I would just like to know the correct interpretation of ASHRAE 90.1 Appendix G. Preheat coil is required in the baseline systems 5 through 8 even if the proposed case does not have it.