Working on a fairly large lab building using DOAS AHUs and a glycol heat recovery run around loop. All exhaust air is blown through the exhaust air handler to heat glycol loop that then is used as a preheat on the AHUs.
Design day temperatures are 0F, therefore making the AHU preheat temperatures 75F in and 30F out to the Exhaust heat recovery. Engineers are worried about frosting the coil in the exhaust since the coil temp could be 30F even though 70F building exhaust will blow against it.
So we are adding extra piping and valves for a AHU bypass to keep glycol temps in the exhaust coil above freezing. Does this make more sense then raising the AHU temp in to 80F or 85F to prevent leaving temps going to below freezing?
I will try to add flow diagram tomorrow to help better visualize.
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