Integrate makeup air with AC ducts and system?
Can I safely bring fan-powered makeup air into the return plenum of our ducted minisplit air-conditioning system? What about into the supply ducts?
I would like to use the ducts we have in our house to supply makeup air, but I do not want the minisplit air handler to be electronically integrated with the makeup system.
Martin’s makeup air article on this site shows the option of a motorized intake damper on the return plenum of an air handler, but we are proposing a different system.
We would like to connect a variable-speed makeup air fan, such as those made by Fantech, to the duct system, to either the return or supply side. The fan would share the ducts but operate entirely independent of the AC system, supplying up to 1200 cfm fresh, filtered outside air based on demand from various air-exhausting appliances in the house. Such appliances will include a kitchen range hood, clothes dryer, wood-burning fireplace, and bathroom fans.
It seems like it would be silly to install a completely separate duct system to supply makeup air to all of the areas throughout our house. We would like to use our well-designed and carefully installed ducts to serve both AC and makeup air!
Can we do this? Will it harm the AC system or risk other problems?
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You have to make sure your ducting is sized to handle the 1200cfm.
The blower in those MUA can do a fair bit of pressure, so you don't want it downstream of the ducted unit as it can cause it to stall out.
If your return ducts are big enough, it should work fine there. If the mini split is off, all the makeup air will come out through your return registers. If the mini split is running, depending on the flow rate of the MUA, most of the air will go through the mini split and be conditioned before distributing through the house. I would check with the manufacturer what the minimum temperature of the mini split is in heat mode. Too cold of supply air in cold climates might cause the ducted unit to fault.
The better option would be to split the flow of the MUA. Part of it going through the ducted unit and part being supplied in the kitchen near the range hood. Search for MUA design guidelines for restaurants, they provide recommended ratios for this.