Supply ventilation via ducted mini-split
Reid Baldwin
| Posted in Mechanicals on
I notice that some of the ducted mini-split models have provisions for a fresh air port. Would using this be a reasonable way to implement a supply-type ventilation system? They say it should be limited to 10% of the flow rate. If that doesn’t satisfy the required ventilation flow rate, how best to supplement it?
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Replies
Reid,
The type of ventilation system you are proposing is called a central-fan-integrated supply ventilation system. This type of system must be carefully designed and commissioned, to avoid problems with underventilation and overventilation.
I have never heard of this type of system being installed in conjunction with a ducted minisplit, but it is theoretically possible. Such a system would need an AirCycler control (also called a FanCycler control) and a motorized damper. The motorized damper needs to be installed in the outdoor air duct.
For more information on central-fan-integrated supply ventilation systems, see Designing a Good Ventilation System.
My understanding of CFIS is that the fresh air flow rate, when operating, is substantially above the required ventilation rate, but the air cycler and fan cycler control the fraction of time that ventilation is provided. That makes sense with a traditional furnace which is designed to cycle on and off and has a big fan. The fact that mini-splits are design to run continuously at lower air flow rates seems to call for something different.
Depending on which rule I use, I need a ventilation rate between about 60 cfm and about 120 cfm. The 12RLFCD has a flow rate that varies between 283-383 cfm. I cannot recall where I saw the 10% limit mentioned. I cannot find that now. If the 10% limit is applicable, then the maximum fresh air rate would be only in the 30 cfm range. If I have two ducted mini-splits in my house, I would have to run the fans continuously in order to even get to the low end of the required ventilation.
The ceiling cassette units require a separate accessory to provide fresh air. The description of these accessories indicates that you need a separate fan in the fresh air ductwork to push the fresh air at the correct rate. The ducted units don't need an accessory. I didn't find any information about whether they are capable of drawing the fresh air in themselves or need a separate fan.
On the one hand, the fresh air inlet doesn't really seem like it is spec'd to be a ventilation system. On the other hand, I cannot think of a different reason for providing the inlet. Is the idea that you would connect these inlets to the fresh air outlets of an HRV/ERV?
Reid,
If you are thinking of installing a ducted minisplit unit that includes a "fresh air inlet," and you have questions about the purpose of the inlet and air flow rates that the inlet can be expected to deliver, you should direct your questions to the manufacturer.
Reputable manufacturers (or, in the case of Japanese minisplit manufacturers, their U.S. distribution agents) provide technical help over the telephone.