Mitsubishi SVZ 18k low airflow – GreenBuildingAdvisor

I was working on balancing a system yesterday and ran into a situation where we couldn’t get the rated air flow out of the system.  Long story short, I ended up calling Mitsubishi tech support and chatting with them.  I told them we were only getting 530 cfm out of a system rated for 675 cfm.  Our total static is .3 in. WC, and supply side is .13 in. WC.  The support rep told me that my duct work was too big.  They also told me that Mitsubishi ducted heat pump systems need to be sized using Manual H, not Manual J – which kind of blew my mind, as I was under the impression that Manual H was more supplementary than prescriptive in nature.  Wondering if anyone has heard anything similar to this from Mitsubishi before, or any other manufacturer for that matter.

The kicker is that I have a near identical system down the hill (same floor plan, same duct design, same installer), and it is producing 700 cfm with .33 in. WC total and .18 in. WC on the supply side.

Of course, I plan to run through some trouble shooting steps to ensure we don’t have a blockage or other obvious issue somewhere in the system.  Mostly just curious if anyone has run into a situation where the manufacturer stated the duct work was “too big”, or had heard of Manual H being required for design.

As a brief aside: in a quick search last night, I found a discussion where Steve Rogers from TEC stated that ECM motor algorithms may have trouble with very low static pressures – if someone has a concise explanation of ECM motors or a resource that explains this, I would love to learn more.

Thanks!

Nat