Hi everyone,
For the last few weeks, I’ve been doing a TON of research (pun intended) into different HVAC systems to replace our ducted 80% AFUE 70k BTU furnace with a new, more efficient system, that could also provide cooling.
After talking to a few HVAC contractors, one of them flat out told me that “a single Mitsubishi ducted air handler for your home size CANNOT be zoned, because the lines from the heat pump will freeze”.
This puzzled me, and I went ahead and tried finding a solution offered by Mitsubishi, but couldn’t really find one until I came here.
There is a post (/vbb/threads/2242573-Damper-zones-on-ducted-Mitsubishi-heat-pump) from 2 weeks ago where user Willoug was in a similar conundrum, and user FlyersFan told them it was indeed possible, and that it is done all the time.
How is the zoning done, and is there a risk of refrigerant lines freezing?
My hot take, no pun intended, is that perhaps the HVAC contractor was not too well versed with Mitsubishi’s newer inverter heat pump + variable CFM technology of the air handler, and just doesn’t want to deal with it.
Perhaps someone here can chime in here with experience? I’ve exhausted my Google abilities by this point.
Ultimately, my goal is to direct my HVAC contractor towards a viable configuration that (a) uses a slim heat pump (limited exterior pathway space, the), (b) has attic ducting (not considering mini splits at this moment), and (c) has two or more zones (we can live with a single zone, but it would not be ideal).
At this point, I’m committed to this post, so I’ll just go ahead and post what I have
We have a home with ~1500 sq. ft. living space and 8′ ceilings.
- ~700 sq. ft. corresponds to an open floor plan for our common areas: living room, dining room, and kitchen (this would be ZONE 1)
- ~600 sq. ft. corresponds to 3 bedrooms and 2 bathrooms (this would be ZONE 2+)
- The remaining ~200 sf corresponds to unconditioned space such as closets, walls, and a hallways (<– this is where the intake would be located)
- We also have an unfinished basement ~ 500 sq. ft. that we might want to condition down the road, but that’s just a bonus consideration for future-proofing the system (and from what I’ve read, this is typically not advised, but we’re just not ready to have it installed just yet)
Based on our floor plan, contractors have suggested units somewhere between 3 and 4 tons of capacity (36k to 48k BTUs), with roughly 9 differently sized vents spread across our floor plan. So far so good.
Our personal preference is to have a heat pump unit with a ducted air handler in our attic routed to 2 or more zones, and not a traditional furnace / condenser unit.
I’ve grown fond for the specs of the Mitsubishi heat pump, either the MXZ-SM36NAM, or the MXZ-SM48NAM, because of their high efficiency, low noise, and slim footprint.
I hear that Mitsubishi quality is excellent, and that both HVAC professionals’ and customers’ satisfaction is quite good. Also on the plus side is that Mitsubishi’s Kumo Cloud control app has much better reviews than say, Rheem’s EcoNet, or Bryant’s SmartWise, but that is beside the point.
I’m considering pairing the Mitsubishi heat pump with their ducted air handler: SVZ-KP36NA, which at the high CFM rate (910) would provide ~4.55 air changes per hour (ACH). Not the best, but not too bad, considering I’d be dividing across the whole floor plan.
If we opt for the larger 4 ton heat pump, this would leave 1 ton, or 12k BTUs, for a near future installation of a basement unit, which could be an additional zone.
So far, everything made sense to me, until that one contractor claimed that the MXZ-SM36NAM + SVZ-KP36NA combination could not be zoned with 2 thermostats because the refrigerant lines would freeze. They were adamant. Any thoughts on this?
I found a video on Youtube (watch?v=wEBb3MykVtw) showing an SVZ air handler being zoned with dampers via a Honeywell HZ432 zone panel:
Tl;dr
Would the configuration below be possible for a 1500 sq. ft. single story home
Heat pump: Mitsubishi MXZ-SM36NAM, or MXZ-SM48NAM (oversized in case basement is added)
Air handler: Mitsubishi SVZ-KP36NA (ducted towards 9 registers across 2+ zones)
Zone panel: Honeywell HZ432 (up to 4 zones using electronic dampers)
Duct dampers: Honeywell ARD8TZ,ARD10TZ, etc.
Writing this all out has really helped. I’ll definitely refer to this for future contractor discussions.