Zoning Variable Speed Ducted Minisplit
daveh4
| Posted in General Questions on
Contractor is pitching a ducted minisplit ducted multi position air handler (SVZ Mitsubishi model). Is there anyway to zone this system? He says no because it’s a variable speed and can’t modulate around zoning.
apparently going with 2 smaller horizontal ducted units is much more expensive.
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You generally don't attempt zones with ducted minisplits. They achieve their best efficiency by ramping their output up and down with demand and running nearly constantly. If you attempted to install zone controls it would seriously mess with the systems controls. The system should be designed based on good Manual J data, followed by Manuals S, D and T. Properly designed, the system should be capable of maintaining very consistent temperatures from room to room. If you need one or more rooms to be maintained at different temperatures, then you're stuck with either a separate system or low-tech intervention (opening windows, using fans, etc.)
Thanks for this confirmation. It does make sense. I read somewhere that you could "partially" zone an area by oversizing the specific supply ducting and then partially damping that ducting (e.g. size to 120% and then dampen to 80%). Apparently the unit can still modulate around this air restriction. I'm confused though how you would control the damper. Definitely sounds like a DIY solution.
You can look at the Airzone system which will work with Mitsubishi units. You definitely want a competent installer to set this up as it is not straight forward.
The simple zoning does work with these as long as you only try to control a small area of the house, thus a fraction of the total air flow. The setup is a normally closed 24V damper driven by a standard thermostat. You wire both cooling and heating outputs of the thermostat together to the damper for control. This way if the zone wants more heating or cooing it will drive the damper open. You do have to select cooling or heating on the zone thermostat when you change the mode on the main thermostat in the house.
Doable as DIY but an installer should also be able to set it up. For the closed position, you want to adjust the stop so that in closed position it still provides the right amount of air flow to maintain the required air changes in the room. For example, a bedroom might need 50CFM for heating. One ACH is somewhere around 20CFM of flow, so adjust the closed position to provide this amount. You want to over provision the zone, so fully open the vent should deliver around 60CFM.
I have three zones with three horizontal ducted all coupled to a single heat pump. The zones are quite separate physically (separate air spaces) such that I can have different temperatures in each zone though all need to be on heat or all need to be on cool (noting that sometimes I turn one of them off).
The all heat or all cool is due to use of a single heat pump.
So you are using 3 air handlers with a single zone heat pump? I did not know that was possible.
Sorry if unclear or I misunderstood the post, it's a single heat pump with built in plumbing for three zones (box is in outdoor unit) thus the single heat pump has three in and three out lines.
THis is commonly called a "multisplit" system. It has a single outdoor heat pump unit with multiple indoor heads. This provides pretty good zone control, but there is still some "crosstalk" between the heads when one is calling for heat/cool and the others are not. Also, the multisplits are not as efficient as one-to-one minisplits, as they have a much lower turndown ratio. This means that they will cycle more frequently as well as being less efficient. Still way better than a combustion system with A/C though.
I think the system-wide equation can in some instances start to shift back to favor a single multisplit when the number of one-to-one minisplits reaches three or more. Installing three or more outdoor units, each with its own 220V, associated defrost/ice generation, noise, minimum power consumption level, etc., can raise some issues.
As to "some instances", as inferred by Peter, a multisplit can be a bad choice when the zones differ significantly in their demands for heating and cooling, the worst being when one zone wants cooling and another zone wants heating. In that instance, depending on how many weeks that may go on, multiple minisplits are likely the right choice.
If the zones do not differ significantly, I do not think the cycling at low demand is too much of an issue for a multisplit as low demand for three zones all calling for heat is more than low demand for a single one of those zones. In some climates (I'm in Zone 6), cycling due to low demand may be limited to a period of weeks - can even be limited to time of day - and is not so likely to occur from Nov through March.
It would be nice to see a full assessment or decision flowchart that considers all factors.
https://www.airzonecontrol.com/products/plenums/
Here you go. No idea if you can get these in the US. I looked at them years ago.
I just installed a two zone (SVZ air handlers) Mitsubishi system with a single MXZ HyperHeat 3.5-ton condenser. Everything is tied together by a PAC branch box that controls the flow of refrigerant to each zone.
It's not VRF, so you can't heat and cool at the same time. But as long as the two zones are in the same mode, it works fine.