Zoning a ducted heat pump system
We have a two story house in Massachusetts (5A) with both the bedroom and office on the second floor. We are trying to replace our forced air furnace (single zone) with a heat pump, and I wanted to also create zones, as I basically always want the bedroom to be cooler than the rest of the house (both in the winter and summer). The HVAC company we got a quote from claims that Mitsubishi’s don’t perform well with zoning, and that the best way to do it would be to have two fan coils with one outdoor unit. The quote is significantly more expensive than I expected, and I wanted a second opinion about how well air source heat pumps on ducted systems generally handle zoning.
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"how well air source heat pumps on ducted systems generally handle zoning"
Lot of the answer to that depends on your existing ducting. For an ideal zoning setup, you want either zone to handle the full airflow of the air handler, this way if there is only one zone calling for heat/cool you are not restricting the heat pump.
Lot of times you are starting from a way oversized furnace, so the ducting can work with the right sized heat pump.
Sometimes, the ducting is too restrictive. In this case you can still do a bit of zoning with some careful controls. What you want to do is only put a zone damper driven by a local thermostat for the 2nd floor. You set up the system to over provision the airflow to the 2nd floor and adjust the damper closed position to reduce the airflow to say 50% of the required value. This way instead of the zone damper completely shutting off airflow it is essentially modulating it which should not create issues with the indoor unit. It also means you don't have to integrate the zoning controls to the factory thermostat.
I started a deep-dive on sizing (because the other thing I'm worried about with this quote is that I think it's oversized) and I'm pretty sure the current furnace is enormous compared to the actual need, and the ducts are huge.
I want to make sure I'm understanding what you're saying: you think that the best way to do it would be to have two separate air handlers for the two separate zones? But if the ducting isn't set up right, the second best way would be to add zone dampers?
Two air handlers is always the best but very costly. My suggestion was adding zone dampers to a single air handler.
We have three Mitsubishi air handlers each paired with an outside unit. Two of the air handlers have two zones, one has three zones. Using Honeywell dampers. Has been running great for years. Also added Honeywell steam humidifiers which are triggered by a humidistat and differential air pressure so they can also run when circulating air, not just when calling for heat. Each supply takeoff also has manual dampers which were set during the balancing and leak testing. So at least in this case, it seems to work. I think some effort was put in during commissioning to set the proper fan speed for each unit in order to correctly perform when only one zone was calling as well as when both were. The dampers are normally open, so the damper for the zone NOT calling closes. I think I got this right - obviously depended on the HVAC installer to get it right.
Thanks for sharing your experience! Did you have a hard time finding an HVAC installer who would put in the zones?
If your house is well insulated, air sealed and the rooms all have about the same percentage of windows, short of opening a window all the rooms on the same floor are likely to be almost the same temp.
Zoning is great when rooms are very different like when you have a sun room or upstairs and down stairs.
Consider getting a bid with two separate systems It may not cost much more and give you much better control.
Avoid the systems with one outdoor unit feeding two indoor units.
Walta
Could you please elaborate on this part?
"Avoid the systems with one outdoor unit feeding two indoor units."
That's exactly what was quoted to me.
Because the outdoor unit is sized for the combined capacity of both indoor units, the outdoor unit is not able to modulate down to the same level as a smaller sized outdoor unit that is sized for a single indoor unit. That is the traditional drawback of multi-split systems.
As the owner of a multi split (one outdoor unit to three indoor units) I cannot overemphasize how much you should heed this advice from Chris in NC....
I believe one:one systems are easier to install so have less chance for install errors, and increase efficiency, comfort and give redundancy. One:one is my current plan for when my system needs to be replaced.
1 Singles system can operate down to 10% of capacity and will be less than half size of the multi that can only get down to 33%. Let say you need 48000 BTUs the multi starts cycling on and off at 16000 BTUs. You turn off one 24000 btu system and operate the other down to 2400BTUs. So the single systems will be operating at peak efficacy much more.
2 The most common fault in refrigeration systems is a leak. Some are next to impossible to locate. The field made joints are the most likely to leak. The single system has 4 field joints. A system with 5 heads and junction box has 24 suspects joints.
3 Sooner or later every system has a fault when it happens with a multi head system any single fault will likely stop all 5 heads.
4 The multi head systems tend to be favored by the installers that stick an oversized heads in every room and oversize the system by 3X so they preform poorly.
Walta