Minisplits vs. Conventional HVAC System
We’re having a 3500 sq ft single story custom home built in the Portland Oregon area. The house will be for myself and my wife, as well as for my mother in law. Our children are grown, but will visit a few times a year. This results in a large house that has many areas that will be used only occasionally.
Attached is the house layout and the different “zones” I envision. In red is the mother-in-law suite; in light green are the guest rooms for the kids; in maroon is the most used common space – great room, kitchen, and the office where my wife hangs out. The primary bedroom is in dark green, used mostly at night.
Our architects are suggesting a series of mini-splits for the HVAC, for their efficiency and inherent zoning flexibility. But I keep hearing that mini-splits are not as well suited for this size home, and that we’d be much better off with a conventional ducted zoned system.
I should add that the house is a slab on grade, with a flat roof and no attic. So any ducting for conventional systems would have to be run through added soffits which we presently don’t have in the design. Also our design is about a month from being submitted for permitting, so we’d like to minimize changes, unless some major error is found.
So I’m really hoping to get some guidance here – will the mini-split approach be a good option here, or should we go back to square one and redesign for a conventional system?
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In any normally proportioned, well designed, and well built new home, the per room loads in private spaces (i.e. bedrooms) will be far too low to warrant an individual mini-split head per room.
Ducted systems make the most sense, and the ducting should be designed in from the start.
Unfortunately, far too many think of mechanicals as an afterthought rather than an essential part of the design. It's really not too different from a designer failing to include a wall because the view is better, and then saying "this would totally be fine if it weren't for gravity."
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Often this question is posed as a binary, when it doesn’t have to be. A combination of a larger ducted system (minisplit or otherwise) plus one or two smaller systems would be easiest I’d imagine. Also, “minisplit” is used as a shorthand for ductless, but there are many ducted versions of mini splits as well. I would not propose a multi-split system (one outdoor unit connected to multiple indoor units), as they suffer from inefficiency because of poor sizing. I would consider using someone besides the architect to design this.
Paul, can you expand on the comment about not using one outdoor unit and multiple indoor units? That seems to be Mitsubishi's suggested method for zoning?
https://www.greenbuildingadvisor.com/article/the-bs-beer-show-making-the-most-of-minisplits
It can be used successfully, but mostly it’s screwed up. What often happens is that a ductless head is put in every bedroom and it’s too large to start, then it’s added to a multi-split which has even worse turn down capabilities. It’s extremely inefficient and uncomfortable so Mitsubishi advises against it in the linked podcast.
For a new build with this layout, it’s not really desirable to use ductless mini splits. A ducted minisplit will perform much better.
It helps me to think of the multi-splits as similar to a central ducted system (the single outside unit being the central blower) but with ducts that can't be adjusted properly for each room. ie each and every room gets a 14" duct with a damper that only adjusts down to something like an 10" duct.
Oh, and if the central unit produces "too much" then it gets distributed around all the spaces equally.
And their software can't simulate the minimum capacities so everyone just waves their hands and says "it's variable so it can adjust". Except it has limits in its adjustments ...
You can find 6k or 9k 1:1 (indoor to outdoor) units that turn down to ~ 1,800 BTU/hr. This is likely more than the load for a new construction bedroom in your climate at the 99% design temperature. This means it will be cycling on and off which can lead to comfort and efficiency issues.
Going the multisplit route compounds this even further. Multispilt outdoor units may only turn down to say 9,000 to 24,000 BTU/hr depending on size and model. So even though your 6k indoor unit could turn down to 1,800 BTU/hr, the turndown is now limited by the outdoor unit. So if only one head is calling for heat, you are getting the full minimum output of the outdoor unit to one room. This can cause even greater comfort and efficiency issues. Also, even when an indoor unit is off it can still have refrigerant flowing through it causing unwanted heating or cooling.
I would look at some combination of ducted mini splits. You can get creative using closets to hide a majority of the soffits. But most importantly, given the spread out nature of your floor plan, I would hire a third party firm to do a Manual J load calculation to properly design the system. This design can then be bid out to local installers. Energy Vanguard is one such firm that is often referenced on this site.
Thank you, that makes sense now, I can see why the single units and multi-splits may be oversized for the individual rooms.
But help me understand the ducted mini-split approach - all rooms on one ducted system would be on one "zone", with no way to adjust temperature within the individual rooms? So in my sketch above, I'd need four ducted systems for the four zones?
Well now comes a decision: how important is this zoning to you? Will these rooms truly be kept at different temperatures? If so, why? Comfort? Energy savings?
Each ducted unit would be controlled by a single thermostat, so one zone. Personally, I think you have two many zones. I think you could have maximum 3, but 2 would probably be ideal.
Do you want different temperatures in the bedrooms compared to living areas? I think one zone with the master bedroom and living area and one zone with guest beds and ADU would work?
Also keep in mind that wallmounts are not zero maintenance. The blower wheel on them needs to be cleaned every 5 to 6 years, which is a not a cheap process.
About the only maintenance on a ducted unit with well sealed returns is changing filters.
Looking at roughly your layout and whishes, the simplest would be:
Slim ducted unit for the 3 bedrooms on the right (there is no need to zone the unused bedrooms, you can just close the dampers a bit when unoccupied).
Single wall mount in the ADU living room with resistance heat in the bedroom and bathroom (floor heat under the tile is great for the bath). This should be on its own outdoor unit.
Multi position air handler (ei standard furnace style) for the living space and rest of the rooms. You can add a passive zone to the side bedroom to avoid any fancy zoning controls. A passive zone is simply an inline duct damper with a thermostat in the room. Since the air flow change through the air handler is pretty small when this zone closes, won't effect the rest of the house, most heat pumps would easily modulate around it.
The bedrooms and the living space could run off the same multi split as now the minimum load on it should be enough to keep it from cycling, but because of the long layout a dedicated outdoor unit for both is probably simpler.
Please have an HVAC engineer size the equipment, you don't want to end up with a ridiculously oversized unit such as the 10 tons of capacity an installer would want to put in.
That seems about right. Ideally, the ducted heat pump unit for the main space would be centrally located. You could make space for a mechanical room in the back of the master WIC, with doors accessible from that room (gym?) above. You can get back the WIC space by eliminating the "hallway" space between the BR door, closet door and bath doors. Bring that wall out flush, move the bedroom door to the end of the main hall, and put the WIC and bath doors facing the bedroom. This approach would keep duct runs and required soffits smaller and shorter. I'd also consider eliminating the door from the master BR to the outdoor sitting area since you've already got one at the end of the main hall. Doors are expensive and waste energy. They also make the room layout harder. I hate it when architects fail to provide space for mechanicals in their designs. Downright negligence, in my mind.
In my experience the blower wheels needed to be cleaned much more often than that. Like each year or 18 months. And it was $400 a head cleaning charge.
Is this possible to do oneself? The HVAC company who installed mine a few years ago never mentioned this. Kind of surprised they're not hitting me up for this business.
I'm still watching that BS & Beer podcast, thanks for the link.
But in the meantime I have a basic question based on the replies here - if I understand correctly, ductless single zone mini-splits are not good for individual small rooms, and even worse when combined into a multi-head/single coil arrangement, because both of these will lead to short cycling.
So that leaves only two "good" options - ducted mini-split, or ductless mini-splits in large rooms. And with a ducted mini-split, I can't vary temperatures in different rooms (can't provide zone control).
It seems that if this is right, the whole "flexible zone control" that is touted for these systems is bogus. Or am I missing something?
It's bogus in that it's very inefficient for low load rooms and can cause comfort issues if you're getting overheating/cooling, but how many flexible zone control options are there for both heating and cooling that can actually raise/lower the temperature, not just throttle the air/water flow?
These are mostly used for retrofits, not blank slate new builds.
"You can add a passive zone to the side bedroom to avoid any fancy zoning controls. A passive zone is simply an inline duct damper with a thermostat in the room. Since the air flow change through the air handler is pretty small when this zone closes, won't effect the rest of the house, most heat pumps would easily modulate around it."
Can you explain this a little more? You're suggesting to have the master bedroom on the same "zone" as the main great room and kitchen area, but with an inline damper to control the air flow to the bedroom, right? I set up something similar in my present home, where a duct damper for the master suite is fully open during cooling (the bedroom has a very high heat gain), but partially closes for heating, since not as much heat is needed.
But as I understand it, this can only "limit" the amount of air coming to the bedroom, e.g. give it less cooling or heating than to the main area. It won't be able to heat the bedroom without also heating the rest of the great room area, which seems a little inefficient for nighttime, when we want to primarily condition only the bedroom?
Yup, it works pretty much as you describe it. If you set the flow rates to the zone higher than what the zone actually needs, the damper will work to throttle it so there is a bit of extra control. Not full control, but pretty good for a couple of extra dollars.
This doesn't mean you have complete control over the temperature there but as long as is near the main space setpoint, it will work. For best results, you want a fully modulating heat pump as these deliver some amount of heating or cooling all the time.
You can dial back the main room setpoint at nighttime a bit if you want but just not too much.
Some manufacturers offer zoning kits for the ducted air handlers (ie Fujitsu Airzone) that would give you full control for each zone.
Generally a well sealed an insulated house with a modulating heat pump has such even temperature that fancy zoning is really not needed. Setting back the temperature at night time also doesn't really save al much if any energy as you can loose most of the savings during the morning recovery by running the unit at full tilt, thus at lower efficiency.
One other option if you really want to micro zone your home is to look at air to water heat pumps. I only suggest this because it looks like you are building a fairly high end custom home and this option will likely not be cheap. You can look at Chiltrix or Arctic heat pumps (as well as others). These units have quarter ton fan coils that are sort of like a radiator with a fan. They would be small enough for a typical bedroom. However you are talking about a lot of units, cost, and good luck finding someone who can install one that knows what they are doing.
As I said, it’s an expensive option, but if you really want temperature control in every room, I think this is the most feasible option.
Does anyone have experience using the Flair vents with ducted mini-splits? If you put a flair in each room then it makes that room a zone by opening and closing the vent.