This post pulls together my experience with ducted heat HVAC / pumps, from the point of view as a customer who has learned a bit along the way. It's intended to help others considering ducted systems. Please ask questions, point out errors, etc. If you have a question about your own heating / cooling needs please start a separate thread, and feel free to tag me and anyone else in this thread who looks like they know anything about these systems :)

Notes:

• Disclaimer: I have no training or professional experience in this area and this is all based on my experience or reading. Any or all of this could be incorrect. Professional advice from an experienced HVAC engineer is highly recommended. Note that HVAC engineers with good knowledge / experience of ducted systems are few and far between in New Zealand, and anything a salesperson says should be viewed as sales rather than technical advice.
• This post will likely be updated in future to add information, correct errors, answer questions, and as I learn.
• My experience is with a Panasonic ducted system (not zoned) and a Daikin ducted unit zoned with Airtouch 4. I've also had a couple of high-wall heat pumps, Fujitsu and Daikin.
• I will not discuss vendors / installers.
• If you have problems with a ducted system the first thing to do is talk to your installer, tell them the problem, and let them come up with solutions. If you don't like their solution or they have no solution then you can make suggestions or ask around.

Summary and Key Points

• Ducted heat pumps can be great, but they're not the answer for everyone. Individual heat pumps for each room or a multi-split unit (one outdoor with multiple indoor units) can be cheaper / simpler / more effective.
• Ensure you get a company who is expert and experienced with ducted heat pumps. Just having done a lot of ducted heat pumps is not sufficient, some companies have high volume but don't do a great job. Ideally you want advice from an HVAC engineer rather than a salesperson - though finding one of those at an HVAC installer may be rare.
• Zones or individual room control is likely to be important for most ducted installations. Fixed airflow to each room is probably going to overheat some rooms and underheat others depending on the season, sun position, room use, etc. Having zones you can enable / disable manually is maybe ok, but will create manual work. Thermostats in each room with automatic airflow control is better.
• When you're looking to purchase tell your vendor what you want to achieve in writing / an email and ensure they reply that they have seen it. This means if their solution doesn't meet your needs you can refer back to it. For example you might say "I would like all our rooms to be a comfortable temperature, summer and winter, whether it's sunny or not, without having to manually adjust the system. We find the north side of the house gets warmer than the south side, and we would like them balanced. We want the system to be quiet at night, we don't want the outdoor unit noise to be intrusive or the noise from the ducts to be loud".
• Brand matters - some are great, some are less good. See below for details.
• My wife isn't a fan of the ducted system. The Daikin 10kw outdoor unit is in exactly the same place as the old Fujitsu 9kw unit is significantly louder and wakes her up, but I suspect all modern heat pumps outdoor units are louder than the old ones. It was also a hassle to put in, then tweak. I like the ducted system though, even with the hassles as the noise doesn't bother me. High wall units are MUCH MUCH simpler though, you turn them on and let them do their job. I wouldn't have a ducted system without being able to tweak and control it with Home Assistant.
• Home Assistant is a great way to control and tweak your ducted system. They're a lot more complex than high wall units. For example, I monitor damper percentage open in my sons room between 7pm and 7am and if it's higher than 40% I shut the damper down, otherwise the smaller room overheats with the high volume of hot air going in.

All things considered, Daikin ducted heat pump with Airtouch 4 controller is pretty good. It works well most of the time, though temperatures do vary more than you might expect especially when it's first turned on. If we were making the decision again though it could go either way - we might choose instead to go with high wall units in each bedroom and a single outdoor largely to reduce cost, complexity, and hassle.

Definition of Terms

• Conditioned - air that has been heated or cooled
• Outdoor unit - the big box outside that has a big fan that does the heat exchange
• Indoor unit - a unit mounted inside on the wall or floor  that pushes warm or cool air through the room, connected to the outdoor unit
• Indoor unit (ducted) - a large box in the ceiling or occasionally under the floor connected to the outdoor unit that has heat exchangers and fans that push air through ducts
• Ducts (ducted) - insulated ducting in the ceiling 100 - 300mm diameter that takes air from the ducted indoor unit to the diffusers, and from the return vents back to the indoor unit
• Damper (ducted) - a mechanical device that controls airflow through a duct. This can be manually set or electric so the airflow can be controlled digitally
• HVAC - Heating Ventilation Air Conditioning

Core Concepts and Considerations
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* How ducted systems work
Ducted systems work by pushing conditioned air into a room through a diffuser, then pulling it towards the return vent so it can be cycled through to be heated / cooled again. High wall units tend to have strong fans with directional vanes that push the air all around the room, whereas with ducted units it's more about pushing air into the room and having it pulled towards the return. I think of this as pushing a "cloud" of air into the room which then drifts towards the return vent - ideally diagonally across the room rather than across one side. While ducted units have big fans that can push air directly to where you need it my experience is it's best to push the air into one side of the room and let it be pulled to the return / room exit.

Ducted systems are best left on for longer periods, rather than turned on and off like traditional heaters. They can be slower to heat or cool a room than high wall units in some cases, partly because have a larger space to work with.

* Return vent
A key feature of ducted units is that the air is cycled through rooms back up into the return vent to be conditioned again. You can have one large central return (probably most common), two medium to large returns, or individual room returns. A good way to set these up is that the return is on the floor with diffusers that supply the conditioned air on the ceiling, or vice versa. This means the air moves horizontally and vertically across the room, and is more likely to condition the key area where people are near the floor rather than just running along the ceiling. This is unlikely to be practical in most retrofit installs, though I've heard that things like centrally located cupboards can potentially be used as a return if the door is undercut and a large vent is put in the top to return air to the main unit.

In practice most retrofit installs will have the diffusers and return air vent in the ceiling. This makes diffuser selection important.

The return vent can be quite large - 1m x 0.6m would be normal enough for a medium system. The larger the vent the quieter it is.

* Diffusers
Diffusers are a visible of the system that supplies conditioned air into rooms. In retrofit ducted installs they tend to be on the ceiling, and are usually round or square. If your return air is also on the ceiling it's important that the diffusers push the air down towards the floor, otherwise the heated air skims across the ceiling causing heat stratification - in winter this would be a hot ceiling with a cold floor. With basic round diffusers I found the floor level stayed quite cool in winter, the ceiling was 5-7 degrees warmer than the floor.

My initial diffusers were Holyoake Eco-A which have a clever passive thermostat system that is  meant to push air towards the floor when warm air is flowing through. In practice I found most air still went sideways across the ceiling, only a small amount downwards. This caused heat stratification, the diffusers were loud when they were pushing air through those small holes. My second set of diffusers were larger square MDO (Multi-Directional Output) diffusers ( see link in resources section ) that let you point the air in any direction you like which are much quieter and much more effective. I can't emphasize enough how important diffuser selection is, and how much better the square MDO diffusers are than standard round outlets.

The larger the diffuser the lower the air speed and the quieter it is, generally. The less of the diffuser that the air touches the quieter it is as well, but then you can't direct the air so well. The Holyoake diffusers were quite loud particularly when the air was going through the small holes pointing downwards. The larger MDO square diffusers are much much quieter.

Diffuser placement is really important. You want the air to be drawn across the room to the return vent or exit point, but without causing more of a draft than is necessary on the people in the room. Some rooms may benefit from multiple diffusers. If you have a system that can't turn off the breeze when the room is up to heat avoid putting diffusers where it will put the breeze onto people - a single central diffuser might be better than smaller diffusers in multiple places.

Some ducted systems (Panasonic) have a small breeze that you cannot turn off regardless of the mode - Daikin can be turned off. Because of this it may be better to locate so this small breeze does not get drawn over people in the room. A larger central diffuser firing in multiple directions may be better in this regard than multiple smaller diffusers as this small breeze may miss the people in the room, but there's probably a trade-off somewhere in there.

* Air Transfer / Return Air Pathways
Once conditioned air has been pushed into a room it has to have a clear path to the return vent. If you don't have a clear exit pathway it can pressurise the room which can be uncomfortable, and the air will escape in a random direction like through floorboards or out windows, which over time could potentially cause damage. There's a number of ways to do air return:

• Undercutting doors is the easy solution but this can result in light / sound getting into rooms, and can look at bit odd since you ideally need 2cm clear at the bottom. Some air gets around the door even in a closed door frame, but can make a whistling noise.
• Wall or door vents are a good solution, if you like the look of them. We purchased 6x12" wall vents from TamTech in the USA, which have the vent cover, a metal conduit inside the wall to prevent dust in the wall getting into the air, and sound / light dampening material inside. An advantage of using wall / door vents if if the supply diffuser is in the ceiling these can be located near the floor, effectively giving you a floor level return.
• Jump ducts put an extra diffuser near the door, and another on the other side of the door, with flexible ducting between them. These work, but you need more diffusers, more ducting, it pushes your air through the unconditioned space and it may look a bit odd with diffusers everywhere.

Recommendation: get the Tamtech return grills, wall or door. I shipped them via Planet Express / FedEx Economy.

* Ducting
In NZ ducting tends to come in R0.6 or R1.0, which isn't really a lot of insulation given these ducts are carrying air that runs through a ceiling cavity that's 50 degrees in the summer and could be freezing in the winter. In the USA they often run the ducts with conditioned air through parts of the house that are conditioned, rather than a hot ceiling, but that's not very practical in NZ unless you're doing major renovations. The ducting tends to be 150 - 300mm, but there can be larger ducts for return air.

I did some really rough calculations that suggested to me that the extra cost of R1.0 ducting would pay for itself fairly quickly through by reducing heat loss in the ceiling space. The calculations are so rough I'm not going to share them. The cost to upgrade from R0.6 to R1.0 wasn't huge but wasn't itemised so I don't know exactly what it cost. From memory it may have been a few hundred dollars for an average sized house with four rooms in the system.

* System Capacity
Ducted units seem to be best left while you're using the conditioned areas, rather than turned on and off as needed. There is typically a higher power requirement when first turned on, but that reduces significantly once the house is up to heat unless the weather is extreme. There needs to be a balance between the maximum output for extreme temperatures / fast start up vs the minimum output to avoid overheating / overcooling the house when it's operating at its minimum output.

We have an old, well insulated house with double glazing and insulation ceiling, walls, and floor. Before the ducted system went in we had a highwall unit in the lounge that put a bit of heat into the bedrooms but it took a lot of time for the air to go around corners. We had 1kw oil heaters in the bedrooms that did a great job of keeping the bedrooms warm, and were only on about 1/3 of the time. That tells me we need about 300 - 500W of heating to bedrooms in winter. The Daikin heat pump we have has 4kw min output. We find that the 4kw of heating is more than we need the house at night two heat the bedrooms even in the depths of winter, so a some of this heat goes to spill zone in the lounge (see spill in the Airtouch section below). Interestingly though, lately I've been finding that with the fan on "low" there isn't much spill into the lounge. I learned this after I connected the AirTouch unit to Home Assistant and was able to graph the temperatures in each room. On the other hand, on really cold evenings in winter we do need the whole capacity of the unit at times, so it really is a balance.

If you get a system with automatic temperature control such as the Airtouch consider getting a system that has a low minimum output, which will probably have to balance with the maximum output. This has to be balanced against the heating needs of the property in the middle of winter. Originally I thought our 10kw heating system was larger than it needed to be, but the house took a few hours to fully heat on a very cold day after it had been off since 8am so I think it's probably a good balance. We use the program / timer / app features and Home Assistant so we rarely have to walk into an overly hot / cold house and turn the system on manually.

* Miscellaneous
Some ducted heat pumps have a small amount of air running through the entire time they're running in heating mode. We found this really annoying. Daikin systems can have this turned off, Panasonic could not. Most of the systems have air running through them the whole time they're turned on in cooling mode.

• The install quality makes a difference to more than just appearance, it can impact how effective the system is and how loud it is. This is what you can see, as well as details you can't see. For example:
• Ducts should never have sharp bends as this reduces airflow and increases noise. This is particularly important near the diffusers. The ducting should ideally be strapped up near the top of the roof space as that makes it easier to have higher diameter bends.
• If a duct is bent or curved near the diffuser it will push more air to one side of the diffuser. This creates additional noise at the diffuser as the air is moving faster and may cause uneven heat distribution. Generally this will push more of the airflow towards the outside wall.
• As well as the noise advantages, it's more convenient to have the ducts strapped up out of the way in the ceiling and properly supported every 1-2m rather than lying about making it difficult to move about in the ceiling cavity

* Ventilation Integration
Some systems can integrate with ventilation systems, such as the Mitsubishi Lossnay. That's pretty fancy, probably very effective, and I guess it's expensive.

A cheaper solution is to have a positive pressure system with an output near the return vent. The fresh air is sucked into the return vent and distributed around the house. We do this, with the positive pressure system on a WiFi timer so it's only on when we want it to be. In summer we have it come on earlier in the day when it's cool, plus for a time while we're cooking and a bit later in the evening for fresh air. In winter it's mostly on during the afternoon when it's warmer and when water is less likely to condense in the air filter.

Choosing Ducted or Highwall
Ducted system advantages: Fairly quiet if you choose good diffusers, mostly out of sight, pay extra for individually controlled room temperatures
Ducted system disadvantages: Expensive, require return air grills or similar, complex to set up, easy for an inexperienced installer to mess up, can result in temperature imbalances unless you have an automatic zoned system. Additional automation using Home Assistant can be required to get good programmability and tweak things to your liking, but if you do that it can be very effective.

Ducted systems can be good, but they're expensive and somewhat complex to install. An install to four rooms took about two and a half days for a team of between two and three people. High wall or floor systems on the other hand are simpler, they keep the conditioned air in a single room within the conditioned space or area which is more efficient and simplifies things, and might be cheaper depending on the number of areas to heat / cool. They do take up wall space though, can be ugly and louder, and typically create more drafts than ducted systems. If there are real extremes in temperature, air return is difficult, or for multi-story houses you should consider highwall or floor based units. You can have multiple highwall indoor units share a single larger outdoor unit.

If one or more of your rooms gets a lot more sun than the rest of the rooms then I recommend against having a ducted system that is not zoned with automatic temperature control. I've spoken to people with this type of system who are happy with them, but I wouldn't choose to do this in any house I lived in.

You can have more than one type of unit. A house could have a mix of highwall units, with shared or dedicated outdoor units, along with a ducted unit for some rooms. We have a ducted unit for most of the house but two area that get more sun have highwall units.

I'm not going to make any other recommendations, other than to say ducted systems are not for everyone and everyone should get advice from a trained, experienced professional. Ideally you want advice from an HVAC engineer, rather than a salesperson.

My Experience

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We have an old, well insulated double glazed three bedroom house with a lounge and a separate kitchen / dining area. The kitchen / dining kept its highwall unit because the area gets a LOT of sun, and we'll sometimes cool this area while we're heating the rest of the house.

* Panasonic Ducted
Our first ducted system was a Panasonic unit with Holyoake Eco Diffusers. The system wasn't zoned as such - each room has a diffuser and each diffuser had a manual damper up in the ceiling. Long story short, we had it taken out after a few months due to issues with noise and the temperature control did not meet the requirements we had shared with the vendor.

The primary problem was the north facing rooms got really hot, so we increased the airflow to keep them cool. However, on cloudy days the rooms then got very cold. I was up in the ceiling multiple times per changing damper settings, which was impractical. It just wasn't possible to get stable temperatures, and when you're paying $10K - $20K you want it to do a good job.

Our Panasonic system outdoor unit was significantly louder than similar capacity units from other manufacturers such as Fujitsu or Daikin for similarly sized heat pumps. The indoor unit in the ceiling caused some vibration through the house when it was working, and you could hear it as well, even though the insulated ceiling, though this may be due to the installation rather than the unit itself.

* Daikin Ducted System
The Daikin ducted system is good. The outdoor unit is a little louder than the old Fujitsu unit which was slightly smaller, but when night mode is enabled on the schedule it's fairly quiet. The indoor unit is quiet and we haven't noticed any vibration. The square MDO diffusers I link to are also quiet - you can't even tell if they're turned on when it's on low airflow, you can hear a little on medium, and they're noticeable on high - but we rarely run the system on high airflow. We don't tend to use auto fan because the Airtouch always seems to select low fan.

The Daikin can be configured so that in heating mode it completely shuts the motors off when it's up to heat, which makes the system more comfortable. This isn't the default because the standard mode has the thermostat in the return airflow and it needs air moving to be able to sense the temperature, but that doesn't apply to us with an Airtouch 4 which has thermostats in each room.

The Daikin has failed twice in the 4.5 years we've owned it. The first time it was making odd noises, and the vendor replaced the whole outdoor unit on the advice of Daikin. The replacement unit took a couple of months to arrive in NZ. The second time it developed a fault with the circuit / control board in the outdoor unit and while it would turn on it wouldn't heat or cool.

Airtouch 4 Review

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Summary: The Airtouch system gives you control over temperatures in each room that has a ducted heat pump output, and is a massive improvement on a basic ducted system. I wouldn't have a ducted heating system without an Airtouch or similar unit.

The Airtouch 4 is a control system that works for multiple brands of ducted heat pumps. It has wall mounted thermostats that look a bit like light switches in each area, and using electric dampers in the ceiling it changes the airflow to keep each area at the temperature you select. It's fairly configurable, has a 7" Android tablet on the wall you control it with, and you can control the basic functions with a phone app inc on / off, mode, fan speed, zone temperatures, and program / schedule. Airtouch directly controls the heat pump, so you rarely need to touch the OEM controller - our controller is in the ceiling. All in all it's a fairly good system. It's MUCH preferable to no zoning or manual zoning. I can't compare it with any other zoning systems as I haven't used them.

The Airtouch has a simple user interface but under the hood seems fairly complex. There's a user area that lets you do all the basics like schedule, mode, fan speed, turbo zone, and basic preferences. There's also an installer area which lets you do initial setup, change which zone gets the spill, and all sorts of complex things. The main reason I go in here is to see the opening percentage of each damper, which helps me understand what the system is doing or why things aren't working as I might have expected. I was also guided by the manufacturer support to change a setting in here to help reduce overheating, by telling the Airtouch to completely ignore the Daikin temperature sensors and to use its own only (economy mode). Previously it would only shut the outdoor unit off in heating mode once the Daikin sensors said the house was up to heat, which isn't what you want when you have individual room controls. To change this go into Installer Settings (password is usually "polyaire"), AC Setup, Detail, change the control sensor to "Economy".

The Airtouch 4 has the concept of a "spill zone". The general idea of this is the heat pump has a minimum power, and if a room needs only a little bit of heating / cooling only a small volume of air is supplied to the room and the extra air has to go somewhere. For example our Daikin unit has 4kw min output. If we have just a small bedroom being heated it needs about 1kw of heat max, which might be 20% of the minimum heating / airflow for the unit. When the room is really cold the Airtouch will route all the heat it can to the room, but as the room approaches the target temperature the airtouch puts only a small volume of air through the room - I've seen it open the room damper as little as 10% when it's almost to the target temp. The rest of the conditioned air is dumped into the "spill zone".

Because the Airtouch prioritises keeping the rooms other than spill zone at the target temperature, it will often leave the outdoor unit on for quite a while, putting a small volume of air into the occupied rooms. In winter this can result in a lot of extra heat that goes to the spill zone. We find that if the lounge zone is turned off at night the room can still be 24+ degrees in the morning. It's a bit of a waste of heat / money. The Airtouch 4 installer manual describes "bypass" as an option instead of spill. My understanding is that instead of spilling conditioned air and overheating / overcooling a room it can open a damper in the ceiling that basically loops conditioned air directly through a duct to the input of the indoor (roof) unit. I've only read about this, I haven't tried it, I'm not sure if it's worth what would probably cost to get it installed and configured.

With an Airtouch controlled unit on cooling mode, it initially puts a fairly high volume very cold air into the room which makes the room very cold, but as the room approaches the target temperature the air volume reduces so the room feels cool but not cold. Heating is similar but to a lesser extent. We address this by having the unit turn on an hour or two before you get home. On the coldest winter days we don't turn the unit off, we just turn it down to 19 degrees or so.

The key drawback of the Airtouch 4 is the spill zone, which as I explained above may be able to be mitigated with the "bypass" mode. I would like the manufacturer to alter the algorithms to reduce the duration of use of the spill zones particularly when in heating mode, putting more air into a room faster so the outdoor unit can be shut off sooner reducing spill. I've worked around this with Home Assistant - if a damper is open less than 20% Home Assistant opens the dampers up to a wider setting until the room is at heat, but this shouldn't be necessary.

I'd also like for it to have a better program / scheduler. You're limited to a max of eight programs, and you can only change temperature - you can't change mode (heat / cool) or fan speed. That would make it more complex, but more flexible. For example, I like to put it into "fan" mode during the day to push fresh air around the house, but I have to do that manually. I considered writing my own system that uses the Airtouch API, but that would be a lot of work and I'm not sure I'll bother since Home Assistant control is good enough.

Airtouch or similar systems are likely able to be added to existing ducted heap pump systems. They're essentially sensors (often wireless), a central controller, and electric dampers that restrict airflow.

Home Assistant can be an effective way to control an Airtouch ducted system. It's not the easiest piece of software to use, you have to be at least somewhat technical to install and use it. There's some information here.

NB: Airtouch 5 is being released late 2022.

NB: A friend got a MyPlace / MiAir control system with a Mitsubishi ducted heat pump. It's a similar thing to Airtouch, but he has had a lot of trouble with it. It took a few visits to get it working ok, but in heating mode it's apparently blowing a lot of cool air when it gets up to heat. Be a bit wary of this combination. Daikin ducted heat pumps can be configured to turn off the circulation / fans when the house is up to heat.

I've put some of my Home Assistant Airtouch 4 automations and other resources into this git repo.

My Recommendation

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Here's what I would do if I had to start again:

• I probably wouldn't get a ducted unit. They work ok, but the Airtouch has many drawbacks that I've had to write Home Assistant automations to get around. I like that it blows less air around than a high wall - I might get a floor unit for the lounge. My wife doesn't much like the ducted system, or any heat pump really.
• Multi-split or individual units in each room. I have a small 2.5kw Daikin in my office that's very, very quiet on low fan and it can go down to very low heating levels. The ducted unit is essentially silent in the bedrooms, which is a positive.

If I decided the house I was in was good for ducted, or perhaps in a new house:

• Daikin heat pump - they have a setting that lets a technician turn off the continuous circulation when the house is up to heat. I'd also consider Mitsubishi if they could turn the continuous air blowing off for heating, and ideally cooling.
• Airtouch 5 or whichever version is current, with individual room temperature sensors and controls. It's important to have the bypass damper installed, rather than rely on spill. That would reduce the number of HA automations I had significantly.
• Use square supply diffusers in the ceiling that push all the air down to the floor, not the round ones that push the air across the ceiling.
• Consider per-room return ducts. Ideally the returns would be at floor level, so supply air is drawn across the room. This would make it easier to keep heating or cooling within a room. Per-room returns rely on a supply diffuser that pushes the air down, or floor / ceiling setup.
• Use a heat recovery or positive pressure ventilation system to provide fresh air to the house. Either integrate it with the system if that's easy, or put the ventilation system output close to the ducted system input so the ducted system pushes the fresh air into each room.
• Automate on / off with Home Assistant.
• Use a smaller lower powered outdoor unit. Ideally you want the system to be running most of the time rather than turning off and on constantly. The minimum power we have is higher than ideal for our house.
• In a new house, consider whether it's practical to run the ducts within the insulated envelope of the house rather than in the ceiling where it's cold. This could be a false ceiling, insulation at the roof level, or (probably) insulated channels the ducts can run in above the main insulation.

References

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There are HEAPS of great HVAC videos on YouTube to show how the USA does this - and it's MUCH better than anything in NZ. It's properly designed and integrated into the house a few videos

• https://www.youtube.com/watch?v=MCKTOAyN9kM 
• https://www.youtube.com/watch?v=Xl9QINETX4I 
• https://youtu.be/q_1RwEopaYQ?t=192 

General references

• Diffuser selection https://www.youtube.com/watch?v=GpXMTL10FGY
• Article on retrofitting bedrooms with air returns https://www.energyvanguard.com/blog/easy-retrofit-return-air-bedrooms
• Door undercutting https://www.energyvanguard.com/blog/can-door-undercuts-work-as-return-air-pathways
• Holyoake Eco-Diffusers https://www.holyoake.com/ECO-Diffusers.html
• Square MDO diffusers https://www.polyaire.com.au/air-outlets-grilles-and-diffusers/mdos-multi-directional-outlets  (Smooth Air used to sell them in NZ but doesn't look like they do any more)
• Tamtech return air pathways https://www.tamtech.com/return-air-pathways-transfer-grilles/
• Ducting guidelines: https://www.proremodeler.com/4-rules-flexible-ducts-remodelers-need-know
• Heat loss guidelines for ducts: https://www.engineeringtoolbox.com/heat-loss-insulated-pipes-d_1151.html