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@TechE2020 you seem to be extremely knowledgeable about this and extremely technical, are you an HVAC engineer or similar? Do you have any thoughts on difffuser selection, both style but diffuser size? I have a higher 2.8m stud, ceiling return, diffusers are not quite over our sofa but not far off, I want to ensure the heat reaches the floor without sitting in a huge downdraft. The downjet would blow air on us, but the square diffuser look like they could blow air down but not directly at us. Blizzard in Wellington seem good but they seem to use more "rule of thumb" than any calculations about diffuser size as far as I can tell. I'm getting a Daikin 10kw with an Airtouch4, it's to heat 3 bedrooms (one usually turned off) and a lounge.
No, not an HVAC engineer. I have just owned 6 different homes all with different HVAC problems. At house number 5, after dealing with about 10 different HVAC companies all with wildly different designs, I ended up getting it designed by an HVAC engineer and that design wouldn't fit physically, so I adapted the design to a medium-velocity system that ran duct velocities of 8 m/s. Worked great. With home number 6, I cheated by skipping the load calculations since it had mini-splits in it, so I knew what sized worked already.
For supply and return registers, I see a lot of poor placement. There are two fundamental approaches to the designs. The first is to forcibly mix the air which is the traditional HVAC approach. The other is to do displacement ventilation which is similar to a heat transfer system. The best option IMHO is a combination of the two. Place the supply vents in front of windows and put the return in a hallway as far away from the supply vents as possible. This will do mixing in the supply room and displacement to the rest of the house. Jet-style vents are only really needed once you get into higher ceiling heights in the 3.5 to 4m range, otherwise they will be uncomfortable when you are close to them. If you need to use them, put them near windows and doors so you are never sitting directly under them.
Have a read through this for some ideas: https://blog.priceindustries.com/improve-air-quality-in-schools-with-displacement-ventilation
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Displacement ventilation is probably a lot easier to do in a new build, and is more difficult to retrofit as you need some fairly large ducts running down into the body of the house.
The problem we have is heat stratification, because supply and return are both in the ceiling. To combat that, in the bedrooms we put in wall grills down low, so the air is drawn across the room, which work quite well. Better diffusers might help a bit there, but not heaps.
The lounge is a bigger problem with stratification. The supply is in two corners, not directly above sofas but only displaced by 1m. The return is about 5m away in the hallway on the ceiling. Because the diffusers push most of the air sideways I can tell with my hand in the air currents any with an IR thermometer that the air goes straight across the ceiling, through the very wide entry way, and out the return vent. Over time the heat does come down a bit, but not far. That's I think we need to actively push the air down towards the floor, so it can then be drawn across the room the whole way from the floor to the ceiling and rise back up.
The idea is really just to have the air travel as far as possible from supply to return so that it mixes naturally. The article was a bit more specific on the location of the vents, but in my experience that doesn't matter much if you have good turbulent airflow and you are blowing the air counter to how it would normally stratify.
The supply location and returns 5m away are good from a location standpoint.
When we took the center of the diffusers out the column of air just missed the sofa, but it did work much better. If we put a downjet in the lounge it will blow directly on us, which is why I think we'll be better off with the square directional diffusers I put in this post, that way I can push the air towards the parts of the rooms we're not sitting in, including pushing air down the walls to be drawn away by the return vent. Based on the displacement ventilation I guess we don't really need to forcably mix the air as such, we just need to make sure the air is pushed down, then as the warmer air is drawn across the room it should naturally mix.
I think the only decisions left for my system are the size of the diffusers (Blizzard said 200mm but I might like to go larger to make them quieter) and the location of the indoor unit and ductwork. I just have to get Blizzard to come out to have a chat about the design. Do you have any experience with diffuser size with regards to noise and air mixing?
You are on the right track. What I would suggest is using a vent that is doing a jet of air downwards -- you want it to reach the floor. That creates nice vortices that mix all of the air in the room from floor to ceiling. The vortices spread out very quickly, so if you place it correctly, you will barely notice it unless you are directly under it. Without any obstructions to cause noise, these open vents can carry a large amount of flow quietly. You can direct the eyeball towards the base of a wall or corner so it doesn't disturb you much. The smaller the hole size the more intense the jet of air. For 5m ceilings, I had the full 100mm duct going into an 90mm vent, so the exit velocity was probably on the order of 10 m/s at high speed (thermostat controlled the fan at either low or high). You could stand under it and vaguely feel the air flow -- it dissipates that quickly. With 2.4m to 3m ceilings you would look like a dog sticking its head out of the car window if you stood under the vent with the same 10 m/s velocity. This is what the vents looked like.
Your duct velocity is probably on the order of 1 m/s and you probably don't want to go above 2 m/s for the vent exit speed. If you can tolerate the looks, something along the lines of this is ideal as you can point it away from where you are sitting to direct the flow.
You can playing around with cardboard to see how it might work before investing in new vents.
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