Ducted Heat Pump Airflow/Air Volume Question

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timmmay

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#2723763 10-Jun-2021 15:35

Paul1977:

The heat loss side of it is still confusing to me. A larger diameter duct has less surface area per litre of air it carries, but the air is moving slower so has more time to lose heat. So which is better, bigger or smaller?

I suspect our design is actually pretty good and should work well (from my limited knowledge), it’s just been frustrating that it doesn’t seem to be performing as well as it expected. Hopefully in the end it will be really good once everything’s sorted.

I suspect you need larger ducts in order to carry the volume of air you need without excessive resistance.

TechE2020

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#2723770 10-Jun-2021 15:55

Paul1977:

I think our two returns have 500mm ducting, and I believe some of the supply ducting is 500mm in the roof before branching off or reducing to a smaller diameter closer to the vents (but I might be mistaken about that).

The heat loss side of it is still confusing to me. A larger diameter duct has less surface area per litre of air it carries, but the air is moving slower so has more time to lose heat. So which is better, bigger or smaller?

I suspect our design is actually pretty good and should work well (from my limited knowledge), it’s just been frustrating that it doesn’t seem to be performing as well as it expected. Hopefully in the end it will be really good once everything’s sorted.

Doubling the duct diameter reduces the friction loss by a factor of 32, so you always want to go with bigger ducting when physically possible. Bigger is more expensive, so there is a cost/benefit analysis here. Bigger is also better for the surface-to-volume ratio which reduces heat loss as you suspected. It does sound like your system is properly sized and ductwork sounds right, so once the final details are ironed out, you should be fine.

Paul1977

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#2723782 10-Jun-2021 16:27

RunningMan:
@Paul1977 those diffusers can also be locked in the hot or cold position (centre holes open or shut) by pushing the small lever all the way to one side until it clicks. If you swap it over for summer/winter (hot air down, cool air across) then you shouldn't get any clicking as the mechanism moves with temperature.

Thanks @RunningMan, but it’s not the mechanism making the noise.

EDIT: I have them in the locked open position already.

TechE2020

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#2723802 10-Jun-2021 17:05

timmmay:

I purchased an anemometer to help calibrate the system myself - not as good as measuring flow volume but good enough to help me work things out with relative speeds. Both our lounge ducts were about 3m/s, with out bedroom speeds more like 1 - 1.5m/s. The bedroom ducts were between silent and quiet, and straightening the ducts in the ceiling made a HUGE different to noise levels.

Standing under the 3m/s lounge duct with the center diffuser removed in the column of air it felt like you were in a waterfall of air - like being in a car with your hand out the window but not at high speed - 50kph maybe. The air column went from the ceiling to the floor and didn't really spread out. Centre out was more effective than the air going across the ceiling, but not quite right. The downjet you used would be about the same, maybe even more focused. Hence needing to spread the air out and direct it away, using the square directional diffuser. Hopefully it works because once that's in putting a round diffuser back in would be quite difficult.

My estimate of the volume of air through each diffuser in the lounge is 200L/s, 466cfm, 720m3/h (according to a quick calculator - L/s is accurate). The spec sheet for 2 way blow (we might use 2, 3 or 4) looks to me like we might want 250mm rather than 200mm. Hard to read though.

Yes, straightening flex duct does wonders for reducing duct noise (and improves airflow). You have good airflow. For the square diffuser, you could try installing it just by the duct itself (it would hang down 50mm below the ceiling) to see how it works before you commit to cutting the square cut-out for the final install. The flow rates for the lounge sound really high. Do you really have a 300mm duct feeding it?

timmmay

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#2723814 10-Jun-2021 17:59

TechE2020:

Yes, straightening flex duct does wonders for reducing duct noise (and improves airflow). You have good airflow. For the square diffuser, you could try installing it just by the duct itself (it would hang down 50mm below the ceiling) to see how it works before you commit to cutting the square cut-out for the final install. The flow rates for the lounge sound really high. Do you really have a 300mm duct feeding it?

Good idea to install before cutting, worth asking them about that. Though our ceiling cavity is FILTHY dirty in a very old house so wife might not ok that!

Flow rates probably high because it's a five outlet system with one outlet turned off (spare room) and two bedrooms running at about 30% airflow to reduce noise. That means the lounge gets about 70% of the air for the system. Given the new system will have Airtouch 4 we will probably still manually turn the bedrooms down a bit but not as much. Because airflow rates are so high is why I'm thinking 250 / 300mm diffuser. I think you need a proper HVAC engineer to work it out properly, but Blizzard said they've never had complaints with 200mm diffusers, though we do put more air through our lounge than average.

We had the old Panasonic ducted system completely removed including ducts so right now I just have diffusers with nothing behind them, and I haven't gotten a plan from Blizzard for the new Daikin system. I don't even know if they do a plan, they might just show up and do it, but I'm trying to get them around in advance. The last system had a 300mm duct to a splitter then 3x200m ducts off that to feed 2x to the lounge and 1x to a bedroom that was always turned off. The bedrooms had a 200mm duct that split into two. Asymmetric plenum with 200 and 300mm outputs. Not sure what the new one will have.

The new system does 10kw heating / 8.5kw cooling. Right now we're just using 2x1kw oil heaters in bedrooms and 1x2.4kw fan heater in the lounge and that's doing just fine but we're still at the start of winter. The ducted system heats everything through better.

Paul1977

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#2723990 10-Jun-2021 19:58

@Timmmay with AirTouch you can set maximum (and I think minimum) damper positions. So if you never want a zone to open past 60% etc you can set that as a rule. That way you can leave the zone on temperature mode but the auto damper will never open more than your set limit.

timmmay

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#2723995 10-Jun-2021 20:20

Paul1977:

@Timmmay with AirTouch you can set maximum (and I think minimum) damper positions. So if you never want a zone to open past 60% etc you can set that as a rule. That way you can leave the zone on temperature mode but the auto damper will never open more than your set limit.

Nice, that's even easier, thanks Paul.

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