Manufacturers could put an
Manufacturers could put an “improperly installed” light on units, and for higher-end units, it probably wouldn’t cost much more since much of the instrumentation is already there. However, many (most?) installers don’t want to know that their duct work is undersized after installation is complete, so they would not want this feature even if it were free. It would be good for the homeowner, but the homeowner doesn’t usually select the specific equipment and options–the installer does. A qualified third-party inspector or system commissioner might be better yet, but no one wants to pay for that either.

What size ducts would you
What size ducts would you recommend then? 250mm ductwork supplying 2 rooms with an outlet to each room.

Manufacturer’s need to make
Manufacturer’s need to make their units more idiot proof. For example, a big red warning light labeled “low flow – improperly installed”.

Bruce
Bruce
As a rep for a ductless manufacturer, the reason we make the short run comment is that, disappointingly, a lot of installers take a 9K low static unit, connect it to a 3′ plenum and then run 6″ flex to 4 or 5 five locations up to 15′ away and can’t understand why it doesn’t work in the end. Ductless systems seem to end up in the hands of a lot of the wrong type of installer. This article by Allison is great, but the installers I have met would blankly stare off into the distance if they had to use the size of ducts required to get good performance.

Have a look at the mid or
Have a look at the mid or high static pressure units, the dimensions are really not much different and you then can use longer duct runs due to more powerful fans.

https://www.fujitsu-general.com/uk/products/split/msp-duct/lhtbp/index.html

I wish I had done this as I could have fitted the larger unit in the space and wouldn’t have needed to use larger oversized ducts to compensate for the slightly smaller footprint the low static pressure unit takes up.

As you see from the links posted there is not much difference…

https://www.fujitsu-general.com/th/en/products/split/lsp-duct/index.html

Hi Allison,
Hi Allison,
I design all my systems using fpm to lighten the load. I usually like to do 800 fpm for all my supplies 600 for return. I want to do my personal house with Fujitsu slim ducts that are rated for up to .33sp but the same thing comes from my supplier, they cant handle long duct runs. It makes no sense to me. At 800 fpm I need a lot of duct and transitions to get over .33. Any advice or opinion would be appreciated

I am from the Uk and I have low static ducted units in my home. Downstairs there are 2 units that literally have a return grill and a supply grill, they are boxed in but there is no ductwork so there is literally maybe 10PA of static pressure if that but probably closer to 0. THe units are a 1200btu and a 9000btu.

The upstairs units however are truly ducted as they are installed in the loft space. Both are 9000btu. The master bedroom unit has 1 x round supply outlet and 1 x round return grill and for this room a very short duct run. The duct is 250mm and approx total is 1 meter duct run on each side of the coil. This unit works equally well in heating or cooling but I have had to boost the static pressure of the fan to say 30PA.

The 2nd unit though serves two rooms (spare bedroom & bathroom), I use the same style and size return and supply grills. The supply plenum off the front of the unit has two outlets (both 250mm round). There is a mixture of solid duct and small amount of flexi duct, totally duct run for this room is approx. 4m. This unit works well in cooling as cold air falls and spreads across the ceiling. The supply grills are these… https://www.koolair.com/en/catalogue/difusores-de-geometria-variable-de-palas-dvp/ and contain a wax element that responds to the supply air temp. In cooling the blades flatten out to throw the air across the ceiling but in heating the blades point downwards to push the warm air towards the floor (so no having to adjust the angles of the outlets as it does it automatically)

However the airflow from this 2nd unit seems to be weak. If I close off the bathroom supply damper fully (normally open say 15%) then this unit performs as well as the master bedroom unit and the air can really be felt. The supply duct for the spare bedroom outlet doesn’t have a damper so it is always fully open as this rooms needs as much flow as it can.

The airflow just doesn’t seem to push down to the floor enough. I have adjusted the static pressure for the fan to the full maximum 50PA but this just creates lots of fan noise and doesn’t seem to do that much.

It seems as if the fan cannot pressurise the duct enough to produce enough velocity for the airflow and in turn the airflow just seems to come out at a slow speed, again fine in cooling but hot air naturally wanting to rise does not help heat the room as well. Ceiling area gets very hot but floor area remains cold.

Would fitting a damper on the other supply duct for the spare bedroom and adjusting this to say 85% open and the bathroom being at 15% open balance it out? Or would this just increase the pressure behind the damper and after the damper the pressure would just reduce and it would be the same? Currently the bathroom only needs a small amount of airflow due to size, but the air flow is very weak as well. Again not sure if fitting dampers on both outlets would achieve anything?

I can’t help thinking that I should have gone for 200mm ducting and supply outlets instead of 250mm. Although I wanted to reduce the static pressure as much as possible and when I installed the system I wanted to reuse some 250mm outlets that I had.

Please help…

You achieved ~1/2 of this. A very rough estimate is that this saves about $35/year in fan energy. Seems OK as long as larger ducts don’t cause installation, aesthetic, variable balance or latent removal issues.

Great article as always!

I am sure that the tightness of the ducts plays a large role in the pressure drop along the length of the ducts. Was there any leakage testing on these ducts?

I think that we compare
I think that we compare ducted vs. ductless on the wrong basis. From my experience and observations, a ductless unit can only do a good job of conditioning one room, and even that is questionable. All of the air supply and return is from the same location, so you need high supply air velocities to get the air distributed throughout the room without short circuiting to the return. Since the return air is typically under the supply on wall units, this can often work in cooling, but is less likely to work well in heating due to buoyancy effects. But for any room bigger than a bathroom, I would rather have multiple supply registers that are located away from the return register or doorway in order to get good air distribution. This requires ducts.

If you have any existing ducted system and want to replace it, you should surely install a smaller unit if your past experience with the existing unit showed that it was over sized. But beyond that, I would seriously consider installing a multi-capacity unit since it will operate most of the time at reduced airflow rate, thus you will get the advantages of lower static pressure with the existing ducts. It will also be quieter most of the time.