Replies

1. 
   user-2310254 | Sep 07, 2022 11:46am | #1

   Kermit49,

   I'll give you a bump.

1. 
   kermit49 | Sep 07, 2022 12:37pm | #2

   Thanks.

2. Expert Member
   Akos | Sep 07, 2022 03:31pm | #3

   I'm assuming you meant 400FPM. At that low velocity, any reasonable ducting will be very low loss. Your ~0.1" is definitely reasonable, I've gotten about half that on installed systems that use very low static air handlers.

   The air return and filter adds to your pressure budget exactly the same way as the supply ducting. There is no difference here. For low static air handlers, you have to watch this as well thus the recommendations of very large filters for most ducted mini splits.

   If your return ducting is well sealed, you can do filters at each return only. A well sealed return and good filters means both your ducting and air handler will be pristine for many years to come. Replacing four filters is not fun, so I would instead use a larger central filter and maybe a washable filter mesh at each return.

   A simpler option is a single hallway return and door undercuts or transfer ducts from each bedroom to the hallway. I've had good luck with converting the ducted unit to bottom intake and mounting a larger filter grill right against the bottom side of the unit. No extra return ducting is needed but it is a bit louder on high speed which is usually fine in a hallway.

3. 
   kermit49 | Sep 07, 2022 04:51pm | #4

   Akos, I've have read your writings here over a period of time and have always been impressed. Not surprised at all now reading your good advice and recommendations. Thank you.

   1. Yes 400 fpm, hey I am a senior and entitled ; > )
   2. You made my day. Akos's blessing on my calcs carries a lot of weight. What a relief.
   3. So my pressure budget includes all components in the return path. Good to know before I continue with the design. Would have been a bad learning experience had I not included that. Remember, all I know is little bits and pieces I read here and there, nothing comprehensive. You filled in the blanks for me.
   4. Got the big filter message from you on your previous posts, good reminder.
   5. Although doable, I would hate having to run another set of metal returns. If I did, yes they will be well sealed especially if I used individual terminal filters. Like sucking a milkshake through straw with a bad split.
   6. I prefer a hallway return and would design accordingly, (using room returns to hallway, etc) however, is this common in the mini-split world? Most all my research here tells me when dealing with rooms/doors to duct back to mini-split. Not looking for extra work but.

   As mentioned, half the house is open and serviced by a mini-wall unit, no return needed as the hallways will take care of that. So Akos, here is my burning question. If the ducted return relies on hallway air using a hallway grill that lead to ducted mini, any chance the combined return (ducted and ductless) cause control problems on the ducted mini? I think I am overthinking this but with zero experience in these matters I just don't know. My gut feeling is that this set up will work fine.

   Akos, thank you for your solid advice, I now know I am not wandering off the correct path.

1. Expert Member
   Akos | Sep 07, 2022 08:30pm | #5

   For the ducted mini splits I've always done very simple returns from hall/open area. About the only time you need to do more complicated return ducting is in a multi story structure where the unit is in the basement.

   Around me, door undercuts are the norm for forced air setups, this also works well enough for a ducted mini split in a high efficiency structure. Typical bedroom heating loads, thus airflow, are low enough that a standard size door undercut won't be restrictive. Undercuts are not the best for privacy, if you care about noise, high/low transfer duct inside the wall is a better option, size so that pressure loss through it at your design flow rate is under 5PA.

   Most (all?) ducted units come with a wired thermostat. Somewhere on the thermostat or in the install menu you can set it to use the temperature sensor in the thermostat instead of the one in the intake of the unit. If you mount the thermostat inside the main bedroom, it will only pick up the temperature there and control based on that. This way it won't matter if there is an extra mini split in the living area nearby.

   Glad to help out, designing a house is not simple and the little details can make a big difference in durability, efficiency and comfort. Good luck with your build.

   P.S. If in your climate you have a hard time keeping winter time humidity high enough in the house, an ERV might be a better choice.

4. 
   kermit49 | Sep 07, 2022 10:47pm | #6

   Akos, your words are a game changer. I trust your judgement and expertise.
   Do I prefer the hallway return method? Heck yes!

   Will wrap up the ducted return design I started, for completeness, and start over with the hallway return method.

   Been considering this method for some time, researching how to balance each room with wall efficient air flow grills, connecting using sound transmission reducing materials. Room to hallway bridge will not leave conditioned space. Undercuts won't pass wife test. We had that conversation some time ago while researching hallway style return.

   Good to know thermostat control won't be an issue. Whew! That's an area I struggled with. Solution is clear now, thank you.

   You make a good point on the ERV. Yes our retirement home will see low humidity (Mohave Desert), practically all year but worse in winter. I am drawing all ventilation lines run now even though the purchase of the E/HRV won't happen for a while. Re-routing badly needed moisture back into the house makes sense. In Florisa, seems that re-routing drier air, than outside, back in the house makes perfect sense. Wonder what climate doesn't benefit from an ERV ?

   Thank you for the well wishes Akos. Our first new home build. Wish we had done this twenty years ago.

   Appreciate your thoughts and real world input.

5. 
   dan_saa | Sep 08, 2022 06:59pm | #7

   FYI the link seemed to have changed to this:
   https://www.mcgillairflow.com/pdf/productlit/ductSysDesign_guide.pdf

1. 
   kermit49 | Sep 08, 2022 07:11pm | #8

   You are correct Dan. Thank you for finding the error. I just corrected it.
   It worked a few weeks ago when I downloaded. Thanks again.

6. 
   rajibroy | Sep 11, 2022 09:00am | #9

   Hello Kermit49,
   I am in the same boat. Using fujitsu slim duct 9k units to support upstairs bed rooms. How do you calculate "critical path supply losses" using constant velocity 400 FPM? The Manual D seems based on friction rate design. Appreciate you attention.

   Regards,Rajib

1. 
   kermit49 | Sep 11, 2022 01:58pm | #10

   Hello Rajib, as I said, I am quite new to all this. I trust someone will chime in if my method is lacking. I will give you a high-level response but can add details later if needed.

   I picked 400 fpm as a reasonable target to keep sound and register air blasts in check especially in bedrooms.

   First check out the Fujitsu capacity tables for the 9k. I wanted to use the 9k but it was short on cooling capacity for my area during the summer season. Too bad as it would have been perfect. I selected the 12k for that reason.

   I assume you did a manual J and know heat load for each room to be serviced. Those load numbers will translate into required heating and cooling room cfms. I recently asked about assigning cfms based on room heat load here https://www.greenbuildingadvisor.com/question/guidance-calculating-room-cfm-requirements-based-on-heat-load#comment-235219?utm_medium=email&utm_source=notification&utm_campaign=comment_notification&utm_content=view

   For each room, I picked the higher of the two cfms (heating and cooling) as the room design cfm. Adding all room cfms will tell you if the 9k fan has the cfm capacity.

   Lay out your ducting on paper from plenum to furthest register. Unless there is another register with high air demand, that will likely be your critical path. You will check other registers later to make sure, right?

   What I did was to size fittings and ducts close to 400 cfm selecting available duct sizes. Some flows were higher some lower using off the shelf diameters in increments of 1 inch starting with 3 inches.

   After passing a serviced room with X cfm requirements, I then reduce the branch duct size to match new lower flow. I kept doing this throughout.

   This seemed simple and logical. Only con will be sourcing fittings that have mixed diameters. Alternative would be to reduce after most fittings.

   Friction rate (FR)

   I calculated the FR using estimated total equivalent length (TEL) on the supply & return critical path. See Manual D Figure 7-6. This was used calculating duct and any fitting with an assigned TEL. Where I could, I used coefficient factor for fittings such as wyes, elbows and tees.
   Got these coefficients from old ASHRAE fitting table. Should be elsewhere.

   Register pressure drops from Hart & Cooley. Flex duct pressure drop estimated 3 x metal duct drop. In my case negligible as these are no more than two feet long. If longer I would calculate more precisely.

   Adding all the critical path losses on the supply and return critical path gave me total system pressure drop. Now calculate system static pressure.

   Sys Static = Critical Path Supply losses – Fan output velocity press (immediately after fan)

   In my case the Total Pressure losses totaled 0.100 iwc.

   My system fan output velocity pressure of 0.007 iwc was obtained from this Ultratech chart
   https://www.kele.com/Catalog/07%20Flow/PDFs/Airflow%20velocity-pressure%20conversion%20chart.pdf

   The resulting System Static press = 0.100 – 0.007 = 0.093 iwc. Now compare this to the 9k's static pressure at the cfm mode it will be programmed to run, there are 4 cfm choices. Quiet to Maximum mode.

   This Rajib this is how I interpreted what is required.