Does Closing All but One Vent on a Ducted, Zoned Airconditioner Harm The Air Con?

tdst-taboo on 02/03/2023 - 13:10
Last edited 02/03/2023 - 19:36 by 1 other user

Hey all,

Just recently had a Mitsubishi Heavy Industries, ducted and zoned air con installed. There's the main zone which covers the living areas and has the return air grill (RAG). Then there's 4 other zones which are just bedrooms and a study.

We have an AirTouch 2 controller which lets us set the airflow to each zone as a percentage out of 100. At night we close the master bedroom door, turn on the zone for only that room and set it to 40% airflow (any higher and it causes the air to whistle as it escapes the door frame).

I recently stumbled across some articles (all for American HVACs) that claimed that turning on only one room can cause extra stress on the air con due to pressure and if you're going to you should leave the door slightly ajar and give the air a pathway back to the RAG. Is there any truth to this? I now worry that having only one zone on with the airflow set so low (40%) is effectively giving little options for the air to escape the duct and potentially causing undue stress on the whole thing.

I'd have initially thought the system would be smart enough to compensate for this and decrease its "air force" but I now assume the air flow setting is just for the zone vent itself and doesn't actually tell the aircon to decrease its force.

Thanks!

ANSWER BELOW:

Ok, I spoke with one of the engineers for Polyaire, the people that make the AirTouch controllers.

He confirmed that yes, running it the way we are is an issue. Running only one room with the door closed will cause a lot of pressure in the system which has to escape somewhere which "at worst could cause things to pop off" I assume he meant ducting/grills. It will also cause extra wear on the fan as it's "like it's running into the wind" (the analogy he gave).

He confirmed that the smartest idea would be to run with all the ducts open but certainly at least the "main house" that has the RAG in it. Given the return air grill is the thing that determines how hard the system runs (and that isn't in our master bedroom) the grill at the moment would be sucking warm air from the rest of the house causing the air con to run harder and so opening vents to the rest of the house and cooling more of the house down will actually cause the air con to be MORE efficient than just trying to cool one room.

So there you go. I learned today.

Home & Garden Air Conditioner

Comments

  • PissLUR on 02/03/2023 - 13:13
    +1

    there should be Constant Zone(s) that are automatically opened.

  • AngoraFish on 02/03/2023 - 13:16

    My mother blew out the ducts by closing air vents in rooms she wasn't using, and it wasn't saving much energy anyhow. She has to leave them all open now, god knows why the vents even include the ability to close them. Ducted heating is super-inefficient.

    • Mugsy on 13/03/2023 - 10:04

      Yeah, I personally hate ducted because of the inefficiencies but aesthetically, people tend to like it.

      Unfortunately, my wife falls into this category so we're getting ducted in our new house.

      I love it when people argue about ducted taking up less space when they don't realise the ducts that run inside the building often takes away from wardrobe/cupboard space.

  • Muzeeb on 02/03/2023 - 13:32

    Which room is the thermostat in?

    set it to only ~40%

    What does this mean?

    Aircon or evaporative cooling? I'm confused.

    give the air a pathway back to the RAG. Is there any truth to this?

    No truth required. It's science, physics, fluid dynamics or whatever you want to call it.

  • tdst-taboo on 02/03/2023 - 13:44

    Not evaporative. A standard air con - Mitsubishi Heavy Industries FDC100VNP-W.

    We got it with an AirTouch 2 controller with motorised zones so we can control the airflow to each room. It basically just acts like a carburetor on each zone to either fully open or restrict the air flow.

    I'm now concerned that if the Aircon unit itself isn't smart and just blasts at full force most the time and we close all zones but one which is only 40% open then that might cause a lot of pressure in the ducts and… I don't know ¯_(ツ)_/¯ cause issues? That's what I'm asking about I guess.

    • AngoraFish on 02/03/2023 - 14:00
      +2

      It will cause a lot of pressure in the ducts, which are typically a flimsy aluminum foil and coil wire held together with… you guessed it… duct tape.

      They are not designed to any particular pressure, or pressure tested in any way after install, they're just rolled out along your ceiling joists or under your floor boards. Once the pressure in the ducts rises you'll learn about any installation or manufacturing flaws the hard way.

      Best case though is that as the ducts are cheap and flimsy to start with these problems can often be the sort of thing you can sort out yourself with a dust mask, a bit of elbow grease and a trip to Bunnings.

      • tdst-taboo on 02/03/2023 - 14:03
        +1

        Thanks.

        So I assume the solution is to either turn on another zone or leave the bedroom door slightly open?

  • Euphemistic on 02/03/2023 - 13:46
    +1

    Maybe ask your installer?

  • Ugly on 02/03/2023 - 13:51

    When we had our ducted system installed we were told to make sure not to close ALL of the vents at the same time.
    As long as at least one is open, you should be ok.

  • JIMB0 on 02/03/2023 - 14:05
    +2

    You'll find that with the door closed most of the air is returning to the inlet by escaping from the house (from gaps, unsealed windows, exhaust fan etc.) and outside air is being sucked into the house to your return air grille. It would be much more efficient to leave the door ajar.

  • beyonddd on 02/03/2023 - 14:07

    We have the same AirTouch 2 controller. It will control and spill out other air vents to prevent this kind of pressure issue if required.

  • netjock on 02/03/2023 - 16:00

    As said, don't close all the vents and don't close all your doors.

    You got sucked into the expensive ducted air con scam.

    • tdst-taboo on 02/03/2023 - 16:01

      I assume you're implying it's a scam compared to an alternative that's better? If so what would you feel is smarter?

      • netjock on 02/03/2023 - 16:03
        +4

        1 outdoor unit multiple heads units. They just recirculate the air within the room. No need for a central return located somewhere causing this draft.

        But you learn.

  • tdst-taboo on 02/03/2023 - 16:08
    +3

    Ok, I spoke with one of the engineers for Polyaire, the people that make the AirTouch controllers.

    He confirmed that yes, running it the way we are is an issue. Running only one room with the door closed will cause a lot of pressure in the system which has to escape somewhere which "at worst could cause things to pop off" I assume he meant ducting/grills. It will also cause extra wear on the fan as it's "like it's running into the wind" (the analogy he gave).

    He confirmed that the smartest idea would be to run with all the ducts open but certainly at least the "main house" that has the RAG in it. Given the return air grill is the thing that determines how hard the system runs (and that isn't in our master bedroom) the grill at the moment would be sucking warm air from the rest of the house causing the air con to run harder and so opening vents to the rest of the house and cooling more of the house down will actually cause the air con to be MORE efficient than just trying to cool one room.

    So there you go. I learned today.

    • Mugsy on 13/03/2023 - 09:58

      Yeah, I made a similar mistake a few weeks back. 36 degrees so I left the aircon on all day whilst at work so the dog wouldn't suffer. He's downstairs and the air intake vent is upstairs… I mistaken decided to leave the windows and doors open upstairs with only the downstairs vent open.

      It was only halfway to work that I realised that the air intake vent would be drawing in extra hot air ALL DAY and the air con unit would be working hard all day long.

      The facts aren't hard to digest… high school level physics and logics will get one there… but after 15yrs of being use to spit system air con in my previous two properties, I sometimes forget how ducted works.

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