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One central HVAC air return in a closed hallway, jump ducts to rest of house?

jonathanb | Posted in General Questions on

Sanity checking an HVAC design possibility…

The question: If an HVAC system has a single central air return in a short hallway that often has closed doors, is it ok to add a jump duct to pull return air from the living spaces? 

The context: We’re looking to use a ducted heat pump in a new house, only 1,200 sq ft, in CZ3C. Based on a Manual J load calc using CoolCalc, with modern energy efficient construction, we will need well under 10,000 BTU in total. (I’m sure a typical HVAC installer would propose 2.5 tons 😉

Based on that, we’re considering the Mitsubishi or Fujitsu 1 ton ducted unit, which are already oversized for our needs. Whichever we choose, it will modulate and never run at max capacity. They’re quiet and efficient and people love them on GBA. We are thinking to install vertically. 

To reach all the rooms, it will take a fair amount of ductwork, and there’s very limited space, based on the architectural design. All of the supply ducts will fit, no problem. Having return duct runs to maybe 3 locations might be possible, but very tight, and it also edges up the static pressure of the system. It would be much simpler to have a single central air return.

The catch is that the single air return would be in a short hallway that may often have a closed door to the main part of the house, mainly for noise reasons. (It’s not the air handler we’re worried about — the Mitsubishi and Fujitsu are very, very quiet. There’s also a heat pump water heater and laundry, and those could add up.) 

It’s possible to add a vent to the bottom of the doors to get more airflow, and maybe that would be enough? It would also partly defeat the goal of reducing noise.

So here’s the question: what if we could put a jump duct with flex duct to get to the other side of that door? That kind of turns the whole hallway into a return air duct. It saves some space from connecting the return air duct to the air handler and having it do a 180 degree turn. 

Any reason this wouldn’t work?

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Replies

  1. acrobaticnurse_Eli | | #1

    I've seen a number of posts on here supporting jumper ducts and it's pretty common to have the hallway as the main return. There's also the option of Tamarack's Perfect Balance in door return air pathway. I've installed one for my daughter's room so far and will likely install one for each of the other bedrooms soon, but a large jumper duct would likely look better, have less noise transmission, and not involve cutting into your door. You'll need to look at how many cfm you need to get to the hallway and what size jumper duct or return air pathway that would require.
    https://www.tamtech.com/product/perfect-balance-in-door-return-air-pathway/

    Also, if it's not a typo and your heat/cooling load is really just 1000 btu your house might be better served by something like the Cerv2 vs a 1 ton mini split.
    https://buildequinox.com/thesystem/

    1. jonathanb | | #3

      (Corrected the typo: it's under 10,000 btu.)

      Thanks for the link to the CERV2. I wasn't aware that some ERVs can also heat or cool. If I might need an ERV, it would be nice to have a single unit that does everything.

  2. Expert Member
    Akos | | #2

    Door undercuts are the simplest. For a low load space even something as small as 5/8" will work which you'll need to clear any carpets anyways.

    If you do want the doors fully sealed, high/low jumper ducts in the wall towards the hallway are the simplest. You can do this with oval pipe to fit into a 2x4 wall but better for sound with flex with a lazy S bend inside a 2x6 wall.

    Avoid running any ducting including returns through the attic.

  3. jonathanb | | #4

    Thanks for the tips!

    The insulation will be at the roof joists, and the roof will be landscaped with 6-12" of soil. There will be about a foot of clearance for ducts between the joist insulation and the drywall of the ceiling.

    I see that the Fujitsu 9,000 BTU Slim Duct heats and cools plenty for this 1,200 sq ft house, so I might rule out the larger 1 ton options, if the extra static pressure isn't needed. The Slim Duct has 0.36 static pressure, puts out 353 cfm at the max setting, and is still only 28 dB. Crazy efficient.

    The ducts will need to extend about 30 ft to reach the farthest room, and there are 3 bedrooms, 2 baths and a living/dining/kitchen area. Kind of a lot to cover with a single 9,000 BTU unit.

  4. acrobaticnurse_Eli | | #5

    10,000 BTU makes more sense ;-) With the house being 1500 square feet I'm wondering about the dimensions involved if ductwork needs to go 30 feet from the air handler. Is it 25 by 60 feet? I would think that with the house being well sealed that hopefully the temperatures would normalize over time as long as there is enough circulation, even separate from the ductwork, and a well sealed house doesn't need to have ducts extending to the farthest edges of the house.

    Also, the Cerv2 is different from ERVs in that it uses a small heat pump rather than an ERV core to get circulated/fresh air to a comfortable temperature. For most homes it wouldn't be enough to serve as the sole HVAC, but some smaller spaces have apparently managed to do so.

    I hope you post pics when your earth sheltered house comes to fruition.

  5. user-5946022 | | #6

    My home has flex jumper ducts for return. They work well, and help control noise much better than undercuts. I put insulation in the walls between the bedrooms and the hall to help control noise, which works well, and the jumper ducts don't compromise this.

  6. Mitsuman | | #7

    That would work just fine, I agree I like to split return high and low when possible for best performance heating/cooling...but it's not necessary if it's not feasible. We build custom return pans with split trunks to achieve this in the walls of new builds (work around the studs). Keep in mind your heat pump water heater is pulling heat from the air and you may not have accounted for that in your load calc. It also needs access to a fair amount of air volume so be mindful of its location and check the spec on the room volume required, they can be ducted to areas with more space. Also, as you indicated, a one ton air handler is not moving much air, there are three fan speeds on the Mitsubishi air handler. The output CFM is 278-381-448 (low, med, high). The ducts need to be sized properly and sealed at every joint, seam, and swivel point to ensure the air has sufficient velocity and is getting where it needs to go. This is not a lot of air to be split between several supply registers. They should design using Manual D. ERV can be ducted into the air handler, there are different configurations that can be used. Some require an interlock so the air handler fan runs with the ERV, some do not. I like to be able to run the ERV separately. If you share return and supply with air handler (back draft damper in ERV supply) it works well and can be run independently. I would not recommend using the ERV being referenced as a source of heat also. Pick the best tool for the job, a tool that's designed to be used for primary heat and give you trouble-free operation and longevity. If you're in a rural area that experiences power outages and brownouts, be sure to protect your heat pump electrically. I like the ICM 493 line monitor, have been using it for a few years now it's an excellent piece of equipment and has identified low voltage issues in some of my customers' houses. They had no idea they were having significant voltage drops until we installed our system with that protection device and it started shutting our system down repeatedly. It monitors voltage and cuts power to the system if voltage varies +/- 10% from nominal, also includes integrated surge/spike protection. Utility ended up having to change/upsize a transformer in one case. All thanks to that little device letting us know there was an issue. Best of luck with your build, sounds like a great project, enjoy the process!

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