Open stairway for air return?
Mauro_Zammarano
| Posted in General Questions on
Hello,
I am building a modular house, 2 floors + basement.
HVAC in basement will be a VRF 3 zones heat pump.
Rheia is designing the duct system and is suggesting that a return central duct is not required because the open stairway provides enough air return. Does this sound right? I understand that stack effect might push against the airflow depending on the season but the house will have a zone per floor and it will be relatively airtight (about 1 ACH50).
Thanks!
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We're doing that on a $2M+ new home with a system designed by a licensed engineer. The return does have a duct, but it's short and the only one for the lower level and main level of the house. The house is well-insulated and air tested at 0.5 ACH50, but I've had similar systems on conventionally-built homes with no complaints, as long as the doors to each room are undercut. (Or, preferably, with a Tamarack return-air grille over the doors.)
Michael, how much of a gap are you leaving under those doors for air return functionality?
Bill
Bill, I should add that I'm not saying that having a single return air grille is best practice. Ducted systems are still rare where I work and the homes where we use them are all high-performance, so airflows are low. The $2M home I mentioned is about 3200 sq.ft. but the second floor, which has two bedrooms and two bathrooms, has its own low-static-pressure ducted heat pump. There are return air grilles in the bedrooms and the bathroom doors are undercut by about 1/2".
There are two bathrooms (with doors) on the lower floors served by the central unit, but they have essentially zero heat load, so we're using a small amount of electric resistance heat instead of extending ducts to them. The rest of the spaces are either open to each other or utility spaces.
At my old office, which was roughly 1500 sf, we did have a pressurization problem due to having no return air grille within the space and a 1/2" gap under the single door. Air whistled through the door and it took significant effort to open it.
The ME spec'ed 1" for my house.
Note that you don't have to cut the bottom of the door, just mount them higher in the frame.
Michael thanks for your answer but I am a bit confused. If I understood correctly the house you are building has an indipendent hvac on the last floor and returns on the other 2 floors floors with a ducted heat pump located in one of the lower floors. It doesn't sound like you are using stairways as central return
There is only one return for the lower two floors, in the finished, walkout basement. The stairway is large and open which allows the air on the first floor to get to the basement return.
In case it's not clear, the stairway itself isn't "the return," it's functioning as an oversized return air duct. The heat pump air handler has a return air intake to balance the conditioned air it pushes out.
OK I believe I understand now. So your main heat pump is in the basement and the stairways acts as a return duct from the first floor to the basement. Is there a zone for each floor or this approach can work well also without zoning each floor?
It's all one zone, which required careful engineering, and even then, the owners will probably have to manually adjust the supply registers between summer and winter.
Ideally it would have been at least two zones, or the whole house on a 3-zone system, but due to the layout--a holdover from the renovation it started as, before finding that we couldn't reuse the foundation--and other constraints, this was our best option.
Code generally requires a return on each floor. So if you have an air handler on each floor than they can have their mini local returns (short run of duct with a filter grill). Since in this case there is no airflow between floors, open staircase doesn't matter.
Thank you Akos I did not know that a return on each floor is generally required by code. Do you know what is the reason for this and why stairways cannot be used as central return? Just curious to understand the physics behind this. I could not really find anything about this and other than stack effect - which I believe can be compensated by single floor zoning - I do not see any physical reason why a stairway would not work as return unless stairway is blocked by a door.
Akos, I don't see that code in the IRC: https://codes.iccsafe.org/content/IRC2021P2/chapter-16-duct-systems#IRC2021P2_Pt05_Ch16_SecM1602.2. Is it in a different code?
I'm only familiar with our code which specifically requires it:
6.2.4.7. Return-Air System
(11) Except for floor levels that are less than 900 mm above or below an adjacent floor level that is provided with a return-air inlet, at least one return-air inlet shall be provided in each floor level in a dwelling unit.
Outside of that, a largish return near the ceiling on the top floor greatly help with reducing stratification and improves cooling in the summer.
Michael thanks a lot for confirming this can be done. I am still trying to understand the drawbacks of this approach and potential code restrictions but I could not find anything specific, especially with single zone per floor.
OK thank you all! So in conclusion even if not illegal in the USA, it is definetly not best practice to use strairway in place of a return duct to the point that is illegal in other countries like Canada. This now let me question the quality of the design I am getting.
Building code requirements don't always make sense for every possible scenario, so codes make provisions for alternative solutions. I can't think of any issue with using an open stairwell as the return. For what it's worth, the following is the "intent statement" that goes with Canada's code requirement for a return on every floor. If you are satisfied that your design deals with those possibilities, then I wouldn't worry about it.
To limit the probability of:
· an inability to maintain the minimum indoor air temperatures [see Sentence 9.33.3.1.(1)], or
· inadequate ventilation.
This is to limit the probability of:
· the inadequate control of airborne pollutants or relative humidity, or
· condensation.
This is to limit the probability of:
· the generation of pollutants from biological growth or from materials that become unstable on wetting, or
· the deterioration of building elements.
This is to limit the probability of:
· negative effects on the air quality of indoor spaces, or
· the inadequate thermal comfort of persons.
This is to limit the probability of harm to persons.
Functionality wise, I don't see the need for return on every floor with an open stair. It is a large opening and much less restrictive than a door undercut.
I would want one largish return near the ceiling on the top floor and the rest can be near the air handler.
Probably a good idea to also have a low return in the basement as well. Not as critical in a well insulated and air sealed basement though.
On the in-between floors, as long as you have a clear bath to one of the two main returns, I don't think it matters.
Supply vent locations make a bigger difference in comfort especially if you have large glazing or rooms with multiple outside walls (ie room with exterior cantilever).
What I am worried about is that return air velocity in the stairway would be extremely low due to the large cross section of the stairway "duct". Stack effect can generate air velocities probably higher than that and push back that return air unless you have a house that is really tight. So return air flow would change depending on several factors and that sounds not ideal but sure we can live with much worse if we need to and do not have any aternative option. I tried to visualize the concept below.