Outside Return Air

Can someone please tell me what would be the deciding factor to have return air coming from the outside in new construction homes with all the HVAC units the same high efficiency condensing size/type and all houses relatively the same square footage? What would be the reason one new construction home would have one and not another? Yes, I’m asking about outside return air not outside combustion air. TIA

Parker, Colorado Home Inspector

Good catch . I wonder could it have been a new installer who forgot to put in the return air.

Depends of the efficiency of the home, high efficiency with blown liquid foam in the attic and exterior walls would need outside make up air for the return but not 100% are you sure there was no return air from the interior

Could it be what is called a hard-connected duct referred to by some as a poor man HRV?

More info at: https://www.cmhc-schl.gc.ca/publications/en/rh-pr/tech/91-206.pdf

No deciding factor, its simply the code in all states from my understanding.

The whole idea is to bring in a small amount of outside air typically with a 4’ duct (50 CFM) to the return air side.
Normally located at far end of the return air ducting so it has a chance to temper prior to going through the heat exchanger.

An HRV or ERV does a similar thing but with precise intake/exhausting of house air and at specific efficiency.

You misunderstood then because its not code in Okla

It is code where I am, but it is also a good idea anywhere, especially with direct vented appliances.
Buyers and builders are concerned about energy used for heat loss or gain, so new homes tend to built tight, which could mean if all windows and doors are closed there is very little air exchange.
A new house could end up being like a jam jar kids put bugs in, with no holes punched in the lid.
A rule of thumb is to exchange all the air inside homes every three hours. Without natural ventilation that was the result of loose construction and conventional draft chimneys this now needs to be done mechanically.
Using HVAC return to bring in fresh air is one way to do that.

I would agree with you on the tight homes but not at a rate of 100% outside return air as I thought was being implied

I’m beginning to think that the deciding factor is wether or not the range exhaust fan is direct vented outside. Could this be right? Everything else is basically identical. The same builder in the same neighborhood with the same HVAC equipment in homes that are generally the same square footage. All these homes have whole house exhaust fans that run constantly to exchange the air because of the “tightness” of the build. There is no identifiable pattern with home models. (i.e 2 different homes that are the same model but one is getting outside return air, and the other is not.) Also the homes have normal return air coming from inside the home on each level.

Parker, Colorado Home Inspector


Refer to Colorado mechanical code for guidance.

As stated above, the fresh air intake into the return air duct whether code or not, is a good idea with current construction being so tight.

The kitchen exhaust hood issue is a separate issue and needs its own make up air inlet if over certain CFM.

ASHRE determines the fresh air ventilation requirements in buildings.

When the house is build too efficiently, outdoor air is required. Generally through an HRV or ERV.

Building negative pressure is also remedied with introducing OA.

If there is not HRV/ERV it must go through the HVAC first.

don’t confuse ASHRE with Code they are not the same

It is the Standard for Weatherization.

Weatherization is a Code (in many areas), and the code quotes and references ASHRE.

Weatherization is not Inspection Standard, and HI’s do not enforce Codes.

OP, if you want to read; look up ASHRE Standards 62.1 & 62.2

Standards vs code
Building code is a* minimum* standard, meeting code is sort of like 50% on the grade 9 math test :slight_smile: OK you pass, but you got half the answers wrong. A building code may be law, if it is law it is because another authority has made it so, usually at the municipal level.
ASHRAE is an industry standard set by an industry association, ie American Society of Heating, Refrigerating and Air-Conditioning Engineers, ASHRAE standards will meet and probably exceed code. interNACHI is an industry association, members have to meet the standard as a condition of membership, membership in industry associations is usually not required by law.
The highest standards are usually the manufacturer’s standards, often they are beyond what is needed and may exceed code and industry standards, which could give the manufacturer an out when it comes to warranty or liability claims. (wasn’t our fault, they did not follow procedures). Think of the tap dance Tesla is doing re their self driving car smacking the semi.

Thanks for all of the feedback! But is there a DECIDING FACTOR? Let’s just say 2 houses are exactly the same except one has outside return air coming in and the other one doesn’t. Both houses are in compliance with code and all that. Neither one is right or wrong. There has to be some kind of building modification or something that requires the outside return air. I’m pretty sure an HVAC Installer is not going to go the extra mile and just do this because they felt like it. I’ve been seeing this a lot in new construction and I would like to pinpoint why one house would have it and not another.

In post #9 you said the only difference is whether the house had a range hood exhausted to the outside. Bingo! That’s your answer. When a specified amount of air is removed by an exhaust fan, an equal amount of air has to be brought in, called make up air, bath fans (aka fart fans) are usually 50 to 75 CFM, ie not very much, range hoods start at 75 CFM and may go up to 800 CFM.

Many years ago I had a blower door test on our Home . I said I have air leak near the electric panel to close .
He said** No stop now **.This is your make up air and it is needed do nothing more.

Took his advice and we have had no difficulties controlling the humidity in our home.

Great string thanks to all.