Fresh air piped into the supply plenum?

[samgevas]

I'm not supper well versed on make-up and fresh air design, but I've only ever seen or thought that if its going to be tied into the central air it goes into the return, preferably before a filter.

A few months ago I did a call on newish tract home and noticed that Chainsaw Chas's boys ducted unfiltered fresh into the supply plenum. I assumed (and told the HO) that they had simply put it in the wrong plenum. I recommend that they move it into the return filter box and add a filter rack. I told them to call CR and check if they were still under the builders warranty and if so have them do it.

Today I ran into a home and system built in 2015 with the same setup and now I'm wondering if this was by design. A brief googling did not get me exactly what I wanted, plus I figured I'm not the only one around here could use the learning.

Details:

5 ton Gas Split horizontal right in the attic
The fresh air is 5" flex (whole system is black pvc flex) run from a vent on the roof.
The 5" flex has an inline booster blowing in toward the plenum.
There is nothing else in the 5" flex no damper that I could see.
If the booster had a damper built into it, it isn't working as I get supply temps and pressure throughout the entire 5" run
It dumps the 5" of conditioned air, minus whatever restriction the fan presents, straight out onto the roof whenever the system is running
I'm pretty sure the control won't let the booster run at the same time as the furnace blower, but if they do, they are fighting each other.
The booster fan, furnace "G" circuit and the laundry room ceiling vent are all tied into a ventilation control that is mounted high on the laundry room wall 2" from the 10' ceiling. I didn't get into the control, but I've messed with them before. It will have timers and control the booster and ceiling vent to try and get the required air changes, taking into account how often the furnace blower is running. I'm not sure, but I don't think any of the ones I've seen are measuring air pressure In my experience they are never commissioned right and I usually just disable all the auto run and timers and set them up as just makeup air for the ceiling vent, or the for the system if the AC is putting the house into negative pressure, but I don't see a bunch of them and fresh air isn't a huge focus for me given our climate.

Anyways, is this a thing? Did they do this right? If not, what do you think they were trying to do? These systems are spec'ed out by the developers so as to meet some requirement for energy ratings if I'm not mistaken, so Chas Roberts must have been following some kind of design plan that at least started out sensical. If it is correct as is, how I test and adjust it to operate properly

I told the customer I would look into it and let him know if we needed to rebuild it or just recommission it

[kdean1]

You're in a hot climate, though dry. I think it's stupid to add hot, unfiltered air to the supply. If you were in a humid climate I would suggest a ventilating dehumidifier, but you probably don't need dehumidification. The fresh air should be introduced to the return so it can be conditioned before being sent into the house.

[pecmsg]

Even in a hot climate a 5" outside air duct to the return will only pull in what it can exhaust. Yes it has to be filtered.

[Zamoramax]

The other thing to consider. Here in norther ca we are required to hard pipe anything that goes to the outside for fire reasons.

Sent from my SM-N960U using Tapatalk

[teddy bear]

I would suggest fresh filtered air be added to a well built home when the home is occupied and the winds are calm. This could be controlled by manual switch or a CO2 meter controlling a duct blower. The amount of air should be a filtered fresh air change in 3-5 hours to purge indoor pollutants and renew oxygen.

In a typical "green grass climate", a small whole house dehumidifier with the fresh filtered air option would be ideal. A unit like the Santa Fe Ultra would be ideal. This combination will keep the home comfortable and healthy when occupied.

Keep us posted on related issues.

Regards Teddy Bear

[PepeLePewPew]

Seems to me if they just tapped into the supply plenum then it would be blowing conditioned air out of the fresh air instead of drawing it in. Although it probably has a one-way flapper to prevent air from being blown out. So then it would do nothing? Only way I can see it drawing fresh air into the supply is if they devised some sort of venturi, which I highly doubt they did.

FWIW this is what ASHRAE says:
The rate at which outdoor air replaces indoor air is described as the air exchange rate. ASHRAE (formerly called the American Society of Heating, Refrigerating and Air-Conditioning Engineers) recommends (in its Standard 62.2-2016, "Ventilation and Acceptable Indoor Air Quality in Residential Buildings") that homes receive 0.35 air changes per hour but not less than 15 cubic feet of air per minute (cfm) per person. as the minimum ventilation rates in residential buildings in order to provide IAQ that is acceptable to human occupants and that minimizes adverse health effects. ASHRAE also suggests intermittent exhaust capacities for kitchens and bathroom exhaust to help control pollutant levels and moisture in those rooms. ASHRAE also notes that "dwellings with tight enclosures may require supplemental ventilation supply for fuel-burning appliances, including fireplaces and mechanically exhausted appliances.

[BBeerme]

I have an old house with old windows and leaks everywhere.

I did change the slider due to cracked glass. That thing was so leaky (nothing to do with the cracked glass), but it probably paid for itself in energy savings.

Seems to me if they just tapped into the supply plenum then it would be blowing conditioned air out of the fresh air instead of drawing it in. Although it probably has a one-way flapper to prevent air from being blown out. So then it would do nothing? Only way I can see it drawing fresh air into the supply is if they devised some sort of venturi, which I highly doubt they did.

FWIW this is what ASHRAE says:
The rate at which outdoor air replaces indoor air is described as the air exchange rate. ASHRAE (formerly called the American Society of Heating, Refrigerating and Air-Conditioning Engineers) recommends (in its Standard 62.2-2016, "Ventilation and Acceptable Indoor Air Quality in Residential Buildings") that homes receive 0.35 air changes per hour but not less than 15 cubic feet of air per minute (cfm) per person. as the minimum ventilation rates in residential buildings in order to provide IAQ that is acceptable to human occupants and that minimizes adverse health effects. ASHRAE also suggests intermittent exhaust capacities for kitchens and bathroom exhaust to help control pollutant levels and moisture in those rooms. ASHRAE also notes that "dwellings with tight enclosures may require supplemental ventilation supply for fuel-burning appliances, including fireplaces and mechanically exhausted appliances.