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Registers are not dampers, shouldn’t be used as such, always at the takeoff dampers

Originally Posted by mgenius33

I agree with Wayne, the mold thing is bogus. Moisture is related to the temperature of the air and the ambient RH% in the home. If you have supply air temperature below dewpoint then you're likely to have sweating supply grilles.

So what do our brilliant manufacturer engineers come up with to reduce indoor humidity???

Yeah! Let's sell overpriced variable speed blowers that decrease air speed which in turn creates a lower supply air temperature! Great, our indoor humidity level is a whopping 3% lower, and now we have moldy registers!

My "Modified Like Button" >>>

Originally Posted by ferd1942

The 10 CFM rooms are water closets in large bathrooms (a room housing the WC, not the fixture inself) large walkin closets, mud rooms, powder rooms, pantries, etc. all with a little exposure to outdoors. Haven't done a Broom Closet yet, but I have seen two 4x10x6 serving a small WIC with no outside walls.

400 CFM/Ton at 1,000 to 1,350 SF/Ton = 0.40 to 0.30 CFM/SF, so 25 to 33 SF.

Thanks for asking.

FYI: I work with one main installer, who's buying 2-1/4x10's, 4" and 5" duct in quantity.

Please define " large bathrooms", ... length, width, height, conditioned walls/ unconditioned walls/ insulated crawl/ Basement/ Insulated attic/ Conditioned Space above.

I'm asking cuz', ... I've never seen a "Large Bathroom" get a S/A that delivered 10CFM only. The "Fart Fan" that exhausts most bathrooms pulls at least three times that amount of air.

MG: The Industry is experiencing an exponential increase in service and repair business created by the high tech stuff installed by guys from the Alpha Hotel, but most contractors (sorry, people) ignore the business opportunity to correct the root causes (oversizing, 0.10" friction, duct workmanship, high resistance fittings, etc.), which would help their customers and generate "Word of Mouth" referrals.

Now if there was an associated exponential increase in Qualified techs who could service the stuff...

I agree with Wayne, the mold thing is bogus. Moisture is related to the temperature of the air and the ambient RH% in the home. If you have supply air temperature below dewpoint then you're likely to have sweating supply grilles.

So what do our brilliant manufacturer engineers come up with to reduce indoor humidity???

Yeah! Let's sell overpriced variable speed blowers that decrease air speed which in turn creates a lower supply air temperature! Great, our indoor humidity level is a whopping 3% lower, and now we have moldy registers!

The 10 CFM rooms are water closets in large bathrooms (a room housing the WC, not the fixture inself) large walkin closets, mud rooms, powder rooms, pantries, etc. all with a little exposure to outdoors. Haven't done a Broom Closet yet, but I have seen two 4x10x6 serving a small WIC with no outside walls.

400 CFM/Ton at 1,000 to 1,350 SF/Ton = 0.40 to 0.30 CFM/SF, so 25 to 33 SF.

Thanks for asking.

FYI: I work with one main installer, who's buying 2-1/4x10's, 4" and 5" duct in quantity.

How small is that room, that only requires 10CFM Ferd?

Are You conditioning a broom closet?

With today's homes running as much as 1,350+ SF/Ton, a lot of sizing issues have arisen: Manufacturers just don't make small enough stuff.

I wrestled with how to deliver only 12 CFM to an outlet (closing a 4" branch damper and its 2-1/4x10 floor diffuser multiblade damper still gets you 15 or 20 CFM) for hours. Then a great tech I'm working with came up with a simple solution, bless him: He uses a 2" hole saw instead of cutting a 4" hole for the 4" adhesive collar, providing the ability to dial in whatever we need.

12 CFM being the ACCA residential balancing tolerance (10 cfm x 120%) for my specing an outlet in small rooms with limited exposures to outdoors (less than 10 CFM required, no outlet - I'm sure everyone here is OK with that).

LOL! That's my last resort when all else fails: The additional 0.03" can make a big difference.

Face dampers certainly are not preferable but in some cases are necessary and are generally not noisy unless the diffuser and branch duct are oversized. Most face dampers are self locking but most used in residential floors are not.
When I ran into a diffuser with a face damper that was short of air I could get on average about 20 CFM more by removing it and I removed several.

I have a friend who inserts round ceiling diffuser dampers in floor diffuser boots where there're no (accessible) branch dampers, says they work fine. I believe that complies with the SMACNA requirement for locking, but the additional PD scares me.

When a system is designed there's typically an ACCA, SMACNA and/or Code requirement for branch dampers. Balancing at the outlet is usually prohibited because of the noise issues and the fact that many devices can't be locked in position. I also don't like them because they're accessible to those "cold" or "hot" occupants.

I don't know who instructed you that mold growth will magically appear when cutting at the diffuser.....but I've never seen it...and I'm in Houston too! Cutting at the spin in to the main trunk will also yield less leakage than cutting at the diffuser (your building static in less duct work). Last but not least, cutting at the diffuser is noisey. Exactly what type of system is this?

I prefer at adjust at the takeoff instead of the register. If there is not a damper at takeoff then you have no other choice.

David, like you I have also worked with systems that were not adjustable. I consider a system with just a terminal adjustment to be in that class. Of course when that's all you have that's what you adjust. Even when the terminal is adjusted the flow pattern
is in many cases a wreak. There are combo grills like the Barber Coleman that has a volume damper directly in back of the grill that has a 4 way pattern adjustment. Pricey though but good results.

Forget mold. If condensation is an issue tweaking a grill won't fix it. The supply air is too cold or wet air is being introduced is another way. The A/C should be able to stabilize the humidity in a short time unless there is a problem.
The air discharge should be what the supply duct is delivering.
There was a job on Johnson Island. It's in the middle of the Pacific and I'm told is the farthest away from everywhere. They had diffusers sweating but it was because the engineer didn't spec insulation on the outside of them. The RH there is often near 100%. Try to find the source of the humidity.
It's the room air that was humid and the condensation was on the outside.

I start with the Volume dampers, then the branch dampers, then complete it with the Register Dampers, in order to balance the Air flow through out the building.
Now, in many cases, where the Installation, was never Inspected, &/or was older than 20 years, .... I've seen many a Residential Installation with out any damper control, other than the Registers.

Another point, is that in a Mobile Home/ Modular, & RV,... The Only Damper Control is always the Registers.

As far as "Mold Growth"? That can largely be cured with the Building Owner having the Equipment Tuned & Cleaned on a Regular Basis, (with Evap. Pan Treatment, & Condensate Drain cleaning if required.) as well as the Contractor informing the H.O., about properly inspecting & changing the Filters.

Look at a grill/register/diffuser air flow profile and how they entrain room air. They are on manufactuer's sites. By closing them the profile will be less than idea.
I don't remember a mold issue before this. If condensation is forming at the terminal you might need to increase the fan speed so the air is a bit warmer or find the supply duct problem.

Making the adjustments at the take off, also will reduce duct losses in the branch runs on systems that are less then perfectly sealed


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It's always best to balance using dampers. If you close off registers it greatly increases your noise level in the space. Also someone else can easily adjust it again and negate the work you just did.

The mold angle is bogus. There are two reasons in residential to balance at the plenum the biggest one being they will hold set point because nobody can mess with them. The other reason is noise potential but that is rarely a problem in residential. Most residential systems are low on airflow and if not the static pressure at the diffusers is almost always below 0.1 inch.