I am intrested in educating my self about my HVAC system and checking and verifying some things. I hope this will not be considered DIY.
I have an older house built in 1980. We had the outisde condensor unit changed a few years ago to a Bryant 113RNA036-C, which I think is a 3-ton heat pump.
We have a 2 returns (ceiling and floor level), but the other day I disovered the floor return was sealed so that no flow would occur. I wonder why this was done? I don't think the tech. that installed the new system noticed this.
I have been trying to verify how much return I need for my Bryant, but the information I see on the net is confusing and inconsistent. The open return in the ceiling is filtered, and uses 20"x30" filter.
I wanted to confirm that this is ok for my present system.
Even better would be to learn in general how a delivery duct system is matched to a return duct system and AC size (tons/btu). A referal for a good book, preferable also a good read, would be greatly apreciated.
couple of factors there. The equation that we use that filter grill would be bare minimum size for a 3t system. As in I wouldn't put anything in front of it or use any sort of restricted filter minimum size. Now what's behind the grill opening, is it a correctly size trunk line, a round duct (metal or flex) is it a wall chase. These will also affect the amount of return going back to the system.
A 20X30 is 600-sq.ins., (I use *1.6 to get close to Manual D's 300-fpm initial filter velocity); 600-gross sq.ins., * 1.6 CFM per sq.in. is 960-CFM; but you need to flow from 1350 to 1200-cfm.
Filtering & airflow would improve with more filtering area, say, 1350-cfm / 300-fpm is 4.5-sf of open-air-area (Ak).
Hart & Cooley engineering data for 1309-CFM; 30X30 is 900-sq.ins., 4.363-sf or (Ak), for 300-fpm initial clean cheap fiber glass filter velocity. Max loaded fiberglass filter velocity is considered 500-fpm.
Last edited by udarrell; 08-01-2012 at 12:40 PM.
I really appreciate you all taking the time to respond.
Cuchulain, to answer your question, I found a 16” diameter metal duct behind the 20" x 30" filter, for an air flow cross sectional area of only 201 sq. inches.
Udarrell, it looks like you have provided some great concepts to digest. I really like that, but I am still a little unclear:
1. I’m not real sure about “*1.6 to get close to Manual D's 300-fpm initial filter velocity”. Is it factor to multiply by to find the startup airflow condition?
2. Why do we divide by 300 here “1350-cfm / 300-fpm is 4.5-sf of open-air-area ”.
3. I’m also shaky about the Hart & Cooley data, but I’ll try to figure that out later and ask if need be.
Again, thank you all.
Also, the sealed off return plenum I originally spoke of is 12" x 36" = 432 sq. inches.
The 16" duct behind the 20" x 30" filter is 201 sq. inches.
201 + 432 = 633.
It appears as though the plenum retrun duct should be reopened?
Do a Google Search for: Hart & Cooley engineering data
Originally Posted by outinaustin
(Copy & paste the search words in browser's address window)
Last edited by udarrell; 08-01-2012 at 08:48 PM.
Reason: square feet area is (Ak) on Hart & Cooley data...
After quite a few hours of reading, I am becoming more familar with the involved subjects, but am also becoming aware I really need to learn more. I've got to take my Engineer in Training Exam in October (Electrical), so going over some of these things is a blessing in disguise. However, when trying to crossreference unfamilar points, I'm encountering confusing and semingly contradictory facts, and paralasses by analysis is setting in.
I think my return line is too small. I know that verifying this is not straight forward for a home owner, so rules of thumb seem valuable for rough estimates at this point.
I have discovered that I only have one return line, a 16" round metal duct, for a 3 ton heat pump. I have another return plenum, but it has been blocked off in 2 places, one of which is drywalled over.
I have read that the 16" duct could be enough, but also that it is undersized. My return line is about 20' long with 3 90 degree turns. I'm headed to the half priced book store to try to find something to learn more.
Okay, for (3-Ton) 1200-CFM of airflow a 16" rd RA metal duct with a 20' foot Run; (3) 90-ells; Friction rate for the 20' run is 0.03; & 860-FPM velocity.
Originally Posted by outinaustin
- 3-Ton @ 1200-CFM
- 20' Run with (3) 90-ells
- 0.03" Friction Rate (FR) over the 20' foot run
- 860-FPM Velocity
If you oversize the Return-Air Grilles &/or Filter Grilles & balance-out the Supply-Side, you might be able to keep the static pressure at a reasonable level; the static pressure & pressure drops should be checked to verify that every component of the air flow system is functioning at minimal pressure drops, including the Supply-Air diffusers.
It's Not Rocket Science, But It is SCIENCE
with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE
Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities
What helps make sense out of HVAC duct design is a few baseline parameters, some of which Darrell has already mentioned, but are worth repeating:
There is lots more to HVAC design, but the basic info above is foundational.
- Feet per minute (FPM): This parameter is a measurement of air velocity into, out of, or through a duct. It is to be differentiated from air volume, which is measured in cubic feet per minute (CFM). FPM has critical importance at any filtered inlet, across any cooling coil or heat exchanger, through ductwork, and finally at any supply air outlet. For residential purposes, 500 FPM is the upper end of velocity one wants to see across filter media. Anything higher leads to excessive noise and reduced particulate entrapment. Supply outlets also need a certain FPM to assure proper CFM but simultaneously keep noise down. If air moves too quickly across a cooling coil, it can dislodge condendate and also lead to excessive coil bypass factor, where too much air moves over the coil without being cooled or dehumidified.
- Cubic feet per minute (CFM): This is how we accomplish total air turnover in a conditioned space. FPM gets the supply air thrown into the room so it can thoroughly mix with the air in the room, but CFM assures that over time the total volume of air in that room will turn over and be "conditioned" by the HVAC system. "Conditioning" means cooled/filtered/dehumidified, or heated/filtered/humidified, depending on what the needs of the space are. Additionally, cooling coils generally go by a set CFM per ton of cooling capacity, more or less around 400 CFM per 12,000 BTUH (400 CFM per ton).
Building Physics Rule #1: Hot flows to cold.
Building Physics Rule #2: Higher air pressure moves toward lower air pressure
Building Physics Rule #3: Higher moisture concentration moves toward lower moisture concentration.
Do you have the evolution air handler and control? It'll tell you static. I believe that system wants 1050 at .5 or below. Are you having noise issues? What prompted "checking and verifying..."?
Which makes more sense to you?
- turning your thermostat back and being uncomfortable. Maybe saving 5-10%
- leaving your thermostat where everyone is comfortable. Saving 30-70%
DO THE NUMBERS! Step on a HOMESCALE.
What is comfort? Well, it AIN'T just TEMPERATURE!
Energy Obese? An audit is the next step - go to BPI.org
, or RESNET
, and find an auditor near you.