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  1. #1
    Join Date
    Nov 2007
    Location
    Livermore, Ca
    Posts
    163

    HP with ducting mismatch, Rheem

    Greetings. New guy. Great info here. Read back 12 pages picking out topics of interest.

    Newer installation. Dec 2005. Outside unit trips. Head pressure issues.

    Rheem 3Ton HP (1200cfm max) in a 1129 sq ft house. Have been told it is too large.
    I can live with it due to future expansion plans. Maybe. 300 sq ft addition.

    Ducting is said to be small. 14" supply and return.
    Six 6" supplies and one 4" in the bathroom.

    Looking for a (rule of thumb) on the supply drops. Am told that a 6" drop is ~100 cfm. So I'm looking at a ~650 cfm duct system and have 1200 cfm pushing through it. Noisy. Slowed the fan down to 1k. Register temp is 80-85*

    Was told that I could replace the 3T with a 2T and all would be well with the ducting and house size.
    Was told that I could increase the ducting and keep the 3T. i.e. 8" drops at the six locations, 6" for the bathroom. Increase the return to 16.

    I'd like to change the ducting so as to try and make the 3T unit work. maybe even adding two more registers. I seek this generic rule of thumb to help plan the new ducting. It is 95% flex through the attic. The unit is in the garage up against the trusses. Hard 90's into the house then flex from there.

    The other option is to remove the HP and replace with a gas furnace and 2T AC

    My question is, is there a rule of thumb for cfm/supply drops to help design the ducting?

    Thanks for reading, Regis

  2. #2
    Join Date
    Aug 2007
    Location
    MN
    Posts
    2,677
    keep the pump and redo the ductwork. Minimal flex if possible, 6 " flex is more restictive than 6" hard piping. Find a good hvac company and get the system designed and installed.
    You can't fix stupid

  3. #3
    Join Date
    Nov 2007
    Location
    Livermore, Ca
    Posts
    163
    Thought I had a good company.

    Still wanna know about the cfm rating for 8" flex drops at the registers. Probably use an 8x14 boot. Two way or three way grilles.

    Present boot size is 6x12 with 6" flex. Three way grilles.

  4. #4
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Many Rheem air handlers require a certain size duct for the first 12 or more inches.

    What model air handler do you have???
    Last edited by dash; 11-27-2007 at 12:11 PM.

  5. #5
    Join Date
    Nov 2007
    Location
    Livermore, Ca
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    163
    Air handler is BHK-21J11NHE with a 10kw heat strip.
    Out door ubit is RPPB-036JAZ

  6. #6
    Join Date
    Jun 2007
    Location
    Metro St. Louis
    Posts
    181
    Quote Originally Posted by regis101 View Post
    Thought I had a good company.

    Still wanna know about the cfm rating for 8" flex drops at the registers. Probably use an 8x14 boot. Two way or three way grilles.

    Present boot size is 6x12 with 6" flex. Three way grilles.
    The general rule of thumb is 2x6" for one 8".

    You need to increase both trunks. 16" might cut it for supply, but not return. 14" is around 800 CFM. I'm assuming we're talking about round trunks here.

    Be careful on your registers. Just because you can push 200 CFM to it doesn't make it right. The register itself needs to be able to handle the flow without making noise. Also, the throw into the room could cause some uncomfortable drafts.

  7. #7
    Join Date
    Nov 2007
    Location
    Livermore, Ca
    Posts
    163
    I'm looking at these issues.

    > The 3 ton HP is oversized for an 1129 sq ft house.

    >>The duct work is too small for the unit.

    So, change the duct work from beginning to end to match the cfm output of the unit. 1200 cfm

    Change the HP to a 2Ton and the duct work is fine. ~750 cfm

    I had a sub come over for a tune up. He said the rule of thumb for a six inch register is 100. Forgot to ask what the R of T is for an eight inch register. Maybe even a ten inch if I need to dump more air.

    It is gonna be easier and cheaper to replace the ductwork.

    The amount of extra available air from the 3ton would allow me to include the garage ( 20'x20' ) for a size comparisson

  8. #8
    Join Date
    Apr 2004
    Posts
    7,680
    first 100 cfm for a 6" is for straght metal pipe, not flex and certainly not with any sags or turns. For a straght 6"flex you might get 70 at best. If it's got dips and sags you could be as low as 50. That said, you have a RBHK air handler, this has an ECM motor. Your operating costs with restricted ducts are elevated significantly but you are probably moving enough air (unless you hear the fan ramping up and down or huffing). This of course is assuming he has the switches configured properly. A decent tech would have a manometer or magnaheilic to verify the system static, he could also test airflow via temperature rise. If your external static is above .5", then you shoud clean up the duct system, enjoy the savings, and listen to noise go by the wayside.

    If your airflow is low, odds are it's been overcharged in cooling to compensate, hence the overcharge in heating that is now exagerated.

  9. #9
    Join Date
    Nov 2007
    Location
    Livermore, Ca
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    163
    Cool. Thanks.
    The duct work is 95% flex across the ceiling truss crawl space. The boots are cut into the ceiling.

    They are 6x12x4 deep. This puts the collar just above rafter (2x4) height. The flex basically lays on the rafter, albeit on some blown in insulation, and 90's into the boot.

    So according to you, not efficient at all. I actually bought some metal 90's and was gonna crawl up in there so as to smooth out the transition from horizontal duct to the vertical flange on the boot. I hope (boot) is the right word.

    I've been whacked by a hack.

    I'm working on a solution to the best of my abilities.
    As previously stated, I could live with 1200 cfm of max air flow if the proper , (mostly flex ) duct system can be designed. The house is a 3-1, 1129 sq ft. One register for each bedroom. One for the bath. Five more for the living-dining-kitchen area. Total of nine.

    So I'm thinking , within my knowledge realm, as a rule of thumb, that 1200cfm/9registers would put each register at 133 cfm.

    Maybe 8" registers would dial me in. Then design backwards from there.
    Have been told that I could keep the 14" supply but really need at least a 16" return.

    One sub stated that if the supplies are in the ceiling that the return should be at the floor to have the greatest temp differential back to the evap coil.
    Can't do this. Well, don't want to do this. I'd have to take away from floor space to bring the return from horizontal attic to vertical drop to the floor and box around it. Eh, maybe disguise it as a book case.

  10. #10
    Join Date
    Nov 2007
    Location
    Livermore, Ca
    Posts
    163

    Hmm

    The HP is capable of various fan speeds. 1200 and 1000 with 10-15% up or down through the dip switches. I have the fan set at 1000 for heat mode. Runs a tad quieter and heat rose a couple degrees.

    My ducting seems so inefficient that I removed the register grilles to allow the air to dump.
    I removed the filter from the return grille and now use the permanant filter at the evap coil. I even lowerd the return grille to expose raw piping. Anything to help take away from restriction.

    I'm trying to push 1200cfm through 6 six" drops and a 4 in the bathroom.

    Not my fault. I'm the victim here. Just want closure.

  11. #11
    Join Date
    Nov 2007
    Location
    Livermore, Ca
    Posts
    163
    This won't turn into a DIY thread. Just looking for direction

  12. #12
    Join Date
    Jun 2007
    Location
    Metro St. Louis
    Posts
    181
    Get some 7" flex and a bunch of 6"x7" increasers/reducers. Replace the 6" flex with the 7" flex using the increasers/reducers. That would provide a bit more airflow. You'll have to get a little creative connecting the reducer to the register boot, but that's what sheet metal snips are for. If the trunk takeoffs are 7"-6" (the trunk hole is 7" with output at 6"), then you could make it even easier by replacing the takeoff with a pure 7" takeoff.

    You could also get 7x12x4 registers. However, this might entail some drywall repairs afterwards (depending on how well attached the existing registers are).

    The key with flex is that it must be fully extended and as strait as possible. What Doc said is right - sags can really bring down the airflow.

    Leave the bathroom smaller. The others at 7" would definately help (vs the 6" flex). The 7" should give you 40% more airflow.

    Finally, add a 7" or 8" bypass duct at your air handler with a damper. With the high static you probably already have, that would effectively remove 150-225 CFM. Then, when you add your 300 sqft addition, you just keep the damper shut.

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