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  1. #1

    Question Square peg in a round hole - Return grill CFM electronic filter sizing question

    Background:
    Recently replaced a 40yo 120k BTU gas furnace w/Coleman TM9V100C20MP11 (Specs: 95% w/2 stage vari speed ECM 2000 CFM).
    The prior blower (which had been upgraded) is not as powerful as this new ECM. (For the curious, the old GE furnace with an iron heater exchanger had no cracks)
    FYI: A licensed HVAC friend did the installation.

    I also want to install an Electronic Filter. (currently "rigged" with a temporary 1" filter) I keep returning to Honeywell but also saw compelling comments for the AprilAire 5000 & Broan HEPA. (open to suggestions)

    My concern is CFM, static pressure, air flow. The dimensions of the furnace sidewall is: ~18" height x 28" deep (the receiving air chamber) which will not accommodate the "properly sized" 2000 CFM electronic Honeywell filter which suggests 20x25. (the 16x25 is rated @1400 CFM)
    The current (and temporary) sheet metal return elbow is 22 x 13.



    Alternatives?
    I could not find any off-the-shelf bases that had a similar footprint as this Coleman that would allow the furnace to be stacked above this "theoretical" base so a wider return (and filter) could mounted from beneath. (the footprint dimensions would accommodate a 20x25 filter).
    FYI: This basement installation is stacked on 16" concrete blocks as this unit was much shorter than prior furnace.

    Also since this furnace has already been installed, I am not really interested in a bottom return as it would require a ghastly patch on the side where the return elbow is currently attached.

    I was wondering, has anyone ever seen or created a DIAGONALLY mounted air filter?
    I was thinking, I could have a local HVAC sheetmetal fabricator build an elbow that allowed the 20x25x? return grille to be mounted diagonally.

    I appreciate any comments or suggestions. Thx

  2. #2
    Join Date
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    That is one large furnace. Did your friend do a heat loss calculation (manual J)? To install a 2000 cfm filter would require a transition section as you already have guessed. You will also note that you actually need more intake air area than you can acquire through just one side of that furnace. It should #1 either be taking return air through both sides of the cabinet which would either require 2 air filters along with some pretty involved ducting or #2 be sitting on a return air box taking air through the bottom. On top of that your 9x22 drop is too small. The question is then, is the 100,000 btu furnace what you really should have had and are you actually moving 2,000 cfm of air with you furnace/duct system combination? Did your friend at least check static pressure and heat rise in high fire? Your choke point of the system from the informatiom you have provided thus far is the RA drop. Your furnace is high enough off the floor for a return air box to be fabbed for the bottom and then have the RA drop redone with the 2,000 cfm air cleaner installed in the vertical drop section. As far as the "ugly hole". A good sheetmetal worker would be able to fab a patch that could be painted and not all that obvious, plus the drop and filter would obscure the view anyway. And why did you friend sit it on double block instead of making a transition, thats what I do. Post a few pictures of the complete install please.
    A good HVAC tech knows how, an educated HVAC tech knows why!

    DEM


  3. #3
    Join Date
    Jul 2000
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    I like your title of the thread by the way.... at least with the size of the square peg you've already pounded into your round hole, there should be plenty of room to pound a lot more smaller ones in around it. Bottom line is though that there is a very good chance that none of your existing ductwork and associated registers in the home are sized properly for the furnace you now have installed.

    You can continue to spend money on this project until you get what you're hoping for, or my suggestion is to stop where you're at and go back to the beginning. Do an accurate heat load calculation and prove that the furnace you've installed is sized correctly. If it is the correct size, then do the calculations on all of the ductwork to insure that it is correctly sized for the new furnace.

    If you find that the furnace is sized correctly for the house and the ductwork is sized correctly for the furnace that's when you do a static pressure test across the furnace and find out how "much" filter resistance you can install with out exceeding the manufacturer's maximum recommended external static pressure. If the test shows the SP is already too high, then you need to make modifications to the ductwork system in the home to drop the reading into a range that gives you "room" to install a filter.

    As stated by heaterman, your return air drop for sure is too small of size to handle the air flow the furnace is capable of delivering. That's not to say that the ECM motor won't try very hard to deliver it........ it's just your electric bill for running that motor is going to be sky high and it will continue to be so until the motor burns it's controls up. This is all conjecture of course on the limited information provided, but the picture speaks a thousand words to a seasoned tech. and this picture and post is screaming.
    Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.

  4. #4
    Thank you Heaterman and Firecontrol for the replies

    re: Manual J - A simple heat load calc was 80K BTU output. (NJ, 2 sty, 2500sq ft w/nominal insulation. Also many windows are still single-pane). Orig 40yo furnace was 120K, guesstimate 65% would equate to 78K BTU.
    No static pressure test was conducted.

    I won't fault the HVAC friend as I knew of some issues. Also attractive (yeah, shake your heads :-) was the "purchase price value" on this left-over unit plus the rebate. That is why it may be a tad larger than necessary. This one has 95K BTU output and I probably could have gone with their 1600CFM ECM motor. (however there may be governor adjustments on this 2000CFM ECM variable speed motor).

    Q1 : Anyone aware of any speed adjustments options ? I know there is a DIP switch setting for AC.

    FWIW, I only notice the highest-speed of this "variable" 2 stage blower kick-in AFTER the burners shutdown. (Likely blowing @high-speed to resourcefully dissipate whatever is remaining on the heat exchanger).

    I will say, the system (even as temporarily installed) provides much better comfort than the previous unit for the entire 2nd floor. (prior unit never delivered much heat to the 2nd floor).

    Observation:
    What surprises me is that furnace mfr's do not offer return-air "bases" with the same footprint as their models. That would be a profitable accessory item for them and still be much less expensive for you than having to approach a local sheet metal fabricator. (this isn't simple ductwork gauge but the heavier stamped steel to support weight of unit)

    Q2: I was hoping someone here may know where to acquire an off-the-shelf RA base with same/similar footprint.
    In addition to the base, if necessary, HVAC friend would replace that last section of 22x9 RA ductwork. (It is easily accessible as 1st and 2nd floor returns combine in basement). However as mentioned, no static test was conducted.

  5. #5
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    Installer should have set up the blower speeds when he installed, and of course use a combustion analyzer to make sure the burner is set up properly. Is it controlled with a 2 stage thermostat, or crippled with a single stage thermostat and using the boards timer to time to second stage.

    What size A/C do you have.
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  6. #6
    Join Date
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    Northern Wisconsin
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    [QUOTE=2wheels;15467101]

    re: Manual J - A simple heat load calc was 80K BTU output. (NJ, 2 sty, 2500sq ft w/nominal insulation. Also many windows are still single-pane). Orig 40yo furnace was 120K, guesstimate 65% would equate to 78K BTU.
    No static pressure test was conducted.

    Comparing output only between units is only one part of the equation. With the high efficient furnaces they get that gain in efficiency by two means. First is a secondary heat exchanger that gets that last bit of heat before it gets vented outside. The second is a blower that moves a LOT more air across the furnace to move this extra heat into the house. When you restrict the air flow your new 90%+ furnace can be reduced to less than 90% right to the point of destroying it and causing major expenses over it's shortened life.

    That is why it may be a tad larger than necessary. This one has 95K BTU output and I probably could have gone with their 1600CFM ECM motor. (however there may be governor adjustments on this 2000CFM ECM variable speed motor).

    That "tad larger" is actually at least 20% larger. Just sayin.

    FWIW, I only notice the highest-speed of this "variable" 2 stage blower kick-in AFTER the burners shutdown. (Likely blowing @high-speed to resourcefully dissipate whatever is remaining on the heat exchanger).

    One thing that will make the fan go to high speed right after the burners go out is....... the furnace's high limit safety switch has opened. This is almost always due to lack of air flow across the furnace and it's overheating.

    I will say, the system (even as temporarily installed) provides much better comfort than the previous unit for the entire 2nd floor. (prior unit never delivered much heat to the 2nd floor).

    More air flow through the system will usually always helps a poor distribution system. Trouble is this can sometimes come at the expense of the equipment providing that larger air flow.
    Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.

  7. #7
    Quote Originally Posted by beenthere View Post
    Is it controlled with a 2 stage thermostat, or crippled with a single stage thermostat and using the boards timer to time to second stage.
    Unfortunately I had just purchased a Honeywell thermostat. It is NOT a 2 stager and yes installer "crippled" the board so the 2 stage is set by a timer. Amazon for the next 45min has a sale on a Honeywell thermostat with the Z-wave feature but will probably skip the sale to compare against others units. I am also considering the Nest.

    Quote Originally Posted by beenthere View Post
    What size A/C do you have.
    Sorry for the delayed response. I had to check the placard. The placard states it's a Carrier 38BRC036320. While I couldn't find the model on Carrier's site, the specs indicate a 12seer 3Ton AC

  8. #8
    Quote Originally Posted by firecontrol View Post
    One thing that will make the fan go to high speed right after the burners go out is....... the furnace's high limit safety switch has opened. This is almost always due to lack of air flow across the furnace and it's overheating.
    I am pretty familiar with that scenario. I had to replace the prior furnace's thermal heat switch. I do not believe that is occurring on this unit.

    Quote Originally Posted by firecontrol View Post
    More air flow through the system will usually always helps a poor distribution system. Trouble is this can sometimes come at the expense of the equipment providing that larger air flow.
    As I mentioned in a prior response, I am wondering if a PARTIAL return duct replacement/enlargement will help. (swapping that last 6' section from 22x12 to a 30x12 would be simple).
    Oh, the variance in dimensions of the previous plenum and the new one is nominal. (The new unit has an identical width and is maybe 1" deeper)
    Unfortunately I didn't receive any valuable assistance from York since "I" am not the HVAC licensee. However York provided the contact info to someone from the wholesaler who assists HVAC pros with design and loads. I hope to get a few answers from him as to how to most economically redesign the distribution system to work without causing future damage to the blower.
    (I also figure if it's a variable speed blower, there should be a way to govern the max CFM speed)
    Ductology - what a concept :-)

  9. #9
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    have your fried disable second stage. You probably don't need it.
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  10. #10
    Join Date
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    re: Manual J - A simple heat load calc was 80K BTU output. (NJ, 2 sty, 2500sq ft w/nominal insulation. Also many windows are still single-pane). Orig 40yo furnace was 120K, guesstimate 65% would equate to 78K BTU.
    No static pressure test was conducted.

    I won't fault the HVAC friend as I knew of some issues. Also attractive (yeah, shake your heads :-) was the "purchase price value" on this left-over unit plus the rebate. That is why it may be a tad larger than necessary. This one has 95K BTU output and I probably could have gone with their 1600CFM ECM motor. (however there may be governor adjustments on this 2000CFM ECM variable speed motor).

    Q1 : Anyone aware of any speed adjustments options ? I know there is a DIP switch setting for AC.

    FWIW, I only notice the highest-speed of this "variable" 2 stage blower kick-in AFTER the burners shutdown. (Likely blowing @high-speed to resourcefully dissipate whatever is remaining on the heat exchanger).

    I will say, the system (even as temporarily installed) provides much better comfort than the previous unit for the entire 2nd floor. (prior unit never delivered much heat to the 2nd floor).
    NJ 99% winter design is 10-F, at 70-F indoors that is only a 60-F temp-differential. I live in SW WI & deal with a -15-F below zero, or an 85-F temp-differential; though less sf area I have a 57,000-output propane furnace that never comes anywhere near using that 57,000-Btuh Output.

    It is always best to do everything that is cost effective to reduce the Btuh size of the furnace so you can keep the airflow close to what the 3-Ton A/C requires. It is extremely difficult to deliver nearly 2000-CFM of airflow with residential systems.

    Some adjustments on the duct system plus longer heating runtimes should help deliver more airflow heat to the 2nd floor.

    All around efficiencies & comfort levels improve as you reduce the rate of heat-loss; & in summer reduce heat-gain.

    I would think, at a low cost, you could add storm windows which would help considerably to reduce the heat loss.

    Then, do the energy saving stuff: RESNET - Big Energy Cost Saving Tips

  11. #11
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    It won't help to enlarge the duct only at the unit, your return and supply duct is probably inadequate and eventually the heat exchanger will fail from running above the spec'd temp rise. Might want to have your hvac friend check temp rise and see if you can get it within the manufactures specs. May have to enlarge your entire return duct, including grill/s, to make your new furnace live a long life and give you close to its rated efficiency.
    Heating/Cooling Services Inc.
    www.andersonhvacservice.com

  12. #12
    Join Date
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    There is a possibility that the input rate of gas to the burners could be reduced.

    Have your contractor check with the mfg'er to find out if the gas manifold orifices can be changed to a lower input rating by reducing the orifice size of the spuds that determine the rate of flow to the burners.

    There is a possibility that this would help efficiency at every level improving overall performance & comfort.

    There will be input reduction limits, based on the heat exchanger & other factors...
    Since it is a very low cost reduced input switch, this might be worth looking into; the mfg'er will determine whether this can be done with your particular furnace. Years ago we used to occasionally do this on standard efficiency furnaces...

    On hi-efficiency furnaces the entire burner may have to be changed out.(?)
    It would be great if you could match the furnace input with the airflow limits of the duct system.

    You could also use supplemental electric heat (baseboard or other) if need be, for those rare short-term extreme weather conditions.

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