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  1. #1
    Join Date
    Aug 2008
    Posts
    416

    Fine Tuning XV95 CFMs

    Have new 80K XV95 thats been running for heat 1 month. Works great.
    So far only runs in 1st stage and is set for NORMAL 1000 CFMs and is quieter than old 1200 CFM heat... Very happy with performance.
    Have run in 2nd Stage by changing temp on tstat and a little noisier since running at 1400 CFMs and quite frankly suspect ducts were sized for older 1200 CFM unit.

    Verified temp rise and ESP appears to be around 0.5 in 1st stage but increase to around 0.7 in 2nd stage based on Service Facts table.

    Would like to consider lowing airflow to make unit even quieter. Before I have contractor out to lower to Medium Low setting (890 CFM and 1260 CFM), I want to understand the pros and cons of lowering CFMs in HEAT mode.

    Based on "Service Facts" of XV95, I can expect a 1st stage temp rise of 50 and a second stage rise of 54 at 890 CFM and 1260 CFM respectively.

    So how will lower CFMs affect:
    (1) Efficiency
    (2) Heat Exchanger Longevity
    (3) Comfort (specifically between floors)
    (4) Noise (I know this answer already)
    (5) Will unit run longer or will output air just be hotter???
    (6) Power consumption of blower motor will go down.

    I don't see the negatives - but there must be some.

    Also,
    What tests should I insist that contractor perform to ensure lower settings are not doing harm (I need to be an intelligent consumer).....I suspect he'll change settings and then simply verify temp rise against the "Service Facts" table. Is that sufficient??

  2. #2
    Join Date
    Jul 2004
    Location
    Massachusetts
    Posts
    6,837
    There's probably nothing to be done for SP unless you're into modifying or totally replacing the ducts. That can get a bit pricey. Naturally the first question is, was a Manual 'J' done before the furnace was replaced? If 'YES' then the duct question should have been answered before the furnace went in.

    It takes a fixed number of Btu's of heat to keep your indoor temperature stable at or near set point for every outdoor temperature. With a 2-stage furnace, you are effectively getting 2 design points. One is the higher design for 1st stage and the 2nd is the coldest day design. At each design temp, the furnace is running continuously without shutting off on the respective stage. For example, if the lowest design temp is zero, the higher design temp might be 19^F. That means that between 60^F (approximately, the start of needing heat in the home) and 19^F, you furnace can satisfy the needs of the home with just first stage. Bu the closer the OAT gets to 19^F, the longer the first stage cycles until at 19^F, it doesn't ever shut off. At 18^F, 2nd stage begins to cycle on/off and those 2nd stage cycles get longer as it gets colder until at 0^F, 2nd stage is also on continuously. At -2^F, your room temperature just went down 2^F but the furnace is running full boat.

    If you decrease the airflow, you'll decrease the noise factor and the temperature rise across the HE will increase. The increase will have a slight negative impact on the HE due to the higher temperature swings between room temp when off and max temp when running. But as long as manufactuers specs are followed, nothing abnormal should happen. It is possible that your 1st stage design in the example above could increase slightly with the lower airflow but I doubt it. You'll be putting fewer CFMs out per hour (that's how the furnace is rated, per hour) but with the higher temps, the Btu output should be about the same. I don't think you'll be able to tell the difference on the run/dwell cycles with the slower speed but again, you need to keep it within mans specs.
    If YOU want change, YOU have to first change.

    If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!

  3. #3
    Join Date
    Aug 2008
    Posts
    416

    Number One Priority is Noise

    Thanks for response. In any tradeoff you have to set priorities, My highest is Comfort-Noise and second is Comfort-Temp. This is why I opened wallet and paid more for XV95 with 2-stage. So far it has made a noticable diff. Now I want to consider further refinement.

    So my first thought is to make system even more desirable is to reduce noise by lowering CFM. Just want to know the pros and cons....and what the tech should test after he/she makes any changes.

  4. #4
    Join Date
    Apr 2008
    Location
    Princeton, NJ
    Posts
    581
    I have the Trane XV95 also (100k btu).
    I had static presure of 0.95 in heat mode 2nd stage (actual measurement) and I now have static of 0.60 in first stage (actual measurement). I have my XV95 locked in 1st stage for heat since I beleive it may be oversized for my house as no load calc was done. So far the lowest outdoor temperature has been 30 degrees and the 1st stage setting keeps the house toasty at 73 no problem. The unanswered question is whether 62k btu will be able to keep my 2800 sq. ft. home warm when the temp gets really cold......if not, I will have to turn the 2nd stage back on which can deliver up to 95K btu....

    My temp rise in 1st stage sits right at 49-50 degrees and the range is 35-65 so I am not worried about harming the heat exchangers.

    I've read heated debates on this site whether or not 1st stage is less or more efficient than 2nd stage with neither side backing down.....

    I also read that there are benefits to running longer in stage one if you have a bypass humidifier because the unit runs longer putting more humitidy in the air.....

    As far as comfort between floors go....I was only able to solve that issue by letting my fan run all the time....


    Key1
    In the land of the blind.....the man with one eye is king....

  5. #5
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,167
    What is your current temp rise in first and second stage.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  6. #6
    Join Date
    Aug 2008
    Posts
    416

    Confirm Where to Measure

    I calibrated my thermometers and made some measurements last night and forgot to bring with me, so I will post back.

    I have been measing the return air at the riser/drop, BEFORE EAC/filter by inserting probe thru joint in flex material.
    I have been measuring the supply air AFTER A-coil at flex joint connection to main duct. Inserting probe thru corner joint in flex duct is conveinent, BUT are these the correct places when comparing to data in "Service Facts" manual for XV95???

  7. #7
    Join Date
    Aug 2008
    Posts
    416

    Sorry, Double post

    DP

  8. #8
    Join Date
    Jul 2008
    Location
    Poestenkill, NY
    Posts
    769
    A notable consideration that was brought up in some of the afore-mentioned heated debates:

    If your ductwork is in an unconditioned space, such as the attic, the lower cfm (and therefore higher temp) settings will loose a bit more heat via the ductwork in the unconditioned areas. Duct leakage (on a percentage of total flow basis) will decline some, but remember, even though less cfm might be leaking, it's higher temp, and therefore similar btu's (when compared to more cfm at lower temp).

    If the ductwork is all in the conditioned space, or in the basement, the above is pretty much moot.

    I also have an XV95. My house is 1,100 sqft (first floor) and we heat pretty much the whole basement. Anyhow, I have a 60k btu unit because that's the smallest Trane makes in the 95. The furnace is oversized. And - like you (and many others) are describing, the existing ductwork is a little noisy in 2nd stage on the normal to higher airflow settings.

    So - my point is - I'm in the same boat, and I am comfortable with using the lower airflow settings, as they are deamed exceptable by Trane, considering the delta t is within their specified range. Especially since the lower airflow settings will match up with my cooling needs anyhow (1.5 ton)...

  9. #9
    Join Date
    Apr 2008
    Location
    Princeton, NJ
    Posts
    581
    Quote Originally Posted by larobj63 View Post
    A notable consideration that was brought up in some of the afore-mentioned heated debates:

    .......the lower cfm (and therefore higher temp) settings will loose a bit more heat via the ductwork in the unconditioned areas. Duct leakage (on a percentage of total flow basis) will decline some, but remember, even though less cfm might be leaking, it's higher temp, and therefore similar btu's (when compared to more cfm at lower temp).
    A further consideration from some of the afore-mentioned heated debates:


    All things being equal I suspect that what you stated in the above quote may be accurate....However in my comparison I am at a lower CFM in stage-1, but I am also at lower BTU's , thus the temp rise is actually lower with the slower airflow compaired to 2nd stage faster airflow(higher CFMs) at higher BTUs.

    My target Temp rise is ~47F in stage 1 on medium speed at my 0.6 static pressure while it is theorhetically ~55F on medium speed stage 2 at the same static pressure. (But since the static actually goes up above 0.9 in 2nd stage the temp rise is above ~60F).

    This is just a practical example of how slower airflow does not neccesarilly translate into higher temp rise when the change of the stage is considered.

    Key1
    In the land of the blind.....the man with one eye is king....

  10. #10
    Join Date
    Jul 2008
    Location
    Poestenkill, NY
    Posts
    769
    Quote Originally Posted by key1cc View Post
    A further consideration from some of the afore-mentioned heated debates:


    All things being equal I suspect that what you stated in the above quote may be accurate....However in my comparison I am at a lower CFM in stage-1, but I am also at lower BTU's , thus the temp rise is actually lower with the slower airflow compaired to 2nd stage faster airflow(higher CFMs) at higher BTUs.

    My target Temp rise is ~47F in stage 1 on medium speed at my 0.6 static pressure while it is theorhetically ~55F on medium speed stage 2 at the same static pressure. (But since the static actually goes up above 0.9 in 2nd stage the temp rise is above ~60F).

    This is just a practical example of how slower airflow does not neccesarilly translate into higher temp rise when the change of the stage is considered.

    Key1

    I wasn't talking about the stage change - I was talking about varying airflow within the same stage. When the airflow is lowered with the same heat input, the discharge air temp rises, and heat loss from the ductwork increases some. Again, moot point if the ductwork is all inside, where it should be.

  11. #11
    Join Date
    Apr 2008
    Location
    Princeton, NJ
    Posts
    581
    Quote Originally Posted by larobj63 View Post
    I wasn't talking about the stage change - I was talking about varying airflow within the same stage. When the airflow is lowered with the same heat input, the discharge air temp rises, and heat loss from the ductwork increases some. Again, moot point if the ductwork is all inside, where it should be.
    Agreed....I just wanted to point out to our faithful readers that there can be instances of lower airflow with lower temp rise....

    Key1
    In the land of the blind.....the man with one eye is king....

  12. #12
    Join Date
    Jul 2008
    Location
    Poestenkill, NY
    Posts
    769
    Quote Originally Posted by key1cc View Post
    Agreed....I just wanted to point out to our faithful readers that there can be instances of lower airflow with lower temp rise....

    Key1
    Gotcha.

  13. #13
    Join Date
    Aug 2008
    Posts
    416

    Now I Have My Temperature Data

    OK I have some data.

    XV95 80K = 49,500 BTU 1st and 76,000 BTU second stage.
    Contractors Manual J indicated a need for 61,000 BTU.

    In 1st Stage
    Supply = 124.0F and Return = 75.0F for Temp Rise of 49.0F (Fan setting = 1000 CFM)

    In 2nd Stage
    Supply = 136.5F and Return = 75.2F for Temp Rise of 61.3F (Fan setting = 1400 CFM)

    Ran these tests multiple times to verify numbers. Return Temp measured at riser and Supply Temp measured at 1' down the main horiz. duct from furnace. So temp is across filter, blower, and A-coil.

    Neither of these rises is on the Service Facts chart for the "Normal" Fan settings.

    I'm thinking I might have a high ESP, so lowering the fan speed might not be an option. Especially since 2nd stage is already at 61F with a 35F-65F operating range. The noise in 1st stage is quite acceptable and better than old furnace (1200 CFM) and given my Manual J I expect to satisy most demand with 1st stage. AS of today, lowest outdoor has been 30 and 2nd stage only comes on in AM in recovery from setback....

    Any explanantion of how this all works would be appreciated...and if its possible to lower fan speed for decreased noise.....

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