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Thread: Newbie seeking expert opinions/advice and answers

  1. #1
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    Newbie seeking expert opinions/advice and answers

    Hi all,
    Lurker seeking your opinions and answers to some questions. Sorry for the long post, but would like to provide as much background as possible.

    Data:
    1. 20 yr old Trane XE 80 and a 3 ton A/C (don't know SEER) in a 20 yr old home with above average insulation
    2. Home in Minnesota. 2 stories above grade + finished basement. 900 sq.ft per floor.
    3. Furnace and A/C working ok - this winter the furnace has been cycling more than usual (5-6 times per hour, average run time 6 mins), but could be due to unusually cold weather we've been having. Heat levels are satisfactory.
    4. A/C doesn't cool the upstairs well when we have 90 degree+ days in the summer, but is otherwise satisfactory.
    5. Aprilaire (don't know model number, but looks 20 yr old) humidifier installed, but doesn't seem to work.

    I'm thinking that these units are past their useful lives and that it may be time to replace them. Received several quotes from reputable local installers and have narrowed it to these two systems:

    A: Trane XV80 mated with XB13 (3 tons), Aprilaire 600 and a MERV 15 5" filter system
    B: Luxaire TM8V mated with TCJD 3 ton system, Aprilaire 600 and a MERV 15 5" filter system

    Both furnaces are two stage gas valves and variable speed blower motors and are 80,000 BTU output systems.

    Not decided whether to install a Honeywell WiFi thermostat (8000 series) or a Ecobee Smart Si Wifi thermostat. I'd like to have the remote control capability. Currently have an older electronic programmable Honeywell that works just fine, but naturally without the WiFi capability.

    Questions:
    1. Am I right in thinking about replacing these systems preemptively or should I wait till one of them fails?
    2. Assuming equivalent installation, is one of these systems preferable over the other either in terms of overall performance or longevity? I'm biased towards the Trane system as the existing one lasted this long without any issues AND I got the quote from the original installer. However, the Luxaire installer comes highly recommended by a contractor I trust.
    3. A manual J-load calculation (I did it using the tools on this forum) puts heat needs at 67,000BTU heating and 2.5 tons cooling. Both vendors said 80,000 BTU heating and 3 ton cooling are sufficient, but neither performed a J-load calculation. Are these systems sufficient / overkill / undersized in your opinion?
    4. Would the Aprilaire600 control the blower in the winter and call for an A/C cycle to de-humidify in the summer if needed? Will it call the fan in the low speed stage or high speed? I read their manual, but it isn't clear on this point.
    5. Any recommendations on the thermostat? Both the above need a C-wire and my current one doesn't use one.
    5. Should the thermostat control the gas stage and fan speed or should the furnace's mother board do that?

    Appreciate any and all pointers in helping me make decisions. You guys rock!

    -zee
    Last edited by zeebanker; 02-22-2014 at 09:57 PM. Reason: Added more info

  2. #2
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    The furnace does N O T need a MERV 15 filter to remain clean.

    MERV 8 is adequate.
    http://www.bobvila.com/articles/merv.../#.UwncfvmThc4

    Heat loss for < 2,000 square foot 20 year old residence should be < 45,000 BTU/HR ( +/- 15%).

    50,000 BTU/HR input at > 90% efficiency would provide heat at -10'F design temperature with 100 CFM infiltration ( about 0.3 ACH).
    Blower door and duct blaster test needed to make an accurate assessment.

    Are you using less than 1300 therms of Natural Gas for heating season?

    3-ton A/C is probably not functioning very well if it cannot cool at 95'F.
    Last edited by dan sw fl; 02-23-2014 at 06:51 AM.
    Designer Dan __ It's Not Rocket Science, But It is SCIENCE with Some Art. _ _ KEEP IT SIMPLE & SINCERE ___ __ www.mysimplifiedhvac.com ___ __ Define the Building Envelope & Perform a Detailed Load Calc: It's ALL About Windows & Make-up Air Requirements. Know Your Equipment Capabilities

  3. #3
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    Thanks Dan; appreciate your response. I did suspect we were oversizing the equipment. You agree that its time to replace them though? I'll check on the therms and report back.

  4. #4
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    On Filters

    I also think 15 merv is a bit much, I am a fan of Aprilaires spaceguard filter or a honeywell filter media. Merv 8 to 10 is fine though.

    Trane would also be my preference but not by much over the Luxaire. Why have you not considered a 90% furnace for estimates?

    3. A manual J-load calculation (I did it using the tools on this forum) puts heat needs at 67,000BTU heating and 2.5 tons cooling. Both vendors said 80,000 BTU heating and 3 ton cooling are sufficient, but neither performed a J-load calculation. Are these systems sufficient / overkill / undersized in your opinion?

    Find someone to do a manual J if you are doubting your figures then compare the yours vs theirs.

    Since things are working correctly you aren't in a rush to do this and slo time will be coming up for HVAC contractors in the North soon .....march and april are the best times to have this done



    4. Would the Aprilaire600 control the blower in the winter and call for an A/C cycle to de-humidify in the summer if needed?

    The Aprilaire Automatic humidistat will be able to activate the blower on a call for humidity but not to dehumidify that would be your IAQ thermostats job if you get one...This from Aprilaire

    :Blower Activation Feature Explanation
    Set the Blower Activation switch to “ON”, see Figure J, to allow
    the Humidifier Control to activate the furnace fan for additional
    humidification. This may be required if the humidifier is mounted
    on a heat pump or a furnace with short run cycles. When using
    the Blower Activation feature, Model 400, 500, 600 and 700
    series humidifers must be plumbed to hot water for effective
    operation.
    When the Blower Activation switch is “ON”, the Humidifier
    Control will turn on the HVAC blower when humidity is required.
    In the “OFF” position, the humidifier will only operate if humidity
    is required and the HVAC system is operating.
    When the Blower Activation switch is “ON”, the Humidifier
    Control may extend the HVAC blower after a heat call for up to
    two hours. If the HVAC system has been idle for one hour, the
    control will turn the blower on for three minutes




    5. Any recommendations on the thermostat? Both the above need a C-wire and my current one doesn't use one.

    Should the thermostat control the gas stage and fan speed or should the furnace's mother board do that?

    The thermostat should always control the stages of the furnace rather than the control board giving you greater control and comfort in your home. New installers should also be able to pull new wires for you to give you extra wires.

    Honeywell Visionpro IAQ and Prestige thermostat lines would be great for this

  5. #5
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    Quote Originally Posted by zeebanker View Post
    You agree that its time to replace them though?
    Let's compare to other expenses.

    Automobile = four times as much $$,$$$.$$ for half the life of HVAC equipment.
    Also, about car use might be 5% to 25% of the time of home occupancy.

    IF one ponders those variables, one should actually be willing to spend
    ___ AT LEAST $80,000 ___ for
    an integrated HVAC which provides IAQ and Comfort.

    [ maybe I should go into HVAC equipment Marketing - L.O.L. ]

    PERSPECTIVE PROVIDED ON " WHETHER it's time to replace system."
    You would most likely NOT be in an Auto-Talk Forum asking if you should trade in
    Your 20 year old vehicle with 333,000 miles on it, Would you..?
    Designer Dan __ It's Not Rocket Science, But It is SCIENCE with Some Art. _ _ KEEP IT SIMPLE & SINCERE ___ __ www.mysimplifiedhvac.com ___ __ Define the Building Envelope & Perform a Detailed Load Calc: It's ALL About Windows & Make-up Air Requirements. Know Your Equipment Capabilities

  6. #6
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    Small change - thanks for the detailed response. 90% would need new PVC venting and that would add a lot of expense for my house with a finished basement (need to go through garage, box the vent to prevent freezing etc.) 80% will connect to existing exhaust venting.

    Dan - Thanks and I appreciate your insight. You are more valuable as a installer than a marketer, is my guess I think its time to replace the systems.

    On a separate un-related note, the car comparison is a bit off IMHO. I've seen this many times in these forums and I think its a bit self-serving of the HVAC professionals to throw this example. My intention is not to offend you, but lets take a look at your example.
    1. I use my car to make a living - it helps me get to work and bring me home and make the $$s needed to own a house and a HVAC system. A HVAC system on the other hand is needed for comfort, not to make a living. If one cannot afford a car, one likely will not own a home or a HVAC system that goes with it. How many people in the good old US of A can afford a home but not a car? Not many, is my guess.
    2. A car is vastly more complex a piece of machinery than typical residential HVAC systems. The life spans of each are a testament to that.
    3. If the car analogy was valid, the market would have driven the prices of the HVAC systems up to the point that people would be spending as much time thinking about HVAC systems as they do about cars.

    Apples and oranges. I'm not saying we should all throw our HVAC systems out the door or install dirt-cheap ones, but they're not the same as cars.

    However, you are right - if we make careful decisions when making major purchase like this, we should take into consideration all factors. Which is exactly what this forum does and folks like me are grateful for that.

  7. #7
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    Well, 20 years ago puts it at 1994; a 10 or 12 SEER.

    SEER Rating History: Before 1980 6 or less; 1980-85 - 7 or less; 1986-1991 - 8 or less; 1992-2005 - 10 to 12-SEER; Jan. 10th 2006 13-SEER or more.

    Data:
    1. 20 yr old Trane XE 80 and a 3 ton A/C (don't know SEER) in a 20 yr old home with above average insulation
    2. Home in Minnesota. 2 stories above grade + finished basement. 900 sq.ft per floor.
    3. Furnace and A/C working ok - this winter the furnace has been cycling more than usual (5-6 times per hour, average run time 6 mins), but could be due to unusually cold weather we've been having. Heat levels are satisfactory.
    Cycling 5 times *6-mins in an hour would be 30-mins burner-runtime / 60-mins is .50% *times your furnaces' Rated OUTPUT Btu/hr; say * 80,000 is 40,000-Btu/hr in coldest weather.

    No way should a furnace be cycling 5 times during one hour; you need a RM-TH with a temp-differential SWING or CPH settings to allow for some temp spread between kick-on & OFF.

    That is very inefficient as the furnace spends way too much time heating-up the heat-exchanger & duct system each cycle, that is a waste of fuel.

    The better the energy-efficiency-factor of the home the longer it will be off between cycles at the same temp-swing-differential setting of maybe a couple degrees F.

    The Big Three for Efficiency are: 1) the Home; 2) proper equipment sizing & excellent install procedures 3) an efficient properly sized & sealed duct system & adequate RA filter sizing...

  8. #8
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    More info:

    Furnace cycled 3 times in one hour between 2:30pm and 3:30pm today. Outside temp was 15 deg and inside was set to 68 deg.

    Each time the blower was on for 6 minutes. The furnace (flames) were on for 3 minutes each. So total blower time was about 18 minutes and furnace time was 9 minutes, or about 30% and 15% per hour respectively.

    80,000 BTU/hr output (100,000 BTU input, 80% efficiency) furnace working 15% of the time => 12,000 BTU of energy leakage per hour.

    So I'm more inclined to believe that an 80,000 BTU input 80% efficiency furnace (64,000 BTU/hr output) should be a sufficient replacement. On nights when its 0 or below outside, it might cycle one or two more times per hour, but a 2-stage gas valve with a continuous motor might actually not cycle that often?

  9. #9
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    Quote Originally Posted by zeebanker View Post
    More info:

    Furnace cycled 3 times in one hour between 2:30pm and 3:30pm today. Outside temp was 15 deg and inside was set to 68 deg.

    Each time the blower was on for 6 minutes. The furnace (flames) were on for 3 minutes each. So total blower time was about 18 minutes and furnace time was 9 minutes, or about 30% and 15% per hour respectively.

    80,000 BTU/hr output (100,000 BTU input, 80% efficiency) furnace working 15% of the time => 12,000 BTU of energy leakage per hour.

    So I'm more inclined to believe that an 80,000 BTU input 80% efficiency furnace (64,000 BTU/hr output) should be a sufficient replacement. On nights when its 0 or below outside, it might cycle one or two more times per hour, but a 2-stage gas valve with a continuous motor might actually not cycle that often?
    One of the important objectives of sizing to the design heat-loss is to increase the burner runtime per cycle.
    If you do an accurate load-calc it will tell you what minimum & maximum Btu/hr furnace you should select for the 'design condition.'

    A fairly accurate load-calc will also reveal how efficient your furnace is delivering heat to the rooms.

    Try this free online load-calc to see how accurate you can get the loads; then print the final calc plus the Sizing page, & see how your furnace Btu/hr use compares to the load-calc.

    Sized solely to the higher 2nd stage, on warmer days the heat-loss, especially on sunny days, will drop way off so the lower 1st stage will easily handle the loss; usually; with a SWING room-stat; you won't need a two-stage system to get sufficiently long burner run-times.

    A 95% 60,000 Input delivering 57,000-Btu/hr Output might well handle your homes' Btu/hr heat-loss; a fairly accurate load-calc would reveal 'whether' that will be the reality.

  10. #10
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    Yup, that's the tool I used and here's the data from the load calc.

    Sizing Calculator based on Manual S

    System Type: Straight Cool w/ gas heat
    Load Calculation Data
    Cooling - Sensible: 19690 btu's
    Cooling - Latent: 1794 btu's
    Cooling - Total: 21484 btu's
    Heating - Total: 45700 btu's
    Sensible Heat Ratio 0.92
    Calculate
    Climate: Heating & Cooling
    Sizing Results
    * Cooling - Minimum: 21484 btu's
    Cooling - Maximum: 24707 btu's
    Heating - Minimum: 45700 btu's
    Heating - Maximum: 63980 btu's

  11. #11
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    Quote Originally Posted by dan sw fl View Post
    Are you using less than 1300 therms of Natural Gas for heating season?
    Yes. Our total usage last winter was 850 therms; assuming 80% is to heat the house, that puts it at 680 therms for heating. This year will be more because of the considerably colder weather.

  12. #12
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    Quote Originally Posted by zeebanker View Post
    Yes. Our total usage last winter was 850 therms; assuming 80% is to heat the house, that puts it at 680 therms for heating. This year will be more because of the considerably colder weather.
    VERY Low heat loss is indicated by use of about 700 therms for a typical 7,500 Heating Degree Day (HDD) season.
    2013 - 2014 Winter may be 25 % more than a typical year.

    Heat Loss is less than 34,000 BTU/HR at -12'F.
    __________________________________--
    ATTACHMENT
    Designer Dan __ It's Not Rocket Science, But It is SCIENCE with Some Art. _ _ KEEP IT SIMPLE & SINCERE ___ __ www.mysimplifiedhvac.com ___ __ Define the Building Envelope & Perform a Detailed Load Calc: It's ALL About Windows & Make-up Air Requirements. Know Your Equipment Capabilities

  13. #13
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    Quote Originally Posted by dan sw fl View Post
    VERY Low heat loss is indicated by use of about 700 therms for a typical 7,500 Heating Degree Day (HDD) season.
    2013 - 2014 Winter may be 25 % more than a typical year.

    Heat Loss is less than 34,000 BTU/HR at -12'F.
    __________________________________--
    ATTACHMENT
    Dan, that's awesome! Thanks for the data and your opinion. Much appreciated!

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