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  1. #1

    Correct HVAC System Sizing

    I'm looking for help in determining the correct HVAC system size for a small house I am rennovating. I've used the HVAC-Calc program to perform the Manual J calcluations. Based on my design conditions Sensible Gain is 20,943 BTU/H, Latent Gain is 2,690 BTU/H, Total Heat Gain is 23,633 BTY/H and Total Heat Loss is 36,130 BTU/H. I've provided this information to multiple contractors and none seem interested in the heat gain/loss figures. They all want to know what size the house is and how old it is. They each have given different recommendations on sizing ranging from a 1.5 ton to 3 ton AC condensor and a 40K to 72K furnace.

    I need an AC condensor that will provide at least 20,943 BTU/H Sensible capacity. Most condensor spec sheets that I can find only provide total capacity. A Rheem model I'm considering does provide Sensible capacity and a 2 ton unit has a capacity of about 16,500 BTU/H depending on the coil, a 2.5 ton unit has the capacity of about 20,500 BTU/H and a 3 ton unit has a capacity of about 24,500 BTU/H. Is a 2.5 ton the right match for my requirements?

    I plan on using an 80% AFUE furance. What is the proper size furnace for my requirements? I've read the output capacity of the furnace should be about 1.25 times of the Total Heat Loss BTU/H. Is this correct? If so I would need a furnace with an output of about 45K BTU/H. At 80% AFUE a 60K BTU/H would seem to be the closest match. How much flexibility in sizing do I have. Would a 80% AFUE furnance with 66K BTU/H and 54K BTU/H output be too large?

    I've found a complete Carrier package consisting of a 2.5 ton AC condensor with coil and 66K BTUH furnace that is a great price but I want to make sure it's a good match before I purchase it.
    cwtdallas

  2. #2
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    Hmm

    Quote Originally Posted by cwtdallas View Post
    I'm looking for help in determining the correct HVAC system size for a small house I am rennovating.

    Sensible Gain is 20,943 BTU/H



    They all want to know what size the house is and how old it is.



    ... from a 1.5 ton to 3 ton AC condensor and a 40K to 72K furnace.

    Most condensor spec sheets that I can find only provide total capacity.

    A Rheem model I'm considering does provide Sensible capacity and a 2 ton unit has a capacity of about 16,500 BTU/H depending on the coil, a 2.5 ton unit has the capacity of about 20,500 BTU/H and a 3 ton unit has a capacity of about 24,500 BTU/H. Is a 2.5 ton the right match for my requirements?

    cwtdallas
    How old is the house? Infiltration is __ ?

    How many windows? Original windows ? Orientation ?

    Floor Area? Location?
    Designer Dan
    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

  3. #3
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    Personally I wouldn't do that 1.25 times heat loss. You'll end up way oversized.

    If you need 45,000 BTUs of heat, find a furnace close to that output. There's usually enough fudge in the load calcs to protect you.

    Remember that the sensible & latent figures a manufacturer publishes are at maximum load. 95° out, 80° in with 67° wetbulb. If you are dealing with a lighter load or lower wetbulb, your sensible & latent figures will be different with more sensible & less latent. Some mfrs give you those figures, some don't.

    Most of us if we saw a 23,xxx gain in residential, we'd look towards a 2 ton system as long as it isn't a "light" 2 ton meaning closer to 21 - 22,000 BTU capacity. Also would vary based on how cold the HO wanted to keep it. My house needs a 1.5 ton but based on the frigid temps I like it in here, I went with 2 ton.

  4. #4
    Quote Originally Posted by BaldLoonie View Post
    Personally I wouldn't do that 1.25 times heat loss. You'll end up way oversized.

    If you need 45,000 BTUs of heat, find a furnace close to that output. There's usually enough fudge in the load calcs to protect you.

    Remember that the sensible & latent figures a manufacturer publishes are at maximum load. 95° out, 80° in with 67° wetbulb. If you are dealing with a lighter load or lower wetbulb, your sensible & latent figures will be different with more sensible & less latent. Some mfrs give you those figures, some don't.

    Most of us if we saw a 23,xxx gain in residential, we'd look towards a 2 ton system as long as it isn't a "light" 2 ton meaning closer to 21 - 22,000 BTU capacity. Also would vary based on how cold the HO wanted to keep it. My house needs a 1.5 ton but based on the frigid temps I like it in here, I went with 2 ton.
    Thanks for your response.

    So for sizing the condensor you would disregard the Sensible Gain and size a system based solely on Total Heat Gain of the house and match with total capacity of the system? It's my understanding that Sensible Gain is more important that Total Gain in order to properly extract humidity and make it a comfortable cool rather than damp clammy cool. Therefore the Sensible capacity of the unit should match as closely as possible to the Sensible Gain of the house at design conditions.

    My design conditions are a summer temperature of 100 degrees wiht 90 grains of moisture outdoor and indoor temperature of 74 degrees with relative humidty of 50.

  5. #5
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    Indoor 74° @ 50% RH.

    You'll likely need a three ton unit.

    Outdoor ratings are usualy at 95°,you'll need to find one that shows 100° or 95 and 100° at interpolate.

    Rating are standard at 80° indoors.Sensible capacity decreases 835 btu for each degree below 80°,per 1000 cfms.

    Typical 3 ton at 95°OD, 27420 sensible,35800 total,at 100° OD minus 1000 total and 500 sensible.

    So we now have 26920 sensible.Now 835 btus X 6° = 5010 btus X1.2(for 1200 cfms)=6012 btus sensible.

    Leaves 20908 sensble for this 3 ton at your indoor and outdoor temps.


    Now half the excess latent is converted to sensible ,so a three ton with similar specs., should do fine.

    Typical 3ton

  6. #6
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    Quote Originally Posted by cwtdallas View Post
    . Most condensor spec sheets that I can find only provide total capacity.
    On average sensible facotor is 72%. Depending on locality it can be adjusted in MJ and fan capacity, for humid or arid extremes.

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    Smile

    Quote Originally Posted by cwtdallas View Post
    Therefore the Sensible capacity of the unit should match as closely as possible to the Sensible Gain of the house at design conditions.

    My design conditions are a summer temperature of 100 degrees with 90 grains of moisture outdoor and indoor temperature of 74 degrees with relative humidty of 50.
    Where do you live ?
    in the desert ?

    100'F at 90 grains = 74'F WB / 64'F Dew Point / 30% R.H.

    Your latent load will be absolutely minimal.

    Match Sensible Load to Sensible Capacity
    at given conditions
    noted in detailed equipment performance table.
    Designer Dan
    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

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    Quote Originally Posted by steve3871m View Post
    On average sensible factor is 72%. Depending on locality it can be adjusted in MJ and fan capacity, for humid or arid extremes.
    Why would one use " an average" after determining requirements based on his design condition? Average sensible load Requirement might be Calculated ~ 85%

    Average rated SHR at ARI conditions might be ~ 74%,
    however at lower Wet Bulb temperature,
    an actual SHR Capacity will be much higher.


    cwtdallas already knows his SHR = 88.6%

    " ... Based on my design conditions Sensible Gain is 20,943 BTU/H, Latent Gain is 2,690 BTU/H, Total Heat Gain is 23,633 BTY/H ..."

    SHR = Sensible Heat Ratio
    Designer Dan
    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

  9. #9
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    Determination of equipment capacity

    Cwtdallas;

    It appears that you’ve invested some initial research into this issue, thus I’ll endeavoring to steer you in an appropriate direction based upon inferences you’ve drawn thus far.

    First and foremost, the methodologies you employed, ACCA Manual J, Seventh Edition do not warrant the degree of precision you’re attempting to carry in the calculation. That said, this methodology is considered industry best practices by residential HVAC contractors, thus this forum should prove fertile ground for testing your hypothesis.

    As noted by a previous contributor, infiltration is a “rouge” variable that can/will compromise your Manual J loads. Too many contractors will dismiss load calculation stating that they are meaningless because infiltration is a gross estimate. This is not a valid inference, since the correct method is to employ a “blower door test” to accurately determine the structure's infiltration

    Next issue… You need to obtain a copy of ACCA Manual S, (1995 Edition, free through interlibrary loan) or if you want to have your own copy, available directly from ACCA.

    Manual S is the industry best practice for the actual sizing of your AC-Furnace combo. Please study the Manual S procedures and become familiar with the limitation of each scenario you’re considering. Typically the cooling load drives the blower size, additionally the cooling load needs to be accommodated precisely, albeit in the case of a heat pump (should you decide to go dual fuel) it can be up to 25% oversized (study Manual S for the specific reasons for this). The fossil fuel heating unit can be up to 100% oversize without an efficiency hit (DOE empirical analysis supports this) but you’ll doom an oversize furnace to less than its design life owing to too many start/stop cycles (other forum contributors can explain this).

    Now that you moving toward selecting the correct capacity for the calculated load, you’ll next need to jump over another hurtle, sizing the conveyance system (ductwork). This is done via ACCA’s Manual D procedure. If the ductwork is already in place you may have to iterate backward to balance capacity with airflow parameters. What this means is your equipment section places “devices” in the conveyance air stream, who’s aggregate resistance must not exceed what your blower can accommodate (measured in inches of water column, owing to the device used to measure the aforementioned) AND maintain velocities acceptable for residential use (approximately 700 to 900fpm max) otherwise noise will become objectionable.

    Thanks for your efforts, rest assured you will be rewarded with the most optimum system if you continue you research. Do not expect the residential contractors in your area to be highly responsive to your request, they most probably do not fully understand the engineering involved in ACCA’s Manual J; Manual S and Manual D methodologies…

    Disclaimer: All the aforementioned ACCA publications are based upon far more precise methodologies derived by ASHRAE (American Society of Heating, Refrigerating, and Air-Conditioning Engineers). The preferred method used is your personal choice, and you must be willing to accept the limitations of using an abridged methodology.

    Respecfully; Faith
    Last edited by faith; 02-06-2007 at 10:11 AM. Reason: add relevant data

  10. #10
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    As Faith said, equipment selection and duct sizing goes with the heat loads. In order to obtain the 20,943 sensible gain you will need to have airflow of about 1,142 CFM. A contractor that doesn't take this into account might size for 400 CFM/ton which would cause more dehumidification that sensible heat removal. The net result would be equipment that doesn't perform properly.

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