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  1. #1
    Join Date
    Aug 2012
    Posts
    11

    New Manual J produced to match installed equipment.

    I posted earlier about issues with a new home cooling. (Thanks for the advice, a company is coming thursday to measure). However, part of my question is that the original Manual J state a 86,0000/71,000 Btuh 80% furance would be installed. Instead a 66,000/54,000 Btuh furance was installed. I paid to have an independent Manual J done and it closely matched the heating requirement of 73,003 Btuh outuput and required an additional 1/3-1/2 ton A/C.

    When I questioned this with the builder, a new Manual J was produced by the builders HVAC company that stated I only need a the 66,000/54,000 3 TON furance and the old Manual J was a mistake and I shouldn't look at it. Yet this new Manual J lowers my cooling from 3 Tons to 2 Tons, yet they will leave the 3 Ton A/C in place.

    I have looked at the new Manual J and to my untrained eye it seems suspect with some of the numbers. If anyone with experiece has time to glance it over I'd appreciate it.

    House:
    Ramber Facing North
    Upstairs 2340
    Basement 2360
    Total footage: 4700
    50% 10' ceiling 50% 9' ceiling on main floor.
    R38 Attic
    R19 2x6 walls.
    7 Bedrooms
    Location SLC, UT

    Questions:
    Missing 40 Square feet of glass on back of house.
    3 Rooms on the main floor have zero heating/cooling load.
    Ceiling dropped from 28% of the heating load to only 6.9%.
    Ceiling dropped from 40% of cooling load to only 14.9%.
    No Latent Cooling is required.
    No GR/lb included on the new Manual J.
    Zero people for latent load.
    Original calculated on MJ8 new one on MJ7 by the same company.
    Sensible heat ratio of 1.00 (Not sure but I keep seeing .75 as the standard)
    etc....
    Attached Images Attached Images

  2. #2
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,967
    Looking at the pdf Design Information: IMO, the 46 gr per lb/air moisture difference - should have been in the cooling design column.

    That is where knowing the grains of moisture per lb/air between the outdoor ambient & the indoor at 50% @72-F counts, because that latent load has to be removed by the A/C system; not in the heating system mode...

    In the heating season you're not concerned about the air moisture per lb of air except for needing a humidifying system to keep the humidity at the best wintertime levels.

    Well, they don't need any latent removed at their elevation & dry climate; ignore my other statements!

    However, COOLING EQUIPMENT SUMMARY: they have 9960 Btuh of latent listed; so how does it match the actual latent load.
    Last edited by udarrell; 09-08-2012 at 03:16 PM. Reason: Cooling Equipment Summary: 9960 Btuh of latent listed

  3. #3
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,187
    I've NEVER seen a gas furnace that was undersized in the real world. All the installs I've seen installed could easily keep up down into the single digits. I don't see a 2 ton A/C cooling a 4700 sq ft house to a 23 degree difference. That's 2,350 sqft per ton. Standard building practice is 500sqft per ton, which is too much for a new house in most cases. I think the 1st calculation for a 3 ton is closer to what will be needed.

  4. #4
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,060
    How much cooling load is there for a basement anyway. So really think of it as a 2300 square foot house. As well built as it is I'm not sure the original loss was accurate. I'd have them put in writing that if it doesn't keep up this winter they go bigger. If it does keep what they have.

    Sent from my HTC VLE_U using Tapatalk 2

  5. #5
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,187
    Even if the basement isn't considered 1 ton per almost 1,200sqft is a stretch. Basement will add some latent load in most situations...

  6. #6
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,272

    Cool Here, we Don't Go aGAIN.

    Quote Originally Posted by 54regcab View Post
    I've NEVER seen a gas furnace that was undersized in the real world.

    All the installs I've seen installed could easily keep up down into the single digits. I don't see a 2 ton A/C cooling a 4700 sq ft house to a 23 degree difference.
    That's 2,350 sqft per ton.

    Standard building practice is 500sqft per ton, which is too much for a new house in most cases.

    I think the 1st calculation for a 3 ton is closer to what will be needed.
    There's DEFINITELY NOT a standard building practice,
    _____________________ unless it's 300 __ to __ 2,600 Square Feet per Ton. _ L.O.L.
    ___________________________ Glass House __ IGLOO.
    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

  7. #7
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,272
    This Manual J certainly seems to be UNIQUE due to BOTH the MANUAL J modeller and the Salt Lake City Utah environment.

    15 MPH is creating a Very HIGH Winter infiltration of 292 CFM.

    The solar heat gain from the windows is signficantly lower than I normally see.
    The S.H.G.C. is not explicitly stated in the window construction features.

    The treatment of Latent loads is simply erroneous IMO, even though they may actually be VERY LOW in S.L.City.

    Selection of a 3-ton is certainly Not based on the analysis as presented.

    1,107 CFM in 4,332 square feet at 4,226 foot elevation will create performance issues and decreased comfort levels.

    ___ IF ___ a 3-ton unit were judged to be appropriate, total air flow might be nearly 1,700 CFM.

    http://www.wunderground.com/history/...q_statename=NA
    5,000 Heating Degree Days
    1,600 Cooling Degree Days
    I would be evaluating use of a heat pump for the shoulder seasons.
    It seems like A/C operating hours would be limited in Salt Lake City.

    A/C unit will actually operate in the SHR range of 0.88 + ( not the normal 0.70 to 0.75)
    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

  8. #8
    Join Date
    Jul 2008
    Location
    Rochester NY
    Posts
    4,721
    Never seen an undersized gas furnace either. Think it's a myth.

    I'd lean towards Loonie on the cooling, think thats a good catch. If 2 tons effectively cools a leaky 4000 sf 1960's 'rancher' in Baltimore THIS summer (and heats to 25), not sure why it wouldn't handle a new build in salt lake. If its anything like Jackson Hole, not much latent load. (Not much sensible either.).

    I think Dans call of 1600+ for 3 tons may also be bang on. REG, per a conversation yesterday with a guy in Nevada, we need to think differently about how folks in dry areas need to design. It's not what we are used to, where people seem to always UNDERESTIMATE managing latent (to TB's perpetual frustration)

    So watch that duct design very carefully, don't let them undersize. Undersized duct seems about as common as oversized furnaces, and both present big energy, noise and comfort control issues.

    Definitely would be going 2 stage heat pump if 3 ton. That pump should carry most of heating load, and if you get solar at some point, you are practically energy independent.
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

  9. #9
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,187
    Quote Originally Posted by dan sw fl View Post
    There's DEFINITELY NOT a standard building practice,
    _____________________ unless it's 300 __ to __ 2,600 Square Feet per Ton. _ L.O.L.
    ___________________________ Glass House __ IGLOO.
    I'm not saying it's right by any means, but that "rule of thumb" is difficult to get contractors to break. Even when "manual J" is done the input numbers tend to be fudged and the result turns out to be the magic 500sqft per ton.

  10. #10
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,272

    Exclamation then a revolution in thinking / standard practice is absolutely required

    Quote Originally Posted by 54regcab View Post
    I'm not saying it's right by any means, but that "rule of thumb" is difficult to get contractors to break.

    Even when "manual J" is done the input numbers tend to be fudged and the result turns out to be the magic 500sqft per ton.
    ... Even when ... FUDGED ..
    _______ With that said, it is no wonder the industry is associated with blatantly deceitful practices.

    INTEGRITY is DEFINITELY the ONLY LONG TERM SURVIVAL CHARACTERISTIC.

    Even in Florida, a rule-of-thumb for New residential construction ought to be in the 700+ range.
    _______ i.e. > 40% higher than the old B.S.

    Northern climates should be ~1,000 square foot /ton or 100% higher than that old legacy un-thinking.
    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

  11. #11
    Join Date
    Aug 2012
    Posts
    11
    How do I make a sound informed judgement? Ideally I would heat and cool the house as cheaply as possible while being comfortable.

    At this point I know the house is stuffy during the summer and I don't want to freeze during the winter. I have three different Manual Js with two different results.

    The last manual j seems manipulated to match the equipment that was actually installed. It may not be, but I'm always leary of documents created after something is done to show it was done right. Where as the manual J created prior to the installation seems more credible.

  12. #12
    Join Date
    Aug 2012
    Posts
    11
    Quote Originally Posted by tedkidd View Post

    So watch that duct design very carefully, don't let them undersize. Undersized duct seems about as common as oversized furnaces, and both present big energy, noise and comfort control issues.
    .
    The original Manual D called for all duct work to be run in sheet metal. It was all ran in air connector (not air duct) and is loose, each run is compressed in multiple places etc. It is supposed to be checked this week for CFM at each supply register, and I'm being told if each supply register is the correct CFM then the system is working fine and they are done.

  13. #13
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,967
    Quote Originally Posted by dan sw fl View Post
    ... Even when ... FUDGED ..
    _______ With that said, it is no wonder the industry is associated with blatantly deceitful practices.

    INTEGRITY is DEFINITELY the ONLY LONG TERM SURVIVAL CHARACTERISTIC.

    Even in Florida, a rule-of-thumb for New residential construction ought to be in the 700+ range.
    _______ i.e. > 40% higher than the old B.S.

    Northern climates should be ~1,000 square foot /ton or 100% higher than that old legacy un-thinking.
    I tend to agree providing there is a performance test proving that the system removes near the Btuh that it's rated to Remove from the conditioned areas.

    Additionally, one could use those numbers even 750-sf per ton in many areas of FL; run the initial mock-up numbers using the way you're going to make those selected load numbers work.

    In other words, do an optimal cost effective mock-up load calc modeling & then do all the things to the home & duct system-airflow using Manual D, to make it get the design numbers the customer originally asks for.

    We HVAC Contractors need to become whole house system designers, because the home is the first order efficiency starting point toward an optimally cost efficient heat & A/C home.

    Here in SW WI & other Midwest states there are a number of +2000-sf homes with 2-Ton cooling systems that do the job perfectly well.

    The mock-up modeling load calc numbers should be run initially so you know the requirements to be performed to reach the desired goal.

    With present day computer technologies this would be a piece of delicious cake, right...

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