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    HVAC calculations (20 Posts)

  • rjjr38 rjjr38 @ 5:45 PM
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    HVAC calculations

    Can anyone recommend a site for figuring the proper BTU sizing of a warm air furnace in an existing home? The 2nd floor does not heat as well as the 1st floor. I'm thinking th old one is under sized. Thanks
  • Zman Zman @ 6:04 PM
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    Heat loss

    Any heat loss program will do the trick.
    Do you have a duct sizing or balancing issue?
  • rjjr38 rjjr38 @ 6:13 PM
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    Right now there are 10 6" supplys for the home. The existing unit needs to go. I need to calculate the proper size new unit for a 1500 sq ft home. Any sites for figuring proper BTU sizing?
  • Zman Zman @ 6:34 PM
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    Slant fin

    Slant fin makes a good heat loss calc.
    I doubt your present furnace is undersized 1500 is a pretty small load. Do you have uninsulated ducts in an attic?
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    Use the Slant Finn program. Your probably going to find the ducts to the second floor are to small, they usually are, You may be able to run additional ducts up through a closet. Air will carry about 1.66 btu/cfm.
     A 6" round will carry 100 cfm or about 10,000 btu/hour
    7" round 150 cfm 15,000 btu/hour
    8 " round 200 cfm 20,000 btu/hour  
    So your existing 10 6" round ducts would handle a 100,00 btu furnace output.
    This post was edited by an admin on January 22, 2014 7:34 PM.
  • Bart Vaio Bart Vaio @ 11:45 PM
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    Cfm vs. btu

    No way you can move 100,000 btu with 1000 CFM of airflow and stay with an acceptable temp rise in the unit. 100k furnace will be 5 tons (2000 CFM) and a 1500 sq ft house of typical build will need a 3-4 ton unit. Likely it is not ducted properly and has restrictions such as poor duct routing, grill losses and definitely an undersized return. So your 10 6" ducts are likely pushing around 700 CFM (just under two tons). I would get a professional to look at your airflow issues before you try guessing at unit sizing. A 6 inch pipe that is perfectly smooth can carry 100 CFM at an acceptable velocity, then put a floor register on that 6 inch boot and you will drop down to around 70-75 CFM not to mention duct losses from unsealed ducting...
    This post was edited by an admin on January 22, 2014 11:46 PM.
  • ProblemSolver ProblemSolver @ 8:01 AM
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    Air Flow Science

    Your on the right track. People should always add their location to help us help them. I'm in the Detroit, Michigan area and I have a 2 stage 80k 80% furnace, derated 10% orphises. We are experiencing single digit temps and my furnace is doing just fine. I am using a toggle switch to jump into high fire (2nd stage) when temps are this low. My house is 1962, 1,500 sq. ft. ranch with a basement. Now here is the man's problem - which nobody addresses - High pressure goes to low pressure every time. Therefore, the rooms lacking comfort are the rooms lacking return air. The blower will pull all the air it can from the closest opening - first; then it will do the same to the next closest opening - and so-on down the line. Most R/A ducts are not balanced, so the blower gets all the air it needs from the 1st floor openings and has no reason to pull air from the 2nd floor openings. The return airs in each room create a negative pressure, therefore the supply air has no choice but to follow that negative pressure. The rooms lacking comfort are the rooms lacking return air / not supply air!!! So, the man has to choke down the 1st floor openings, and force the blower to pull from the 2nd floor openings. As a test; put a magnetic sheet over the 1st floor grills covering all but 2 inches of the grill.
    This post was edited by an admin on January 23, 2014 8:50 AM.
  • rjjr38 rjjr38 @ 5:57 PM
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    High fire

    I'm not familiar with the toggle switch for high fire 2nd stage. Can you elaborate?
  • Zman Zman @ 12:25 AM
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    Why in the world are you guys talking 100k btu?
    Do a heat loss calc, balance the system and move on.
    The heat loss for a 1,500 square foot home is no where near 100k.
  • Bart Vaio Bart Vaio @ 12:54 AM
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    Didn't want to seem to advocate 100k you couldn't do it if you wanted to. Single or Two stage 3ton 50-60k is more likely. 4 ton if you want to do cooling but the duct work will have to be completely redone. Airflow is the issue not unit sizing, slap a return on the top floor while your at it. Likely has only 35 to 40k of load depending on windows and insulation etc. Correct airflow, balance and properly size.
    This post was edited by an admin on January 23, 2014 10:10 AM.
  • rjjr38 rjjr38 @ 12:46 PM
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    Heating Calculation

    Hey Bart,
    Thanks for all your help. My customer does have a return on the 2nd floor. I need to install some damper on the 1st floor supplys to help balance things. I;m looking at installing a 117,000 btu unit. No A/C here. What are your thoughts on adding a humidifier.We have some very cold and dry air here in Maine during the winter. Thanks again, Ray
  • Bart Vaio Bart Vaio @ 12:55 PM
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    Airflow not btu's

    I would address airflow first by upgrading the ductwork 117k will be overkill. Look at the Cfm of that unit, I guarantee it will exceed your duct capacity and heat loss. Humidifiers are fine as long as they are installed in conditioned space as they can freeze up.
  • Stephen Minnich Stephen Minnich @ 5:31 PM
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    4-5 tons???

    That much cooling for 1500 square foot home? No chance unless he wants to hang meat in there.
    It's all in the details.
  • Zman Zman @ 5:42 PM
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    Why are you asking about heat loss calculators if you are just going to WAG it?
    I mean as long as you are going to throw 62BTU/Ft at it why not put in a 200k unit just to be sure. Be sure to round up just in case.
  • rjjr38 rjjr38 @ 5:53 PM
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    Heating Calculation

    I plan on putting in an 85,000btu unit for heat only. The home is 1500 sq. ft. I'll be adding additional dampers on the 1st floor to try to direct more heat to the 2nd fl. There are returns on both floors. I feel piping size is correct for the job. It seems to be a balancing issue. Also offering the customer an optional humidifier.
  • Bart Vaio Bart Vaio @ 6:04 PM
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    Big mistake, if you are using an 85k unit that means 4 tons / 1600 Cfm which is roughly twice what your duct work can handle if you have 10 6" runs. Your going to create more problems than you will solve by over sizing. Are you licensed warm air contractor? Refer to ACCA Manual J for sizing
  • rjjr38 rjjr38 @ 6:15 PM
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    Heat Calculation

    Hey Bart,  What size unit would you use?

    Thanks, Ray H.
    Master Oil Tech
  • Stephen Minnich Stephen Minnich @ 6:20 PM
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    I disagree

    Just looked at a random 90k input furnace I offer with a 3 ton drive, 4 speed blower. On low speed it would deliver 927 cfm at .5 static pressure.
    It's all in the details.
  • Stephen Minnich Stephen Minnich @ 6:26 PM
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    4 ton drive...

    Is 1600 cfm. 1600 cfm at 0 static which doesn't exist. It's also only 1600 cfm at high speed. Most 85k furnaces are offered with 3 ton drives.
    It's all in the details.
  • Bart Vaio Bart Vaio @ 7:38 PM
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    Tinman is right you can often manipulate airflow by changing speed settings and we haven't even discussed Multi stage units, but trying to solve a problem by just buying a bigger unit sounds like trouble to me. I just fixed a 2100 sq ft. house with a 5 ton variable that wouldn't heat even on high speed, we put the flow hood on the registers and were getting 43 CFM per 6" boot due to sanded down inset hardwood grills. You could have put the biggest furnace in the world in there and it wouldn't have helped. Also we see a ton of duct velocity noise issues due to over sizing. If I was going to pick a furnace for a customer that has been having heating problems I would definitely take the time to do a proper heat loss and look at airflow as well. This situation is a lot different than a customer that has a failed furnace but has been happy with the systems performance till it broke.
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