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  1. #1
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    New Furnace and duct design problems...HELP PLEASE!

    Hello Everyone,

    First I want to thank all of the regular posters on here because I have learned a lot in the crash course of a few weeks since getting my new furnace installed. I am prepping to finish my basement and seal up all of the HVAC ducts into soffits, but I think I have run into concerns about the whole system (ducts and new furnace), and it has led to a pretty stressful few weeks because I just got a new furnace installed.

    First, all of my supply is made of 20 year old Fiberglass Ductboard reducing trunk (60ft) and branching flex (5-30 ft). I am realizing now after some research that the ductboard is not the best for health reasons, and the flex runs are way too long. So I am going to have someone install a new all new insulated metal duct system. But before I go that route and start talking to contractors, I am concerned that the furnace system I just had installed will not be sufficient for the new ducts (or the ducts that are there now). I imagine this is what contractors will tell me, but I want to confirm from others first. I normally either do all of my work in my house myself, or heavily research before purchases like this, but circumstances dictated otherwise this time and I had to trust my HVAC company that I have been happy with for the past 7 years. They said this system would be sufficient for my set up, and I do not think that is the case. Now I don't even feel confident approaching a contractor to rework my ducts until I have a better grasp on the capacity of my system.

    The new furnace installed is a Daikin DM96VC (Variable speed ECM, 2 stage, 100k/70k BTU, 3.5 Ton, 96%). I feel that I now have a system that will need to exceed the maximum ESP described in the manual (0.5 heat and 0.8 cooling).

    My house is an open floor plan 1800sqft ranch (60x30ft). Because the furnace is on the opposite end of the house as my furthest supply and return runs, I am looking at TEL of 600 feet for my ductwork as is. At the furthest supply and return registers, I am at 80' each for actual lengths in my reducing trunk branch setup, 160' TEL, without even adding the effective lengths in the runs. I can't change the placement of my furnace.

    I have calculated my ASP to be 0.18 (ESP-component static pressures = 0.5-0.32 =0.18). My filter alone is 0.23. The static pressure while running my heat, before my filter is -0.15, after filter -0.038, after AC coil = 0.20) I did not get a reading inside my furnace before the heating coils to measure my current ESP. I did't want to drill into my new furnace just yet.

    As things stand, I believe my friction rate is 0.03 (ASPX100/TEL=0.18x100/600), which seems very low. Even if I can somehow get my TEL down to 400' (which I don't think is possible), with my ASP at 0.18, my friction rate would still be 0.045 which seems well below where it should.

    Unless I am missing something, I don't think I can have anyone do anything to my ductwork to alleviate this problem based on the distances I have in my house and the low ESP rating of my system.

    So my question I have now is what do I do.

    1) Leave things the way that they are with my new furnace and have a contractor try and install the new metal ductwork with the low friction rates I have with this system. Will my system compensate and run at an ESP above the recommended, but put stress on the blower, but effectively work? Right now all my registers (12) seem to get air they are designed to and it does not seem to be short cycling.

    2) What are the problems with a system running at ESPs of 0.7 or 0.8 when they are designed for 0.5.

    3) Should I see if the company will install/retrofit a blower with a higher rated ESP (is this even possible?). Will this void the warranty of the system? Most energy efficient systems seems to max out at 0.5 ESP for heating before installing new metal ducts? Is there any way to decrease ESP without adding a bigger blower or decreasing TEL?

    4) Is there another option I am missing?

    I very much appreciate someone else's insight into my situation. In retrospect, I wish I would have known that I needed new ductwork as well before having a new furnace put in so I could have had a full Manual J, S, and D run and had someone redo the whole system, but this is where I am...

    ~Dave

  2. #2
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    Your TESP calculation is wrong. Ignore the negative sign. Just add the numbers. If I understand your readings, the TESP is .58" W.C.

    I suggest you get a technician who has experience measuring static pressure do the tests while you watch.
    AOP Rules: Rules For Equipment Owners.

    Free online load calculator: http://www.loadcalc.net/


    There = not here. Their = possessive pronoun. They're = they are
    It's = contraction of it is. Its = the possessive form of it
    Too = also. To = expressing motion. Two = 2
    Then = after that, next. Than = indicates a comparison.
    Questions should end with a question mark "?" Statements end with a period "."

  3. #3
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    Thanks for the quick reply, my measurement was higher up in the supply plenum, ABOVE the heater and AC components. So wouldn’t my total pressure reading be higher if I took the pressure below the components, right above the blower, and therefore increase my TESP? Isn’t there a significant drop in pressure between the blower and the heating and cooling elements (even if the AC is dry) of around 0.2? I agree I need to have some who knows this better get the pressure readings though, but as I mentioned, I just want to better informed before I make the mistake of having a contractor come out with me ignorant of the system. I did just get a second tube so I can just measure the differential pressure tonight with the monometer instead of single spots, and will update when I have those values.

  4. #4
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    If you have concerns about the ducts and the system then I strongly advise you to simply get a technician who can perform a combustion analysis and measure static pressure. I think you’ll understand it much better while watching someone do it. I would hope they could explain it to you at the same time.
    You do not need to learn how to do this stuff. You need to know how to find a capable technician who has spent years learning this stuff. Then you can trust his findings and decide on a course of action.
    AOP Rules: Rules For Equipment Owners.

    Free online load calculator: http://www.loadcalc.net/


    There = not here. Their = possessive pronoun. They're = they are
    It's = contraction of it is. Its = the possessive form of it
    Too = also. To = expressing motion. Two = 2
    Then = after that, next. Than = indicates a comparison.
    Questions should end with a question mark "?" Statements end with a period "."

  5. #5
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    Quote Originally Posted by Davemiller View Post
    Hello Everyone,

    First I want to thank all of the regular posters on here because
    I have learned a lot in the crash course of a few weeks since getting my new furnace installed.

    I am prepping to finish my basement and seal up all of the HVAC ducts into soffits,
    but I think I have run into concerns about the whole system (ducts and new furnace),
    and it has led to a pretty stressful few weeks because I just got a new furnace installed.

    I normally either do all of my work in my house myself,
    or heavily research before purchases like this, but circumstances dictated otherwise this time
    and I had to trust my HVAC company that I have been happy with for the past 7 years.

    They said this system would be sufficient for my set up,
    and I do not think that is the case.

    Now I don't even feel confident approaching a contractor to rework my ducts
    until I have a better grasp on the capacity of my system.

    The new furnace installed is a
    Daikin DM96VC (Variable speed ECM, 2 stage, 100k/70k BTU, 3.5 Ton, 96%).
    I feel that I now have a system that will need to
    exceed the maximum ESP described in the manual (0.5 heat and 0.8 cooling).

    My house is an open floor plan 1800 Sq Ft ranch (60x30ft).

    So my question I have now is what do I do.

    1) Leave things the way that they are with my new furnace and have a contractor try
    and install the new metal ductwork with the low friction rates I have with this system.

    Will my system compensate and run at an ESP above the recommended, but put stress on the blower, but effectively work?

    Right now all my registers (12) seem to get air they are designed to
    and it does not seem to be short cycling.

    4) Is there another option I am missing?

    I very much appreciate someone else's insight into my situation.

    In retrospect, I wish I would have known that I needed new ductwork
    as well before having a new furnace put in so I could have had a full Manual J, S, and D run
    and had someone redo the whole system,
    but this is where I am...

    ~Dave
    AN INSTALL WITH A SO-CALLED PLAN without a basis = STRESS

    STRESS Avoidance NOW is start with an appropriately accurate Manual J calc.

    ZIP _ _ _ _ _ ???? _____ 30'N or 52'N Latitude ??
    House built in 1999 ?

    1,800 Sq Feet may require < 40,000 BTU/HR Heating at ~ 8'F
    …. " ……." ….. "...…… "...…… < 22,000 BTU/HR Q. Sensible Cooling

    Response to 4) : Use 2 filters .:. 0.23" would be << 0.10"

    TEL may actually only be < 360
    Duct layout schematic is required.

    My gmail address; racingdan11
    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

  6. #6
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    Thread Starter
    Quote Originally Posted by kdean1 View Post
    If you have concerns about the ducts and the system then I strongly advise you to simply get a technician who can perform a combustion analysis and measure static pressure. I think you’ll understand it much better while watching someone do it. I would hope they could explain it to you at the same time.
    Maybe I am wrong, but I don't think I misunderstand the process...it does not seem that complicated to get ESP, I just don't want to physically take the measurement between the heating coil and AC component. I am having someone come out today to confirm my suspicions about high static pressure though.

    You do not need to learn how to do this stuff. You need to know how to find a capable technician who has spent years learning this stuff. Then you can trust his findings and decide on a course of action.


    As per my initial post, I had a person that I thought they knew what they were doing and I have trusted to do good work, but because I did not take time to learn things on my own, I was not able to vet their process. Now I have a system that does not have the static pressure capacity to match my pre-existing duct and layout. This is why I posted so that I can know they are doing it right, and not just "Trust" them.

  7. #7
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    Thread Starter
    Quote Originally Posted by dan sw fl View Post
    AN INSTALL WITH A SO-CALLED PLAN without a basis = STRESS

    STRESS Avoidance NOW is start with an appropriately accurate Manual J calc.

    I agree, I need to have this run (or see if my installer did this)

    ZIP _ _ _ _ _ ???? _____ 30'N or 52'N Latitude ??
    House built in 1999 ?

    Zip is 19063. House was actually built in 1999, 9.5' ceilings, and many large windows on all walls ( the 900 sqft great room has 6 (5x6' windows). Overhanging roof of 2 ft, and surrounded by trees. I get very little sunlight. Also there is a 1800sqft basement below with 10 foot ceilings that is half underground, and half walkout.It is unconditioned at the moment, but well insulated. There is not insulation between 1st floor and basement. The heater does have a first stage 70k BTU.

    1,800 Sq Feet may require < 40,000 BTU/HR Heating at ~ 8'F
    …. " ……." ….. "...…… "...…… < 22,000 BTU/HR Q. Sensible Cooling

    Response to 4) : Use 2 filters .:. 0.23" would be << 0.10"

    I have one of the larger 5" filter cartridges installed (20x24") on this new system.

    TEL may actually only be < 360
    Duct layout schematic is required.



    My gmail address; racingdan11
    Thanks for your response Dan!

  8. #8
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    Thread Starter
    So I had a technician come out today, and we confirmed that the ESP of the new heater is running at about 0.71 w.c. in 1st stage, low blower mode (~1300cfm). We forced the system into 2nd stage at ~1750cfm and the ESP jumped up to 1.25 w.c.

    I plan to have a contractor replace my ductboard and flexible runs with all metal to reduce static pressure, but I don't think that will be enough to drop my ESP to an acceptable level in the system. Do I need a different furnace installed? Do I not worry because the system won't run in high stage very often? All 12 of my registers get even air and the correct cfms (I confirmed with an anemometer)... I am just concerned about the high static pressure burning out my new system.

    Thanks again for those that have responded.

  9. #9
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    There are fittings and duct design strategies that can be used to reduce TEL of the duct system. I would start with Man J & S to see if that furnace is sized properly for the house. Maybe the duct system is the correct size (unlikely) and the furnace is too big.

    Also, might be a different AC coil that could be used to lower SP.
    Climate Control Solutions for your Home or Office

    Serving Northeast Philadelphia and Surrounding Areas

  10. #10
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    Quote Originally Posted by Davemiller View Post

    So I had a technician come out today, and we confirmed that
    the ESP of the new heater is running at about 0.71 w.c. in 1st stage, low blower mode (~1300cfm).

    We forced the system into 2nd stage at ~1750cfm and the ESP jumped up to 1.25 w.c.

    I plan to have a contractor replace my ductboard and flexible runs with all metal to reduce static pressure, but I don't think that will be enough to drop my ESP to an acceptable level in the system. Do I need a different furnace installed?

    Do I not worry because the system won't run in high stage very often?

    All 12 of my registers get even air and the correct cfms
    (I confirmed with an anemometer)

    ... I am just concerned about the high static pressure burning out my new system.

    Thanks again for those that have responded.
    WHY does your house Ever require > 1,100 CFM?

    DO YOU set your thermostat to < 74'F in the Summer?

    Analysis of your Monthly electric and natural gas use provides VALUABLE SIZING GUIDANCE.
    EVALUATION shall include HDD, CDD AND ACCA MANUAL J with associated Energy Estimate & Correlations.

    HDD - HEATING DEGREE DAYS
    CDD - COOLING DEGREE DAYS

    YOUR PREVIOUS ACCA Manuals J & D Calculations should be available.
    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
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    A.C.H. NATURAL IS NEEDED.

    AIR CHANGE PER HOUR

    Reference: BLOWER DOOR TEST Procedure & Results
    _________ ------------------------
    https://www.energy.gov/energysaver/blower-door-tests

    _______
    Double pane glass is presumed for a 1999 PA residence.

    Total Glass Area: ____ Square Feet ( < 360 SF presumed max / 20% of Floor Area)
    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

  12. #12
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    Thread Starter
    Quote Originally Posted by comfortdoc View Post
    There are fittings and duct design strategies that can be used to reduce TEL of the duct system. I would start with Man J & S to see if that furnace is sized properly for the house. Maybe the duct system is the correct size (unlikely) and the furnace is too big.

    Also, might be a different AC coil that could be used to lower SP.
    I agree, I am going to do everything I can with the contractor to reduce TEL, even so, I don't think I can get below 450. We are going to make a better transition our of the plenum, maybe add turning vanes, avoid hard 90s, use all metal (no flex). Also, the originally installer has two small flex runs coming off the main supply plenum, instead of off the trunk.

    I am hopefully meeting with the company today to see if they did a Man J and S, or just matched the system, or used rule of thumbs.

    I think it is likely my ducts are too small. My main supply is only 9x20 ID, for 1300-1800cfm. I am going to evaluate with my contractor what my options are to increase duct size, but we are trying to figure out where we will be with friction rate first.

    Also, I never thought of different coil... or maybe it needs to be cleaned.

    Thank you for this reply @comfortdoc, it was very helpful.

  13. #13
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    Nov 2019
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    Quote Originally Posted by dan sw fl View Post
    WHY does your house Ever require > 1,100 CFM?

    DO YOU set your thermostat to < 74'F in the Summer?

    Analysis of your Monthly electric and natural gas use provides VALUABLE SIZING GUIDANCE.
    EVALUATION shall include HDD, CDD AND ACCA MANUAL J with associated Energy Estimate & Correlations.

    HDD - HEATING DEGREE DAYS
    CDD - COOLING DEGREE DAYS

    YOUR PREVIOUS ACCA Manuals J & D Calculations should be available.
    We use smart thermostat set the temp to 72 degrees when we are home and at night and 76-78 other times we are not there. My house has high humidity based on its location (bottom of valley in the woods).

    To answer your question about cfm, I am not entirely sure because I am not sure if a schedule J, S and D were done...which I am hoping to get a final answer on today. I will say that I have 9.5' ceilings, lots of windows, and a 10' tall basement below with windows and walkout. This basement is not currently conditioned. At the end of the day I need to have the proper Manuals run.

    Thanks for your reply Dan.

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