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  1. #1
    Join Date
    Jun 2005
    Posts
    42
    My furnace and evaporator coil are in a closet in my house. I have a 5 ton system. An A/C contractor measured my external static pressure (between the entry to the blower and the top of the furnace) as .99 in. w.c. which is over the maximum static pressure on the furnace info plate of .8. From what I was told, I have the following static pressure readings:

    .07 return air
    .25 furnace mounted media filter (almost new)
    .29 evaporator coil
    .38 supply air

    From what I understand, the .38 in. w.c. for supply air is high. It was measured just above the evaporator coil, near the start of the supply air plenum. I can take out the media filter at the furnace and put some filters at my return air grills. I have 1224 sq. in. of return air grill. However, I would still be pretty close to the maximum.

    After the supply air plenum, the ductwork Ts to the 2 sides of my house. I have tried to make a fairly accurate drawing of the ductwork with lengths, fittings, locations of turning vanes, etc. I want to find someone who can troubleshoot where the source of the high pressure is and fix it. I have a 40+ year old house with hard metal ductwork (with turning vanes) that is wrapped in foil covered fiberglass. Most of the lines are rectangular with the Ys being round metal pipe. I dont see any place where the ductwork is collapsed. The contractor who took the measurements wants to replace all of the ductwork. I would like to see if there is a less drastic option since Ive been told by others that its hard to get better than hard metal ductwork.

    Is there any way to isolate where the problems might be?
    Can the static pressure be measured at the beginning of the 2 lines after the Ts?
    Can someone determine if the problem is in the supply air plenum or in one of the 2 lines going off of the T?
    When I am interviewing contractors, what kind of approach should I be looking for?
    I have read about using Manual D on this forum. Would or should that be used to look at possible problems since I have the basic layout info?

    Thanks.

  2. #2
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Find a contractor well versed in Man. D,check static ,before and after various fittings,the difference is the "drop" thru the fitting,this will show where the largest increases in static are.


    If you already have turning vanes ,it may be that the duct system is just to small.The equipment may be larger then needed,and a smaller system might be the best solution.


    1244 sq. in of return grille is a lot,using the same filter type,or a little less restrictive might solve the problem.


    One thing to remember is that,you are likely getting less then required cfms ,due to the static,so if for example you used a different filter,say .10 PD,the cfms will increase,and so will the static readings in return and supply,due to moving more air.

    If you have temperature differences in some rooms,a Man. J should be done to determine the cfm(airflow ) needed in each room,for redesigning the ducts.

  3. #3
    Join Date
    Nov 2002
    Location
    VA
    Posts
    296
    Yes ACCA's Manual D publication is a stellar source of information on your issue. Please take the time to study this publication in its entirety. With the proper duct geometry you should be able to calculate the theoretical pressure loses throughout your system. However empirical measurements are always preferred.

    In civil engineering the time-honored mantra is:

    In theory there is no difference between theory and practice, however in practice there is

    This reality becomes more painfully obvious the longer your tenure in the industry, enough said.

  4. #4
    Join Date
    Jun 2005
    Posts
    42
    I forgot to say that my furnace has a variable speed motor. From what I have been told, it has a 'brain' that makes sure it reaches the desired amount of cfm's according to some settings inside the furnace, which is supposed to be 1985 cfm for the 5 ton unit. I don't know what happens when the external static pressure exceeds the one rated by the equipment. Does it still reach the target cfm amount?

    Also, would someone get the 'real' pressure drops by drilling holes in my ductwork at different locations and seeing what the readings are? I assume the holes could be sealed off afterwards.

  5. #5
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    5,987

    Lightbulb Near Perfect

    Originally posted by apowers
    An A/C contractor measured my external static pressure (between the entry to the blower and the top of the furnace) as 0.99 in. w.c. which is over the maximum static pressure on the furnace info plate of .8. From what I was told, I have the following static pressure readings:
    0.07 return air
    0.25 furnace mounted media filter (almost new)
    0.29 evaporator coil
    0.38 supply air

    From what I understand, the .38 in. w.c. for supply air is high.
    WHY fool around with a system that is " Perfect " or nearly so?

    Is your house cool with low ( < 53%) relative humidity?

    The EXTERNAL Static Pressure is Actually 0.70"
    .07" R.A.
    .25" Filter
    _.38"_ S.A.
    0.70"

    Evaporator Coil is NOT EXTERNAL.

    E.S.P. is NOT TOTAL Pressure.

    What MERV or efficiency rating is the filter?
    0.25" is HIGH for a NEW filter,
    so I would guess you might be using > MERV 8 ( ~40%).
    Lighten up on the efficency rating a little.

    The Variable Speed Air Handler is very likely NOT running at 100% air flow capacity for extended periods.

    [Edited by dan sw fl on 06-26-2005 at 11:31 AM]
    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
    Join Date
    Jun 2005
    Posts
    42
    I thought the evaporator coil was separate. It was not part of the furnace package. The coil is a separate part that was added. This ia all Lennox equipment.

    The filter is the Honeywell F100 series, a 20" x 25" x 4" pleated filter with a MERV rating of 10. I know I don't need a MERV 10 filter but that is what came with the filter kit. Maybe I got the media filter static pressure value wrong.

  7. #7
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    On a furnace the evaporator coil is external,lets not start that debate again.

    The mfr. does not know what coil will be matched with what furnace ,so there is no way the coil is included in their fan data.


    Common problem in Florida ,with natural gas being more available lately,some think the same duct system they used for an air handler is correct with a furnace,most times it's not.

    [Edited by dash on 06-26-2005 at 12:24 PM]

  8. #8
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,389
    Only thing I see is high supply duct static. All the other numbers look pretty typical. Of course the days of shooting for ESP are over with media air cleaners and 1" pleated filters but if you could get your supply static closer to .1" you will be better off. Many VS systems can maintain close to their rated capacity as static gets up to .8 or so but could have trouble at .99".

  9. #9
    Join Date
    Apr 2005
    Posts
    36
    I had to weigh in on this one, because it hits close to home for our business. There are Lies, Damn Lies, and then Statistics. What you haven't said is how the contractor measured the various Static pressure readings you've given us. Nor have you said why you had the contractor out to make these measurements in the first place.

    Generally speaking you determine total SP by measuring the SP of the Return, a negative number like: -.07 as you've stated. Then you measure the Supply duct Static Pressure, a positive number like: +.38. You add these number together ignoring the positive and negatives and that is the total static pressure the blower is "seeing". The Filter, heat exchanger, and coil are then accounted for in that number. If you do like your contractor has stated, you've double counted your filter and coil pressure drop. Now it may be that he measured this and already accounted for it and he is correct, but I doubt it.

    You can read the SP in various duct runs by drilling holes and using a manometer and a pitot tube or static pressure probe with holes on the side, not a tube like a straw.

    This information will help you determine if the actual duct sizing is appropriate. For example if you have two supply trunks, and one has a much higher SP than the other, then the place to look to make repairs is the one with the highest SP.

    However, before you tear out ducts and redo them, make sure all the registers are open and that you are using high airflow registers and not the Home Depot decorative Air Flow Proof type grills.

    As for other issues you've raised, Variable Speed technology uses an DC motor called an ECM. ECM technology generally will provide the designed CFM regardless of the Static Pressure, the higher the static pressure, the more amp draw to accomplish the designed CFM, thus the higher the electric bill to run the system, and the shorter the life of the motor due to higher operating temperatures.

    On the other hand, a traditional AC motor will not be able to produce the designed CFMs against a high static pressure, but it also will not work any harder or draw additional amps to try to deliver more cfms.

    As for weather the coil is External, the word external refers to external to the box the fan comes in. Therefore, a coil is NOT external in the case of a heat pump, but is external in the case of a furnace.

    So, in summary, based on the assumption that your contractor did not account for the pressure drop at the return grill and the filter seperately, and the coil and supply duct seperately, he is double counting your SP. I would say your SP is actually 0.45 and is well within the stated limits of your furnace. Generally I assume .5 is a maximum allowable SP when making recommendations to my clients.

    Your actual PD of your Media filter is probably .25 - .07 = .18 Not bad for a thick media filter.

    Your actual PD of your coil is .09, which is about normal for a wet coil. I usually assume .1, any higher than .12 and I'm cleaning the coil for a customer.

    One of my little tricks is to account for 72 sq. inches of duct per ton of A/C on supply and return. Therefore, a 5 ton A/C needs a minimum of 360 Sq. Inches of Supply and Return duct attached. Based on your description, you're Return is in great shape, and your supply is probably close to that, but based on your overall SP, you're fine.

    Measure the size of your supply ducts and calculate the SQ. Inches and if you're over 360, you're good. Quit wasting your time on this, and focus on sealing your ductwork against air leaks and make sure your rooms are comfortable and within 3 degrees of what the Tstat reads. If they aren't then you probably have a design problem that is due to improper amount of CFMs being delivered to each room. In that case a Manual J & D will help the contractor redesign the duct to improve comfort. You never actually said why you had this contractor out there making these readings in the first place, but I suspect you have a comfort problem. That problem is different from a SP problem. I would make sure my ductwork does not leak due to poor construction or damage. If it is accessible , then have your contractor paint every metal to metal seam with mastic (aka Duct Butter) on square duct he is to paint heavily the drive pins in the corners and where the takeoff comes out to run to the room vent (assuming they used flex or hard round pipe) paint that connection thoroughly) so you can feel no air on the back of your hand.

    A ductblaster test can tell you how much duct leakage you have to supply and return and sealing that duct leakage may be enough to fix your comfort problem that I assume you have.

    One more thing, since you have square duct, I assume you have an older home. If the home has been remodeled and the A/C duct was not significantly changed to account for that and the remodelers just added some vents to blow air into the new room(s) they probably stole air from other rooms to make that happen and thus created a comfort problem I see that all the time. In that case, your contractor may be right that you need to tear out your ductwork to fix it. He may be using the SP thing to show you he knows what he's talking about, and you may have misconstrued the meaning of his numbers, or he is not interpreting his own measurements correctly. If you are in fact looking to solve a comfort problem, you will need a Manual J & D done by someone you can trust to tell you what size ducts you need going to each room.

    If you want to contact me, I'll be happy to talk to you about this live. Mon - Fri at 601-540-3001.

    Mike









  10. #10
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Originally posted by apowers
    My furnace and evaporator coil are in a closet in my house. I have a 5 ton system. An A/C contractor measured my external static pressure (between the entry to the blower and the top of the furnace) as .99 in. w.c. which is over the maximum static pressure on the furnace info plate of .8. From what I was told, I have the following static pressure readings:

    .07 return air
    .25 furnace mounted media filter (almost new)
    .29 evaporator coil
    .38 supply air

    From what I understand, the .38 in. w.c. for supply air is high. It was measured just above the evaporator coil, near the start of the supply air plenum. I can take out the media filter at the furnace and put some filters at my return air grills. I have 1224 sq. in. of return air grill. However, I would still be pretty close to the maximum.

    After the supply air plenum, the ductwork Ts to the 2 sides of my house. I have tried to make a fairly accurate drawing of the ductwork with lengths, fittings, locations of turning vanes, etc. I want to find someone who can troubleshoot where the source of the high pressure is and fix it. I have a 40+ year old house with hard metal ductwork (with turning vanes) that is wrapped in foil covered fiberglass. Most of the lines are rectangular with the Ys being round metal pipe. I dont see any place where the ductwork is collapsed. The contractor who took the measurements wants to replace all of the ductwork. I would like to see if there is a less drastic option since Ive been told by others that its hard to get better than hard metal ductwork.

    Is there any way to isolate where the problems might be?
    Can the static pressure be measured at the beginning of the 2 lines after the Ts?
    Can someone determine if the problem is in the supply air plenum or in one of the 2 lines going off of the T?
    When I am interviewing contractors, what kind of approach should I be looking for?
    I have read about using Manual D on this forum. Would or should that be used to look at possible problems since I have the basic layout info?

    Thanks.

    If you look at the way the statics are presented above ,it's resonable to assume they were taken correctly,but we can't know for sure,if they were taken correctly you have a problem.


  11. #11
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Originally posted by pfemike
    I had to weigh in on this one, because it hits close to home for our business. There are Lies, Damn Lies, and then Statistics. What you haven't said is how the contractor measured the various Static pressure readings you've given us. Nor have you said why you had the contractor out to make these measurements in the first place.

    Generally speaking you determine total SP by measuring the SP of the Return, a negative number like: -.07 as you've stated. Then you measure the Supply duct Static Pressure, a positive number like: +.38. You add these number together ignoring the positive and negatives and that is the total static pressure the blower is "seeing". The Filter, heat exchanger, and coil are then accounted for in that number. If you do like your contractor has stated, you've double counted your filter and coil pressure drop. Now it may be that he measured this and already accounted for it and he is correct, but I doubt it.

    You can read the SP in various duct runs by drilling holes and using a manometer and a pitot tube or static pressure probe with holes on the side, not a tube like a straw.

    This information will help you determine if the actual duct sizing is appropriate. For example if you have two supply trunks, and one has a much higher SP than the other, then the place to look to make repairs is the one with the highest SP.

    However, before you tear out ducts and redo them, make sure all the registers are open and that you are using high airflow registers and not the Home Depot decorative Air Flow Proof type grills.

    As for other issues you've raised, Variable Speed technology uses an DC motor called an ECM. ECM technology generally will provide the designed CFM regardless of the Static Pressure, the higher the static pressure, the more amp draw to accomplish the designed CFM, thus the higher the electric bill to run the system, and the shorter the life of the motor due to higher operating temperatures.

    On the other hand, a traditional AC motor will not be able to produce the designed CFMs against a high static pressure, but it also will not work any harder or draw additional amps to try to deliver more cfms.

    As for weather the coil is External, the word external refers to external to the box the fan comes in. Therefore, a coil is NOT external in the case of a heat pump, but is external in the case of a furnace.

    So, in summary, based on the assumption that your contractor did not account for the pressure drop at the return grill and the filter seperately, and the coil and supply duct seperately, he is double counting your SP. I would say your SP is actually 0.45 and is well within the stated limits of your furnace. Generally I assume .5 is a maximum allowable SP when making recommendations to my clients.

    Your actual PD of your Media filter is probably .25 - .07 = .18 Not bad for a thick media filter.

    Your actual PD of your coil is .09, which is about normal for a wet coil. I usually assume .1, any higher than .12 and I'm cleaning the coil for a customer.

    One of my little tricks is to account for 72 sq. inches of duct per ton of A/C on supply and return. Therefore, a 5 ton A/C needs a minimum of 360 Sq. Inches of Supply and Return duct attached. Based on your description, you're Return is in great shape, and your supply is probably close to that, but based on your overall SP, you're fine.

    Measure the size of your supply ducts and calculate the SQ. Inches and if you're over 360, you're good. Quit wasting your time on this, and focus on sealing your ductwork against air leaks and make sure your rooms are comfortable and within 3 degrees of what the Tstat reads. If they aren't then you probably have a design problem that is due to improper amount of CFMs being delivered to each room. In that case a Manual J & D will help the contractor redesign the duct to improve comfort. You never actually said why you had this contractor out there making these readings in the first place, but I suspect you have a comfort problem. That problem is different from a SP problem. I would make sure my ductwork does not leak due to poor construction or damage. If it is accessible , then have your contractor paint every metal to metal seam with mastic (aka Duct Butter) on square duct he is to paint heavily the drive pins in the corners and where the takeoff comes out to run to the room vent (assuming they used flex or hard round pipe) paint that connection thoroughly) so you can feel no air on the back of your hand.

    A ductblaster test can tell you how much duct leakage you have to supply and return and sealing that duct leakage may be enough to fix your comfort problem that I assume you have.

    One more thing, since you have square duct, I assume you have an older home. If the home has been remodeled and the A/C duct was not significantly changed to account for that and the remodelers just added some vents to blow air into the new room(s) they probably stole air from other rooms to make that happen and thus created a comfort problem I see that all the time. In that case, your contractor may be right that you need to tear out your ductwork to fix it. He may be using the SP thing to show you he knows what he's talking about, and you may have misconstrued the meaning of his numbers, or he is not interpreting his own measurements correctly. If you are in fact looking to solve a comfort problem, you will need a Manual J & D done by someone you can trust to tell you what size ducts you need going to each room.

    If you want to contact me, I'll be happy to talk to you about this live. Mon - Fri at 601-540-3001.

    Mike








    The likelyhood of .38 being the ESP of the supply ducts and a wet coil ,is very unlikely,I'd bet the readings are correct as stated,and you have a problem.

  12. #12
    Join Date
    Jun 2005
    Posts
    42
    I looked in the manual that came with my evaporator coil. There is a table titled "Coil Static Pressure Reading (Dry Coil)". It has the following info for my model at 1600 cfm (doesn't show any other cfm value):

    .23 in. w.c. for Dry
    .29 in. w.c. for Wet

    From what I understand, my unit runs at 1985 cfm at the maximum speed for cooling.

  13. #13
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Okay,so the question by some,is where did the contractor take they static readings.Can you tell us,as there is some question ?

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