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.
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.