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Re: static pressure issues

Was this a S _ _ _ S job ?

Re: static pressure issues
jled96 wrote:

#5 Today, 10:41 AM
Junior Member Join Date: Nov 2011
Location: boston,ma
Posts: 1

Re: Return Air


I think I have the same problem with not enough return air flow. I had a carrier infinity 59k btu 3-stage ics furnace and 2-ton ac installed last year and kept having the change filter message pop up everytime we use a,c or high heat is activated. All brand new ductwork and My only return duct is 20x6 runs 15 feet to common hallway and the grill size is 20x8. The suppy is 14x8 runs 10 feet and goes to 7 six inch flex ducts with 4x10 otlets and grills.The house is only 1,080 sq. feet.The original installer has left the state, and now the new contractor i hired has found a few issues, he wants me to add another return off the return plenum 8"pipe up inside wall to second floor hallway, where there are 2 bedrooms to a new 8x14 grill and says to only use a merv 8 filter not the carrier merv 11 that i bought 4 off the original installer. he says the staic pressure .89 is to high on high speed and i will burn out the esc blower motor, he has also turned the cfm's on the infinity controller down to 800, it was at 950. He says its ok for now but I should do the return duct before the next cooling season, does any of this sound correct? the original installer states he does theses every week and has not had any issues for the last 15 years and my 1 year install warranty is up, I paid a lot for this system i cannot believe it would not be installed cooectly. any advise would be much appreciated. Thanks!

Around these parts we use this general rule of thumb........

For heating with a gas or oil furnace, 100 CFM represents approximately 8000 BTU per hour.
For air conditioning, 100 CFM represents 6000 BTU.

The standard eight inch deep duct is designed around this 100 CFM standard. Every two inches of width equals 100 CFM. A six inch round duct also equals 100 CFM. So for every two inches of width of an eight inch trunkline equals a six inch round.

Based on this ----
your supply trunk at 8 X 14 = 700 cfm

your return trunk at 6 X 20 = 900 + cfm

Now, if the blower was set to deliver 950 cfm you can see the supply truck can't handle the volume effectively --- creates a bottle neck ( restriction ) --- even though the larger capacity of the return which can handle the volume easier.

In other words -- the supply and return trunks are unbalanced from the get go.
What goes out must come back, so the return system must be the equivalent.
Returns can be fewer in number and of larger diameter but must equal CFM total.

Sounds to me the original installer does the quick and easy route for him and charges premium prices. Maybe he thinks putting in a huge return will create enough negative pressure to draw air down from upstairs --- who knows --- just a guess what he thought.

The second HVAC guy seems to on the right path by reducing the CFM output closer to what the supply trunk is rated for and suggesting adding the second return. Having only one large return down on the lower level will not circulate the air properly on the second level at all. You also need to balance what the supply and return volume on the main and second level independantly.

For example : let's say there supplies are 500 cfm on the main level and 200 cfm on the second level --- then returns should also refect the same to be a balanced system.

Other than that --- having the six inch lines with 4X10 boots and vents should be fine for velocity , volume and diffusion.

Then and again, up here we always have supply and returns in each room --- even in retrofits in older homes supplies and returns are run to each room. Far better and more even air distribution.

2 cents worth.

Re: static pressure issues

And a fine two cents worth is is, it's always better to have returns in every room as you know your recirculating the air but there are times when it's not feasible. Cost is a major player in most problems that occur in Heating and A/C, a subcontractor that deals with a specific Retailer gets the job with a price already attached. Sometimes they will also tell what and how to do the job and if it isn't done the way they say, (per there engineers or whatever you want to call them) you don't get paid. The job that is described by the original poster looks like a job that would fall into this category. The HVAC Co. that does the work won't or can't decline any work because then the work goes elsewhere. I live pretty close to Boston and without saying to much, read between the lines.

Re: static pressure issues

Yes your reading on this situation is correct it was a sub job, and original installer for deep energy retrofit program has left mass and went to northern Maine to build a huge retirement sub-division, that is why I am having so much trouble contacting him for correcting issues, he said job is at spec and thats what I paid for, now I am stuck with the bill for repairing the system as well as the huge loan I took out to do the heat and a/c,doors, windows, and insulation for over $30k.

New contractor i hired said different and just did a few minor changes for me before the cold weather set is. He just added 5" supply for bathroom where there was a shared boot between small den and bath off a 6" supply run. Also he changed 6" run to upstairs master bedroom to 7" because the room is 11x16 and says 6" is too small for cooling, and ran this run right to supply plenum ,not off the 8x14 supply trunk, he ran the 5" for bath off the exising 6" take off with a reducer on trunk. Now in the spring i am going to add 8" return to upstairs hallway wich is shared by both bedrooms second bedroom is 11x12 so 6" is fine for that room i have been told.He also added a supply vent 8" to the basement off the supply plenum that I can open and close iff it gets too cold in the basement, which there was no heat in basement before, he says only thing left to do is add return to second floor and it should work much better with less static. I hope this is all i have left. Thanks for the replys!

Re: static pressure issues

Your not supposed to take any supply runs off the supply plenum, this will cause you to lose pressure within the system but, if it works for you then OK. I've seen things that really make me wonder how some HVAC Cos. stay in business, if it's done right the first time then there's no callbacks. Good Luck

Re: static pressure issues

I agree with you about the supply plenum feeds,because now things are starting too look very congested looking at the furnace area, once I add that other return I think it will be the last thing I can cram in that small area, but this is now a repair after install and I have no other options short of ripping down the nice looking insulated ductwork, That is the only part of the deal that came out nice, they spent too much time insulatining the ductwork and sealing every nook and crevice that I think they forgot about everything else (the way that it functions) although these were just the installers , so I guess they have to make it right to pass inspections. The way it is layed out looks great , too bad it doesn't function as good as it looks.I wish they double checked there numbers on the cfm's before they installed everything. I would think the gas company energy people would know thsi stuff, but they told me they are not responsible for any botched installs , it is all on the homeowner. I should have looked up the duct sizes myself before they installed, but i figured they were on the energy conserv. list, so they must know what they were doing.


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