View Full Version : low pressure at vents

09-23-2010, 03:37 PM
we have a 2400 sq ft house in missouri, when we first bought it new it had a builder grade furnace and ac. The furnace always ran and our bill was rediculously high. So we replaced with a bigger furnace and a/c furnace is a 30 kw and I believe the a/c is a 4 ton unit. We have never had alot of pressure at any of our vents before or after our new system was put in. Our main trunk line is 48' long and its 8"x20" the whole length. the plenum is 16.5"x22
the return air is (the box the furnace sits on) 28"x36"x10" then connects to a 10"x25" that goes up to the return trunk line which is 8"x20"

I wanted to know if I stepped down to 8"x16" or 18" for a section or two on the main trunk line for my supply, would that up my cfm to all my vents? I have asked several hvac guys nd noone will give me an answer or is willing to come out.
thank you for your help!! sorry if Im not using the right terms. thanks again, shawn

09-23-2010, 04:06 PM
A few questions. Do you have an electric heater section (30KW) or gas furnace?
If you have a 4 ton, you should have a nominal 1600 CFM. Your supply opening off the unit should transition to something on the order of 20"x14" or 20"x12" main trunk line, for straight cool w/ electric heat and probably something on the order of 20"x16" for a true gas furnace. It really depends on what pressure your unit can work against.
What size unit did you replace..... 2.5 ton?
Sounds like you have a duct size problem.
Stepping down duct sizes for your existing duct system only add external static, more pressure drop for your system to overcome. Even your return duct of 8"x20" is to small if I understand you right.
Can you do a sketch of your existing duct system system with sizes ands lengths and post it? Include the manufacture and model number of your new unit.

09-23-2010, 05:23 PM
The air handler can only move so much air through a specific duct before the back pressure, noise, and resulting friction get to the point where you've maxed things out. Making the duct(s) smaller won't help.

09-23-2010, 05:31 PM
I believe the ac unit before was a 2.5 or 3 ton and the furnace was a 20kw (electric) the supply is 48' long and the return is 28' long. all supplys ran off main trunk line is 6" pipe to a 4x10 register. I made a sketch and scanned it but I dont see an upload file butto. thanks

09-23-2010, 05:48 PM
I think I figured out how to upload

09-23-2010, 05:50 PM
How many 6" take offs do you have? I need to see how to send a message also. Not sure if I can post an my email address or not but will check.
Do you have the model and manufacturer of the new system and is it a package unit or split system. Sounds like a split (indoor unit and outdoor unit)

09-23-2010, 05:52 PM
I couldnt draw it exactly how it is but the supply and the return are runningside by side like most homes. thanks again!!!

09-23-2010, 05:55 PM
whoops sorry I forgot to post that part the ac is an arcoaire model #nac048aka1 and the furnace is suppose to be a goodman ( thats what guy told us he was putting in) model# nfcp4800d1 there was also another similar one right below that #abf4800d1

09-23-2010, 05:56 PM
its a split unit, 14 vents upstairs and 10 vents down stairs

09-23-2010, 06:17 PM
Looks like for what you are telling me that the 6" vents are OK. It appears that the main supply and return are undersized.
Note: you indicated a townhouse which with one unit you are always going to have a problem satisfying both upstairs and down stairs at the same time without always adjusting the grills. My email is fdraa@yahoo.com.
Are you able to increase the supply and return duct sizes?
30KW is a whopper is that what the requirement calculated out to.

09-23-2010, 06:28 PM
we have a ranch house, I guess thats the same as a townhouse, normal house with a basement. one half of the supply and the return is in an unfinished area while the other has drywall around it. it was suppose to be wired where the furnace would run at 25kw and on colder days 25 degrees and under it would use the remaining 5kw to warm the home faster. but they never did it. in fact for the first 4 years the furnace was only running 20kw because they didnt run extra power when the new furnace was installed. the 30kw does heat our house well but still were back to the low vent pressure. I know the ac is still too small because it will never cool the house to 74 until the sun goes down

09-23-2010, 06:30 PM
when they put in the new furnace and ac they didnt change any ductwork at all either, which didnt make any sense to me either. so now im trying to find some answers. thanks again

09-23-2010, 07:12 PM
From what you have told me so far I believe you will see an improvement in cooling and heating.
One way to look at it is the unit is trying to satisfy the T-stat but you don't have enough air movement to satisfy the T-stat by bringing the temp to it set point therefor the unit runs and runs. If you could increase the duct size Looks like the trunk line, you would get more air to the rooms during heating and cooling season. Do you have the room and access to the duct system to enlarge the main supply and return? I am surprised you have not had the electric furnace cutoff with the internal safety switch (clixon we used to call them)

09-23-2010, 07:22 PM
there is maybe 15-20 feet of supply and 10-15 feet of return that could be changed the other portion is under drywall. but if it comes down to it I might be able to tear that down to hang new duct, I just didnt want to have to go through all that work and mess if something else could be done.
would it help if I made the plenum bigger and the supply trunk line in the unfinished area. and make the return bigger also.

09-23-2010, 07:53 PM
I just measured 14' of supply duct and 5' of return that I have access to, also the plenum looks like it cant go any bigger, it almost covers up the whole top of the furnace

09-24-2010, 03:52 AM
Doesn't look like there is much you can do without a major renovation of the duct system. Anything made bigger would help but I doubt if it would ever achieve optimum. Next time you up grade consider two systems. Sorry Shawn

09-24-2010, 07:24 AM
You know, it might be something as simple as the fan speed not being properly setup. Is it a model that automatically adjusts the fan speed or is it a fixed speed setup (one speed for AC, another speed for heat)?

Did they do a manual J calc to figure out the heat load before changing the size of the unit? There are a lot of factors, but the 4 ton seems like it might be too big. I live in a similar climate, larger house (~3100 sqft), and probably older house (1968). Anyway, I have a 3 ton AC and 24 kW electric furnace. It is a 2-storey house w/ finished basement. There are two zones (controlled by motorized dampers, only single air handler). Upper floor on one zone, the main floor and basement on the other zone. Like I said, there are a lot of other factors too, so the only way to know what size of unit to use is to do a manual J.

If they did do a manual J and figured that you did need 4 tons, then they should have done an additional calc to make sure that your ducts were sized to support a 4 ton unit.

In terms of the electric furnace, the larger (or smaller) unit won't save you any money. With electric, heat is heat, so a larger unit will run less but will also use more power when it does run. You might save a tiny amount with the blower running less, but the power that the blower uses is nothing compared to what the actual heaters pull. I am surprised that you didn't go with a heat pump or gas furnace. The only reason I am still on the electric furnace is that the AC unit is still good. Once it dies, I'll go for the heat pump. We live in pretty mild climates, so a heat pump does pretty well most of the time. As you know, the electric furnace is very expensive to run.

Some other things to look at:

- check your filter(s). Even if clean, some types cause a large pressure drop. You might want to try a cheapie filter (the ones that are under $1) to see if that changes the flow
- besides the supply size, the return also has to be large enough to support the flow. Look at all of the returns (grilles) and check their size. Make sure that none are blocked. Sometimes, they will also install filters at large returns. Maybe there is a clogged filter in the system that you never knew about?

- get a tech to check the blower speed, static pressure, etc. This will tell you if the air handler is setup properly and if the ducts are sized correctly.

09-24-2010, 03:32 PM
the blower has 3 different speeds but it is set on high

09-24-2010, 03:35 PM
the guys who installed it was my brothers friend and his dad, which the dad has been doing for like 20 years. he basically just told us what we needed and no calculations were ever made. I know now these guys dont have a clue what their doing especially since they didnt changed any ductwork.

09-25-2010, 09:43 AM
Pull the blower panel off & run the blower see if that makes a difference most of the time i find the returns are undersized that will tell you if it's supply or return problem unless you can get somebody with a flow hood to actually test how much air is moving.

09-27-2010, 04:17 PM
I removed the cover from the blower and it doesnt seem that anymore air pressure is at any vents

09-27-2010, 05:00 PM
That says the problem is in the supply an 8x20 duct at 1600 cfm for 4 tons of cooling is .25 static which is o.k but leads me to believe it's the taps off of the supply. how many runs come off & what size are they?

09-27-2010, 06:24 PM
there is 25 total runs and there all 6"

09-27-2010, 07:02 PM
Based on what you said 1600 CFM and 25 six inch vents the six inch ducts are fine.
(~64CFM thru a 6" duct is sufficient, assuming you unit is OK, it is your supply and return size.)
Note: the mention of .25 SP is .25" per hundred feet of 20"x8" straight duct. This does not count the grill pressure drop, filter loss, elbows,transitions, 6" duct, plus the similar losses for the return system. I doubt if your unit puts out 1600CFM at a .75" external SP but something less.

09-27-2010, 08:19 PM
how exactly would I know what cfm my furnace puts out, the guy who installed it said it should be 2000cfm, but Im not sure and like I said before these guys didnt know what they were doing as I know now. but as far as for the sp how would I know what my system is at. is there a meter that would test that. Im not sure if the meter I plan to buy will tell me that but Im going to get a meter that tells me how much cfm each vent is putting out. thanks again

09-28-2010, 04:02 AM
Try searching for "measuring external static pressure" on the web. One page that gives you an idea is http://contractingbusiness.com/enewsletters/cb_imp_70061/

To give you an example for air volume of air look at the following: http://eetd.lbl.gov/ie/pdf/LBNL-51550.pdf#search=%22%22garbage%20bag%22%20%22air%2 0flow%22%20%22homeowner%22%22

The contractor should have paperwork similar to that for the Trane equipment below, giving you an idea of what your unit is capable of. As an example look on pg 8 of this web page for a Trane unit http://www.trane.com/CPS/Uploads/UserFiles/DXUnitarySystems/Split%20Systems/Small%20Splits/22-1774-17_10012009.pdf (static pressure vs cfm, note the heating section if external to the AHU also has a pressure drop assciated with it that has to be overcome by the AHU).

09-30-2010, 04:28 PM
so what size duct would you reccomend for the supply and the return? or do you need a drawing of what my setup looks like

09-30-2010, 06:20 PM
Need a plan view rough sketch of the trunk lines and the approximate location of each of the 6" duct in reference to each other and approximate length in feet between the 6" ducts totaling the 48' of trunk line. (Some of the 6" ducts may be acrossed from each other or off set.)
You still have my email if it would be easier. I can accept pdf older versions of autocad or smartsketch.

09-30-2010, 08:10 PM
I emailed the sketch to you and I will also post it here, it might be too big a file to upload but I'll try. thanks again!!

10-02-2010, 06:29 AM
25 supplys at 6" apiece adds up to (28" x 25= 700 inches of air. But the main is only 8x20= 160 inches. That means the MAIN DUCT is much TOO SMALL to handle this amount of supply takeoffs.
You need a pro to come in and size the supply AND return properly.

10-02-2010, 08:01 PM
25 supplys at 6" apiece adds up to (28" x 25= 700 inches of air. But the main is only 8x20= 160 inches. That means the MAIN DUCT is much TOO SMALL to handle this amount of supply takeoffs.
You need a pro to come in and size the supply AND return properly.

I have already had 4 supposed pro's out here, I cant afford anymore pro service calls!! That's why Im here. Do you think if I had the money I would come to a forum for help?? Everyone I had out here said everything was fine, what a waste of my money.

10-02-2010, 08:04 PM
it seems to me that gator37 is the only one that has truly given me any real answers, and didnt give me the run around like all the other service pro's I had out here. So thanks again gator37, I really appreciate it

10-03-2010, 07:20 AM
Yes ,Gator37 has given good advice. it's the same advice given by me and some others .
Your MAIN TRUNK duct is either not large enough to supply those 25 6" takeoffs.
Or it also could be that those 6" takeoffs are not required. Pehaps 4" or 5" would be sufficient to supply these rooms.. i have been in Hvac for almost 50 yearsand I know what i'm talking about.
Get a real pro to come in and look it over. this time call in an hvac rep, not any old plumber/carpenter or butcher, but a real live hvac (heating/ cooling)pro, not a "supposed" pro.

10-03-2010, 03:33 PM
Again, if I could find a real pro I wouldnt be here. The first one was the guy who put the system in, and the other 3 I found out of the phone book, all local guys too. You act like Im looking through the newspaper or I'm just calling anyone who thinks they can do hvac work, Im calling straight out of the phonebook. Isnt that people normally do, use the phone book?? I got more info from emailing gator37 than here is what I was trying to say. I know the 6" ducts arent the problem because I have shorter runs that are also 6" and they still dont get enough pressure, so it has to be the supply duct and or the return duct