Air handler / duct work problem.

[Mikebarone]

I am working at a client’s house where I think I have discovered that the return air duct, (on her five ton split system) is too small for the size of the air handler itself. I also have a good friend that is a licensed HVAC contractor and he agrees that it wasn’t sized sufficiently for the unit. I am going to add another air return to the air handler and see if that is the over-all fix.
I noticed on the tag, to the air handler, that it has been static tested at the factory, at 0.5 inch water column. With the amount of suction the return air duct has now, I know that it would suck the water right out of my water column tester!
My question is…in a perfect world…if the correct duct size was installed, (both in coming return air and out going cool air) what kind of pressure should it have in the air handler in the intake and out going side of the blower motor. Could this pressure be measured in water column inches, as compared to the static test performed at the factory?
I'm also wondering, (before I add this extra return) how much pressure should be at the air handler, so I can size the new return correctly.

Thanks much,

Mike

PS. Any other thoughts on this would be greatly appreciated!

 

[Jadnashua]

Do they still have the installation instructions or can you download them? They often list the min/max values. The power consumption, noise, wear, and performance will be degraded if it is not right.

 

[Sponsor]

 

[Mikebarone]

My client may still have the manuals on the unit; she keeps all of that stuff. She really has two units, (out of the three on her house) that are doing the same thing. The first problem she had on one of the units was that the platform that the air handler sits on, (that is also part of the air return duct work) was not sealed inside of the platform, and it was sucking attic air from the wall next to it. My A/C buddy knew something was wrong when there was only a 7 degree split for the return air, to the out going air. When I sealed up the air from the attic, I then turned back on the unit, and I could hear the ridged return air ducting, besides the air handler, being sucked into itself. That’s when I realized that it wasn’t getting enough air. All of the air filters have been changed and that didn’t help the problem at all.

 

[Jadnashua]

It sounds like the return is quite undersized. The installation manuals typically give some quidance on sizing.

 

[Mikebarone]

Thanks

I will see if she still has the manuals on her A/C unites. I just though there may be a standard rule of thumb on the pressure on either side of the blower…like negative 3 pounds on the return side and a positive 3 pounds on the delivery side. I’ll post it if I find something in those manuals.
I’m about to do my third blower door test, (that I do in reverse…blowing in) and then using smoke bombs, I track down leaks in the duct work. I just got done testing my dads double wide modular home, and he is loosing about 200 feet per minute through a 12†round duct. (I haven’t converted it to cubic feet yet). Modular homes are tough, because the duct work is in the belly of the home, and covered up with a canvas. I haven’t used the smoke bombs at his house yet, (the supplier I use is out of them).

Thanks for the good advice,

Mike

 

[Mikebarone]

Well I think I found the answer to my question.

Well I think I found the answer to my question.

Well I did find a Blower chart that lists different Carrier air handler models. What I did notice on the chart is that the water column pressure starts at 0.10 and goes up to 0.5, (This now make sense that the air handler itself is static tested at 0.5 in W. C.). I also noticed, (and it only makes sense) that the higher the water column, the lower the CFM. It seems a little strange to me why the factory wouldn’t equip the air handler unit with a attachment, (like a valve stem or something similar) that a water column test could be done by the installer, to make sure that the air flow going through the unit is within specifications. They also stated, (not shown) that the CFM’s should be between 350 and 450 per ton.
I’m going to install a tap into the unit and see what W.C. the unit is running at now, and then another result after I add the 20â€x 25†air return, (with a 14†flex duct).

--------------------------------------------------------------
Blower Performance Chart
Air Handler CFM @ ESP. – in. W. C.
Model Blower Speed

in W.C. .10 .20 .30 .40 .50

18 Low (Red) 754 716 674 626 569
High* (Black) 1020 958 886 800 689

24 Low (Red) 998 938 868 786 681
High* (Black) 1042 978 905 817 704

30 Low (Red) 1146 1091 1029 959 877
High* (Black) 1349 1289 1224 1150 1066

36 Low (Red) 1407 1341 1268 1186 1090
High* (Black) 1561 1481 1392 1290 1169

42 Low (Red) 1785 1705 1616 1516 1402
High* (Black) 1938 1846 1744 1629 1495

48 Low (Red) 1870 1789 1699 1599 1484
High* (Black) 1943 1858 1765 1660 1540

60 Low (Red) 2077 1986 1886 1774 1646
High* (Black) 2161 2066 1962 1845 1710

 

[Mikebarone]

If anyone’s interested.

I did perform a water column test at the base of the air handler, (return side) yesterday and I came up with a 0.80 water column. According to that chart I found, a 0.5 w.c. is the maximum they even list, and even at that 0.5 w.c. a lot of volume is lost. I haven’t hooked up the extra supply return line yet, to see how much it improves the w.c. test.

Mike

 

[NHmaster]

Here's my rule of thumb. With the unit running, remove the door to the blower compartment. If it sucks back in and almost takes your fingers off you need more return air.

 

[Mikebarone]

Thanks

Good point! I did that with the air filter, but doing it there at the air handler door is a no brainer huh!

 

[Mikebarone]

Hot Damn!

Hot Damn! I added that 14” flex duct return to the bottom platform of the air return chase. The water column test I did prior, (on the suction side of the air handler) went from 0.80 down to around 0.2 to 0.3. Not only that but one of the cold air delivery vents, (the only one I tested on) went from 550 feet per minute to 660 feet per minute. The temperature difference, (between the air going into the coils, and what’s coming out) went from 18 degree split to 20 degree split. And best of all, there is not more sounds of duct work caving into itself when the unit comes on.

Just had to tell you guys,

Mike