Temporarily increasing return air, or will I regret this?

I was having dust problems. HVAC guy said I had high static pressure & suggested adding a bigger return air duct. It does sound like an airplane is in the basement near the furnace when the furnace is on.

Please let me know if this is a bad temporary idea that I may regret:

Can I cut a hole in the return air box, connect a flex duct that just runs the length of the basement (to get it away from the water heater so it doesn't backdraft) & just let it sit on the floor? I thought my basement wasn't heated but I discovered there is a supply but I didn't find any returns in the basement.

I figure if it doesn't work, I'd just take the flex duct out & cover the hole with some sheet metal like nothing ever happened, but I'm not an HVAC pro.

I know it'll be ugly, but a bigger return air trunk means taking down drywall, increasing the trunk soffit framing & all that other construction stuff. I want to verify this temporary solution works before doing the more expensive fix (which I'm incapable of doing myself). And no, I don't plan to make the temporary solution the permanent solution!

Thank you for your input!

 

A quick way to determine if return ducts are inadequate would be to ensure all doors are open to the space where the furnace room is located and then carefully remove the blower door.
This will give you as much return as possible and test his theory.
Another rough indicator of inadequate return duct work is how much suction there is pulling at the motor compartment door.
IOW if you try to open the motor compartment door and there is a lot of
suction pulling the door closed he may be right.

Are you sure that when he said "high static pressure" he was not maybe referring to the supply side?

 

Thanks for that quick test. Here are the results:

1) Turning the fan on, trying to open the blower door: yes, the blower door pulls pretty tight. The "airplane" sound does get lower in pitch as I open the door more, but the fan eventually cuts off (I'm guessing a protection mechanism).

2) Did the same thing, but without the filter (16x20x1) installed. The filter is installed on the side of the furnace in about an opening the same size of the filter. The blower door does not pull tight at all when I try to open it and I don't hear the high pitch wind noise.

Filter I use is a "Rheem 16 in. x 20 in. x 1 in. Basic Household Pleated FPR 4 Air Filter" from Home Depot, which I figured would be the least restrictive as compared to all those other high-MERV ones they have.

Is this just stating the obvious? Filter is too restrictive? For a couple bucks, I'm obviously gonna start with that, but how can I be sure if I do in-fact have a return air problem?

 

Do you have a door or removable panel over the filter slot? If yes, did you replace it when you tried without the filter or did you allow air to enter through the filter slot?

If you DID allow air to enter through the filter slot then do the test again using something to close that slot, a piece of still cardboard if there is no panel or door. Does the blower whine or is the blower compartment door hard to open when testing in this manner? If the test is the same as when the filter is installed then you have a too restrictive return duct system. If it ONLY does the whine and tight compartment door when the filter is installed then it is the filter itself.

Post a couple of pictures of the installation showing the filter slot. Not too close as I need to see if there is room to install a larger filter.

 

What size unit? Most 5 tons require return air from at least two sides

 

If you remove the filter and you loose the high static pressure then the filter is the problem and it would also mean there is possibly adequate return ducting.
Many older duct work set ups can be marginal when installed and are pushed over the edge by using any pleated filter.
Are you sure your filter is a Merv 4?

If you are ambitious you could make one of these, drill a small hole in the fan compartment and measure the actual return static pressure under different conditions.

I'm surprised your "HVAC guy" didn't do this.

 

Furd, I will have to try again and block the filter slot - I left it open. It's just a slot that I foil tape shut after putting the filter in it. Wouldn't installing a thicker filter increase the pressure even more?

GregH, I just looked that filter up @ home depot, it says FPR 4 which is the lowest one they have. But then looking at the fine print specs, it says MERV Rating 8. Since then I put in a true fiberglass filter which the fine print says is MERV Rating 4. The blower doesn't whine now, but the door still pulled a little bit. The company I called, I presume they heard the airplane in the basement in instantly knew what the problem was - I don't know enough to question an expert's opinion.

 

Right.
"Trust but verify"
The original set-up was probably with a fiberglass filter.

How I would proceed is to compare the "pull on the door" with the filter installed and removed.
If the "pull" is reduced greatly without a filter then airman's observation of perhaps needing an additional opening at the furnace for a second filter will be what you need to do to fix your problem.
IOW a second filter that is the same size will half the velocity moving through it and also decreasing the filter compartment resistance by half.

If you were to make yourself a manometer and take some measurements you would advance your troubleshooting to the level of an "advanced amateur"

 

Not necessarily. A four-inch thick filter is pleated so it has many times the surface area of a one-inch thick filter. Even with a filter with a higher MERV rating the increased surface area makes the pressure drop less.

Biggest downside is that you may not have room to fit the larger filter frame. Second biggest downside is that the filters themselves are significantly more expensive.

 

Sorry I missed airmain's post - the air conditioner is 2 tons. Is the filter opening not big enough for 2 tons? 16x20? There's room to increase the thickness of the filter slot to maybe 4 inches. The whole design of the return air into the furance is like a "U" with the filter being the bottom of the "U", the left leg the return air mainline and the right leg the furnace.

I'm curious why you would want to decrease the air velocity being pushed through the ducts even if that means the static pressure drops. When the fan is on, it is loud and we have to talk over ourselves a bit & turn the TV up, but doesn't that mean the heat delivery to the rooms is better? Would a slower fan actually mean better heat delivery?

Sorry for all the amateur questions, I just want to understand how it works before I have to spend thousands redoing ductwork!

 

This will require a long explanation and I am going to bed in a few minutes. Perhaps I can answer the questions tonight.

 

GregH, I took some measurements with a cheap $50 Pyle manometer. I only had one hose connected (though it has two ports). So my guess is that the measurements are with respect to the room pressure and can only be used in a relative-to-each-other sense.

Pressure before air filter: -0.34 inH2o
Pressure between furnace and A/C coil: +0.24 inH2o
Pressure above A/C coil: +0.14inH2o

Is the total pressure just "out" (above A/C coil) minus "in" (before air filter) = 0.14 - (-0.34) = 0.48 inH2o?

If correct, that number's not too bad, right?

 

You will get a bit more accuracy if you use both ports across a measuring point as opposed to separate readings.

You can measure before and after the filter to get the pressure drop of the filter.
Another useful measurement would be a differential measurement before the filter and on the supply plenum after the furnace.......Take one with and without the filter.

You would do well to get the specs for your furnace.
As far as I know most furnaces are normally equipped to work at about a maximum .50"wc total static pressure which would be the readings before and after the furnace.
.35" wc total would be a fairly unrestricted reading.

 

You would take a static pressure reading after the furnace and before the furnace.
Ignore the negative reading and add the two numbers together.....This is the total system static pressure reading.
If you open the blower door and the total reading drops significantly you have a return problem.
If you replace the blower door, remove the filter and your reading puts it near the ballpark, your return duct work cold still be inadequate and your installing a pleated filter made this apparent.

 

The hoses on the manometer are only 4 inches long so I'm trying to source some to take a dual/accurate measurement.

In the meantime, I found the furnace manual in our stack of manuals. I saw that you could adjust the speed of the fan depending on which wires were connected. Correct me if I'm wrong, but based on the chart, it looks like going from a high fan speed to a lower speed implicitly reduces static pressure?

My fan was set on the second lowest speed (for heat). I put it on the lowest. I made sure the temperature rise was still within the 65deg limit. It's about +55deg. With the higher fan speed, it was about +40deg. Pressure readings (relative to room) read lower than before. Though it's relative, the speed of air coming out the vents seems to be a little bit reduced, but not significantly reduced, but the noise is significantly reduced. Set-point temperature maintenance is a lot better in terms of comfort. Honestly with the faster fan, when the furnace would come on for 5 minutes every 10 minutes, just felt like it was blowing cold air.

Also, when I was messing with the wires on the blower, I saw that the filter was almost sucked into the blower.

So, since my original issue was concerns about the high amount of dust, I'm wondering if the real reason was just a fan that was set too high that caused too much pressure.

Once I get longer manometer hoses, I will collect some better numbers with the faster fan to still determine the return air / filter pressure drops. But I'm curious what you guys who actually know what you're doing think about my lowering of the fan speed.

Note: I think I figured out why my summer electric bill is so high - the fan speed is set to the highest for cooling mode (it's set for a 3.5 ton unit) and I've only got a 2.0 ton unit. This is a small house with 4"x8" ducts to the rooms. The furnace is 41 btus.