New HVAC Woes (Rumbling Noises)

[Aikouka]

I recently had my AC replaced among a few other changes such as laying down a new vapor barrier in the crawlspace. (They didn't want to call it encapsulation, but it pretty much is.) The problem is that ever since they installed the new unit, you can hear a very low frequency tone (i.e. humming with some wub-wub) when you're in a room that's close to the unit itself (i.e. the dining room) and even by some vents that are further away. It's also possible to hear it on the other side of the house where the HVAC line comes in from the crawlspace and enters the two-story portion of the house. If you close the closet door (the HVAC line is right above the closet), you can hear the humming, and if you open it, the humming goes away.

Now, of course, I brought this up to the installers and flat out said that I didn't like it and the previous unit didn't have that problem. At first, they tried to pass it off as normal ("It's close to the unit"), but I refused to let that slide. They sent out their head of installations, and whew... this guy was a piece of work. At first, he called me up and asked when he could check out the items while commenting that I'm never around. This really irked me given that I've taken off days of work to be there for them, and then I watch as this "head of installations" proceeds to tell me how big vents are by measuring the register vent cover. "Well, this looks like a 14x6!" "No, that's a 4x10...." His guess was that the removal of the insulation from the crawlspace combined with the fact that the previous AC was installed by knocking a large, square hole for the ducting rather than holes specifically for the ducting was causing the reverberation.

So, they went through and put up foam board to cover up the area between the crawlspace and the unit, but as evident by me writing this, that didn't solve anything. I told them that my thoughts was that this was a reverberation being sent through the HVAC ducting. The installers told me that they came off the unit with a 16" duct and connected it about a foot later to my 14" ducting. This raised a bit of a red flag to me, because only a few minutes prior, the installers said you typically step down a duct size (e.g. 16" -> 14") after about 10 feet. Wouldn't this immediate reduction in flow create a backdraft, which would result in turbulence and potentially reduced efficiency? I mean... this is stuff that we deal with when working on computer airflow. My thought was that the turbulence could be causing the reverberations, which is what I'm hearing.

All in all, I'm just wondering if anyone has any thoughts or has seen this sort of thing before. I'd like to gather some more information before I complain to the HVAC company again. ...and before anyone asks, this company seemed rather reputable given their ratings, and their salesman was rather thorough, available, and worked with me a lot to get a good deal. However, once I'm all done, they're going to get a rather scathing review from me. I mean... I requested that my two-stage unit be setup properly with the thermostat (i.e. 7-wire instead of 5-wire), and what did they do? They used wire nuts to tie W1 to W2 and Y1 to Y2. :| "This is what we usually do." :| :|

 

[mindless1]

Doubtful it's turbulence. You wrote you had the AC replaced. What exactly was replaced?

Compressor unit outside and the coil inside? Was there an existing furnace/stack that the new coil went into, or at least stack if you don't have a furnace attached? Newer coils are designed for higher efficiency and can be larger than those of yesteryear to achieve that.

If your old fit in a 14" riser column but the new needs 16", this is not a problem per se, there might be a little turbulence dropping those two inches but that won't create enough noise to travel anywhere, is not as loud as the blower itself. If they did a poor job installing it, that could vibrate, but I'd think that would be obvious with you standing in front of it observing what is happening.

The efficiency difference is not much, remembering that you had used 14" previously and that's still present. The far larger source of flow inefficiency is the length of the run and the bends in it, especially the vertical changes going into or out of walls.

What seems most likely is that it is the compressor itself vibrating. That is normal, but it should be mounted on a base that absorbs most of that, but if it's set on poles coming out of a wall then more vibration is transmitted vs sitting on a detached slab, and it may also vibrate the refrigerant lines going in from outside.

Check those lines, they could be poorly isolated and vibrating against the wall, and yet you don't want them completely immobile as that puts a strain on them while the compressor vibrates, just not rubbing against a rigid portion of the wall that transmits vibration.

 

[Aikouka]

Doubtful it's turbulence. You wrote you had the AC replaced. What exactly was replaced?

I have a gas package, so I had the entire thing replaced. It seems a good bit of the rest of your post is in reference to a split unit, so I'm not 100% sure if it would be the same? I can tell you that my unit is sitting on what looks like metal rails on top of a concrete slab. The slab isn't new as it's the same thing that my previous unit was on.

Here are a couple of photos of the unit. Apologies since I had to take them just now in the dark, so they may not be as clear as desired:

Top/Front
Front
Corner
Back Corner w/ Flashing

If you'd like me to take photos of anything else, just let me know, and I'll go take some. I can tell you that they didn't replace the trunk, but they did replace the entire return. They did add one new vent, moved the vent in the kitchen (new flex line to it), and reopened a vent in another room (the previous owner covered it with flooring). They also added a new return downstairs (accessed via the crawlspace). I would've preferred it upstairs, but it'd be a huge amount of work to get one upstairs.

 

[mindless1]

I have not worked on this type, but the general principle would be to isolate the source of the noise. What you described as hum with wub-wub is usually the compressor itself. I would imagine that with the duct leading straight off, there should be some kind of partition to isolate that noise some, and is that duct insulated well in the inside? A heavy weight insulation will deaden sound transmission through it.

Those rails it sits on, they are just separate, on the concrete slab also separate, not set in the brick wall, and the slab is not on contact with the wall, correct? That is what I was referring to in order to keep vibrations lower.

Ultimately since it's a new install they're going to need to make any changes. DIY and they might have an excuse to void the warranty. What you can DIY is when it's making noise, put pressure on the ductwork with your had and see if that quiets it down. Could be that introducing slight bends in the duct (kicking or punching it - I am not kidding but don't go overboard) could resolve that. Sort of sideways twisting it so the seams slide some then taping it together with metal foil tape in that position with slight tension can also alleviate some duct noises.

Beyond that I can't give much advice remotely.

 

[monkeydelmagico]

Does closing off some registers change the sound?

 

[Aikouka]

I have not worked on this type, but the general principle would be to isolate the source of the noise. What you described as hum with wub-wub is usually the compressor itself. I would imagine that with the duct leading straight off, there should be some kind of partition to isolate that noise some, and is that duct insulated well in the inside? A heavy weight insulation will deaden sound transmission through it.

The ducting is fully wrapped with insulation, but I'm not sure if it would count as heavy weight or not. There definitely isn't any sort of partition though. The trunk is just a straight shot all the way to the end. A different HVAC company suggested putting in a slight curve before the trunk gets to the end wall. I think the guy compared it to filling a balloon. Not sure if that would be useful though.

If it's worth noting, I do have a bypass HEPA filter on the return line.

Those rails it sits on, they are just separate, on the concrete slab also separate, not set in the brick wall, and the slab is not on contact with the wall, correct? That is what I was referring to in order to keep vibrations lower.

I believe the rails are attached to the unit itself, but I'm not 100% sure. The unit is definitely on them, but whether there are bolts or anything attaching the two is unknown to me. The slab is from the original unit, so it's definitely separate from the unit and the rails. I'm not sure whether the slab comes in contact with the brick wall, but I can check later.

Ultimately since it's a new install they're going to need to make any changes.

Ugh... I'm wondering if they'll even do anything. Honestly, they seemed to just not care about the problem. When I pointed out that it didn't go away, I got a response along the lines of, "Uh... let's just see if it gets any better." I also need to get on them, because my contract calls for a vent in the crawlspace to keep it conditioned. I mentioned that it hadn't been done, and the response was, "Uh... we don't really do that."

Does closing off some registers change the sound?

I'm not sure, but I can give that a try tonight.

 

[Micrornd]

That looks like a "packaged unit" (complete AC unit with only AC air duct out and return air duct in)

You didn't say how they made the transition from the unit to your duct work, but it should have been made with a flex connection, otherwise you will hear the "drumming" of the packaged unit in the metal ductwork.

Even if that was done, you may still be hearing "drumming" through the ductwork, if the packaged unit is "drumming" excessively.

You should really only hear the sound of the condenser fan when standing next to the unit outside, anything else is excessive and needs to be checked.

BTW - it is rather strange to use a "packaged" unit on a house, they are generally used for manufactured homes and mobile units.

 

[KK]

Down at our farm, it has a lennox AIO unit...Biggest POS,but thats another story. We had to replace the squirrel wheel blower on it. Actually the motor went out on it, and then in my replacing the motor, I didn't tighten the set screw tight enough and I fucked it up the squirrel wheel. Maybe check that.

 

[Aikouka]

That looks like a "packaged unit" (complete AC unit with only AC air duct out and return air duct in)

...

BTW - it is rather strange to use a "packaged" unit on a house, they are generally used for manufactured homes and mobile units.

Yes, it's a gas package unit. They're actually not all that uncommon around here, but I think you tend to find them on older homes. I did have the option to go with a split system, but the cost increase was about 40% for the same efficiency rating.

You didn't say how they made the transition from the unit to your duct work, but it should have been made with a flex connection, otherwise you will hear the "drumming" of the packaged unit in the metal ductwork.

I'm not 100% sure what they did, but I took some pictures.

Wide View
Close Up

It looks like there's a zip-tie in that close-up photo. It makes me wonder if there's a sort of "soft connection" between a flex tubing and the rigid tubing that I have for my main trunk line.

Even if that was done, you may still be hearing "drumming" through the ductwork, if the packaged unit is "drumming" excessively.

You should really only hear the sound of the condenser fan when standing next to the unit outside, anything else is excessive and needs to be checked.

Maybe it could be the unit itself? Like you say, when dealing with an HVAC unit, I'm used to just hearing the fan noise, but when I was outside yesterday, I could hear the wub-wub coming from the unit from the other side of the fence. I might see if I can record the audio.

Down at our farm, it has a lennox AIO unit...Biggest POS,but thats another story. We had to replace the squirrel wheel blower on it. Actually the motor went out on it, and then in my replacing the motor, I didn't tighten the set screw tight enough and I fucked it up the squirrel wheel. Maybe check that.

I'd probably have the company do that since the unit should be in good, working order as it's brand new. At least I would hope it is.

 

[Scarpozzi]

I'd start by setting a glass of water on top of the unit outside and see how bad it vibrates when it's running.

I've only had Split Systems and currently have a Dual-fuel Heat Pump System...
What you have is what's called a Packaged HVAC System (That'll help you find answers via Google). What I see in the picture is the square rail the system appears to be resting on...I would expect since it's just sitting on the concrete pad that it could be vibrating and since there's nothing between the metal and concrete to dampen the vibration, it's going down the ductwork in the form of noise. If you could slide a thin piece of cardboard under that rail on a dry day, you might be able to prove whether or not you should look into some kind of rubber feet or pads to go under the unit.

When I had my latest split system installed, they actually brought a pre-poured concrete pad and set it on top of the old pad to start with a good surface....it appears they skipped that step on your installation. The glass of water on top will also help identify how bad the case may be vibrating...otherwise, you could be right...it might just be the sound of the air handler causing more noise. Just remember all of those units make different levels of noise and that can vary significantly due to a lot of outside factors. Start with the outside installation and then start looking internally to make sure the fan isn't having problems.