Are my ducts noisy because of heat or too much air?

[Dom]

I posted before about my noisy ducts -- crash, bang, pow, all night. I'm about to call a home improvement guy and have him remove the dry wall on the ceiling, replace or tighten the duct, then repair the dry wall. I don't think it will be a big job, but someone told me that the problem might be too much air going through the ducts because of the new more powerful furnace (two stage, variable speed, 95K BTU). Can someone advise me on this?

I think it is the heat because: 1) It happens at only two ducts, 2) I get a noise when it cools down and sometimes long after it cools down.

More importantly, I got the furnace from a VERY reputable firm, and I don't think they would make a mistake like that, and I got two other quotes from other reputable firms, all of whom were ready to sell the same furnace. Surely, they couldn't all make the same mistake.

So, any advice?

[bob haller]

call the company that installed your new furnace and tell them of the issues, there may be a simple solution...

[Dom]

Oh, i did that long ago. They say it is just oil-canning, and not their problem. But what they don't know is that in cold weather it is intolerable. And whether it is oil-canning or not, I want it fixed.

[chaniarts]

if it's oilcanning, it could be caused by simply expansion and contraction. tightening them could even make it worse. you want a bit of looseness to allow the expansion to not force the ducts to bend. however, that makes them leak air, which also isn't good.

you'd need to go to flex ducts to get rid of all the noise i would expect.

[EXT]

Where are you located, a 95,000 BTU input gas furnace is huge unless your house has no insulation, is also huge, or way up north. I have a large house in Canada, and my furnace is 2/3 the size of yours.

You may have too much pressure, but also why is your furnace going into the second stage all the time, this is normally used when it is very cold outside and stage one cannot keep up.

With the blower on full and the heat on second stage, your old air ducts may not be able to handle the extremes and be oil-canning. How old are your ducts, old ones were made of heavier metal, and should be able to handle the new equipment. You don't want a "handyman" messing with your ducts, he may be good to drop the drywall but get a good sheet metal duct installer to fix/replace your problem.

[Dom]

Good point. I'll be sure to tell the home improvement guy about the flexible ducts. Do you think I'm safe in saying it is not just the amount of air flow?

[chaniarts]

that would be pretty far down on my list of possibilities. you'd have to have one heck of a blower to bend the metal. the major cause of this is heat/cool cycles causing metal expansion and flexing.

flex ducts aren't as efficient as smooth metal ducts (they're rougher inside), so you'll get lower velocities i would expect.

[TomR]

I agree.

To the OP (Dom), how closely are you able to pinpoint where the popping sounds are coming from? Are you able to tell which duct, what part of the duct, etc. And, if so, is that duct accessible or maybe right behind a certain section of sheetrock? If so, I wonder if you could just cut a small hole in the sheetrock where the noise is located, then put something in between the sheetrock and the duct to see if that fixes or changes the noise problem.

[Paul Drahn]

Can you gain access to either of the two ducts? If so, see if they have the crease that goes from one corner of the duct to the opposite corner. These are there to strengthen the sheet metal and prevent what you are hearing.

Paul

[Dom]

Thanks for the info. You really put my ind at ease. I had heard that flex ducts weren't efficient, but it is only one or two ducts that I want changed, so I think I'll go with the flex ducts less the home improvement guy recommends otherwise.

One more question. Do you think it is a good idea to go through a home improvement guy? I assume he build additions, so he must have good subcontractors for duct work. Good idea?

[Dom]

Yes, I can tell the duct and, unless there is a loud echo, I can tell the part of the duct. It is within a foot or so of the vent. In fact I took real heavy tape, and just taped the sides of the duct to the "boot" that ends in the vent. I did that on all four sides, and most of the noise ended. I'd still replace the last part of the duct just to get rid of the last remaining boom!

[Dom]

I can't acces it except through the vent. I need to remove the sheet rock, and I'd rather go through a home improvement guy for that.

[trader4]

95,000 isn't so huge.. They make them up to 120K, which is what I have here in NJ in a 3100 sq ft house. The one it replaced was 150K. A 100K would be OK too, but 120 warms the house up faster if you have it set back and come home early, which is nice.

Did he say it's going to 2nd stage all the time? Typically most two stage are going to go to 2nd stage if it's recovering or you move the themostat higher. If it's just maintaining, then first stage is usually enough.

Even on full, the blower in heat mode is moving only about 60% of the air that the same blower would move in AC. I asked if he had AC and what happens with that on or just fan only weeks ago, but I don't think it was answered. That would go a long way toward figuring out if it;s a temp expansion noise or the duct flexing from air pressure.

How old are your

A new furnace isn't putting any unusual demands on the ducts. The air moved is about the same as a 30 year old furnace.

You don't want a "handyman" messing with your ducts, he may

I'd say if you can see it flex and figure out where to brace it so it doesn't move, no reason anyone can't do it.

[trader4]

Just make sure you have everything in writing, including the cost and don't get hosed by a home improvement guy.

[Dom]

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Trader4, thanks for the info. You actually remembered me from my last post? Wow. You're right on a few points -- no, 95k is not too large at all, others around here (I'm in central jersey) have more, but I did some research and I figured for my size home anything larger would cycle too frequently. And no, I did not say I go into second stage. Usually it is only the first stage, and I have it set so it stays there for 12 (max) minutes.

I never did the fan only test because my thermostat does not have that setting. Don't know why. I'm going to assume it is temp, and not air pressure, because, like I said, the hvac company is very reputable, and three other reputable companies tried to sell the same system.

[Dom]

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Why did I say central jersey? I'm in south jersey.

[trader4]

e quoted text -

That's where I am too. As I said, one advantage to being a little bigger is that you can heat the house up faster. I can go up about 6 deg an hour. That way if you don't always come home at the same time, you can have it set back and recover faster.

have more, but I

Where is that set? Sounds like at the furnace and you don't have a two stage thermostat? Ideally the thermostat should make the selection because it knows what the temp is and where it needs to go.

Well, it must be an odd thermostat, because every one I've ever seen had it. What about AC? What does it do with AC on, or haven't you had a day where you could try it? On heat the blower should move around 900 -1200 cfm. On fan only typical would be maybe 1400. On AC, 1800-2000.

 I'm going to assume it is temp, and not air

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I don't see how the reputation of the HVAC company is a factor. The blowers are what the blowers are. It's not like Rheem is 2000 cfm and Trane is 800. For the same size furnace they are about the same. And going from say a 75K furnace to 110K, doesn't change it all that much either. It is possible that they moved something slightly, or didn't put screws, hangers back where they should be. But I assume the noise isn't near the furnace, right?

You should have a pretty good idea if it's temp or air pressure. You can hear the blower ramp up. If the noise occurs in the first 30 secs or so, it;s likely air pressure, because it's going to take longer than that to heat it up. Also, the bangs I've heard from air pressure occur both when it starts up and when it shuts off. Does it bang right when the blower stops?

[woodchucker]

I don't think that is a good idea. If this is a rectangular duct, then flexible ducts will restrict the air flow... Sounds like you need the insulated duct that I have. It is not metal, and it also keeps the heat in the duct. Mine are fiber insulated with metalic foil. They are very easy to make and I am sure any reputable HVAC firm will be glad to do it, if you are opening the wall or cieling.

[Dom]

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e quoted text -

Lots of good points, trader4. Btw, I'm from around cherry hill. Now, your points.

The time between the first and second stage is set at the dip switches in the furnace, not the thermostat. I do have a setting for fan only, but nothing turns on when I set it. Time to read the manual, I guess. I didn't start the AC yet, too cold for that. I assumed a good hvac company would not sell me something if they knew the ducts couldn't handle it. No, the noise is not at the furnace, it is at one of the ducts that is farthest from the furnace, and another that is about half that distance from the furnace. No noise at the closest ducts. I assume pressure would get those first. The first bang is well after the start of the ramp up. And I get several bangs long after it stops, never immediately after it stops. Remember too that I was able to get rid of the start up bangs by using heavy duty tape to hold the duct open so to speak.

Thanks for all your help, btw.

[Dom]

Jeff, I'll keep that in mind when I talk to the home improvement guy. Thanks