Did the old furnace cause the duct sheet metal to flex? If it did not then the problem is either the furnace is over sized or the duct work is under sized. If the old furnace kept you warm on the coldest of nights then the furnace is probably over sized. Do you know the sizes of the old and new furnaces. Also what make and model number is the new furnace?

There is also a possibility the blower motor speed can be adjusted so that is pushes less air through the duct work. But there is a limit how slow it can be set. He has probably tried that and finding out he can't compensate for the fact the furnace is too big.

Magic 8 ball says: hazy need more information.

Does the sound happen only when you run the furnace? Does the sound happen if you run the fan only? Does it happen when you run the AC?

What was replaced? Furnace only, (no duct work?)

Realize we are only playing guessing games here.

The banging noise comes right after the fan shuts off. The fan comes on when the heater comes on to heat the house. First the heater shuts off then shortly after the blower shuts off, then the banging noise.

There is no AC.

Everything but the ductwork in the actual walls was replaced. The old furnace wasn't even hooked up to the ductwork, there was about a 12" gap between the furnace and the ductwork above it. The old furnace technically seemed to work but the former tenant was using a pellet stove for heat. He said it was cheaper - which it very well may have been given the old heater was not properly connected to the ductwork. The old heater was 30 or 40 years old.

Its too dark to be able to read the label on the new furnace - the hall light is disconnected because I am painting. But when it gets lighter I'll read off the make and model and whatnot.

I have been concerned about this for awhile now because the guy didn't get the work permitted. My son had specifically asked about permits and the guy told him "all my work passes inspection". Which my son took as "yes, permits will be pulled" when in fact no permits were pulled and the work was never inspected.

Also, the venting that goes through the roof - the flue - is attached oddly and runs in front of the furnace before going up through the roof and I nearly burned myself on it because it is right in front there and low to the ground - within reach of children in fact.

When I asked about this after noticing how hot it gets and nearly burning myself, the guy pretty much treated me like I was an idiot for having even come close to touching it. When I pointed out the danger to children, he sneered about putting a door up there - there isn't one there due to the odd opening size, its about 48" wide by 67" tall so I have to have custom interior shutters installed because that is the only thing I've found to close up that opening that doesn't involve me building something. After I'm done painting I'll have that done.

The former tenant had this Rube Goldberg thing up there that basically consisted of a 2x4 frame attached to the opening with drywall screws, a piece of iron grating that was set up sort of as a "gate", and a piece of fiberboard that was screwed to the rest of the kludged 2x4 frame with (again) drywall screws. The building owner was up for renewal of the occupation certificate that allowed the home to be rented and it would never ever ever have passed the inspection. In fact I suspect money must have changed hands for it to ever have passed inspection the first time. That's an overall home inspection for purposes of establishing the residence is safe and meets code standards for rental, not just an inspection of the furnace. So instead of fixing everything that would have needed to be fixed, they sold the place.

We were aware of code violations and expected the workmen hired to pull permits properly. This guy definitely did not do that and my son did not realize he had been baffled by weasel words (in regard to permitting and inspection) until it was too late.

As of now, one of the workmen is supposed to come out yet again, this time to replace that flue with double-walled flue instead so as to reduce the danger of burn injuries - which IMNSHO should have been done in the first place given the placement of the flue. That's going to cost us nearly $200, for something we shouldn't have to correct. I'll tell him its banging again when he shows up here but I have little hope of remedy by now.

What you are describing used to happen with my old furnace. When the furnace turned on the main duct would flex outward but it did not seem to make any noticeable sound. But when the furnace blower stopped the sheet metal would flex back and make this loud bang. This no longer occurs with my new furnace since it is smaller and moves less air.

I suspect this is what is happening at your place. The fact that the furnace was not properly connected means the much much less air was passing through the duct work. Now that you have a better connection the duct work is flexing and making noises.

What you have described above is a very bad situation. Is this guy a licensed HVAC contractor? If not then that is why he did not apply for any permits. You need to have everything inspected for your safety. You probably need an electrical, gas, and fire permit. You can check yourself with the code official of your municipality as to what exactly is need.

Nov/Dec 2016 issue of This Old House has an article about noisy ducting. Look for any places where ducting is touching wood, the supply duct should have a flexible expansion joint, and the blower should be correctly sized.

We thought he was licensed. I wasn't too enthused about this guy but he had apparently been recommended by a friend of my son's so HE wasn't worried. He claims to be licensed, his website says he's licensed, and his Yelp page states that he is licensed.

I'm attaching some pictures of the current setup, which worried me as soon as I saw them cutting a hole in the side of the furnace for the air return which is mounted in the ceiling of the hallway. I have never seen this done before and do not understand why the cold air return wasn't mounted below the furnace itself, which I HAVE seen done before. And the furnace is clearly short enough to have fit on top that way.

I do not know how it should have been done properly, but I'm pretty sure the way it IS done is not proper and therefore most likely also unsafe.

It is a Bryant Model 310AAV/JAV

How it is hooked up:

Label from interior:

The manual which you can see next to the furnace on the floor there states that it should be "affixed" to a surface near the furnace. It looks like it has gotten wet and then was walked on, which is sort of minor except in the face of everything else that seems to be wrong. When the fan comes on it is VERY loud. I have to turn up the TV when it comes on. I don't know how to get at the fan to tell what size it is or whether there is one fan or 2 - one the normal blower motor and the other in the ceiling to suck in "return" air. I have checked the filter and while I wouldn't say it looks pristine after 2 or 3 months of use, it surely doesn't look anything like needing replacement. Given that its 8' something from the floor, how much crud would it really be likely to suck in up there anyway?

I had hoped to put the water heater in the space now being taken up by the air return venting, but the guy had not told us he intended to take up the entire closet space that way so the water heater remains where it ought not to be, eg in the garage. The old furnace did not take up all that space.

I will note that his yelp reviews (there are 34) are quite high. But the fact that he did not pull any permits, even in the face of my son's question about that, and the Rube-Goldbergian nature of the installation, has me worried.

Get a second opinion on the situation and to double check things. Also, considering that you have to turn up the TV now and you didnt before then the fan speed is too high or unit oversized. He may or may not have changed the fan speed yet.

How many sq ft is your house and what state? Here is a general guide for calculating furnace size. Does not take all factors into consideration, just a general guide.

http://www.acdirect.com/gas-heat-learning-center-furnace-sizing-calculator

Return into the side of the unit is normal. What you ask to use that space for a water heater is not feasible because the ductwork has to be large enough to accommodate proper return CFMs. Approx 200 sq in per ton or 200 sq in per 400 CFM.

Technical info on your unit.

http://dms.hvacpartners.com/docs/1009/public/0a/pds310a.45.3.pdf

Factory setting for your unit is 1375 CFM. More than likely can be lowered as long as temperature change (delta T) isnt over 70. If we think 400 CFM per ton that is around 3.5 ton. Would need a decent sized house for that.

Nevada, NW of Reno, and something between 1350 and 1400 sq ft. - Checked and it is 1394 sq ft according to the realtor listing. It is a single story ranch on a slab.

I am at something between 5000 and 5200 feet in altitude.

I looked the furnace up on the Bryant website and it is 80% efficiency but I don't know which of the listed BTUs on the label should be used to determine the proper size of the furnace. The ceilings are insulated but the walls have little or no insulation to my knowledge.

The calculator to which I was referred gives a result of approx 70k BTU.

So if the highest listed BTUs on the label (88k) is the correct one to use for the size calculation, that results is 80% of 88k = 70,400 BTUs - so it would seem the furnace is properly sized.

If that is NOT the correct value for BTUs, all the others listed on the label are less than that and the furnace must then be too small.

We do normally get a lot of solar exposure here which may or may not enter into things. I have never set the heat higher than 68F and it is usually set to 60F at night and whenever I am gone. I'm on a fixed income so I watch those bills like a hawk and just wear more clothes.

I did find a registry to look the business up as far as licensure, and it states he is licensed and the license is active.

Keep in mind there is no AC so until it got quite cold here the furnace - or more correctly the blower - was not being used. I turned it on sometime in late Oct or early Nov. So before that there was no fan noise because the unit was not being used at all.

The furnace is centrally located so that duct runs are shorter than usual. These houses were military housing for Air Force Brass and whatever else you could say about them, the floor plan and layout are quite efficient.

@Vith - the problem is that the furnace could have been vented from below or at the very least through the closet to the right (as you face the furnace) with the air return on the wall of the hallway. Thus no need for taking up the totality of the space with the return ducting.

Or are you saying that furnace should ONLY be vented through the side? That there was no other venting option? The document to which you referred me seems to indicate otherwise. Unless I misunderstand, it seems that it COULD have been vented from below. There may be other reasons why the water heater couldn't go in there - but it doesn't seem like running the return air the way they did is one of them. Am I wrong about that?

I will again note that his Yelp reviews are overwhelmingly positive - there are actually 59 of them including the "not recommended" reviews, a category that has absolutely no value on Yelp. I have never seen a fishy review in that category and Yelp's determination of what constitutes "not recommended" is entirely arbitrary.

In any case, there are only 3 or 4 reviews of less than 4 stars and those seem to mostly center around poor communications rather than actual poor service.

So possibly I am being paranoid, but considering the lack of permitting and inspection despite our having asked for it, and his snotty reaction when I called to report the hazard of that flue being so close to the front and so low (well within the reach of children), AND my feeling that the install was unnecessarily complicated, perhaps I have reason to be concerned.

Hopefully my fears will be unfounded, but in the meantime - I have a furnace that bangs every time it shuts off and that you can't hear the TV over when it comes on.

If your bold enough to take off the panel cover and take a picture of the control board, we might have an idea on the fan speed setting.

It is possible to have the air come in from the bottom, but its a more complicated install. Which will cost more to do. And he might not be able to use the hallway on the other side of the unit if the air return is only located directly above where it currently is. Depends on where the main return trunk is and how it runs. If it is above where you wanted the return to drop then it would be do-able. I am guessing you would want to box-in the return in the hallway at some time in the future. Would need an access to change out the furnace filter.

Now that I look at it, I dont see a furnace filter on the return in your pics. Unless it is above.

Well I DID take off the outer panel - but I presume you are talking about some kind of inner panel? I would do it if I knew exactly what needed to be removed, but even the repair guy didn't delve that deep into the innards. What would I be looking for?

On the front, there are two panels, top and bottom, control board is in the bottom.

A 90,000btu furnace is probably 50% oversized for you unit. The noise is likely from the 1375 cfm airflow which is probably twice that of your old furnace. "the supply duct should have a flexible expansion joint," This may help the banging if it's the new plenum that is making the noise. The actual install doesn't look that bad, but you can't always tell from pictures.

Some localities do not require a permit for certain things. Pulling a permit will increase cost, just as much as how you hook a return to the furnace. A permit will not prevent noise problems.

This problem could be a bad blower motor bracket and may have nothing to do with the install. Manufacturer's in their infinite wisdom install proprietary motors with welded brackets onto the motor itself. While it's rare for one of these brackets to break it does happen.

It could be any number of things.... something that is found by diagnosing the problem. It has to be inspected to find the problem.

The issue appears to be solved now - I waited a week to make sure it didn't come back.

The way it was described to me is that they installed the furnace over the void in the floor where the old furnace sat so there is a void under the furnace now.

Please note that I had SPECIFICALLY asked that they remove the furnace and then give me a day to lay down some tile to fill that in, and to clean and paint the closet, and the guy refused. Said it was a 1 day job. Then it took them nearly 2 full days to do it - they literally hung around until I had to ask them to leave about 7 or 8 PM. And had to come back the next day to finish anyway. Really ticked me off. Anyway.

I have no idea what they did but it has apparently worked, at least for now.

I could care less about the "cost" of pulling permits. It is illegal to do work like this without it. At this point its a catch 22 for me as turning the guy in will only result in increased cost to me even over and above what it should have cost in the first place. We SPECIFICALLY asked about permits, the guy quoted the work as if he would pull permits, and then he didn't pull a single permit.

Also, what with putting the air return in the ceiling, the fan is ridiculously noisy when it does come on. There HAD to be a better way to install it without having to use all that run of ducting to pull air in from the ceiling. This furnace could have had the air return mounted beneath it and why they did not do it that way I will never understand. It sure would have been a lot quieter than having to suck air in through 10' or more of ducting.

I guarantee there will be a yelp review about the lack of permitting.

Like I said before, some localities do not require permitting. If your locality doesn't have an inspector who would inspect the work for the permit? Cost is the other issue... and if cost wasn't the issue why didn't you buy a better furnace that makes less noise. (2 stage furnace with variable speed fan)

As far as how the furnace is attached to duct work, it has to be spelled out in the contract. The contract is the specification as to what and how things are to be done. If you don't have that, the contractor is only on the hook to do the job as he see's fit.

If you have the specifications on a contract (how things are to be done) and the contractor doesn't do it as laid out in the contract, then you could sue them for 'BREACH' of contract.

This is how contract law works. You probably won't like it because it most certainly doesn't benefit you one bit.