resonating noise from new furnace

[rhombeus]

We have just had two new forced air gas furnaces installed. Both are two-stage variable speed units, with single stage t-stats. One furnace services the bottom two floors of the house and is 75,000 btus, and the other furnace services the top two floors and is 50,000 btus. The unit for the bottom two floors works perfectly. Unfortunately, the unit for the top two floors has been creating a loud resonating noise, which can be VERY annoying. The installer has been very good, and he has offered to come and replace the motor if necessary. But first he wanted to try some trouble shooting. Makes sense to me. He spoke with the tech support people at the manufacturer, and they suggested adjusting the some dip switches to change blower speeds, so I have been working with the installer to do just this. The factory default for the furnace had a Lo heat speed of about 625cfm, and a hi heat speed of about 825cfm (both at 0.1 of static pressure). This was the setting that was originally producing the loud resonating sound. Over a few days I have tried lower and lower blower speeds in an attempt to eliminate the noise. I have now gone as low as it will go, with a Lo heat speed of 500cfm and a hi heat speed of about 675cfm. At this point the noise at lo speed is completely gone, but instead there is some noise at hi speed. I should note that with the factory settings the noise occurred at low speed. This leaves me to believe that the "noisy" range is from about 675 to 825cfm. Unfortunately, it is not possible to adjust the lo heat and hi heat speeds independently, so it doesn't seem like I can come up with a totally quiet combination, although the way it is set now is considerably better. My question is - is a new motor likely to fix the problem? If we opt to leave it as it is now (ie. 500cfm/675cfm), will it function properly? It seems that at lo speed there is very little air movement out of the vents. Should I be concerned about this? Am I compromising efficiency at this low blower speed? Any input is much appreciated.

[t527ed]

may have a blower wheel out of balance or bad motor, i would replace both if noise is coming from heater and not duct work.

[Black Adder]

Did the tech try repositioning the blower wheel on the motor shaft? I have seen a few applications where this will solve the problem. I doubt that changing the motor will fix your problem if the noise is not coming from the motor itself. Changing the motor may make accomplish the same thing as repositioning the blower wheel and therefore solving the problem, but I would try adjusting the wheel on the shaft on the old motor first.

[1fatcat]

Personally, I would try to apply hand pressure on the duct work at all possible locations and see if the pressure eliminates the noise. If/when you find the problem area, then focus attention there.

[rhombeus]

Thanks for all the info. The noise is definitely coming from the furnace - not the duct work. One question, that will make this decision easy. By adjusting the blower speeds am I really just masking the underlying problem? In other words, is there any scenario where a noise like this would be expected if the blower speed was not configured properly? Or should I be able to run at any speed with little or no noise?

[Steve Wiggins]

Is this a noise that only women can hear?

[t527ed]

only noise you should hear at any speed is air moving.

[rhombeus]

Thanks t527ed,

So nice to get clear and concise answers! (Not to mention intelligent) Mr. Wiggins. I am not quite sure what you mean by "noise that only women can hear". I can only assume you are suggesting that this noise is so negligible, as to not be worth mentioning. This is not the case. At factory settings, the noise is very loud, and you can almost see the furnace shaking.

[mark beiser]

How does the return air enter the furnace?

What brand/modle is the furnace?

[1fatcat]

Originally posted by rhombeus
and you can almost see the furnace shaking.

Is the furnace leveled? A furnace is a working machine, it's going to have some vibration to it. The one thing that I understand needs to be done is the furnace is leveled with an absorbant material (thin pieces of wood work pretty good) and the duct work needs to be isolated from the unit with a flexible link...pretty standard stuff from what I have read.

Leveling is very important if it is anything like a washing machine?

[docholiday]

Originally posted by rhombeus

At factory settings, the noise is very loud, and you can almost see the furnace shaking.

The factory settings are not "factory settings". The factory connects the wires and the switctes are either on or off. It is up to the installer to configure both properly EVERYTIME. The factory has no clue what size a/c is going to be installed and the heating speed is dependant on temperature rise.

As for your noise, I also suspect a clearance issue with the blower wheel to the housing or a wheel that is out of balance.

[troyorr]

Is the noise present all the time the furnace is operating or just when the inducer is running? Had a similiar problem with a 90% furnace. Problem was the gas line to the unit was jammed against the floor joist and when the inducer came on, the vibration would transfer to the joist. try to determine when the noise is present.

[rhombeus]

The furnace is level, and there is no play between the furnace and the ground (ie. it can't be rocked back and forth), so I don't think leveling is the issue. Seems like the consensus is the blower wheel alignment, so I will follow up with this. Just for my curiosity. How do you go about configuring the proper blower speed? In this case there is no a/c hooked up to it yet, so I am just concerned about the heat settings. I know there are a lot of factors involved here, including static pressure etc. But my understanding is that the higher the cfm the faster the air will move over the heat exchanger and therefore the less chance it will have to be heated and therefore the lower the temperature rise (temp diff. between air going in and coming out). Is this in the ballpark? If this is the case, how do you go about setting the cfm? Is there a particular temperature rise you want to achieve?

[MadeinUSA]

You need to call your technician back out to perfrom and check these critical measurements. We can not give step by step instructions such as adjusting heat rise on this site.

[rhombeus]

I was not asking for step-by-step instructions for adjusting heat rise. I have already called my installer, and he is coming out to investigate and adjust the settings. I am just interested in conceptually how you go about setting the blower speed.

[jacob perkins]

Yes,you have the basic idea of it...the rest of the info (specifications) are published by the manufacturer,and should be available.

[trane]

I know you say its in the furnace but it sounds like a noise you get when return grills are undersized. Try taking the return grills off and see if it goes away. It would seem to me that any problem with the blower wheel would be obvious at start up.

[tinknocker service tech]

does the unit have a central return grille. i have seen many cases where the grille humms and sounds like it is coming from the blower. if this is the case changing the angle of the finns may help. this may sound strange to you but it is not uncommon to think this noise is in the blower because of echoing in the duct

[propmanage]

Check the vent and flue, sounds like it could be restricted. Has it been cleaned?

[coolrunnings]

I have heard of this problem occurring with many different manufacturers. You should NOT have noise like that at any speed. Blower wheels are not made like they used to be and subsequently they can literally "vibrate" apart because they have a resonant frequency too close to the torsional resonance of the motor itself. Solutions include moving the motor shaft axially to alleviate the resonance or sadly replacing the wheel. It is important to note that if there is extremely high static pressure that it can cause resonance regardless of motor position and must be addressed. Paste the address below into your browser to read more about this problem and how it is remedied.

http://www.sandvmag.com/downloads/0105zeng.pdf