First the intake makes a grinding noise, then air flow starts on low (which is ok) then ramps up to high (which is awful).

The first noise you hear is the inducer motor. If you hear a low flow of air, followed after a few minutes with a high flow, then I suspect you have a 2-stage furnace. I would also suspect it is over sized for the 950 square feet of space and duct work.

There may be a simple solution and you don't have to live with this annoying problem. If you do have a 2-stage furnace, I suspect you don't have a 2-stage thermostat and the furnace always go into the second stage after a fixed period of several minutes. It may be possible to install the correct 2-stage thermostat so that the furnace stays in the lower quieter stage for longer periods of time, or in many cases shuts off before the high stage is needed. Another solution may be to lock out the second stage completely. This assume the low stage is sufficient to heat the upper floor on the coldest night of the year.

Can you post the make and model numbers of the furnace and thermostat? Once that information is available we may be able to figure out a solution.

Hi Mike and thank you!

I'm afraid I can't find the model (or manual) for the furnace, but the make is Amana and in case it's helpful the efficiency rating was 96.0 (at installation). I could likely contact the installer to get the model # if necessary.

We just had the coldest nights of the year here the past 2-3 days - with temps to minus 11 C (I am on the west coast and we don't get many days below freezing) and my furnace was only using the low stage as the filter was so clogged (as mentioned, I've been living away and neglected to replace as soon as I got back). At lower stage, my house was only heating to about 16 or 17 C. I put in a new filter yesterday, and now I have heat but also the air flow noise.

The thermostat was not replaced in 2008 along with the furnace. It is a brand called MagicStat and doesn't appear to have 2 stages. Sorry - again no model number is apparent.

Locking stage 2 would be great but may not work for extra cold weather. i can look into the 2-stage thermostat. Let me know if I'm on the right track, and thank you again!

In my opinion, you're dealing with a hack. You need a new HVAC contractor.

We need to establish you have a 2-stage further. Everything you have said so far leads me to believe you do, but you should verify it. If you can get the model number we can search for the installation manual to determine if my idea will work.

You are going to have to invest in a new thermostat. One other complication to all of this is that you need an additional wire (W2) to control the second stage. Hopefully there is a spare wire you can use if installing new wiring is too difficult.

The model number also tells you the size of the furnace. For that you will know how much heat is delivered in the low stage and whether locking out the second stage is a viable solution. Installing a 2-stage thermostat and reconfiguring the furnace control board to work with it properly is the best solution.

Hi again,

I did some digging, and I am pretty sure it's this make and model: AMANA GAS FURNACES AMEC96 High Efficiency two-stage multi-speed furnace.

I’m no expert, but we recently replaced our furnace and AC for our 2800 sf two-story. I have a hunch you were way over-sold for your house. It’s also possible your fan has become worn a bit, making it noisier. It might be time for an overhaul. Find a trustworthy local hvac to advise you. Nextdoor.com site for your area will help you find a good one and avoid the thieves. If you don’t have that, see about asking for referrals from trusted sources.

Do you know the size (BTU) of the furnace?

High efficiency furnaces have higher airflow requirements than older furnaces. In addition, if the furnace is oversized, you have a double whammy! Hence the question about BTUs.

Consider getting a second opinion. We have a big operation in our community that is famous for selling new units unnecessarily, claiming that it’s broken and unsafe. They tried it on us, but we saw through it and only needed a small electronic part. If you’re going back to the same people that potentially over-sold to you, then I would treat them with a healthy dose of skepticism. Sadly, not all outfits are trustworthy or as knowledgeable as they need to be. Good luck!

Ps, when they inspect it, it should take more than 5 minutes. They should be measuring airflow on your registers in most, if not all rooms. If they come in and jump right into a sales pitch, politely show them the door.

1. The AMANA AMEC96 is a multi-speed gas furnace as described in the Technical Manual.

2. The furnace can be connected to a single-stage thermostat or a two-stage thermostat. If the former is installed, it is wired as shown on Figure 24 of the Installation Instructions.

3. With a single-stage thermostat, the furnace switches between stage 1 and stage 2 as described in the paragraph at the top of page 23:

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"SINGLE-STAGE HEATING THERMOSTAT APPLICATION
"A single-stage thermostat with only one heating stage may be used to control this furnace. The application of a single-stage thermostat offers a timed transition from low to high fire. The furnace will run on low stage for a fixed period of time before stepping up to high stage to satisfy the thermostat’s call for heat. The delay period prior to stepping up can be set at either a fixed 10 or 20 minute time delay or a load based variable time between 1 and 12 minutes (AUTO mode). If the AUTO mode is selected, the control averages the cycle times of the previous three cycles and uses the average to determine the time to transition from low stage to high stage."

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4. As stated in the paragraph above Figure 24, the furnace dip switches are set to AUTO mode at the factory; the furnace automatically switches between stage 1 and 2.

5. You can change the stage-1-to-stage-2 delay to 10 minutes or 20 minutes by moving the dip switches as shown in the table at the top of page 23.

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Note: If you connect a two-stage thermostat to the furnace, switching between stage 1 and stage 2 will be controlled by the thermostat if you also set the dip switches correctly.