What is the size of this furnace and the size of the old furnace? Was the old furnace as loud as the new one? Did the installer measure the duct work when he quoted the new furnace and was there any discussion of increasing the size of duct work?

Both furnaces are the same size, but my old one wasn’t a variable speed. I probably wouldn’t have questioned the sound had I not been reading everywhere these described as whisper quiet. They did not measure ductwork as far as I know, and had no discussions with me about it. I already voiced my opinion about that. Perhaps if I selected the lower grade option that was more comparable to my existing furnace, I wouldn’t be in this situation now. Then again, perhaps my old furnace has a static pressure issue and I never knew to ask.

Did you look at the specification panel on the furnace - some installers will get a great price on a leftover unit and will install it no matter the size! Also, modern furnaces run at higher fan speeds than older furnaces for improved efficiency (lower temperature differential). The old furnace was probably a fixed speed, blowing a reduced CFM against the duct resistance (reducing noise). Not a lot you can do about that.

Can you post the model numbers of the furnace and thermostat? Manufacturers will decribe a furance as variable speed when in reality the speed is set at the time of installation and does not vary when operating. The other thing that might be happening is you do have a variable or multi-stage furnace but either the thermostat or how it has been wired is not allowing it to operate in the low stage.

A static pressure of 1.25 is not acceptable in my opinion. The installer should have addressed this at the time of installation.

It’s the Rheem Prestige Series Modulation R98V Upflow Gas Furnace. 70,000 BTU & 3 ton (same as my last one). It comes with an Eco Net smart thermostat. From what I’ve read is that if variable speed furnaces are operating loud, it’s because the return vents are insufficient. I’m asking if the solution the installers gave sounds like a reasonable fix.

Oh, another thing about the static pressure, when they tested it at installation, it was around .6. It was when I called them back to figure out about the noise that he forced it at high and got 1.25. I have been curious as well if something is set wrong where it isn’t using the lower speeds, but the reality is that it’s winter in Wisconsin.

How does the noise of the new furnace before locking it to 60% maximum compare to the old furnace? If the old furnace was also 70K BTU and the duct work is undersized, then I would expect the noise level to be about the same.

What is the size of the return filter and the main return duct? You may have an undersized return and main supply. It is hard to say what the installer is proposing is reasonable without getting more information.

The installer should have measured the static pressure with the highest air flow on the day of installation. This is a basic installation test. I don't get a good feeling about this installer.

I have two Rheem prestige modulating firnaces. They invariably run at a low speed (only on higher speeds if I crank up the thermostat). I'm not sure why you're having this problem unless the speed setting is wrong (there are speed adjustments on mine but it's not identical to yours).

If the 3 ton (36,000 BTU) sizing came from the specification plate on the furnace then you should be good. If not, look at the panel on the side of the furnace.

If the old furnace was a single speed, it was most likely running at 1200CFM (the same speed as the new Rheem on High).

I don’t know the answer to all this stuff. I’m sorry. It’s all too technical for me, and I don’t understand them.

Sktn77a - I have speed options on my thermostat. Did you set yours to low, or was this something the installer does? Or is it something it just does on its own?

Not sure if this helps. My furnace is running now, and this is what the thermostat shows. When capacity switched to 0%, I watched the RPM reduce, but the CFM stayed the same until it stopped completely, then went to 0. When it says variable speed, is it the CFM or RPM that is variable? Or am I way off base and completely confused?

RPM stands for revolutions pre minute. This indicates how fast the blower motor is spinning to push the air flow. The blower is a variable speed motor. Varying the speed of the motor will vary the air flow. CFM stands for cubic feet per minutes. This is the volume of air moving through the furnace at the output of the blower.

You thermostat has a default 90 second blower off delay. When the thermostat stops calling for heat, the blower with continue to run for 90 seconds to push out the remaining warm air. When the thermostat is satisfied I assume the capacity will go to 0% and eventually the RPM and CFM will go to zero. There is probably a time lag when the thermostat actually reports the data to the display screen. You will probably hear the furnace become quiet before all values go to zero.

The static pressure shows 0.83 when capacity is 40% and the air flow is 567 CFM . That is not a good reading. You have an undersized supply and/or return.

You never responded how the noise of the new furnace on the day it was installed compared with the old furnace. If the noise is about the same then noise is not due to the furnace in my opinion.

I’m really not sure about the difference in noise actually. I never paid attention before. It wasn’t anything alarming on my old furnace. This new one being so loud despite being described as whisper quiet is what got me concerned with this. But also, on my old furnace I never looked into static pressure either. If the old was loud and there was high static pressure, I never would have known.

I’ve been watching this stays box at different times the furnace runs. The RPMs do vary, so I’m guessing the variable part of the furnace must be working, meaning not locked at a single blower speed.

I was noticing that 75° for return temperature. What is that referring to? Would that have anything to do with why I set the thermostat to 72° but it warms to 73° or sometimes higher? The 75° under return temperature doesn’t change.

The fact you are noticing the noise more than before makes me wonder if there are multiple noise sources.

I am not an expert for your furnace and thermostat, so I will explain what I believe to be correct. The return temperature is the temperature of the air as it returns to the furnace. It is probably a few degrees warmer than the air temperature near the thermostat. If you set the thermostat to 72° , it is not unusual to see the display show 73 ° . Digital thermostat temperature displays typically round up or down in the direction it is trying to reach.

You have a top of the line variable speed furnace with a modulating gas valve. You also bought the manufacture's communication thermostat. You spent a lot of money on this furnace and its installation. A good installer would have measured the size of the duct work and advised you about making modifications so your furnace can work quietly with proper air flow.

For a permanent fix, he is recommending putting new return vents in each of the bedrooms and the dining room, widening the other existing vents, and replacing all supply vents with larger ones.

Can you post photos of the return and main supply ducts? Without any additional information it is difficult to comment on whether adding returns where none existed and increasing all supplies is a reasonable solution. My thinking is it may be overkill to reduce the noise to an acceptable level.

Your thermostat image shows the furnace operating at 40% capacity and 567 CFM. If noise is bothering you at these settings then you have a MAJOR problem.

Is the noise coming from the vents or the furnace?

I’m not sure what kind of photos I can send. The supply ducts go all over the basement ceiling. The return, do you just want me to send where it goes down next to the furnace?

@sktn77a It doesn’t bother me at 40%, but it’s at 40% because my service tech locked the system to not run as hard as it was.

What is the size of your house and where are you located? This will give an idea if the 70K furnace is too large for your house and duct work. You stated the original furnace was the same size. Are you sure about this? Do you have documentation that indicates this?

In an earlier post you indicated the service tech limit the furnace to 60%. Have you seen the furnace reach the 60% limit? The output of the furnace is about 68,600 BTU (98% of the 70,000 BTU input). If you multiply the output by 60% it becomes 41,160 BTU. At 40% it is down to 27.4K BTU. The question then becomes is that enough to heat your house on the coldest nights of the year? It the answer is yes then you can operate with a reduced output and probably don't have to do all the proposed duct work replacements. I am not an expert on Rheem modulating furnaces so I can't advise you if limiting the output is a good long term solution and does not violate the warranty. You need to talk to the installer about this.

A photo of the return as the enters the furnace, and the main supply as it leave the furnace is a good place to start.

"@sktn77a It doesn’t bother me at 40%, but it’s at 40% because my service tech locked the system to not run as hard as it was. "

Understood. At what capacity (60%, 80%, 100%) does it get noisy?

My house is just under 1200 sq ft plus full basement (unfinished) in Wisconsin. I’m sure the furnace is sized right, but regardless, this is what I have. I saw the specs on the old and new furnace, and that’s how I know they are same. I no longer have documents on the old though. The tech might have limited it to 50% since that’s as high as I’ve seen it get. It won’t likely be enough to heat my home in a normal Wisconsin winter. This winter hasn’t been normal at all. My installer said it might not heat to my set temperature on the coldest days. Also, right now to keep static pressure down, I have to have supply vents opened in the basement and the door to the kitchen kept opened. This isn’t ideal long term either. I’ll send photos when I’m home.

@sktn77a When it was loud before locking it out to lower speed, I didn’t know how to check the stats, so I couldn’t say what it was running at when it was loud. But even now it does wake me up in the morning sometimes when it starts.

Is this the right things in the photo?

Sounds like your furnace is oversized and/or the ductwork is undersized. If the old furnace really was a 3 ton then it was probably set on the lowest fan speed setting (but that would still have been higher than your current 40%-50% lock).

Everything looks fine on you picture so I don't know what more to say. You can try getting a different company in to evaluate the noise problem.

I’ll get a couple other estimates in September and have the ductwork done in October. The static pressure is high when the furnace runs normally, and the only solution I’ve found appears to be increasing return and supply vents.

Hi, Marci,

If you purchased a furnace that is advertised as "whisper quiet." I understand a performance expectation was created. To enable an objective comparison of your old and new furnaces, we'd need performance data--including sound level data--for both the old and new systems. I'm guessing your HVAC contractor didn't gather sound data so we're limited by that.

Noise in an HVAC system can come from a variety of sources, e.g., equipment noise, duct noise, air noise, etc. If the air flow rates are the same, you shouldn't blame your replacement contractor for air noise produced by a duct work system he inherited. That said, there are a couple of things you can try before investing in a complete overhaul of your duct work. While they won't substitute for a properly designed duct system, they may make your existing system tolerable. I can't tell from the photo, but it looks like the air handler is installed on masonry units of some sort. Vibration isolating supports--or a layer of vibration absorbing material between the unit and the supports might help. The duct work should include a flexible connector to reduce the transmission of vibration. I can't tell if your system includes one, but I consider that something every system should have. Air noise at supply and return registers can often be reduced by replacing them--especially the decorative ones-- with registers that have more open area or different vanes. You could experiment by replacing registers in a single room and seeing if it makes a difference. You may find registers designed for commercial applications will be quieter, especially if they have no dampers.

I don't understand why your old 70K BTU furnace did not create the noise problems you are hearing with the new furnace. The air flow at the highest setting would have been about the same. You stated in a previous post: " I have to have supply vents opened in the basement and the door to the kitchen kept opened. This isn’t ideal long term either. " With the old furnace you had vents and doors closed and feel the old furnace was not as loud. Your main supply and return are not ideal, but good enough.

You are going to have a noise problem when running the air conditoner. You can't reduce the air fllow to a 50% rate. If the air flow is too low the coil will freeze and form a block of ice.

@Charles Ross Homes Will your suggestions reduce the static pressure?

@mike_home As I said before, I’m really not sure about the old furnace because I never expected it to be quiet, so if it was loud, I didn’t pay attention. This new furnace is more energy efficient than the old, so there must be some difference to create that. Also, if the main supply and return are good enough, how do I reduce the static pressure then?

Also, I’ll check back with my hvac guys before summer and running the air conditioner to see if something else needs to be done. My understanding wasn’t that the 50% was air flow but rather fan speed. I don’t fully understand these things. I don’t want my air conditioning running full blast though. That’s not comfortable.

Hi, Marci,

The suggestion to replace registers is an attempt to reduce air noise; any reduction in static pressure is likely to be small. You can test that by simply removing the registers in one room and seeing what the effect is with regard to noise and static pressure.

All supply vents need to be fully opened and unblocked for lowest staic pressure. Then increasing the duct work is the next step. I suspect you only have one undersized main return grille. Adding another return will help.

As I explained earlier fan speed and air flow are in direct proportion to each other. The fan speed is reduced by 50% to cut the air flow by the same amount.

"Also, I’ll check back with my hvac guys before summer

I'm not getting good vibes with your existing HVAC guys. Get a second (and maybe third) opinion sooner rather than later.

@Charles Ross Home I need a big reduction in static pressure. It’s too high. When I closed the basement register, it increased the static pressure by a lot. I had to reopen it. Also the air noise and the static pressure are related.

@sktn77a Yes, I was going to get other estimates at the point I’m ready to do the work.

@Marci,

You're not likely to see a big drop in static pressure by replacing the registers, but doing so can have a significant effect on noise which is one of your objections. It's simple to test whether it will make a difference. You can do the test yourself at no cost.

How do I drop the static pressure? That’s the more important issue. I don’t want to ruin my furnace.

In order to drop the static pressure in a significant way you'd need to redesign and modify the ductwork to handle the design air flow rate.

Ok, thank you. So it sounds like the suggestion by my heating and air company is what I’m going to need to do.