80% Furnace exhaust silencer?

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16,002
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Silicon Valley
You probably heard about my noisy furnace (Rheem Criterion II, 100K BTU, 4 burner, with a 3 ton Carrier AC) that vent through the wall between 2 bedrooms. It was throttling for about 1 min every 15 mins because it is over powered, waking people up at night, and drying the air when it is on. I've replaced the exhaust draft fan and it helped the noise problem a lot (probably at least 10-20 dB), but it is still a bit noisy and it is still very dry. So, I'm thinking about replacing it to quiet down and make it more comfortable. I'll never make the money back in terms of efficiency, but at least I can sleep better. Here are the 2 approaches I can think of: 1) A new 90%+ that vent to the side of the house, the new exhaust would be the expensive part, the furnace is probably going to be just a cheap 40k BTU 1200CFM dual stage Goodman. I won't be able to do anything and will like hire a pro. I'm guessing it would be a $4k-5k job consider how expensive labor is here. 2) A new 80% 40K BTU 1200CFM dual stage Goodman. I might be able to swap it out myself but I do need to make an adapter (i.e. cabinet width going from 19" to 14") between the furnace and the existing inlet and outlet. I should be able to just dip switch my way to hard wire it to 32K BTU and max airflow. The exhaust draft motor is still blowing through between the wall, so there'll still be noise, but the 66% reduction in heat will likely make it more comfortable and throttle a lot less. If exhaust vent muffler exist, I'd like to install one before it reaches the bedroom. So far I see a lot of these thing on pot growing hobby site but I'm wondering if there are furnace specific muffler instead. The furnace, adapter, and exhaust will likely cost $1k for parts, and I might be able to install them myself. Suggestion?
 
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Messages
228
Location
Litchfield, Ohio
Going from 100k btu to 40k btu is a big drop. I don't know your square footage but I would suggest a 60k or 80k two stage. Also stay away from Goodman. I do HVAC work for a living and hate goodman with a passion. Their equipment is junk!
 
Messages
167
Location
NJ
For drying you can add a humidifier or place trays of water at the output vents. If the noise is at start and stop it might be duct flexing. Find the ducts that flex and add L brackets/bracing to stop the flexing duct. If it is motor noise add noise baffling around furnace, but not touching furnace.
 
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PandaBear

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16,002
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Silicon Valley
The big jump is the intend. The house was build initially with a way over powered furnace (demo home, probably need to warm up fast in the morning to demo). It basically kicks in for 1 min and then turn off for 15 mins. That 1 minutes is very uncomfortable dry heat. We usually turn on the furnace when it is about 58F inside, and set the thermostat at 64F. I thought the 1 on 15 off is called throttling, I guess I was using the wrong word (maybe it is short cycling?). The sound is actually air flow within the flue / exhaust. It is within a thin wall so there's not much I can do to slow it down. Maybe going from 100k BTU to 32k / 40k would help.
 
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PandaBear

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Silicon Valley
Originally Posted by JC1
Is there any way to soundproof the walls around the furnace? I would try to use some Roxul insulation (Mineral Wool - Safe and sound) to kill the noise.
Unfortunately no, the vent is a flat vent (adapter on both in the garage and the attic) in the wall and it is almost touching the drywall already. Come to think of it, maybe a sophisticated muffler to slow the air down, then split from 1 vent to 2 inside the wall (to slow down the air flow) then merge back into 1 and go out of the roof, would be a solution. Then again, if I were to spend so much effort, I'd rather just replace the whole furnace with a side vent 90+. The sqft is 1454, but it has a cathedral ceiling in the living room so the "volume" of air to heat is about 400 sqft more had it been the same house in 2F all over.
 
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Would it be possible to bleed off some of the heated air to outside by disconnecting a duct and running it out a vent? Kind of bogus but it would lengthen the heat cycle and effectively lower the BTU of the furnace. Any way to shut of some of the burners? Doesn' fix the noise but might help the short cycling. Probably the safe way to slow the velocity of the exhaust gas is to open the wall and put in a flue with much larger volume, like your 2 pipe idea.
 

PandaBear

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Silicon Valley
The duct flow is not what's noisy, or I'd complain about AC in the summer too. If I am going to bleed the exhaust to the side of the wall, I would just do it right by going 90+. Side venting a 80% furnace is bad for your wall (heat) and you still need permit and all sorts of other works, so why bother?
 
No, bleed off heated room air so house doesn't heat up as fast. You'd need make up air from outside so not as simple . Still think if you could cut the burners in half you could put in a much lower CFM draft fan and lower exhaust velocity that way.
 
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9,373
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Canuck living in California
The air handler speed can be varied on the control board a little bit. I would check it out and see if you can set it on the lowest possible setting. That would be my first step, since it doesn't require any money.
 

PandaBear

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Silicon Valley
Plugging 1 or 2 of the 4 burners may be an idea (at least if it crack after a couple years it will be worth the money already), and then reducing the exhaust draft fan speed (instead of the blower speed I assume)? That'll be a completely new draft motor I guess?
 
Originally Posted by JustinH
New 80 percent furnace 40k is probably 600 bucks. I'd go that route before messing with an old 100k unit. Not sure if 80 percent is still legal in California.
I'd mess with the old one a bit just to see if it could be done safely. Then probably change it out.
 
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2,137
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Texas
Originally Posted by JustinH
Not sure if 80 percent is still legal in California.
80% is still allowed for retrofits in California. 92% is required for new construction.
 
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