Improving a Suburban Furnace

My living room furnace is nice and quiet, but the bathroom one is a jet engine. Recently had the motor replaced so I'm not sure what happened there.

Haven't gotten to it yet, but my DIY is going to be a large resistor to try and slow down the fan speed. No, I'm not going to slow it down so much it'll overheat the furnace.

Simple answer is you can't.

You can't put a slower fan or fan motor (which would be quieter) in there.

You can't slow down the motor as the furnace WILL over heat and not to mention the same motor serves a dual purpose. One side of the fan blower is for the inside and the other side of the fan blower is the draft inducer for the burner.

Alter either and now you have a non safe or non functional furnace.

Additionally the furnace has a safety switch called the sail switch which will prevent the burner from firing if there is not enough air movement, don't bypass that..

2oldman, that is a pretty dangerous path to go down and I highly doubt you are going to be able to pull this off safely or reliably and not to mention it would take a considerable drop in RPM to reduce the fan noise.. Any drop in RPM will affect not just your inside fan but the draft inducer fan for the burner since both fans SHARE the same motor.. If I were you, I would PM the resident RV furnace guru Dougrainer BEFORE attempting. I suspect he will pretty much say the same thing I have though. Not to mention it will be one whopper of a resistor as far as wattage goes..

Sometimes oil can help. Depends what kind of noise were talking about. Things that dont move get gummy.

Add additional ducting for more airflow. Fan does not work as hard, should draw less power per BTU moved, and the RV will heat faster to give less run time.

Cleaning fan blades may help. Both combustion and circulatory.

Your model uses a 3" fan motor (232682/with gasket or 232650/without gasket)
Rated at 8.5A at 300 CFM

The room air wheel is 350129

The combustion wheel is 350189


Noise could be motor bushings, room wheel dirty/vanes missing, combustion wheel dirty/vanes missing
OR one of the wheels loose/scraping against housing

Only YOU can find out by disassembly and inspecting


SF20 Installation Manual

If you are hooked up to 120 volt outside power most of the time you might consider installing a toe-kick heater, and install it under the sink. Hook it to the same circuit as the air condoner.

My furnace is pretty noisy, but it gets used so infrequently that I haven't bothered to do anything about it. It is in a large cabinet under the fridge with lots of room on all sides of the furnace. One day I'll put some acoustic duct liner insulation on the cabinet walls & top & a baffle between the furnace & return air grill. There are several inches between the two, so airflow won't be adversely affected.

A noisy furnace is not a problem -- it's an advantage! The noise is so unpleasant that it reminds us not to use it unless absolutely necessary (like at shower time every evening). That thing is an electricity hog!

Yes, that means that the interior of our trailer is cold, especially when we are snow camping. As the Eskimos supposedly say, "There is no bad weather, only bad clothing." Bundle up! ;)

Seriously, though, if there were a way to make it both quieter and more energy efficient, I would jump at the chance to do so.

Old-Biscuit wrote:
Your model uses a 3" fan motor (232682/with gasket or 232650/without gasket)
Rated at 8.5A at 300 CFM

The room air wheel is 350129

The combustion wheel is 350189


Noise could be motor bushings, room wheel dirty/vanes missing, combustion wheel dirty/vanes missing
OR one of the wheels loose/scraping against housing

Only YOU can find out by disassembly and inspecting


SF20 Installation Manual

OP didn't specifically say what KIND of noise their furnace is making, but if they are wanting to slow down the motor, I guarantee that it is not from "noisy bearings" or bushings or dust/dirt stuck to the wheel.

Pretty much ALL RV furnaces make a very loud sound from the wheel spinning fast creating a high velocity air movement, sometimes causing whistling in the air vents.. Think like when you turn a big fan on at high speed or like your roof A/C unit the difference in sound level from low speed to high speed..

Slowing the fan down on a RV furnace may help some of the sound level but in doing so may have unintended consequences like overheating and damaging the heat exchanger, overheating the furnace cabinet which potentially could cause combustible materials near the furnace to ignite. Overheating internal wiring of the furnace, overheating the furnace control board due to heat build up internally.

In reality, to slow the fan down safely, you would nearly have to re engineer the furnace to produce less heat from the burner.

Over time, I have gotten used to the noise, they all make it, mine has from new and sometimes the sound can be soothing and even drown out outside noise..

Worst comes to worst, try a set of in ear hearing protectors, cuts sound levels by 29 DB, pretty good chance you won't even notice the furnace noise with those in..