A few years ago, I converted my heating system from an oil-burning hot-air furnace (which had rusted out and was putting combustion products into the air) to a natural gas furnace. This was easy to do, since the house already had gas for cooking and hot water. One does need to be careful with gas plumbing -- it's different from water plumbing; if you don't know what you are doing, hire an experienced gasfitter or plumber.
The new furnace was made by DME in Canada. It has been basically trouble-free for the past 17 years, with a few minor problems. I had to replace a pressure switch fairly early on. Grainger provided one. I’ve had to replace hot-surface igniters every few years. The igniters could be considered a consumable, but having spares on hand is recommended, because they will burn out on Friday night in January, and the parts stores are closed until Monday morning.
The only real annoyance I have had with this furnace is the combustion blower. It sucks the hot combustion gases out of the furnace. The pressure switch mentioned above monitors the pressure in the blower scroll housing. The startup sequence is:
- The combustion blower starts.
- When the pressure (or vacuum) in the blower reaches the setpoint, the hot surface igniter is actuated.
- After a time delay, the gas solenoid valve is turned on.
- When the heat exchanger temperature reaches a setpoint, the main air blower starts, sending hot air through the ducts to heat the house. There are various safety lockouts and other timing functions, but that's the basics.
The combustion blower is a relatively small -- 1/30 HP -- squirrel-cage blower with the blower wheel in the exhaust gas stream and some provisions to keep the 400F exhaust gases from overheating the motor. This means that the blower wheel is inside the blower scroll housing, and the motor is outside. The motor shaft enters the housing through a hole not much larger than the shaft. The blower wheel is held to the shaft by a setscrew which bears on a flat on the shaft.
The original blower motor was a ball-bearing unit, and after a good number of years, the balls wore down so that the blower sounded as if it were running on square balls. Re-lubrication was probably not foremost in the mind of the blower manufacturer -- Emerson Electric. It was possible to get oil to the rear (cool) bearing, but lubing the front (hot) bearing was a matter of squirting oil into the general area and hoping some got into the bearing.
Replacing the motor was in order. Unfortunately, removing the motor requires removing the blower wheel, and all trying to loosen the set screw got me was a bent Allen key -- neither penetrating oil nor heat would get that blower wheel off. So motor replacement turned into blower replacement, at a greatly increased cost. Unfortunately, the replacement blowers had a differently shaped housing, which interfered with the exhaust plenum on the furnace. What was supposed to be an exact replacement, wasn't. Being stubborn, I decided to try to place the new motor and blower wheel on the old blower housing.
Getting the old motor out required the removal of the blower wheel from the motor shaft, which I eventually wound up doing by slipping a hacksaw blade into the gap and cutting (painfully) the motor shaft. To remove the motor and blower wheel from the new housing, I had to (again) remove the blower from the motor shaft. Ah, but this is a new unit, shouldn't be any problem at all. So I backed off the setscrew and tried to remove the blower wheel. It moved -- maybe 1/16 inch -- and jammed.
Turns out that the cheapskates who made the replacement blower didn't put a flat on the motor shaft, so when they tightened the setscrew, it raised a nice burr on the shaft and locked the blower wheel in place. After a certain amount of effort, which to my amazement didn't wreck the motor (sleeve bearings on this one), I got the blower wheel off and (after filing down the burred shaft), made the swap.
I can certainly understand why an engineer or mechanic might, on a prototype or one-off piece of equipment, neglect to provide a shaft flat on a motor to accommodate a setscrew, but for a company making thousands of units to do so is inexcusable.
This entry was submitted by Brooks Lyman and edited by Rob Spiegel.
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