A quarter century ago, one fifteen year old was trying to find something that made a fair amount of money and didn't take a college education to get into. It also helped that the majority of work was on deceptively simple pieces of equipment. It sure didn't hurt that this young man didn't mind fixing stuff, or getting his hands dirty. Same time later, that young man is now 40 and the work has become a bit more complicated. I still love it and praise the Lord I have a job. However, like everything else, something as simple and mundane as a furnace has become a bit more complicated. This is the same for air-conditioners. There are also many more choices than there were just a decade or two ago to heat and cool your home. This is great for the consumer (until they need to get it fixed) and provides a steep learning curve for the person trying to work on this stuff.
Today was a reminder on just how complicated furnaces really are. I'm no fan of Carrier or Bryant, and the ones made in the 1980's can drive a tech to distraction. The 80% efficient furnaces are a bit of a bear to work on, as BDP had the idea to add a relay to the vent motor as well as a control board to this component separate from the main board. I sat in front of one these for about 30 minutes today with a homeowner over my shoulder. The furnace was in pieces, literally. The doors were off (never a good sign) and half the wiring was disconnected. The customer's hopes of this being a simple clean and check evaporated as I fished through the wiring, getting things to a semblance of working order.
In frustration, the customer had tried to work on his own piece of equipment. Worse, he had someone work on it who used parts off an old unit instead of buying new (which he should have done). It was a D.I.Y. install on top of that and since nothing was put back, it created an electrical hazard for me. My pliers are a testament to that fact as well as a tripped breaker. I took a deep breath and went to figure this out.
A word of caution here, this is for entertainment purposes only. Do not attempt this if you are drunk, or don't know exactly what you're doing as you can mess up expensive furnace parts or even more expensive people parts. Call a qualified heating contractor and offer him a soda or cup of coffee. I'm partial to Vitamin water or jerky myself. The money you spend will be well worth it.
All furnaces made since 1986 (by my reckoning) have at least one control module or board. These may or may not have relays which are replaceable. There are also ignition modules which control the spark or hot surface ignition (the latter on early 1990's equipment). Today, most have one control board, but some also have a personality module to simplify the need for different models of furnaces. One board will fit most, but these need a personality module to make everything fit. This is where the HVAC industry is headed. However, this Bryant didn't have this feature, nor any modern way to diagnose the problem. It was me and my meter.
Nearly every board is going to have two sets of input, 120 and 24 volts. This is with the former running the fan, vent motor and sometimes the igniter, while the latter controls the relays and thermostat. The high voltage wires are usually white and black, while the low voltage wires from the transformer to the board are red and blue. I traced power going in the board, high and low volts, and checked the thermostat terminals. However, there was no low voltage to the relay for the vent motor, or to get the spark box clicking. One rarity with these boards was a blade type fuse usually found on later model equipment. A look at this determined a possible cause of trouble. Someone put a 20 amp fuse in a spot meant for 3 or 5 amps MAX! This furnace was going to need a new board. Further investigation showed that the only low voltage coming from the board was to the limit control and back to the board itself. Apparently, the customer tried to fix his own furnace and created more of a problem than if he just called me in the first place. Instead, he messed it up and then tried to get me or a competitor to pick up the pieces (literally).
First of all, whether on a car, boat, airplane, or even a relay satellite, a part doesn't "just fail." If a fuse blows, there's a reason. If an igniter, blower or vent motor quits, there's an underlying cause. Either the furnace was over sized and short cycled all the time, the venting was wrong, a bird got caught in the intake, the flame sensor was never cleaned, the manifold pressure was never checked, you used one of those infernal allergy filter in a 1 inch filter slot (one of my pet peeves) and the furnace took a time out. So replacing the part, even if you are successful isn't going to fix the underlying problem and you'll be replacing it again; sometimes as soon as the same day. I've replaced several blower motors on one Carrier (there's that name again) until I realized the customer was using a high density filter that was killing the blower motors. I told her the next replacement was on her and to use a standard density filter, changed monthly from then on. I've had customers complain bitterly about "a lack of quality," all the while killing their equipment as surely as pouring battery acid into the blower compartment.
So please, leave this kind of thing to the pros. Change the filters if you like, but use the low density stuff and if you need more filtering, get a four inch media filter installed for $400. Otherwise buy the low efficiency 1 inch pleated paper filters and change them monthly. By all means, don't try working on something you don't understand unless you at least take a community college course on what you want to work on. I don't fix my cars myself nearly as much as I used to with good reason. The stuff is too complicated for a casual mechanic. Sure, I'll tackle the starter or alternator and maybe a water pump or thermostat, but I'm aware of the risks. If you're in the same boat, be aware there are risks to doing your own furnace or air-conditioning repairs. You can be electrocuted (happened to me several times) burned (this too) suffer cuts (a whole lot) and damage expensive parts (I've fried one board.
The guy trying to be the technician is going to be out of at least $500 to replace a part on a 20 plus year old furnace, whether a vent motor, control board, or igniter. This doesn't make trying to be cheap really worth it. Maranatha!