Or, in this case, the joys of just being responsible for a house. At least the applicances that we have to deal with are fairly simple and pretty reliable. You certainly had your hands full with this sequence of problems! Nice job.
The level of expertise in dealing with the problems detailed in our Sherlock Ohms and Made by Monkeys blog never cease to amaze me. Oftentimes, it just shows what one can accomplish with a little patience and common sense.
Fairly simple appliances? Front loader washing machines with microcontrollers in place of mechanical cycle timers? I actually bought one with a mechanical timer as they tend to be more reliable than the electronic controllers that get zapped by power line spikes. But even my "simple" front loader has a variable speed DC motor controller for the drum with some sort of logic. Fingers crossed that it survives as long as the mechanical timer.
I recently replaced Honeywell mercury thermostats and furnace zone and aquastat controls with their computerized system to squeeze out some more savings on heating oil. Although the old T-stat wiring was only two wire for each zone, Honeywell makes smart replacements that establish 2-way communications and power on the same pair. Every room thermostat displays the outdoor temperature as well as the room ambient temperature, set point, icon for occupied and unoccupied setback, icon for calling heat and an LED backlight. The controller down cellar was easy to install but for one quirky design element. The controller has built-in relays to directly operate 120VAC zone pumps, but nothing for the oil burner! You have to connect a conventional Honeywell zone relay box to the low voltage control terminal if your oil burner does not have its own low voltage control loop. And most don't.
Anyhow, household appliances are becoming more owner unserviceable, just like automobiles, as their complexity expands due to availability of "cheap" silicon.
It seems that quite a few of the problems are related to those two thermostats that monitor the temperature of the heat exchanger. The interesting part is that a few years earlier there were far fewer problems, using switches of an earlier design. The very most reliable switches used a mercury switch bulb as the switch element. Those switches never failed under most conditions, since the mercury contacts were new for each operation. The mechanical life was much longer as well, since there were no stressed elements needed to create a snap action to quickly open and close the contacts. But then somebody came along and pointed out that mercury is toxic, and must be banned from all places forever. The result is switches that fail, both open and closed, usually at inconvenient times. How many of those steel cased mercury switches ever released their little blob of mercury into the envirronment? None of them that were burried without being crushed, and my guess is that even when most furnaces were mashed down, the steel structure of the switch housing protected the glass capsule. How long does it take a glass capsule to leak when it is buried underground? Can anybody answer that one?
I did have an interesting furnace service problem this past fall. A less technical co-worker who was renting a house called and said that the furnace would not run. The flame would come on, then go out after a few minutes, but the blower would not run. My first service guess was that the unmarked switch for the blower, mandated by the safety code, was in the wrong position. That was not the problem. When I opened the service cover of the furnace to look at the problem, I observed a very cheap switch installed as an unneeded blower safety interlock. The concept being that some fool would plut their hand into the spinning blades of the blower, and be injured. Unfortunately the metal of the housing had flexed just enough to allow the switch to open. I bent the metal back the other way, and very carefully replaced the service cover. This time, a short while after the flames came on, the blower started, and my co-workers house began to warm up. So the rental agent did not need to be called, and the problem was solved. At least until metal creep released the switch tyhe next time.
Since the furnace did not have a standing pilot, the flame senor most likely does not sense temperature. Depending on the age of the furnace, the flame sensor either senses the resistance of the flame, or checks the diode properties of the flame.
Either way, cleaning the sensor, the insulator, and perhaps the flame hood would have been a better fix than bending the sense probe. These flame probes are usually made of hard and brittle materials to stand up to the heat of the pilot flame. Very lucky that the sensor did not break.
Just to qualify, years ago I managed the test equipment department at a company that makes furnace ignition controls. In the early 1980's, we switched from raw flame resistance to the more reliable flame rectification method of sensing the presence of the pilot flame before turning on the main burner.
As to the plenum sensor for the blower motor, the standard for years was a coil wound bi-metal spring that operates the on and off switches. The coil does stretch out over time, causing the exact problem you had.
Electronic thermostats are another issue. Some of the electronic thermostats draw power off of the 24 VAC that runs the furnace controls. If the power supply circuit draws enough power, the furnace control will turn on and run the furnace until the overtemp switch (part of the plenum sensor) opens the circuit and shuts down the furnace until it cools. The overtemp shutdown usually ends up with the house temperature being in the upper 80's to low 90's.
Of course, all of this is why you should have your furnace serviced every fall as regular preventive maintenance. Newer furnaces that use a hot surface ignitor to directly light the main burner and have powered exhaust are even more critical to have regular service.
I agree with "thechas" that adjusting the fame-on sensor is a bad idea, not because it will likely break but because the flame characteristics rather than the sensor position are the problem. I have seen this many times especially on LPG/NG fired furnaces - obstruction(s) in combustion air intake; excessive fuel; damages/obstructed burner and/or flame shaping assembly can independently or combine to change the flame characteristics. Simply moving the flame-on sensor defeats the purpose of it being in the correct OEM position and will often result in subsequent secondary failures.
If the flame characteristics are not correct and the unit is forced to continue operating such as described here, the three most common subsequent failures will be damage to the burner assembly. the fire chamber and/or the heat exchanger assembly. A flame running too high above the burner assembly not only wastes fuel but more often than not will eventually cause the heat exchanger to rupture allowing deadly and dangerous combustion gases to enter the domestic air plenum.
Flame characteristic issues are not limited to gas-fired units, I ran into a similar situation with a waste oil fired furnace being used to heat a commercial paint booth. An experienced plumbing & heating technician was called when the unit began shutting down before the paint booth was up to temperature. Said technician "adjusted" the flame-on sensor which forced the unit to run with the incorrect flame characteristics. Said technician was called back a few hours later when the unit again shut down before set temperature was reached in the paint booth. Technician determined the problem to be a malfunctioning high-limit t-stat on the heat exchanger assembly which he "corrected" by using a couple flat washers to create an air-gap between the t-stat and heat exchanger housing. The furnace continued to operate in this condition for approximately two months until the paint booth suffered a fume flashover explosion.
Post incident investigation discovered the "corrective actions" taken by the alleged technician allowed the flame to directly impinge on the heat exchanger resulting in the structural damage that eventually allowed the combustion flame to penetrate the heat exchanger.
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