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Some Like It Hot

Your winter comfort depends on your furnace; learn what makes it  work  and  keep  it  in  repair
 
It was after six o'clock in the evening, and Joe noticed that snowflakes were blowing against the small window of his washroom as he scrubbed his hands with a stiff brush.

"Good night to be home," he thought to himself, just as the office phone started to ring.

"Oh, no, not at this hour," he groaned. He let it ring a dozen times, then could resist no further. "A-One Heating Service, Joe Oily speaking."

The woman's excited voice meant only one thing, trouble. "Mr. Oily, you must come right over. The house is getting colder every minute and the furnace doesn't come on. I've set the thermostat to 80 and we have plenty of oil in the tank. By the way, this is Mrs. Smith."

Remembering that Mrs. Smith was a steady oil customer and also a subscriber to his annual service plan, Joe did not mention that his own wife had supper waiting for him, but instead he said, "I'll be over in ten minutes."

Joe started his "service" routine without even thinking.  First he  checked the  oil level in the 275-gallon tank and saw that it read about ¾ full. With a lamp (the "Lightester") he checked the furnace line in the meter room and noted that it was alive. He went up the cellar stairs to examine the thermostat in the living room. He moved the adjusting arm back and forth and heard the contacts click softly as they opened and closed.

An actual cause of oil-burner "failure"—the AC power switch was turned off. In a house with small children, tape the switch in "up" position.

"Well, there goes my supper," he said to himself as he started back down the cellar stairs to get his tool box out of the car. When he reached the bottom he stopped suddenly, a puzzled expression frozen on his face. Then he laughed quietly, did a quick about face, bounded back up the stairs to the top landing and looked closely at the wall switch with the bright red cover and the white lettering: "Oil Burner Emergency Switch." It was the delayed mental picture of this switch that had stopped him a minute earlier. He looked at the switch in disbelief, but his eyesight was good and there was no question about it: THE HANDLE WAS DOWN, IN THE OFF POSITION.

This is a completely true incident from the records of a heating contractor in a typical residential community. It is by no means an unusual one. In any house with children between the ages of about fifteen weeks and fifteen years, unusual incidents are usual. A bright red oil burner switch, so markedly different from all the other switches in the house, is bound to attract the eyes of inquisitive youngsters, but it's not the only source of trouble. In most oil-burner installations, there is a cut-off valve in the pipe directly off the tank, and another just before the oil pump at the furnace.

Because the heating system of your house is by far the largest, most complicated, most expensive and most important single element in it, you should become thoroughly familiar with it. Your "bible" is the manufacturer's instructions. These were packed with the original equipment and should have been left for you by the builder. If the installer threw them out with the crating materials, as often happens, obtain another set from the manufacturer. Copy off all the identifying model and serial numbers you can find on the equipment and the name and address of the maker, and write directly to the latter. Be sure to mention that you would like to have a complete wiring diagram of your system, among other things. Most furnace manufacturers realize that homeowners don't monkey with heating plants just for the sake of playing around, but only to keep them in good operating order. Some of the literature they send back is very elaborate. The "dope" on the combination warm-air furnace and air-conditioning unit in my own home consisted of 48 letter-size sheets.

Heating Systems by Type

Home heating systems are classified according to the fuel they burn and the method of converting the heat of combustion to room warmth. A generation ago coal was king because it had only to be chipped out of the ground and shipped off to distribution centers. However, all members of a household so thoroughly disliked the heavy, dirty work of shoveling in coal and taking out ashes that "automatic" systems using oil or gas as fuel became popular as soon as they were introduced. Hundreds of thousands of old coal burners have been converted to oil or gas, and virtually without exception new houses built since the end of World War II use these clean, convenient fuels.

In the "steam heat" system, probably the one in commonest use, a quantity of static water in a boiler is heated by an oil or gas flame to the boiling point. Steam is generated, and this pushes its way through connecting pipes to radiators, which become hot as the steam whirls around inside. The outer surfaces of the radiators warm the air in the room. As the steam gives up its heat this way to the radiators and the room, it condenses back to water, which dribbles back to the furnace to be reheated into steam.

In the "hot water heat" system, water is again heated in a boiler, but not to the point where it turns to steam. The hot water itself is circulated through connecting pipes to the room radiators, usually with the assistance of a motor driven pump, and returns to the boiler for reheating.

In the "hot air" system, the oil or gas flame heats up the air in the belly of a furnace. This warmed air rises by itself, or more usually is pushed along by a blower fan, through large sheet metal ducts or pipes which merely open out into the rooms of the house; the openings are called "registers." In older houses the warmed air gradually leaks out through windows and doors and only fresh air enters the furnace (from a partially opened window or unsealed door) to be warmed and pushed up. In newer houses, especially those designed for central air cooling as well as heating, return ducts as well as entrance registers are built into the rooms. Most of the air is circulated back to the furnace and reused after being mixed with a little fresh air to keep it from going stale.

The screwdriver points to heating resistor that accelerates the action of the contacts of a typical thermostat.

Left, a standard-type thermostat with wheel adjustment at top; right, a matching "night shut-off" with a mechanical wind-up and controlling clock.

The methods have their advantages and disadvantages, a discussion of which is beyond the scope of this book. What they have in common is a sensitive electrical control system, upon which the entire operation depends.
 
Although at first glance the wiring diagram of your particular installation may look like that of a television receiver, a little study of it and of the actual wiring will make it understandable.

The mechanism for turning the heat on or off is relatively simple. What makes a complete system a bit complicated is the presence of interlocking safety devices designed to prevent one thing: accidental fire or explosion.

Figure 1 is a simplified version of an oil burner system. The basic parts consist of the room thermostat TH; the transformer Tl, which steps down the 115 volt A.C. line to about 24 volts; the relay R, which is an electrically controlled switch; the motor M, which turns the pump that brings oil from the storage tank and forces it through an atomizer nozzle in the furnace; the ignition transformer T2, which steps up the A.C. line to about 10,000 or 12,000 volts; and the spark gap SG, across which the high voltage jumps to form an intense electrical flame.

The thermostat TH is a temperature-sensitive switch, which can be set to close and open within narrow limits. A normal winter setting would be 70 degrees. Suppose the air in the vicinity of the thermostat cools just below this value. The two contacts of the thermostat close, and the device is said to be "calling for heat." The 'stat is directly in series with the 24-volt secondary of Tl and the winding of the relay R. The contacts Rl and R2 of the relay are normally open. When current from the 24-volt transformer flows through the contacts of the thermostat and the relay winding, the action of the relay is to pull the contacts Rl and R2 closed. These are in the 115-volt circuit to the pump motor M and the ignition transformer T. Oil is vaporized into the furnace  and this vapor is ignited by the sparking across the gap SG. The action is very much like that of an automobile engine. Actually, ignition is needed only for a few seconds, after which the oil flame maintains itself.

Figure 1: This is a simplified diagram of an oil-burner electrical system as controlled by a thermostat, marked by the symbol TH. Full details of the current cut-off and supply are given in the accompanying text.

The flame heats or boils the water in the boiler, or the air in a hot-air furnace, and the room warms up. When the air in the vicinity of the thermostat reaches 70, the 'stat contacts open, breaking the relay circuit and causing the relay contacts to open; this in turn cuts off the motor and the oil supply, and the flame dies out.

A complete diagram of an actual oil burner installation is shown in Figure 2. Let's follow this through and you'll get a pretty good general idea of how systems of this class operate.

Figure 2: Detailed schematic diagram of an actual oil-burner installation  as  found  in  many  homes.

A separate branch fuse F usually feeds the furnace line. SI is the red-plated line switch previously mentioned. Normally it is kept on. Ignore S7 for the moment, except to assume that it is on. The step-down transformer Tl and one side each of the motor M and the ignition transformer T2 connect to the 115-volt line, as before. The thermostat TH now has three terminals, marked R, B and W, for red, black and white, respectively, the colors of the three-wire cable that connects the unit to the furnace. HR2 is a resistor, which provides a small amount of heat inside the thermostat case for a purpose to be described. S2 is a thermal safety switch. Its contacts normally are closed and furnish a path between the left end of the 24-volt winding of Tl and the winding of the relay R. HR1 is a thermal element that causes the contacts of S2 to open if current passes through it for more than a predetermined time. The relay now has three switch units, S2, S3 and S4, which operate together and are normally open. The switches S5 and S6 are part of a mechanism that is rotated by a heat-sensitive arm stuck into the exhaust stack of the furnace. With the latter cold, the central arm of S5 rests against the C or cold contact, and the H or hot contact is open. Switch S6 is a tiny glass tube containing two contacts and a few drops of mercury. It is so angled that the contacts are immersed in the mercury, or closed, when the furnace is cold.

The primary of Tl is connected permanently to the 115-volt line, just as in the case of a bell-ringing transformer. If the room is warm and the setting of the thermostat is satisfied, the B and W contacts are open. Since W goes to the right end of the Tl secondary and nowhere else, the entire 24-volt circuit is dead. This means that S2, S3 and S4 are open, and since S4 controls both the motor and the ignition the whole furnace is at rest. When the room cools down, the B and W contacts of TH close. This establishes the 24-volt circuit as follows: right end of transformer, W contact, metal arm of thermostat, B contact, lower contact of S3, center arm of S5, C contact of S5, heater resistor HR1, relay R, switch S2 and back to left end of transformer Tl. Current passing through R causes its magnet to pull S2, S3 and S4 closed. For the moment, the closing of S2 has no effect, because the arm of this switch goes only to contact H of S5, which is still open. The closing of S3 establishes an auxiliary circuit to the heating resistor HR2, which at the moment is unimportant. The closing of S4, however, is most important. It turns on the motor and the ignition through the closed contacts of S6.

The Stack Control

If everything is normal, the oil flames, and hot exhaust gases start going up the stack to the chimney. In flowing over the heat-sensitive arm to which switches S5 and S6 are mounted, they cause this arm to twist. After several seconds, the movement is enough to move the arm of S5 from contact C to contact H; this opens the circuit to HR1 of the safety switch and keeps the relay circuit closed through switch S2. The same movement tilts S6 so that the internal wires are freed from the pool of mercury, and the circuit to T2 then opens, cutting off the ignition. The furnace is now running full blast.

When the room warms up, the B contact of the thermostat opens first. This does not open the 'stat circuit, as you might think. It is still intact but this time from the W contact, through the metal arm and the heating resistor HR2, through the closed contacts of S3, S5's contact H, switch S2, relay R, safety switch S2 and back to the transformer. Current flowing through HR2 heats it up, and some of the heat passes to the bi-metallic strip of the thermostat. This artificial heat accelerates the action of the latter and causes the contact W to open sooner than it would without it. With R and W both open, the 24 volt circuit is broken, the relay is deactivated, S2, S3 and S4 open, and the motor stops. As the furnace cools down a little, the heat-sensitive arm of S5-S6 twists back to starting position; S5 moves to the C contact and S6 tilts to close.

Modern thermostat with electric clock timer that runs on 24 volts has two temperature adjustments on right side of the case. The upper one is for the daytime, and the lower one is set for night-time lower temperatures.
 
Heat acceleration is now more or less standard with room thermostats because it eliminates a lag that seems to be characteristic of unheated 'stats.

Preventing Oil Flood

Suppose now that for some reason the ignition system is faulty: S6 might be bad, T2 open or short circuited, the spark gap fouled. When the thermostat calls for heat the circuit includes S5 and HR1. The motor starts pumping oil, but it doesn't ignite. The chimney stack remains cold. This means that switch S5 doesn't twist, but remains against contact C. Current flows through the heating element HR1 of the safety switch S2. After a predetermined time, usually a maximum of 120 seconds, HR1 causes the contacts of S2 to pop open, as they do in a thermal circuit breaker. With S2 open, R is de-energized, S2, S3 and S4 open, and the opening of S4 particularly shuts down the motor. If S2 did not open, the motor would continue to pump oil into the furnace, and it would quickly overflow onto the cellar floor and cause one grand mess. S2 stays open until someone resets it by hand.

When the Flame Fails

If the flame should fail after the burner has gone on properly, the quick drop in stack temperature causes S5 to untwist. The instant the arm leaves contact H, the relay circuit opens and again shuts down the burner. S2 is not affected. A momentary loss of power will also shut down the system. However, as the stack unit cools down further, S5 closes against contact C, and if the thermostat is still calling for heat the burner will start itself again, or "recycle," after a cooling off period of one to two minutes.

Initial failure of the burner to ignite, causing the safety switch S2 to lock open, might be due to air in the oil line, a bit of dirt lodged in the pump or something else that can readily clear itself in time. If you find this switch open when you are checking a cold furnace, always reset it for another trial before going further. When the motor starts this time it might blow the obstruction clear.

Now suppose a burner keeps running. If you leave it alone long enough, in the case of steam and hot water systems, it can build up a lot of steam pressure and eventually blow itself up. Most such furnaces have mechanical safety valves, but long before one of these starts whistling a pressure   switch   in   the   water  chamber should open. Connected into the power line as switch S7, this merely cuts off all power and the system shuts down.

Here is the inside view of the combination thermostat shown at the left; small pointers on round center dial are set for desired periods. To clean thermostat contacts pass a piece of white paper lightly between them.
 
In most oil burner installations the furnace also is the domestic hot water heater A coil of heavy copper pipe carrying cold water is immersed in the water jacket of the boiler. The cold water is heated by contact with the boiler water, and passes on to a storage tank. In "tankless" systems the copper pipe is big enough to act as its own reservoir. In winter, when the furnace is on a great deal, hot water is plentiful. In summer, an independent thermostat in the water jacket turns the burner on for short periods to bring the water up to 140 or 150 degrees, as desired, but not high enough to bring on heat. This thermostat merely parallels the room thermostat's connections. Gas-Fired Systems

Gas-fired steam and hot-water systems are somewhat simpler than oil systems because they do not require a pump motor or an ignition system. Instead of the pump motor, there is a magnetically operated gas valve, and for ignition there is a small, permanently lighted pilot flame in the fire box.

Any failure of the gas supply causes a thermal safety valve to cool down and lock the main gas line shut. It cannot come on again by itself until the pilot is relighted and allowed to reheat the safety. This is a primary requirement in any gas system. The bi-metallic shut-off devices found in gas furnaces are extremely rugged and reliable and should be left strictly alone. Cases of failure are virtually unknown.

Inside views of standard (right) and night shut-off thermostats show (pencil) locked contacts of shut-off. Controls enable user to time the furnace turn-off.
 
Forced warm air furnaces, particularly gas-fired jobs, are becoming increasingly popular because they are clean, easy to maintain, and fast in action, in addition to lending themselves to combination with a cold air unit. The wiring is easy to follow once you know the functions of the parts.

 
Figure 3: Schematic diagram of typical gas-fired furnace system.
 
See Figure 3, which shows the actual connections of a typical furnace. Simple, isn't it?

The branch fuse F and the main line switch SI are familiar. T is again a 24-volt step-down transformer. There are three switches between the 24-volt winding and the electromagnetic valve V, which regulates the gas to the burners. TH is the usual room thermostat, which might or might not have heat acceleration. When the room is cool and the 'stat calls for heat, its contacts close. Before it can actuate the valve V and turn on the gas, two other switches must be closed. The first is S2. This is controlled by a pushrod which is part of a thermostatic element exposed to the pilot light in the gas chamber. If the pilot is properly lighted, the switch is closed. If the gas should fail at any time, the element cools off and quickly locks the switch open. When the gas comes on again, it cannot do any damage, as the valve V remains closed as long as S2 is open. If the pilot is relighted, S2 snaps closed after a minute or so.

The second safety switch is S3. This is normally on, and is called the limit switch. It is usually built in combination with S4, the blower motor switch. With S2 and S3 normally closed and S4 open, let's pick up from the room thermostat. This called for heat, so current from the 24-volt transformer flows through TH, S2, S3, V and back to the transformer. Energized by this current, the valve V clicks wide open, and immediately the gas is ignited by the pilot light. However, the motor M does not start at the same time, a fact that puzzles new owners of warm air systems. If it did, it would send cold air up through the ducts and that would make everyone unhappy. Instead, a thermostatic element in S3-S4 keeps S4 open for a period of three to five minutes, while the air in the furnace chamber gets good and warm. Then S4 closes, and a blast of warm air is pushed through the system. When the room thermostat is satisfied it opens the circuit to V and cuts off the burners. However, the fan switch remains closed for another few minutes while it empties remaining warm air out of the furnace, and then the thermostatic element flips it open.

If the room thermostat is turned up rather high and keeps the furnace on for a long time, the furnace and the air it sends out can get too hot for comfort and safety. Here's where the limit switch S3 goes to work. It merely opens when a predetermined temperature is reached, thus opening the circuit to V and de-energizing the gas valve. The fan continues to clear the hot air into the ducts until its control switch S4 is thermostatically opened.

Night Shut-Offs

Under most circumstances it is advantageous to lower the house temperature during sleeping hours and to bring the thermostat up to a higher value about an hour before the family arises, so that the house is warm when they get out of bed. This is done by means of a double thermostat in one case, with a small electric clock timer as part of the integrated mechanism; or by a "night shut-off" with the night thermostat and the timer in a case separate from that of the main thermostat.

The controls, which can be set for any desired "night" and "day" periods, are arranged so that the two thermostats are connected in simple series. The day unit is always in the circuit, but the night 'stat, during the day hours, is merely short circuited. The timer opens this short circuit, and puts the night 'stat into action at night and takes it out again in the morning. During the night, the burner is controlled by the lower setting. For example, suppose the normal day setting is 70 degrees and the night adjustment 65 (those five degrees make a big difference), and the room temperature falls to 68. The day 'stat calls for heat but it doesn't turn on the burner because the night 'stat is still satisfied. When the room drops to 65, the heat will come on. Accidental misadjustment of the night shut-off is responsible for as many cold houses as is an open main switch. It's one of the last things a frantic homeowner usually looks into, but it should be among the first. Push up both day and night controls to 80; if that doesn't energize the heater, look elsewhere.

Ceiling view of separate transformer to run the clock of combination thermostat; it carries only 24V.
 
In typical gas-fired hot-air installation, step-down transformer  is  accessible  with  furnace  front  off.

Troubleshooting

Before any trouble develops, familiarize yourself with the voltage distribution in the electrical system, with the aid of the Handi-tester or any similar voltmeter. In most installations, the step-down transformer, relay and stack control are combined in a single box, fitted with a cover that comes off easily. Leave the line power on, and don't touch any bare metal with your fingers; use the test probes of the tester. Study the technical literature from the manufacturer, identify each part and check the wires from point to point by their color.

Remembering Joe Oily's experience with the main switch, start at the fuse box and work on. With the tester set for A.C. volts on the 300 scale, you should read the full line voltage of 115 or thereabouts between the grounded side of the line, marked G in Figure 2, and the following points in the typical oil burner system illustrated: 1, fuse; 2 and 3, main switch; 4, transformer primary  and pressure  switch;   5,  motor switch contact of relay. These readings will be the same whether the burner is on or off.

The R, B and W leads from the room thermostat usually terminate at binding posts with the same letter markings, somewhere in the furnace control box. With the meter on the 30-volt A.C. scale, use W as the common test point, and touch the probe to other points to obtain 24 volts with the burner off: 6, transformer secondary and one contact of safety switch; 7, other safety contact and one end of relay; 8, other end of relay, one end of heating element HR1, and fixed contact of S2; 9, other end of HR1, contact C of S5; 10, S5, S3 and B.

Absence of a reading between W and 7 is a sure sign that the safety switch contacts are open, a common but not often suspected cause of trouble. No reading at 8, after a normal reading at 7, means only one thing: an open relay winding. With the burner running, the only change is that point 11, S2 and H of S5, previously dead, now also read the 24 volts.

To make continuity and grounding tests on individual parts, set the meter for LO OHMS and proceed exactly as indicated elsewhere in this book in the safety, tools, and motor sections.

With a gas-fired air system, the checks are kindergarten stuff. See Figure 3. With the furnace in full operation, get 115 volts between Gl and points 1, 2, 3, 4 and 5; get 24 volts between G2 and 6, 7, 8, 9, 10 and 11. If the thermostat is not calling for heat, only G2-6 is alive with 24 volts, and points 7 through 11 read nothing.

Many cases of jittery furnace operation are due merely to loose connections, rather than to defective parts. Pump and blower motors sometimes run for hours in cold weather. Go over all leads with a socket wrench or pair of pliers. ·

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