Ultimate Fix-It-Yourself Manual: Large appliances--Central heating systems

Central heating systems in most modern homes have a forced-air distribution system like the one shown at right. Typically, a gas or oil furnace heats air, and the blower propels it into supply ducts that carry it throughout the house. Return ducts carry the air back to the heat source. To heat a house evenly, a forced air system may need to be balanced by adjusting the dampers inside the supply ducts that control air distribution. A dirty filter, poorly adjusted dampers, and obstructed registers are the usual causes of inefficient heating by a forced-air system.

Most other central heating systems use pipes to carry hot water or steam to convectors or radiators throughout the house. Easily solved problems, such as trapped air or insufficient boiler water , are the most common causes of complaints.

Adjusting and fixing a gas furnace is relatively easy. Except for changing oil and air filters and restarting the unit in an emergency , oil furnace repairs are best left to a professional.

One or more thermostats control a central heating system. You can clean and adjust a troublesome mechanical thermostat; replace an electronic model that tests faulty.

For safety, keep the area around a furnace clear of combustible materials. Cut off both fuel and electricity to a furnace whenever you shut it down for repairs. Don’t try to remove the asbestos coating found on some older ducts or pipes; that’s a job for an asbestos-abatement contractor.

ENVIRONMENTAL HINTS: Turning down the thermostat 10 degrees at night and when you leave the house for longer than 6 hours can cut 15 percent or more off your fuel bill. The catch is remembering to do it. To make these adjustments routine, install a programmable thermostat. On the popular digital models, you can punch in a week’s schedule. For example, you can lower the daytime heat on workdays but not weekends and have the nighttime setback occur later on nights that you stay up late. The program can be overridden manually on non-typical days. Such a thermostat may pay for itself in a season.

For the sake of clarity, the wiring in this section has been simplified or largely deleted, as in the cutaway view abve.

Note: Details of repair and disassembly may vary, depending on the heating system. If your system differs markedly from this one, consult your service manual or contact your local utility, fuel sup plier, or a service professional.


  • Fan-and-limit switch
  • Return duct
  • Power switch
  • Transformer
  • Gas burner or oil burner
  • Exhaust stack
  • Damper handle
  • Exhaust stack
  • Damper handle
  • Damper
  • Supply register
  • Supply plenum
  • Blower panel

To gain access to either the blower or the burner of a forced-air heating unit, make sure the unit is off; then rotate the screws or knobs on the access panel 90 degree. and lift it off. Some panels just lift off.




Won’t run


Poor or no heat; furnace on

Blower squeals or rumbles


No heat; furnace on

Poor or uneven heating

Runs nonstop

Gurgling noise


Furnace won’t run

No heat; furnace on

Banging pipes

Hissing radiator

Power off at service panel or at switch

Thermostat too low

Faulty thermostat

Pilot out or faulty thermocouple (gas burner)

Faulty electric ignition (gas burner)

Tripped reset (oil burner)

No fuel (oil burner)

Clogged filter (oil burner)

Clogged nozzle (oil burner)

Blocked supply or return registers

Closed or poorly adjusted dampers

Dirty filter or blower

Loose or broken blower belt

Faulty fan-and-limit switch

Faulty blower relay

Faulty blower motor (split-phase)

Loose blower belt

Motor or blower needs oil

Foreign object in blower

Bad blower or motor bearings

Faulty circulator motor (split-phase)

Faulty pump

Faulty aquastat

Air trapped in radiators or convectors

Waterlogged expansion tank

Faulty pump

Faulty pressure gauge or reducing valve

Faulty aquastat

Air trapped in radiator or convector

Low water in boiler

Air trapped in radiators

Return water blocking steam

Clogged air vent

See General troubleshooting; check switches.

Set to above room temperature.

Test and adjust or replace.

Relight pilot or test and replace thermocouple.

Have serviced.

Reset once.

Measure fuel; call supplier if needed.

Replace cartridge (see owner’s manual).

Have serviced.

Clear obstructions. Open supply registers fully.

Open or adjust dampers.

Clean or replace

Tighten or replace

Test and replace.

Test and replace.

See General troubleshooting. Or have serviced


Oil motor. Or oil blower (see owner’s manual).

Remove object

Have serviced.

See General troubleshooting. Or have serviced

Have serviced.

Have serviced.


Drain. Or recharge.

Have serviced.

Have serviced.

Have serviced.


Refill boiler.


Slope radiator

Replace vent


[Degree of difficulty: Simple; Average -- Complex • Volt-ohm meter required:]


Clean or change filter monthly during heating season or as directed by owner’s manual. Don’t remove filter while unit is operating. For more on cleaning filters, see below.

Vacuum blower every year or more often if dusty. Clean blower blades with brush. Lubricate motor and bearings with 3 to 5 drops SAE 20 oil if oil ports are present. Avoid overfilling.

Check blower belt alignment yearly or if blower is noisy. Make sure belt is perpendicular to motor shaft and that motor and blower pulleys line up. Loosen setscrew to move motor pulley.

Check blower belt tension yearly. Belt should deflect ½ to ¾ in. To tighten belt, loosen motor mounting bolts and move motor back. Replace frayed, cracked, or shiny worn belt.

Check for loose ducts yearly. Refasten flanges with sheet- metal screws; seal joints with duct tape. In unheated spaces, wrap supply ducts with foil-faced batts rated R-1 1, foil out.

Testing a fan-and-limit switch:

Forced-air systems usually have a combined fan-and- limit switch mounted on the supply plenum or on the furnace near the plenum. The switch reacts to the furnace temperature, turning the blower on and off and shutting down the furnace before it overheats. If a furnace blower short-cycles—turns on and off often— adjust the switch’s fan control to widen the spread between the blower’s start and stop temperatures. But don’t set the temperatures too low. Below 90°F or so, the blower may turn on on a hot summer day.

1. Turn off power to furnace and remove cover. Test limit switch with VOM on RX1. Release a switch lead and probe switch terminals. Look for zero reading when setting is low. Gently move cam to above 200° F; look for infinity reading. (Some limit switches share one terminal with fan switch.) Limit switch terminals

2. To test fan switch, release a fan lead. With VOM on RX1, probe fan terminals. Look for infinity reading when temperature setting is low. Gently move cam higher and look for zero reading. Replace entire switch unit if either fan or limit switch tests faulty. Fan switch terminals.

Balancing a forced-air system:

A forced-air system is balanced when each room is as warm or cool as you want it to be. Balancing involves partly closing some ducts’ dampers while leaving others open; the results are gauged with thermometers. A trial-and-error process, balancing may take a day or so. When you finish, mark each damper handle’s position. If the system also cools, balance it separately for the cooling season.

1. Distribute thermometers throughout the house, putting one in the center of each room at table level. Use identical thermometers or ones that all read the same when side by side.

2. Open all registers and dampers. Set thermo stat tor 68° F and start furnace. After 30 mm., partly close dampers for warmest rooms. Readjust dampers until each room is at desired temperature.

Common variant: Blower relay

Forced-air systems that both heat and cool may have a multispeed blower governed by a relay. When the furnace is on, current from the fan-and-limit switch to the relay energizes the blower’s low-speed windings. When the system is cooling, current from the thermostat to the relay energizes the high-speed windings. If the thermostat circuit tests faulty, check for a faulty transformer, wiring, or thermostat.

To test thermostat circuit, turn off power to system. Set VOM on ACV scale, 50-volt range; clip probes to relay terminal (G) and common terminal (C). Without touching probes, restore power and set thermostat for cooling. Blower should go on; VOM should read about 24 volts.

Test relay coil for continuity with power to heating system off and wire to either G or C terminal disconnected. Set VOM on RX1 and touch probes to 0 and C terminals. Look for a low ohms reading. If reading is infinity, replace relay.

Gas furnaces:

Whether it uses natural gas or liquefied petroleum (LP), a gas furnace works in a simple, straightforward manner. Gas enters by way of a thermostatically controlled valve and goes to burners, which are ignited by a constantly burning pilot flame or by an electric spark that lights a pilot or the burners directly. For safety, a device called a thermocouple senses heat from the ignition system and allows gas to flow only when enough heat is present for combustion.

Gas furnace troubles usually stem from ignition problems and clogged burners. Clean the burners on an older furnace yearly. Burners on newer unit require cleaning only every few years. Have the furnace inspected once a year by a gas service technician.

CAUTION: If you smell gas, turn off the gas at the main shutoff valve. Extinguish any open flame; don’t operate electrical switches or the phone. Evacuate the house. Call the gas company from a neighbor’s house.

Adjusting the pilot and burner flames:

If a pilot won’t stay re-lighted or keeps going out, make sure that the flame is lapping over the end of the thermocouple. If not, adjust it, and if necessary, clean the pilot opening. A defective thermocouple or dead transformer may also result in no pilot light. Too much yellow in a pilot flame may indicate an air or fuel-line obstruction; call a gas service technician. Have a malfunctioning electric igniter serviced also. To burn efficiently, burners may occasionally need their air intake adjusted, usually by moving a plate or sleeve on a shutter. Have burners with fixed shutters or without shutters serviced. The flame colors shown below are for natural gas; LP flames may differ.

CAUTION: When adjusting a pilot flame, never remove the adjusting screw. Always replace the screw’s cap.

Clean pilot opening with a toothpick if flame flickers or won’t stay lit (also test thermo couple, below). Some models require removing pilot gas line to access opening.

To adjust burner flame, loosen any lock screw and open air shutter until flame lifts from tube, then close shutter until it settles back and is correct color (right). Retighten screw.

Correct burner flame reflects the right proportion of gas and air. Adjust air shutter (left) until flames on burner resemble the right” flame, above.

To adjust pilot flame, remove any cap covering the pilot adjusting screw on a combination control and turn screw counterclockwise to increase flame or clockwise to decrease it. Flame should envelop thermocouple bulb by ½ in. and be dark blue with slight bit of yellow at tip.

Cleaning the burner tubes---Burner tubes in most furnaces can be removed for cleaning. Shut off gas and power. Unscrew tubes from supporting bracket (some tubes are not screwed on); then gently twist and lift tubes free. Brush or vacuum tubes to clean them; avoid damaging burner ports. Carefully clear any clogged ports with a stiff wire.

Testing the thermocouple---To test thermocouple, unscrew it from control valve. With VOM on DCV scale, lowest setting, clip one probe to thermocouple’s unscrewed tip below insulation and clip other probe to its housing. Press and hold in reset button (or gas control knob on some models) and light pilot. Any reading above zero means thermocouple is OK.

Testing the transformer--- To test transformer, turn off power to furnace. Remove leads going to combination control. Set VOM on ACV scale, 50-volt range. Clip probes to terminals. Without touching probes, restore power. Look for a reading of 24 volts. If reading is zero, turn off power, open junction box, and replace transformer (-. Leads to combination control.

Relighting a gas furnace:

To relight the pilot on a furnace with a combination control, turn gas knob and main electric switch to Off; lower thermo stat. Let gas dissipate then turn gas knob to Pilot. Depress and hold in reset button (or gas knob) and light pilot. Release but ton after 1 minute. If pilot goes out, repeat, holding in longer, before calling for service. If pilot stays lit, turn gas knob to On. Turn on current and raise thermostat.

On older furnace, lower thermostat and close main and pilot gas supply valves. Wait 5 minutes for gas to dissipate; then open pilot gas valve only. Depress and hold in thermocouple reset switch and light pilot. Release switch after 1 minute. If pilot goes out, repeat, holding knob in longer, before calling for service. If pilot stays lit, open main gas supply valve and raise thermostat.

Oil furnaces: Restarting in an emergency

If raising the thermostat doesn't start an oil furnace, check for blown fuses or tripped circuit breakers at the service panel. Then follow the steps below.

CAUTION: Never press a reset button repeatedly. Excess fuel oil pumped into the furnace could explode.

1. Make sure the emergency switch is on. There may be two switches, one on the furnace and another on a nearby wall or at the head of the stairs. 2. Check the £ fuel oil level in storage tank. If tank has no gauge, put a clean 8-ft-long dipstick into the filler pipe to check for the presence of oil. 3. Look on the burner motor for a thermal over- load button. If you find one, press it once. 4. A Press the reset button on the primary control once. If furnace doesn't start, turn off power and call for service.

Steam heaters:

Most steam systems are one-pipe systems. Steam enters a radiator, condenses to water as it cools, and returns to the boiler through the same pipe. If a radiator develops a noisy knock, it's usually the result of water buildup that is blocking the steam; in this case, the radiator needs to be angled so that the water drains. If the noise is in the pipes, check for a sagging pipe that needs to be re-secured. If you have no heat, check the boiler water level. If it's not halfway up the glass gauge, add water. If the water level is all right, sediment may be blocking the low- water cutoff and should be removed.

CAUTION: Turn off the system, let it cool, and let the pressure drop to zero before making any repair.

To replace drippy air vent, unscrew it and screw in one of the same size. Also replace a vent that spits or that fails to hiss as radiator heats.

To fix a leaky inlet valve, tighten packing nut on valve. If leak persists, close inlet valve, remove knob and nut, and replace packing.

To stop knocking, tilt radiator toward the inlet valve so that water won't block incoming steam. Always open inlet valve fully when turning radiator on. To remove sediment, open boiler's low-water cutoff valve and let water run until clear. Then refill boiler until glass gauge is half to two-thirds full.

Hot-water systems:

Convectors and radiators heat poorly when air is trapped inside them. Bleed them yearly or when the heat is poor or uneven. An expansion tank above the boiler provides an air cushion that lets the hot water expand safely, but it can fill with water. If water drips from the boiler's safety relief valve and the expansion tank feels hot all over, the tank is probably water logged. Drain an older expansion tank. If a diaphragm tank has an air valve, try recharging it. If it doesn't have an air valve or can't be recharged to the correct pressure, replace it. Because the tank is filled with water and heavy, replacement is a two-person job.

Bleed radiators and convectors to purge internal air when furnace is on. Starting on top floor, slowly and carefully open the bleed valve on one unit at a time until hot water flows without sputtering. Capture the water in a cup or thick towel. Bleed valves usually require a screwdriver. Some radiator valves can be turned by hand.

To drain old-style expansion tank, turn off system and let tank cool. Close the shutoff valve on pipe from boiler. Connect hose to the combination drain valve and open it. A combination valve lets air into tank as water drains out. After tank drains, close drain valve and reopen shutoff valve. Bleed radiators or convectors (left).

To recharge a diaphragm tank, turn off system and let tank cool. Close shutoff valve on pipe from boiler. Use tire air-pressure gauge to take a reading on the air valve, If it's less than 12 psi (pounds per square inch), add air with bicycle pump. If you can't get correct pressure, unscrew tank from pipe and replace it.

Central heating systems --Thermostats

A thermostat is basically a switch that turns a furnace (or central air conditioner) on or off at a preset temperature. Try cleaning and adjusting a mechanical thermostat before replacing it. An electronic thermostat has fewer moving parts to cause trouble.

Replace the backup batteries with new alkaline ones when the low-battery indicator goes on.

If an electronic thermostat tests faulty, replace it with an identical model. Replace a defective mechanical thermostat with an electronic model for greater furnace efficiency. This may require having an isolation transformer or relay (or both) installed.

Most thermostats have extra switches for selecting fan operation and for choosing heating, cooling, or Off settings. Such switches operate on 24-volt current.

Single-range mechanical thermostat; Temperature lever; Leveling post; Thermometer coil; Mercury- switch; Bimetal coil; Pointer

The heart of most mechanical thermostats is a metal coil with a glass-bulb mercury switch. The coil is a sandwich of metals that expand and contract at different rates when heated. When the coil tilts the switch, mercury covers the contacts, completing a circuit.

Dual-range mechanical thermostat: Clock leads; Night temperature lever; Day temperature lever; Mercury switch; Anticipator scale

Electronic thermostat; Base-plate; Backup battery; LCD readout

Cleaning and adjusting a mechanical thermostat

Take off the thermostat cover by gently pulling on it. Remove the front assembly and check that the wall plate is level; if not, loosen the mounting screws and level it. Reattach the front assembly and clean all the parts with a small soft brush. Clean the switch contacts. If necessary, adjust the thermometer reading and the anticipator, which prompts the thermostat to cut the furnace off early to allow for residual heat.

Clean contacts with coarse paper such as bond typing paper (never use sandpaper). Slip paper under each lever and clean by moving lever and sliding paper around.

Adjust anticipator if furnace turns on and off too often or too seldom. If too often, move pointer a bit toward longer setting. If too seldom, move pointer the other way.

Set thermometer reading to match an accurate room thermometer, using a small screwdriver or hex key. The thermometer is often a bimetal coil in the thermostat cover.

Testing a thermostat

Jumper wire To test thermostat, remove the thermostat body and clip an insulated jumper wire (-5) to the R and W terminals on the base plate. If the furnace goes on, the thermostat is faulty. Replace an electronic model; try cleaning and adjusting a mechanical one (left) before replacing it. It the furnace doesn't go on, the problem is with the furnace relay, the furnace itself, or the transformer.


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