The two boilers most seen in residential use are made from cast iron or steel. And are either wet based (fire chamber surrounded by water), or dry based (boiler sits on top of the fire chamber). The boiler is a sealed pressure vessel which circulates hot water from the boiler through the header pipe, past the flow vale or zone valve, then through the baseboard / radiators and back into the boiler with the help of a circulator pump to be reheated. The system operates at a water pressure between 12-15 PSI. This is accomplished thru the use of a pressure reducing valve. It reduces street pressure to 12-15 PSI and also in most cases has a back flow preventer built into it to prevent boiler water from contaminating your domestic water. The boiler temperature is regulated by an aquastat (control that senses water temperature). The circulator is also regulated by an aquastat preventing the circulator from pumping water that is too cold. Most of the time these aquastat's are a combined unit and are labeled high and low. High is the boiler temperature setting low is the circulator setting. A boiler is first wired from the circuit breaker box to a wall switch (sometimes marked emergency switch) and back to the aquastat. The power is then split in the aquastat supplying the high and low sides. The high side feeds current to a safety relay and then to the oil burner (gun) which ignites and heats the water when the water temperature falls below the high limit setting. The water then gets hot and turns off when it reaches it's high limit setting. When the water heats up it expands creating the need for an expansion tank. This takes up the expansion of the water keeping the boiler water pressure between 12-15 PSI. At 30 Psi the pressure is relieved via a boiler relief valve this prevents the boiler from creating excess pressure and possible rupture. The low limit is wired to the thermostat and then back to the circulator. When the room temperature falls below it's setting it turns on the circulator thus circulating hot water through the baseboards or radiators. When the thermostat is satisfied it turns off the circulator. Their are several different methods of wiring boilers using switching relays , low voltage, line voltage, etc. But they all control the boiler in the way described above.

A warm air furnace consists of a sealed heat exchanger surrounded by a housing containing the blower , system controls and duct connection's (supply and return plenums). The heat exchanger consists of the fire chamber and channels or passageways for the combustion gases to exit the furnace while extracting the heat from the combustion gases. The blower circulates air around the heat exchanger and delivers it to the rooms of the house via duct work connected to the housing. This system is controlled by a fan limit switch. This is a control that senses air temperature controlling both the oil burner (gun) and the blower. Their are two settings high and low. High controls the oil burner (gun) turning the oil burner off when it reaches its high setting usually 180 degrees. The low setting controls the blower. Turning it off when the temperature goes below it's setting usually 120-160 degrees The system is wired first from the circuit breaker box to the wall switch (sometimes labeled oil burner emergency switch) to the fan limit then to the stack relay or cad cell relay (heat and light sensing controls detecting wether the oil burner fired) and then to the oil burner ( gun) and blower. This can be done several different ways combining switching relays, etc etc. But usually using a switching relay of some type. When the heat exchanger gets a hole in it carbon monoxide can be circulated throughout the house causing headaches or even in bad situations death.

Purge system of air, or bleed radiators Close shut off valve above circulator to isolate the zone, connect hose to drain valve Usually Above that shut off valve. Open Drain valve until you get a steady stream of water comming out of the house with no sputtering. If their is a lever on the feed valve push it up to increase boiler pressure while purging, and return to down position when done.

Boiler aquastat setting's (Basic)
Winter    : High limit 180 degrees low limit 160 degrees
Summer : High limit 160degrees low limit 140 degrees

Fan Limit High limit 160-200 degrees low limit 120-160 degrees

Safety relay's Their are two types. Cad cell relay which senses light, if no light is detected it shuts off the oil burner preventing excess oil from being pumped into the combustion chamber

Stack relay is usually mounted on the flu pipe it senses heat via bi metals if no heat is detected it shuts the oil burner off preventing excess oil from being pumped into the combustion chamber
These relays have a reset button that your told to push . For your safety only push this button 3 times if the burner does not come on call a oil burner technician.

Switching relays allows the use of low voltage controls such as low voltage thermostats running a line voltage circulator, zone valves, etc.

Aquastat controls the boiler high and low temperature settings their are many types single, double, triple acting. A triple acting for instance would have an aquastat for both high and low temp settings a cad cell relay built in and could also power zone valves and the circulator.

Wall thermostats they are either line voltage or low voltage and come in many different styles.

Delayed oil valve this is mounted on the oil pump and delays oil from being pumped into the combustion chamber for a set period of time.

Vent dampers Close off the draft from the boiler or furnace keeping the heat in the boiler or furnace

Their are two types of expansion tanks one is just a tank that needs to be drained once a year the other is a pressurized tank with a diaphragm needing no maintenance, except when it's diaphragm ruptures and fills with water the tank would then need to be replaced.

Tank with air bleeder valve :         To drain the expansion tank turn off the shut off valve on the pipe connecting it to the boiler. Connect a house to the drain valve mounted on the tank and open it you will first get water coming out at pressure then tapering off then open the air valve connected to the drain valve to let air in the tank so the rest of the water can drain out. Let all the water out then close both the air bleeder valve and the drain valve reopen the shut off valve. The system will then refill and stabilize at 12-15 PSI.

Tank without air bleeder valve  :   Connect house to any drain valve on the boiler lower than the expansion tank. Turn off the boiler feed valve(pressure reducing valve). Open the drain valve when the pressure is relieved open the drain valve slowly on the expansion tank (have a small bucket ready to catch drips) the valve should not have to be open all the way just enough for air to enter the expansion tank. You will hear it bubbling up thru the tank. As soon as the bubbling stops shut the drain valve on the expansion tank, then the drain valve the house is connected to. Reopen the boiler feed valve and the system will refill to 12-15 PSI.

For systems with a domestic hot water coil. ( a coil of copper bolted into the boiler supplying hot water to the home). Hot water for the home is made by cold water flowing thru a copper coil being heated by the boiler water. Reasons for the hot water being to cold could be several different things. Boiler high limit setting to low, mixing valve open to much (valve between cold water coming into the coil and the hot water exiting the coil, clogged coil. For water that is to hot open mixing valve more, lower boiler high limit setting.

Parts of the oil burner (gun)
Burner motor
Burner motor fan
Oil pump
Pump coupling (connects motor and fuel pump together)
Transformer (creates spark to ignite atomized fuel oil)
Transformer clips (makes contact with the electrodes )
Electrodes (porcelain with buss bar in the center delivering the spark to ignite oil sprayed by nozzle like a spark plug)
Nozzle assembly (connects to the fuel pump holds the electrodes and the nozzle )
Air gate (air adjustment located next to the fuel pump on the oil burner housing)

Reasons for water leakage from boiler relief valve

1. Expansion tank full  (Drain tank or replace pressurized type)

2. Boiler temperature. should be no higher than 200 degrees. (its a hot water boiler not a steam boiler).    This condition creates excessive pressure due to expansion of the water.

3. Bad feed valve (check all above first) wont shut off.

4. Hole in domestic hot water coil. (Check all above first)

1. No pressure in boiler . Check gauge should be 12-15 psi

2. Air in baseboards / radiators (bleed or purge)

Circulator pump not working when system calling for heat.

(Thermostat turned up, boiler up to temperature, circulator has power to it)
A. Motor burned out.
B. Coupling broken or loose.( connecting cic motor to bearing assembly)
C. Seised bearing assembly or circulator , Impeller broken.

Tip: For seized circulators one piece such as grundfos or taco (look for screw on the back of the circulator some unscrew revealing the circ shaft. turn the shaft with a screw driver. a lot of times this will free up the motor and it will run fine (they have a bad habit of seizing) tacos are another thing give them a rap with a large wrench or hammer not but not to hard to damage it . Sunstrand has what looks to be a cap nut loosen it pull it out and turn.

Tip: For systems with a broken circulator tempory heat can be supplied to that zone by opening (unscrewing the bobber) on the flow valve. Or manually opening the zone valve. This will supply a gravity flow of water throught the baseboards or radiators. Their will be no control of the room temperature however.