Boiler Maintenace

Basic Maintenance for a residential boiler
Residential boiler maintenance

Scotty McGregor
HVACOracle.com
2005-08-07

Pressure Relief Valve

Hot Water Boiler Preventive Maintenance The following only refers to boiler system in general and is for educational purposes only. Refer to your owners' manual or consult with your local boiler technician for proper maintenance procedures and operation of your heating system. The procedures described here should only perform by a qualified technician. Serious injury or death could result if improperly handled.

Boiler Pressure: Look for a temperature Pressure Gauge on the front of your boiler. Normally just above the burners or on side manifold. For most residential and small commercial systems this pressure should be 12-15 pounds. Read the gauge when the boiler is cold and the circulating pump is off. This will show the static water pressure of your boiler set by the water feed regulator. If the pressure is too high the regulator may be set too high or leaking through the regulator. If the feed regulator bypasses even a small amount of water from scale or sediment trapped in the seat the pressure on the system will creep up to the pressure relief set point and blow off water.

Expansion tank: As water is heated it will expand and occupy a larger volume. The expansion tank should allow sufficient volume to allow for expansion. There are two types of expansion tanks. A Diaphragm Tank, which uses a bladder, filled with compressed air, or a steel holding tank, which traps air in the top, half. The diaphragm tank is charged with air to the same operating pressure of the boiler set by the feed regulator. Do not release or add air to the diaphragm tank is connected the boiler system or it will not function properly. Tap on the tank and it should sound hollow through out. If the bladder has leaked the tank will completely fill with water. If the tank is not properly charged with air, the diaphragm tank will fill 1/2 way with water. For the steel holding tanks there should be a sight glass on the front of the tank to indicate the water level in the tank. The tank should be about half full. If the tank is filled near the top or completely filled the water will have to be drained off. Turn the boiler off and attach a hose to the drain valve of the expansion tank. Close the valve that connects the expansion tank to the boiler and open the drain valve. Drain the tank until it is empty. Then close the drain valve and open the valve to the boiler system. The expansion tank should filled to the half way mark or less and trap air in the top of the tank. If the tank fills again after a few days then there is an air leak in the tank.

Pressure Relief Valve: Every boiler has at least one pressure relief valve. If the pressure climbs too high on your boiler it will open and blow off the excessive pressure and close again when the pressure falls below the setpoint. There should be a tag on the valve to tell you at what pressure it will open. Most residential boilers will be set to 30#. If your boiler is operating close to this setpoint the pressure relief valve will leak slowly. The cause of the high boiler pressure needs to found. At least once a year this valve should be open to blow off water and make sure it has not plugged up with sediment and scale. Check the boiler pressure before you begin, if it is close to the set point the relief valve will not close right away. The outlet of the relief valve should be piped to 6" above the floor and to a place where it will not cause water damage if opened. This makes annual blow down on the relief valve easy. It can be piped directly to a floor drain, but the outlet should be open so any leakage can be detected. If the unit is not piped to a deain use a bucket to catch the water if needed. Make sure you know where the boiler feed water shut off is, just in case. With the boiler off and cold, lift the manual lever and blow off some water for a few seconds and let the lever snap back. There should be a good gush that comes out and be fairly clear water. If the valve has small leaks after wards, just open it again and let some more water out, sediment can become trapped in the seat causing minor leakage. If the valve will not close at all shut the boiler water feed off and remove and replace the relief valve. If no water comes out, then the valve is plugged or there is no water in the boiler. In either case do not use the boiler until the problem is identified and repaired.

Circulating Pump: Most boiler pumps require lubrication for the motor and the pump. The motor normally requires a few drops and the pump bearing should have and oil port. This needs to be filled to keep the oil wicks wet. Refer to the manufacturer instructions for lubrication procedure for you pump. There maybe more than one pump on you system. Some of the smaller cartridge style pumps do not require any maintenance. The volume of water circulated by the pump is very important to the boiler operation. Too little water will boil inside the heating coil and plug it with scale. This will sound like a crackling sound inside the boiler when the burner is operating. A single pressure gauge piped across the pump inlet and outlet is the best way to monitor pump performance. As the pressure difference increases across the pump the lower the volume. The lower the pressure differences the higher the volume. Another way to check water flow is by temperature. When the burner is operating check the temperature rise through the boiler. Most are designed for a 10%F TD with nominal water flow. If the TD is higher 25%F or more the water flow is too low and can start to boil off in the heating coil.

Temperature Set point: There could be up 4 temperature controller on a hot water boiler system. Room Thermostat, Boiler Water Temperature Control, Circulation pump temperature control, and the safety high limit control.

Operating set point is normally an external control to the boiler. It controls the temperature of the circulating water. This can be a fixed set point set around 160°F to 180°F or preferably an Outdoor reset controls will raise the boiler water temperature the colder it gets outside and lower the water temperature the warm it gets outside. Outdoor reset controls can save a lot of money on a boiler system. It also has a minimum boiler temperature of 130%F to prevent condensation inside the boiler and will shut the boiler off it get too warm outside.

Room Thermostat can be used to open a zone valve, which in turn enables the boiler, or enables the boiler directly.

Circulation thermostats are used to start and stop the circulation pump and can be combined in the same control as the high limit. Normally the circ pump should be set around 90°F-100°F. On atmospheric boiler with draft hoods after the burner shuts down the continued draft starts to cool the water .Air from inside the boiler room is cooler then the water temperature in the boiler system. By shutting down the boiler circuit pump after the water cools helps to reduce this heat loss. An Automatic draft hood vent damper can also prevent heat loss.

High limit control is a safety limit switch that will shut the burner off if the water temperature gets too high. Check with the boiler manufacturer for the proper set point for your system. These are normally around 200%F but can be higher or lower. I have often found these being used as the operating control for the boiler because they are shipped with the boiler and the installer does not add the external operating control leaving the boiler without a safety high limit control.

Coil Cleaning: Atmospheric boilers are used extensively in residential and small commercial applications for space heating, potable hot water and for pool and spas. The finned coils on these units plug up externally from the flue gas passing between the fins. This reduces heat transfer and causes flue gas to spill out the front of the boiler. Every few years the draft hood and casing need to be removed and the coil needs to be cleaned. If you notice staining on the face of the boiler or can feel hot flue gas spilling out the front of the burners then coil is likely starting to plug up.

Automatic Draft Damper: Many of the new atmospheric boilers come equipped with automatic draft dampers. These dampers save a lot of fuel by stopping the draft over your heating coil and cooling the water off when the burner is shut down. The damper is interlocked to the burner and is plugged directly into the ignition control. The damper should drive closed when the burner is off and drive open before the burner starts. Over the summer this damper can seize and not open. Stopping the burner from coming on or it may stick in the open position and still allow the burner to operate but not closes off when the burner shuts off. Use a flashlight and have a look in the vent for the damper. Check and make sure it is doing it's thing.