Guide to Heating System Boiler Controls & Switches
InspectAPedia® -
Troubleshooting guide for heating system boiler controls and switches
What are the basic components of heating systems?
How to inspect & repair central heating systems
An owner's guide to heating system controls and switches
Our site offers impartial, unbiased advice without conflicts of interest.
We will block advertisements which we discover or readers inform us are associated with bad business practices,
false-advertising, or junk science. Our contact info is at
InspectAPedia.com/appointment.htm.
This article describes the operating and safety controls on a Heating System Boiler - central heating systems to aid in troubleshooting, inspection, diagnosis, and repairs.
The photo above shows a modern cad cell relay on an oil fired heating boiler - one of the safety controls which we discuss in this article.
Here we describe the key operating and safety controls on heating boilers used to provide hot water heat through baseboards, convectors, radiators, and radiant heat flooring or ceilings. Because some controls are used in common on hot water heat, hot air heat, and steam boilers, readers should see these other articles: see BOILER PARTS LIST for a detailed list of heating boiler controls, other heating system components, parts such as circulator pumps & draft regulators. If your building uses steam heat see STEAM HEATING SYSTEMS. If your building uses warm air heat see FURNACE CONTROLS & SWITCHES. See ELECTRICAL POWER SWITCH FOR HEAT.
Heating System Controls Inspection, Peripherals, Key Components
How to Identify, Reset, or Adjust Hot Air Heat Furnace Controls and Switches
While going through the detailed sequence in the operation of the heating boiler, watch for and inspect the condition of the heating
boiler controls and safety devices (as required by ASHI 9.1.A.3 automatic safety controls). Here we provide a list of most of the switches and controls found on heating systems. Each item is defined and we provide links to one or more detailed articles about the inspection, testing, setting, and use of each of these devices.
Air Bleeder Valves on Hot Water Heating Systems - the basics
Air bleeder valves, manual or automatic, are installed on hot water heating systems at several places in order to remove unwanted air from the heating system.
Air trapped in hot water heating lines, radiators, baseboards, or convectors is not only responsible for a bubbling and gurgling noise, it can also actually prevent hot water from circulating, thus preventing the heating system from working.
Aquastats: Combination or Multi Function Primary Controls (Aquastats) for Heating Boilers - A Guide
Combination control or primary control on heating boilers: this control, such as a Honeywell R8182D combine High Limit and "Low Limit" boiler controls
(The dial marked "low limit" on a combination control may or may not be in use depending on presence of a tankless coil).
This primary control, the most common type on modern heating boilers,
controls the oil burner operation, turning the burner on or off as the boiler low limit or high limit temperatures are reached respectively.
This control may
switch on and off a single circulator pump, and if a tankless coil is installed on the boiler, it may also turn the oil burner on and off as needed to maintain
temperature in the boiler to provide domestic hot water as well.
Combination Control High Limit: On a typical combination control, the "Hi Limit" is the cut-off temperature for the
heating boiler on a call for heat. The cut-on temperature is hard wired in this control at about 15 degF below the "Hi Limit" setting.
Combination Control Low Limit: On these
controls the "Low Limit" is NOT the "cut on" point for heat but rather it is a setting which is intended to maintain heat inside the boiler in order to
assure that the boiler can produce hot water when a tankless coil is installed.
The "Low Limit" is normally set at least 20 degF below the "Hi Limit" to
avoid a "lockout" condition on this control which we discuss just below. During warm months when the boiler is not being called-on to heat
the building itself, the "Low Limit" keeps heat in the boiler for the tankless coil.
So the "Low Limit" is actually a "low range" operating upper limit on boiler temperature that applies out of the heating season.
Combination Control Differential: On this control, the "Diff" or differential control dial specifies the amount below the "Low Limit" to which boiler temperature can
fall before the boiler should turn on to keep the boiler warm for making domestic hot water through the tankless coil.
Details about the control settings of boiler aquastats and how to set these devices to save money on heat, remain safe, and function properly, can be read at Aquastat Functions
Cad Cell Relay Switches on Oil Fired Heating Boilers & Furnaces - the basics
Flame sensing devices on heating boilers: modern heating boilers using an oil burner for heat source use a Cadmium Cell sensor, usually located inside the oil burner tube, to "see" the
presence of flame and thus to assure that the oil burner assembly stops pumping oil into the combustion chamber if flame ignition is
unsuccessful.
Some older heating boilers, steam boilers, and water heaters may use a stack relay switch to confirm oil burner operation.
Heating Boiler Oil-burners use either a cad cell or stack relay to confirm that the furnace oil burner is operating properly and to avoid flooding
the combustion chamber with un-burned oil.
Boiler Cad Cell Relays Explained Modern heating boilers use a Cadmium Cell sensor, usually located inside the oil burner tube, to "see" the
presence of flame and thus to assure that the oil burner assembly stops pumping oil into the combustion chamber if flame ignition is
unsuccessful.
The cad cell is wired to a relay switch (usually a gray box with a red "reset" button located on top of the oil burner)
which switches the oil burner off when a flame is not established in the burner.
Resetting the Oil Burner Cad Cell Relay: if the red button on the cad cell relay is sticking up and the oil burner has shut down,
the homeowner is permitted to try ONCE to "reset" the system by pressing the red reset button.
If the oil burner does not turn on and run normally and continuously (no smoke,
no loud noises, etc.) for at least 5 or 10 minutes after resetting the relay or pressing the reset button, DO NOT keep resetting the system since
doing so can flood the combustion chamber with un-burned heating oil - a dangerous condition.
Details about Cad Cell relay switches, how to inspect, test, and reset them can be read at Cad Cell Relays
Circulator pumps & circulator relay switches on gas or oil fired heating boilers - the basics
Circulator pumps & relays on heating boilers - an older and by some heating service technicians, a preferred method to control the distribution of heat to individual
building areas uses a individual circulator pump to force heating water through each individual heating zone piping.
This system too is usually found
on hot water baseboard heating systems.
Two or more circulator relay switches, one relay for each circulator pump, will be installed to turn on and off
each heating water circulator if more than one heating circulator is installed. Individual low-voltage thermostats located in the living area will respond to
a call for heat by switching on the circulator relay which in turns on the (120V) circulator pump.
Details about circulator pumps and circulator relay controls can be read at Circulator Pumps & Relays.
Electrical Switches that Control Heating Systems - the basics
An electrical on-off switch is located out of the heating equipment room and serves to turn off electrical power to heating equipment in an emergency. This switch should be one of the first things you check if your building has no heat, as someone may have turned it off on purpose (such as for safety reasons) or by accident.
A second electrical switch is located on or close to the heating equipment.
A third electrical switch or fuse turns off power to the heating equipment service at the building electrical panel.
High Limit Controls and Low Limit Controls on Heating Boilers - the basics of single limit switches
Individual High Limit and Low Limit relay switches on heating boilers: these may be provided on older heating boilers.
A separate high limit control, usually mounted near the top of the heating boiler monitors boiler temperature and shuts off the oil or gas burner when that limit has been reached.
A separate low limit control (that looks like the high limit unit), usually mounted lower on the heating boiler monitors boiler temperature and turns on the oil or gas burner when the heating boiler internal water temperature reaches the low limit.
Details about individual limit switches and controls on boilers can be read at Limit Switches, Boilers.
Stack Relay Controls on Oil Fired Heating Boilers & Furnaces - the Basics
Oil Burner Stack Relay Switches Explained Older oil burners may use a Stack Relay
to accomplish the same purpose (turn off the oil burner if the flame is not established).
The "stack relay" is a bimetallic spring
inserted into the flue vent connector located usually quite close to the heating boiler between the boiler top and the chimney.
The bimetallic spring warms in response to hot oil burner exhaust, confirming that combustion is taking place.
If combustion is not
occurring a timer inside the stack relay turns off the oil burner to prevent flooding of the combustion chamber
with un-burned oil.
Low water cutoff on steam heating boilers and on some hot water heating boilers: on both some hydronic (water) and all steam type heating boiler systems a sensor is installed on some modern heating boilers (and on virtually all steam boilers) to turn off the oil burner
should the water level or pressure in the system fall below a safe level.
Pressure and Temperature Relief Valve on heating boilers: a TP valve is installed on all modern heating boilers to release hot water and pressure should
the boiler's internal pressure or temperature rise to an unsafe level.
The relief valve should be piped to a few inches from the floor with
the end of the discharge tube always in a visible location so that if it is leaking or open the building owner or manager can observe
that (unsafe) condition. Some very old heating boilers may not have a relief valve installed.
These systems used a pressure relieving overflow
tank located high in the building, above any upper floor radiators or baseboards, often in the building attic.
The attic pressure tank was
open to the atmosphere and often itself included an overflow pipe which would permit any excess water (or pressure) to flow out of the tank
and out of the building, perhaps through a building wall to the outdoors. While these systems worked well for decades, placing a temperature
relief valve right on or very close to the heating boiler is a safer installation.
Pressure and Temperature gauge on heating boilers: this gauge displays the heating boiler internal pressure and temperature.
Typical pressure for a residential boiler serving a two story home would show 12 psi cold, and less than 30 psi hot.
Over 30 psi boiler pressure will cause the pressure relief valve to open.
Typical hydronic heating boiler operating temperature
settings are LO-120-160 HI-180-200 degF. At 200 degF. we'll see should see pressure under 30 psi.
Typical operating temperature observed at the gauge will be below the high, and can be as low as
nighttime room temperature in non-heating season if no tankless coil is in use. The temperature/pressure gauge may help in checking for
normal conditions before and during boiler operation. However the gauge can be wrong!)
Tankless coil use on heating boilers: This device, basically a coil of finned copper tubing which is inserted into the heating boiler, is used to provide
domestic hot water to some buildings. Watch out for leaks at piping fittings or more seriously the coil mounting plate which bolts the coil to the
boiler (leaks at this location can destroy a steel boiler).
Watch out for missing a mixing/tempering valve which mixes cold in with the outgoing hot water to avoid scalding temperatures
at nearby taps.
Some building jurisdictions require a separate temperature/pressure relief valve on hot water piping at the boiler.
The photo shows a pile of tankless coils found in a building basement next to the heating boiler. We suspected that high mineral content
in the building's water supply was causing frequent coil clogging.
Water Feeder Valves on Hot Water and Steam Heating Systems - the basics
Automatic water feeder and Expansion Tank on heating boilers - these are often controlled in a single unit on modern heating boilers which use an Amtrol or
similar expansion tank. The water feeder is the brass assembly at the bottom of the expansion tank on these units.
On older heating boilers
the expansion tank and water feeder valve are separate physical units. On these older systems the "automatic water feeder" is often a
bell-shaped device which opens and sends makeup water into the heating boiler and its piping whenever the heating system's internal
water pressure falls below a normal level (perhaps 12 psi when the boiler is cold).
Some older heating systems may not have an automatic
water feeder and may only provide a manually operated valve to add water to the boiler.
Systems without an automatic water feeder are
less safe and risk serious boiler damage should boiler water be lost and should there be no low water cutoff installed on the system. See WATER FEEDER VALVES for details about water feed valves for hydronic boilers. Steam boiler automatic water feed valves are discussed at Water Feeder Valves, Steam.
Zone Valves on Hot Water Heating Systems - the basics
Zone valves on heating boilers - heating water piping in a building, particularly where hot water baseboard heat is used, may be divided into separate heating zones (different floors, or different areas on a single floor) to
permit more detailed control of heat distribution in a building.
The control of heating water through these different heating zones may be accomplished
by use of zone valves (one per heating zone or area or "loop" of heating piping) which in turn are connected to individual thermostats.
When the thermostat calls for heat in a particular building area, the thermostat switch causes the zone valve to open, to permit hot water to flow through that zone.
When the zone valve is fully open, an "end switch" inside the valve tells the heating system's circulator to begin operating, causing hot water to flow
through the zone.
Typically heating systems using zone valves will have two or more zone valves (usually but not always located close to the heating
boiler) and a single circulator pump (usually located on the return end of the hot water piping close to the heating boiler).
Details about zone valves on hot water heating systems can be read at Zone Valves.
Critique, contributions wanted: Contact Us to suggest corrections or additions to articles at this website, and if you wish, to receive online listing and credit as a contributor. Particular thanks are due to the many experts and also consumers who read and critique technical articles at InspectAPedia.com.
Additional technical contributors & reference sources for this article are listed below.
Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.
How to Identify, Reset, or Adjust Hot Water Heating Boiler or Steam Boiler Controls and Switches
For details about the controls, components and switches commonly found on hot water heating systems see the articles listed below in which we explain how to identify, set, re-set, repair, replace, or avoid problems with the components of a hot water heating system.
Relief Valves - TP Valves: A guide to temperature and pressure relief valves & safety controls on hot water and steam heating systems
Stack Relay Switches: Guide to finding, resetting, maintaining stack relays on oil fired furnaces or boilers as flame sensors & safety devices
Tankless Coils: A Guide to tankless coils used to produce domestic hot water on heating boilers: hot water quantity, safety, temperature control, leaks, corrosion, repairs
Thermostats & Heat Controls for furnaces & boilers, oil & gas fired, heat pumps or electric furnaces or boilers
Water Feed Valves: A guide to water-feeding/pressure-reducing valves on hot water and steam heating systems
Zone Valves: A guide to zone valves for heating zone control on hot water heating systems
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building design, inspection, and repair, and about indoor environment testing, diagnosis, and cleanup are at the InspectAPedia Bookstore.
Domestic and Commercial Oil Burners, Charles H. Burkhardt, McGraw Hill Book Company, New York 3rd Ed 1969.
National Fuel Gas Code (Z223.1) $16.00 and National Fuel Gas Code Handbook (Z223.2) $47.00 American Gas Association (A.G.A.), 1515 Wilson Boulevard, Arlington, VA 22209 also available from National Fire Protection Association, Batterymarch Park, Quincy, MA 02269. Fundamentals of Gas Appliance Venting and Ventilation, 1985, American Gas Association Laboratories, Engineering Services Department. American Gas Association, 1515 Wilson Boulevard, Arlington, VA 22209. Catalog #XHO585. Reprinted 1989.
The Steam Book, 1984, Training and Education Department, Fluid Handling Division, ITT [probably out of print, possibly available from several home inspection supply companies] Fuel Oil and Oil Heat Magazine, October 1990, offers an update,
Principles of Steam Heating, $13.25 includes postage. Fuel oil & Oil Heat Magazine, 389 Passaic Ave., Fairfield, NJ 07004.
The Lost Art of Steam Heating, Dan Holohan, 516-579-3046 FAX
Principles of Steam Heating, Dan Holohan, technical editor of Fuel Oil and Oil Heat magazine, 389 Passaic Ave., Fairfield, NJ 07004 ($12.+1.25 postage/handling).
"Residential Hydronic (circulating hot water) Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
"Warm Air Heating Systems". Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
Heating, Ventilating, and Air Conditioning Volume I, Heating Fundamentals,
Boilers, Boiler Conversions, James E. Brumbaugh, ISBN 0-672-23389-4 (v. 1) Volume II, Oil, Gas, and Coal Burners, Controls, Ducts, Piping, Valves, James E. Brumbaugh, ISBN 0-672-23390-7 (v. 2) Volume III, Radiant Heating, Water Heaters, Ventilation, Air Conditioning, Heat Pumps, Air Cleaners, James E. Brumbaugh, ISBN 0-672-23383-5 (v. 3) or ISBN 0-672-23380-0 (set) Special Sales Director, Macmillan Publishing Co., 866 Third Ave., New York, NY 10022. Macmillan Publishing Co., NY
Installation Guide for Residential Hydronic Heating Systems
Installation Guide #200, The Hydronics Institute, 35 Russo Place, Berkeley Heights, NJ 07922
The ABC's of Retention Head Oil Burners, National Association of Oil Heat Service Managers, TM 115, National Old Timers' Association of the Energy Industry, PO Box 168, Mineola, NY 11501. (Excellent tips on spotting problems on oil-fired heating equipment. Booklet.)
InspectAPedia® Home & Site Map - Building & Environmental Inspection, Testing, Diagnosis, Repair, & Problem Prevention Advice: In-depth research & advice on diagnosing, testing, correcting, & preventing building defects & indoor environmental hazards. Unbiased information, no conflicts of interest.
The Mold Information Center: What to Do About Mold in Buildings, When and How to Inspect for Mold, Clean Up Mold, or Avoid Mold Problems
Environmental Inspection, Testing, & Diagnosis On-Site IAQ, Gas, Air Testing, Mold Investigation, Sick Building Diagnosis, Lab Services, & Remediation Plan Preparation - indoor air quality testing, problem source determination, supporting lab work, written remediation plan addressing removal of environmental and other hazards and prevention of their recurrence.
Mobile View 
InspectAPedia ® Home & Site Map 
InspectAPedia® Home & Site Map
