Question? Just ask us!
Free Encyclopedia of Building & Environmental Inspection, Testing, Diagnosis, Repair
InspectAPedia ® Home
AIR CONDITIONING & HEAT PUMP SYSTEMS
BACKDRAFTING HEATING EQUIPMENT
BACKUP HEAT for HEAT PUMPS
BANGING HEATING PIPES RADIATORS
BLOWER FAN OPERATION & TESTING
BOOKSTORE - InspectAPedia
BTU USAGE MONITORS
CARBON MONOXIDE - CO
CIRCULATOR PUMPS & RELAYS
DIAGNOSTIC GUIDES A/C / HEAT PUMP
DIAGNOSE & FIX HEATING PROBLEMS-BOILER
DIAGNOSE & FIX HEATING PROBLEMS-FURNACE
DIRECT VENTS / SIDE WALL VENTS
DRAFT REGULATORS, DAMPERS, BOOSTERS
DUCT SYSTEM & DUCT DEFECTS
ELECTRIC MOTOR DIAGNOSTIC GUIDE
FAN, AIR HANDLER BLOWER UNIT
FLOODED HEATING EQUIPMENT REPAIR
FLUE SIZE SPECIFICATIONS
GAS BURNER Flame & Noise Defects
GAS PIPING, VALVES, CONTROLS
GEOTHERMAL HEATING SYSTEMS
HEAT PUMPS, DIAGNOSIS, REPAIR
HEATING COST SAVINGS METHODS
HEATING OIL PIPING TROUBLES
HEATING OIL TANKS
HEATING SYSTEM NOISES
HEATING SYSTEM TYPES
GAS LP & NATURAL GAS SAFETY HAZARDS
MANUALS & PARTS GUIDES - HVAC
MIXING / ANTI-SCALD VALVES
MOTOR OVERLOAD RESET SWITCH
NOISE, HEATING SYSTEMS
ODORS FROM HEATING SYSTEMS
OIL FILTERS on HEATING EQUIPMENT
OIL FILL PIPE LEAKS
OIL SPILL CLEANUP / PREVENTION
PLASTIC Plexvent / Ultravent RECALL
PUFFBACKS, OIL BURNER
RELIEF VALVE LEAKS
Reset Switch - Heater Primary Control
RESET SWITCH - ELECTRIC MOTOR
Reset Switch - Stack Relays
SAFETY, HEATING INSPECTION
SAFETY RECALLS CHIMNEYS VENTS HEATERS
SOLAR HEATING SYSTEM DESIGNS
SOOT on OIL FIRED HEATING EQUIPMENT
STEAM HEATING SYSTEMS
THERMOSTATS, HEATING / COOLING
VIDEO GUIDES: Heating System Videos
WINTERIZE A BUILDING
WOOD, COAL STOVES & FIREPLACES
WOOD STOVE SAFETY
ZONE VALVES, HEATING
Heating system controls guide: This article describes the operating and safety controls on a Heating System Boiler - central heating systems. Start here to learn how to identify the different controls found on heating equipment. & 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.
Green links show where you are. © Copyright 2014 InspectApedia.com, All Rights Reserved.
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 CONTROLS & SWITCHES, and also see BOILER COMPONENTS & PARTS for a detailed list of heating boiler controls, other heating system components, parts such as circulator pumps & draft regulators.
[Click to enlarge any image]
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 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.
See AIR BLEEDER VALVES for details.
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.
Backflow Preventer Valves on Heating Boilers - A Different Check Valve
Backflow preventers are a different type of check valve found on heating systems.
The backflow preventer is installed to keep hot, high pressure water in the hydronic heating system from flowing backwards through a boiler water feed line into the building water supply - a sanitation concern.
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.
Check Valves Prevent Un-wanted Gravity or Convection Circulation of Hot Water
If electrical power switch to a hot water heating boiler is in the on position (see ELECTRICAL POWER SWITCH FOR HEAT) and the room thermostat is set above room temperature, the heating system (hot water or steam) boiler or (warm air) furnace should not run.
And hot water should not be circulating through the building's radiators or baseboards.
But for hot water heating systems (baseboards, radiators), check valve settings (in the forced-on position) or other internal defects could cause or permit hot water to circulate through the heating system by "gravity" (convection, that is warm water rising on its own through the heating piping) even though the thermostat is not calling for heat.
The problem, if this is occurring, is usually that a check valve (photo above-left) (found internal to some circulator pumps, or external as a physical device) intended to prevent hot water from circulating on its own - when the circulator pump is off - is either set to a "forced open" position, or it has become defective.
If that's the problem (diagnosed by a heating and service technician) then the valve or circulator needs to be replaced. While waiting for that repair to be made, you can still turn off the heat, by turning off electrical power to the boiler.
See CHECK VALVES, HEATING SYSTEM for more about various kinds of check valves and flow-control or flo-control valves on heating boilers.
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.
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.
Differential Bypass Valves - BG
Differential bypass valves are not usually found on residential heating systems, but you may encounter one. According to B&G:
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.
See ELECTRICAL POWER SWITCH FOR HEAT where we explain these electrical switches on heating systems in more detail.
In some jurisdictions, an additional thermally-fused electrical safety switch is required at oil fired and possibly gas fired heating equipment. At below right we an older-style Fire-o-Matic fusible link electrical safety switch. Details of this switch and its newer successors are at FIREMATIC FUSIBLE ELECTRICAL SWITCH
Heating & Air Conditioning Thermostats where are thermostats usually located, what types of thermostatic controls are used on various heating and cooling systems, how do we find all of the thermostats, how do we set, adjust, and use thermostats
The round Honeywell (R) wall thermostat shown in this photo has been locked in a plastic enclosure to prevent people from changing its set temperature.
Set temperature and how the thermostat works are described in excruciating detail below.
Room thermostats for air conditioning work and are set the same as when these devices are used for controlling heating systems.
We provide complete detail about how heating & cooling thermostats work, are installed, wired, diagnosed & repaired: see THERMOSTATS for complete details.
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.
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).
For details concerning this device see Stack Relay Switch
For details see LOW WATER CUTOFF CONTROLS
Pressure and Temperature Relief Valve on heating boilers:
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.
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.
Pressure gauges for steam boilers are discussed at STEAM PRESSURE GAUGE.
Details about gauges on steam heating equipment can be read at GAUGES ON STEAM BOILERS
Tankless coil use on heating boilers
A tankless coil is 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).
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.
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 VALVE, 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.
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).
Green link shows where you are in this article series.
Frequently Asked Questions (FAQs) about controls on heating boilers
Question: where to find info about the fan limit switch
I am in HVAC school trying to find info on the fan limit switch and limit switches - Paul Gibson, 5/24/11
Paul you are looking at boiler controls on this page.
Hi we have a 1950 to 60 old boiler in our home which we want to remove. All gas has been turn off at the meter. This furnace has not been operational for at least 10 years now, and everything is more than old to say the least.
In order to empty the baseboard style radiators in each room where do this.
The release valve is totally broken on the actual boiler itself. Is there a page on your website which deals with questions like this? We plan to have a new gas forced air system installed in our home once this has been removed. Anon; 6/26/2011
Reply: how to drain an old heating boiler & radiators when converting to forced hot air heat
Anon, just to clear up what we're discussing,
a "boiler" uses hot water or steam to heat a home through radiators or convectors or baseboards or radiant floor tubing
a "furnace" uses warm air to heat a home by circulating air (not water) through supply and return air ducts
You are describing a heating boiler - as you mention a broken (what sounds like) pressure-temperature safety relief valve. And you say you are converting to forced warm air - a furnace system.
To drain an old boiler including all of the radiators or in your case what sounds like convector units, you can usually drain everything from the boiler drain; that's because typically the individual convector or radiator valves have been left in the "on" position. Indeed draining will be a LOT faster if you can let air INTO the system.
Question: is it legal to bypass a heating boiler control?
Isn't it illegal to bypass a flow sensing device and leave the boiler in operation? - Chris, 8/14/11
Chris I'm not sure what flow sensing device you refer to, but
Watch out: in general it is absolutely unsafe and improper to bypass or subvert a safety control on heating equipment. If at an inspection I found that boiler safety controls had been removed, bypassed, subverted, I would turn off the system, red tag it, and notify the owners/occupants in writing of an unsafe condition.
Question: Can I increase the cold water feed pressure to my heating boiler?
will it damage an operating hot boiler if I increase the pressure on the water feeder valve by turning the top screw clockwise?. - Joey Butter, 12/31/2012
Joey indeed after loosening the lock-nut on many automatic water feeder-water pressure regulators for heating boilers, loosening or tightening the screw adjusts a spring that sets the water feed pressure down or up.
Question: Sudden loss of water pressure in the heating system, clanging pipes
I woke up last night hearing this clinging sound from the pipes. I checked out the heater and the pressure valve used to be at 10 is now at 0. I have oil radiant floor heating in a 1500 sq. ft. 1 story house. Did a pipe break in my floor and that's why I lost pressure? - Jeremy 1/20/2012
Jeremy, a sudden loss of water pressure sure sounds like a leak to me, but check everywhere. If you're lucky the leak is not in the floor but at an accessible, repairable place - start by checking the pressure/temperature relief valve.
Is your Aquastat works with Solenoid valve ? - Amit Prajapati 6/21/12
Questions & answers or comments about controls used on heating system boilers: adjustments & settings, diagnosis, function, maintenance, repair or replacement
Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.
Search the InspectApedia website
HTML Comment Box is loading comments...
Technical Reviewers & References