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Guide to Heating System Boiler Controls & Switches
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• Troubleshooting heating system boiler controls and switches
• What are the basic components of heating systems?
• How to inspect & repair central heating systems
• Cleaning & maintenance guide for heating systems

This website answers most questions about Heating System Boiler Controls on 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. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution. © Copyright 2008 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use the links at page left to navigate this document or to go to Other Website Topics. Green links at left show where you are in our document & website.

Heating System Controls Inspection, Peripherals, Key Components

How to Identify, Reset, or Adjust Hot Air Heat Furnace Controls and Switches

For details about the setting, re-setting, or function of the controls and switches commonly found on hot air heating systems see these articles:

• Hot Air Heating Furnace Basic Operating Steps
• Guide to Heating System Components:
• Air Bleeder Valves for Hot Water Heating Systems: Radiators, Baseboards, Convectors
• Aquastats Heating Boiler Primary Controls: A guide to Aquastatus for control of hot water heating boilers and tankless coils
• Cad Cell Relays on oil fired furnaces or boilers as flame sensors & safety devices
• Check Valves Guide to check valves and backflow preventers on hydronic (hot water) heating systems (gas or oil fired)
• Draft Hoods on gas fired heating equipment, function and safety
• Draft Regulators & barometric dampers on oil fired heating equipment
• Expansion Tanks Guide to pressure control and expansion tanks on hydronic (hot water) heating systems (gas or oil fired)
• Fan Limit Switch on hot air furnace heating systems (gas or oil fired)
• Flue Gas Spill Switch on gas fired heating equipment sense combustion and protect from blocked flues
• Gas Piping, Valves, Controls including gas regulator assembly and other controls
• Mixing Valves: Guide to mixing valves on hot water heating systems and radiant heat systems
• Power Switches to turn on or off heaters for furnaces & boilers, oil & gas fired, heat pumps or electric furnaces or boilers
• Relief Valves - TP Valves: A guide to temperature and pressure relief valves & safety controls on hot water and steam heating systems
• Stack Relay Switches Explained on oil fired furnaces or boilers as flame sensors & safety devices
• 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

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).

Guide to Combination or Multi Function Primary Controls (Aquastats) for Heating Boilers

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.

Since tankless coils on heating boilers, especially modern small high-efficiency boilers, have rather limited ability to deliver much hot water to the buildng, most users set the "DIFF" control to its smallest number, assuring that the boiler will come on as soon as possible when it's needed to make hot water out of the heating season.

What Settings To Use on a Heating Boiler Aquastat Combination Control

Lots of people, even some service technicians are confused about the actual function of combination controls and aquastats like the Honeywell R8182D. Some don't know how to set the "HI", "LO" and "DIFF" dials on a combination control. Or more commonly, we were taught to just use a "rule of thumb" which set the "HI" to 180 degF and the "LO" to 160 degF. This was certainly a "Safe" setting for this control, and it's in use on millions of heating boilers. But with a little thought, we can adjust these settings and set the heating aquastat control so as to save a bit more on heating cost.

Guide to Setting the HI Limit on a Honeywell R8182D heating boiler aquastat & Similar Controls:

The "HI" on a combination heating control like this aquastat is usually set by the heating service technican to a spot between 180 degF and 200 degF. Or the "rule of thumb" fellows just park it at "180" degF as you can see in our photo.

Generally a heating boiler is more efficient if we operate it at a higher temperature. The thermal conductivity of heating water inside of finned copper tubing baseboards or through radiator surfaces is exponentially greater at higher temperatures. In other words, hotter water actually transfers heat into the occupied space more efficiently than cooler water. So you may actually save money on heating costs by running the boiler at a higher temperature.

Since this is not lab-grade equipment these settings are not precisely accurate. For this reason we like to set our HI limit at around 200 degF. We let the boiler run through a few heating cycles, watching the temperature and pressure gauges on the boiler to see what temperature we're actually reaching. And we watch the boiler rellief valve to be sure we're not causing leakage there.

Setting the "HI" too high on the aquastat can result in over-temperature in the heating boiler and can result in dumping water and temperature and pressure at the relief valve.

Guide to Setting the "LO" Limit on a Honeywell R8182D heating boiler aquastat & Similar Controls:

The "LO" is set at least 20 deg F. below the "HI" limit on a combination control. If we set the "LO" any closer top "HI" it is possible that the control, trying to give priority to making domestic hot water for someone in the shower, will simply lock out the circulator pump entirely - the heating circulator will never run.

In our photo at left you'll see where the heating service tech left this "LO" limit control after cleaning and tuning the heating boiler.

What happens if you "cross the controls" and set "LO" above "HI" or if you set "LO" too close to "HI"? The circulator will not run.

We've inspected homes at which the owner, for decades, observed that the heat in the building was very slow to come up in response to the thermostat, and where the problem was that the LO was set close to or even above the HI - the circulator pump never ran and hot heating water circulated but only very slowly by convection.

Guide to Setting the "DIFF" (differential) on a Honeywell R8182D heating boiler aquastat & Similar Controls:

Where a tankless coil is installed on a heating boiler to make domestic hot water (for washing and bathing) we prefer to set the differential to its smallest number (usually 5 degrees) so that the boiler will come on as soon as possible to make domestic hot water when we're out of the heating season. (In the heating season the boiler is usually hotter due to calls for heat and the "LO" and "DIFF" are basically asleep.).

If a tankless coil is not installed on a boiler where this combination aquastat control is installed, this combination control may still be in use. In this case, the "LO" has almost no use whatsoever and, if you read the instructions provided by the control manufacturer you'll probably see that the manufacturer calls for the "LO" to be electrically disconnected entirely - it's simply a matter of moving a jumper wire (usually the blue wire pointed-to by our pen, next to the DIFF control.

If you do not disconnect the "LO" limit switch on a combination control on a heating boiler where no tankless coil is installed, during the summer you may wonder why once in a while you hear your heating boiler running. Now you can solve this mystery. In a few cases, certain cast iron boilers may cool off and begin to leak between their cast iron sections during the summer. This may be a reason to keep the "LO" wired and active on just these heating boilers.

More Heating System Controls

• Individual High Limit and Low Limit relay switches on heating boilers: these may be provided on older heating boilers.
• 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).
• Circulator 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.
• Flame sensing devices on heating boilers: 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.
• 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 (ususally 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.
• 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.

  How to Reset the Oil Burner Stack Relay

  Knowing how to reset the stack relay can avoid a costly "no heat" service call or it can keep the heating boiler running sufficiently to keep the building warm while waiting for the heating service technician. There are actually two resets that can be performed on an oil burner stack relay. A gray box with a red reset button housing the stack relay and its reset switch will be found mounted on the flue vent connector if this control is in use. The first is simply to press the red reset button that protrudes through the cover of the stack relay box itself. As with cad cell relays and other heating system "reset" buttons, if the oil burner does not turn on and run continuously and normally (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.

  The second and more thorough reset of the stack relay can be accomplished by removing the cover on this control box (SAFETY WARNING LIVE VOLTAGE electrical connections are inside this box - a shock hazard). The stack relay bimetallic spring assembly can be "reset" by removing the cover of the assembly and pulling the (visually obvious) reset lever out away from the flue vent connector, and then carefully and gently releasing it again. Watch out for sensitive stack relays: sometimes these switches trip off during oil burner testing or operation and they can be trick to re-set.

• 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.
• ow water cutoff on heating boilers: 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 tempearture. 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 operating temperature settings LO-120-160 HI-180-200 degF. 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!)

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More expert information on this topic

More Information on Inspecting and Repairing Heating Systems

• Carbon Dioxide Gas Toxicity
• Carbon Monoxide Gas Toxicity, exposure limits, poisoning symptoms, and inspecting buildings for CO hazards
• Dust from HVAC? An Investigation of Indoor Dust Debris Blamed on a Heating/Cooling System Reveals Carpet Dust
• Goodman Furnace High Temperature Plastic Vent HTPV safety recall US CPSC notice
• Home Heating System Should Be Checked [for proper venting and for CO Carbon Monoxide Hazards - DJF]
• Inspection Procedures for Oil-Fired Heating Systems Detailed step by step approaches for inspecting complex systems]
• Lennox Pulse Furnace Safety Inspection/Warranty Program: Carbon Monoxide Warning
• Oil Tanks - The Oil Storage Tank Information Website: Buried or Above Ground Oil Tank Inspection, Testing, Cleanup, Abandonment of Oil Tanks
• Oil Tanks Above Ground, UL Standards, guidance for home owners, buyers, and inspectors
• Plastic Heating Vent Pipe & Other Heating Safety Recall Notices
• Weil McLain Model GV Gas Boiler/gas valve CPSC recall/repair
• 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 Steam Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
• "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.)

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