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Guide to Heating System Boiler Aquastats, their Settings & Wiring
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• Guide to the functions of a typical heating boiler combination control aquastat like the Honeywell R8182D, the Honeywell L8124A, and the Honeywell L8151A.
• Guide to the Honeywell 6006 Single Function Boiler Limit Control
• How to set the HI LO and DIFF on a heating boiler control
• Links to common heating control aquastat installation and adjustment instructions
• Heating efficiency and boiler temperature explained
• How to save money by setting the Honeywell R8182D and similar controls optimally

Here we discuss the functions and best settings for a heating boiler aquastat - the combination control that sets boiler temperature and may also control hot water production via a tankless coil on the heating boiler. 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 Honeywell R8182D combination heating control, also called an "aquastat".

Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

© Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links 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.

Guide to the Reset Button on Heating Boiler Controls - Aquastats & Primary Controllers

Our photo (left) shows an older aquastat type primary controller on an oil fired heating boiler. You can see the gray box covering the control in the right in our photo.

The coiled "wire" leaving the upper right corner of this control is routed to a thermal sensor mounted in a well inserted into the water of the heating boiler.

Unlike the aquastat shown at the top of this page, in the cover of this unit you'll see a red "reset button" in the center of the upper portion of the gray control box.

If this heating boiler has turned itself off on SAFETY OFF you'll see that the button will have popped up higher out of the control cover.

If the red "reset" button on the this safety control 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. Please be sure to read our more detailed instructions for using the reset button on heating equipment at our article: Cad Cell Relay Switch Flame Sensors.

Where are all the heating system reset buttons? If you are looking for the main reset button on heating equipment you'll want to see: Aquastat Functions and Cad Cell Relay Switch Flame Sensors (hot water boilers and some water heaters), Stack Relay Switch on older oil fired boilers and furnaces, SPILL SWITCHES (gas fired equipment), and also Low Water Cutoff Controls on steam heating systems. At ELECTRIC MOTOR OVERLOAD RESET SWITCH we discuss the thermal overload switch and reset button that is found on many electric motors including those operating air conditioning fans, heating system oil burners, and furnace blowers and motors.

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

What are the Three Controls & Dials We See Inside the Cover of the Honeywell R8182D Aquastat?

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 building, 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.

More about How the HI and LO Limit Controls Function on an Aquastat

This is a little repetitive but is written in response to questions from our readers. On primary controls (aquastats like the like the Honeywell R8182D, the Honeywell L8124A, and the Honeywell L8151A) the HI Limit control (and dial) operates at a heating boiler water temperature range defined as follows:

HI sets the boiler Cut-off temperature or burner turn-off temperature on a call for heat - the temperature to which the HI dial is set.

The boiler Cut-on temperature or burner turn-on temperature for the boiler, AS LONG AS THE THERMOSTAT IS CALLING FOR HEAT is fixed at 15 deg.F. below wherever the HI is set.

So if HI is set to 200 degF that's the cutout temperature, and the cut-on temperature for the boiler, as long as the thermostat is asking for heat, will be (200 - 15) = 185 deg.F.

The instructions tell us to set the HI at least 20 DegF above the LO. So during the heating season as long as the thermostat is calling for heat the HI is doing all the work.

LO sets the heating boiler Cut-off temperature or burner turn-off temperature when the thermostat is NOT calling for heat - in order to keep the boiler hot for making domestic hot water with a tankless coil.

During the summer, because the thermostat never calls for heat, the HI control will never turn on the boiler. The HI limit operates a relay that is turned on by the thermostat and turned off by the boiler reaching the HI or cutoff point.
Out of heating season, your thermostat stops asking for heat, so the HI goes to sleep.

Out of the heating season, the LO gets to work - keeping some heat in the boiler so that the tankless coil will be able to produce hot water. Unlike the HI whose Cut-on is hard-wired, the Cut-on for the LO range is adjustable by the DIFF or differential dial. The DIFF ONLY talks to the LO control, it has nothing to say to the HI control.

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 technician 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 relief 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.

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 removing a jumper wire (usually the blue wire pointed-to by our pen, next to the DIFF control. Next to each control wire push-in connector is a small rectangular slot.

WITH ELECTRICAL POWER OFF pressing a tiny screwdriver or even a small nail into the slot will release spring pressure so that the wire can be pulled out of its connector. As you can see (photo at left), we placed a small twist-on connector electrical cap ("wire nut") onto the un-used blue wire tip after the wire was disconnected.

Short boiler "on" cycles? 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.

Cast iron boiler leaks? 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.

Guide to Single-Function Heating Boiler Limit Controls

Use of a single function limit switch on boilers & tankless coils

Here (above left) is a photo of a more traditional single-function heating boiler limit controls. In this example the aquastat or limit switch is being used on a tankless coil that happens to be on steam boiler. If you click the image to enlarge the photo, you'll see that the temperature limit on the control is set to about 140 degF.

Our second photo (above right) shows a limit switch used on direct vent Energy Kinetics System 2000 boiler installed in 2004. Click to enlarge the right hand photo and you'll see that this limit control is set to keep the boiler temperature between 96 degF. and 102 degF. A dab of red paint show at the 5'o-clock position on the dial enables the service technician to detect if someone has adjusted the control off of its factory setting.

When a heating boiler uses a tankless coil to produce domestic hot water, a third single-function control may be installed for that purpose. In this photo a Honeywell limit control switch is being used to monitor hot water temperature at the tankless coil which is in turn mounted on a steam boiler of an older home in Portland, Maine. You can see the black-handled mixing valve in the lower right of this photo.

Cold water from the building is entering the tankless coil via the bottom pipe (green corrosion) and hot water, heated by the coil is leaving at the upper part of the tankless coil, where it turns downwards to enter the left side of the mixing valve.

Additional cold water is permitted to enter the bottom of the mixing valve, and tempered (non-scalding) hot water then leaves at the right side of the mixing valve in this photo.

More information about tankless coils, how they work, what goes wrong, and their controls, is at Tankless Coils.

Use of Single Limit Control Switches to Control Heating Boiler Hi and Low temperature settings

On heating boilers where individual limit controls are installed, you will find two similar devices, but mounted at different locations on the boiler, controlling the boiler's HIGH limit, or cut-off on a call for heat, and the boiler's LOW limit or cut-on point as the boiler water temperature falls.

Strap-on Aquastats

An older type of aquastat used to set boiler temperature limits was strapped to the hot water riser pipe close to the top of the heating boiler. This device is shown in Carson Dunlop's sketch at above left, and in our photograph (above right)

Rather than a temperature probe inserted directly into the heating boiler's water (as used by modern aquastats) the strap-on type aquastat relies on metal-to-metal contact between the control and the boiler water riser pipe.

If your heating system is controlled by a strap-on aquastat such as shown here in Carson Dunlop's, you'll want to check that it has remained securely in place. If the aquastat becomes loose its contact with the hot water pipe is lost and the system is unsafe.

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• 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.

BOILERS, HEATING
  BOILER OPERATION DETAILS
  BOILER CONTROLS & SWITCHES
  Air Bleeder Valves
  Aquastat Functions
    Aquastat control HI LO settings
    Aquastat control DIFF settings
    Single Function Limit Switches
  BOILER LEAKS CORROSION STAINS
  BOILER LEAKS, HOW TO LOCATE
  BOILER OPERATING PROBLEMS
  BOILER COMPONENTS & PARTS
  Cad Cell Relay Switch Flame Sensors
  Circulator Pumps & Relays
  Expansion Tanks
  Gauges on Heating Equipment
  Limit Switches, Boilers
  Low Water Cutoff Valves, Boilers
  Mixing Valves
  Relief Valves - TP Valves
  Reset Switch - Primary Control
  Reset Switch - electric motors
  Spill Switches
  Stack Relay Switch
  Thermostats
  Water Feeder Valves, Hydronic Boiler
  Zone Valves
  WATER HEATERS
ZONE VALVES

• We provide links just below to several aquastat installation, setting, and adjustment documents in response to reader requests and comments that people sometimes have difficulty finding this information. But readers looking for specific aquastat control information should always first try the control manufacturer.
• Armstrong 24-hour time and aquastat control accessory: provides on/off pump control according to preset operating times, designed only for use with specified Armstrong Astro series wet roto circulators installed in indoor hot water re circulation applications. Typical timer applications will cycle the pump at preset times, allowing the user to select operation of the circulating pump during high peak usage periods in order to decrease the delivery time to outlet fixtures and reduce wasted water.
• Honeywell Corporation, control installation guides for Honeywell aquastats and other heating and cooling controls: see http://customer.honeywell.com
• Honeywell L4008E Aquastat high limit,manual reset, aquastat controller instructions
• Honeywell L4041A,C Pool Heater Aquastat Controller Instructions
• Honeywell L4006, Manual Reset Strap-On Aquastat (high limit control, 100 to 240 degF) [we consider strap on controls less accurate and reliable than immersion type sensor controls; these were commonly found on older heating boilers including the GE series downfired boilers]
• Honeywell L4026B Aquastat control instructions (contact Honeywell Corporation) Here is some basic description of the Honeywell L4026B control
• Honeywell L4029 Reset Limit Control
• Honeywell L4103A,B,C Combination Aquastat and High Limit Controller Instructions
• Honeywell L6006A Aquastat Controller installation instructions
• Honeywell L6006 and L6007 Aquastat Controllers, installation instructions for the trained service technician, these controls combine low or high limit and circulator control, with or without an immersion well depending on the control model A,B, C etc.
• Honeywell L6191 Dual aquastat immersion thermostat
• Honeywell L7224 Aquastat Controller, oil electronic controls allowing high limit, high limit differential, low limit, and separate low limit differential settings - set to the specifications of your oil heating equipment manufacturer - 69-1957-1 L7224 Series controls
• Honeywell L7224U Oil Electronic Aquastat Controller - 69-1720-2 series instructions. These newer aquastat controllers are often used to replace the older Honeywell L8124A series that was and remains widely used
• Honeywell L8124A,B,C,E,G,LO,M Aquastat Relays immersion type controllers for forced hydronic heating systems that include domestic hot water service (such as a tankless coil)
• Honeywell R8182D,H combination protectorelay Primary Control

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.
• 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 Aquastats 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 back flow 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
• 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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