Guide to Heating System Boiler Limit Controls & Strap-On Aquastats, their Settings & Wiring
LIMIT CONTROL, SINGLE - CONTENTS: Guide to single limit controls on heating boilers and water heaters. Guide to strap-on aquastats or limit controls on heating boilers and water heaters, functions, applications, test procedures. Links to common heating control aquastat installation and adjustment instructions. Heating efficiency and boiler temperature explained
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Single function boiler limit control switches, strap-on aquastats or limit controls on heating equipment: here we explain the functions and typical settings single function limit controls and for strap-on limit controls or aquastats used on both hot water heating boilers and on water heaters
This article series 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.
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 °F.
[Click to enlarge any image]
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 °F. and 102 °F. 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 Two Individual 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.
We noticed three individual limit controls on this steam boiler in a photo contributed by reader H.H. who was discussing expansion tank draining.
There are two strap-on limit switches on the riser pipe at right, and an immersion-type limit switch that has been added over a plumbing tee atop the boiler behind and to the left of the steam gauge. I'm not sure that that immersion limit switch is installed according to the manufacturer's instructions.
They may want the switch mounted directly on the boiler not on a tee and riser above the boiler - just speculating.
Guide to Strap-on Aquastats as Heating Boiler or Water Heater Controls
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 and in our photograph above. [Click any image to see an enlarged version.]
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.
How to test the limit control switch
Reader question: How do I test my Operator Limit Control.on my Lochinvar boiler to see if it is bad? - Kelley 12/15/2012
Most single function limit controls do not have owner-accessible test functions, buttons, or devices. Here are some simple tests that a trained heating service technician might do with a single function limit switch:
Compare the actual boiler temperature (on the boiler gauge) to the setting on the limit control. If you see the boiler temperature above the setting on the limit control we suspect the control is not functioning or is not properly mounted or installed. For example poor contact between the control sensor and the heat source can cause misreading of boiler temperature. Don't panic if the difference is just a degree or two - as these controls are not that precise.
With the boiler operating at its normal maximum temperature - after it has been running continuously for five minutes or more, notice the actual boiler temperature on the boiler TP gauge. Now set the limit switch to 5-10 degrees below the actual current temperature. That should cause the oil or gas burner to shut down. I must say that while this procedure should work, it is not a part of normal heating system service and inspection, and it's something I never did during my years of work as a service tech.
Watch out: don't keep cycling the control setting back and forth, turning the oil or gas burner on and off repeatedly. Turn off the boiler, wait five minutes, then restore the limit control to its original setting. Then turn the heating boiler back on.
Inspect the control for visible damage or worries such as leaks onto or into the control, loose mounting, or signs of overheating of wiring or electrical components, bent components, tampering. And for strap-on controls, see our warnings about proper control mounting.
If you give us the brand and model of your limit switch we (or you) can ask the manufacturer for a copy of the installation and maintenance instructions for that specific device. Often the instructions contain specific advice about testing the control. In general, information about Lochinvar heating equipment is available from the company - give them a call at 615-889-8900, or contact Lochinvar Technical Service at 1-800-722-2101.
Reader question: where to buy a Klixon replacement limit control for an American Standard heating boiler
I am working on an american standard boiler that has two klixon limits for high limit control, Stemco K4, one at 198 the other at 220. are they still available 2/25/2014
Reply: Klixon - Sensata Snap-Action thermostatic sensors for HVAC/R applications
Todd, I don't know if the original Klixon limit controls you describe are still available, but the company is certainly around. You can contact the manufacturer at:
529 Pleasant Street, MS B19
Attleboro, MA 02703-2964
Sensata is a huge company, with offices world-wide, so you'll want to make clear what you want so as to be directed to the proper supply outlet without a lot of arm waving. The Klixon type limit controls rely on a snap-action temperature sensitive electronic switch that may be replaceable if that's all that's failed.
Send me sharp photos of the parts you seek and I may be able to comment further - use the CONTACT link found at page bottom or top.
For forced air furnaces the company describes their HIGH TEMPERATURE LIMIT THERMOSTATS -
1NT Series “Stat-on-Stilts”™
Extended Airstream Mount for
Electric Furnaces . I found this by searching the company's site for "furnace limit control".
Designed as a safety switch for gas and
electric furnaces the Sensata “Stat-on-Stilts”™
may be used anywhere a temperature switch
must be mounted remotely from electrical
terminations. The most common application
is as the primary temperature limit in
gas furnaces and unit heaters.
At below left you can see Sensata's Klixon snap action thermostat switch examples and at below right their intresting "Stat-on-Stilts" sensors. But to choose the correct product you'll need to consult the limit switch manufacturer directly.
For hydronic boilers the company describes Thermal cutouts includine the MA10, TH10, TH11, & TH21 and heating line break thermostats include the 20600 Series and 20490 series.
Also see FURNACE FAN CYCLES DURING HEAT where we discuss an example of using a snap-action heat sensing switch in furnaces, the Termo-o-Disc fan limit control. In that article we list sources of snap action type thermostat switches and controls for HVACR equipment.
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Home Inspection Education Home Study Courses - ASHI@Home Training 10-course program. Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on these courses: Enter INSPECTAHITP in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
The Home Reference Book, a reference & inspection report product for building owners & inspectors. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
The Home Reference eBook, an electronic version for PCs, the iPad, iPhone, & Android smart phones. Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter inspectaehrb in the order payment page "Promo/Redemption" space.
The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors. Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
The Horizon Software System manages business operations,scheduling, & inspection report writing using Carson Dunlop's knowledge base & color images. The Horizon system runs on always-available cloud-based software for office computers, laptops, tablets, iPad, Android, & other smartphones.
 Thanks to Bottini Fuel service manager Ron Thomas for discussing aquastat functions, low limit controls, oil burner short cycling causes, and boiler maintenance, reliability, and service contracts 4/13/2010. Bottini Fuel is a residential and commercial heating oil distributor and oil heat service company in Wappingers Falls, NY and with offices in other New York locations. Bottini Fuel, 2785 W Main St,
Wappingers Falls NY, 12590-1576
(845) 297-5580 more contact information for Bottini Fuel
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, also available at www.honeywell.com
 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 L4006, Manual Reset Strap-On Aquastat (high limit control, 100 to 240 °F) [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, also available at www.honeywell.com
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., also available at www.honeywell.com
 Honeywell L6191 Dual aquastat immersion thermostat, also available at www.honeywell.com
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, also available at www.honeywell.com
 Lochinvar High Efficiency Water Heaters, Boilers & Pool Heaters, Lochinvar, LLC World Headquarters, 300 Maddox Simpson Pkwy., Lebanon, TN 37090, Tel - Customer Service:
615-889-8900, contact Lochinvar Technical Service at 1-800-722-2101 web search 12/15/2012, original source: http://www.lochinvar.com/
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
Or choose the The Home Reference eBook for PCs, Macs, Kindle, iPad, iPhone, or Android Smart Phones. Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter INSPECTAEHRB in the order payment page "Promo/Redemption" space.
Carson Dunlop, Associates, Toronto, have provided us with (and we recommend) Carson Dunlop Weldon & Associates' Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
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.)