Limit switch on a steam boiler tankless coilGuide 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
  • POST a QUESTION or READ FAQs about temperature limit controls used on heating boilers: strap-on type limit controls & aquastats - diagnosis, repair & replacement advice

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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.

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Guide to Single-Function Heating Boiler Limit Controls

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

Limit switch on a steam boiler tankless coil Limit switch on direct vent Energy Kinetics Sysetm 2000 boiler

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.

Multiple single limit controls on an old gas fired converted coal boiler (C) HH

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

Strap on aquastat (C) Carson Dunlop Associates Strap on aquastat (C) Carson Dunlop Associates

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.

Loose aquastat is unsafe (C) Carson Dunlop Associates

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:

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.

Reader question: where to buy a Klixon replacement limit control for an American Standard heating boiler

todd said:
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:

Sensata Technologies
529 Pleasant Street, MS B19
Attleboro, MA 02703-2964
Phone 1-888-438-2214

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 Gas and 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.

Sensata Klixon fixed temperature snap action thermost switches - Sensata Klixon Stat on Stilts snap action thermosat switch examples -

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.


Continue reading at LIMIT SWITCH, BOILER or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see AQUASTAT CONTROLS used on heating boilers & some water heaters

Or see FAN LIMIT SWITCH for limit controls on forced warm air heating systems.

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