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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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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 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.
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:
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