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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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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.
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.
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.
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
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 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.
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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Sept 30, 2014) Jim Sambrook. Email: firstname.lastname@example.org said:
I am looking for a strap-on temperature switch to fit to the boiler outlet water pipe that switches the boiler on at a lower temperature and off at a higher temperature. Can you help.
NO Jim, to avoid conflict of interest and maintain reader trust we don't sell anything. Stop by your local heating supplier for an aquastat controller but better, install an immersion well type if you can.
(Oct 18, 2014) Mark said:
I have an Addon Harmon wood boiler and a peerless gas boiler. I already have Johnson controls on the wood boiler and I was thinking of adding a Honywell 4006 strap on Aquastat like there was before. What shot I set these controls to. 160-180 for the main circulator? And 140-160 for the strap on so the Gas comes on when the Wood boiler goes out?
Please see WOOD-OIL COMBINATION HEATERSfor a discussion of the tricks of controlling these systems.
(Nov 14, 2014) Tony said:
I have two AquaStats snap on, recently one has been rusting due to a water leak on the Actuator's valve. The leak happens when when the AC is on, during Heat mode, the Actuator closes one of the valve and there no leaks. Are both of the AquaStats snap on suppose to be on the same pipe (Heat). Is there a way I can send you pictures as I have model numbers and comparisons pictures of new replacement parts and picture of my parts.
Not necessarily: where the aquastats are mounted depends on what each is controlling. Use our page bottom CONTACT link to send me system photos for comment.
(Nov 20, 2014) WadeMc said:
I have set up my wood/gas boiler, I included an emergency bypass N/O held closed by pass valve incase of a runaway wood boiler. I was to use a clip on RTD to attach to a 1" pipe. What kind of set up, or aquastat will work at a tripping temperature of 180 degrees F?
Watch out: I don't have the whole picture but this does not sound safe to me. Normally one protects a boiler from explosion using a properly sized and located TP relief valve not an electrically operated zone valve. Solid fuel systems also include damper controls to avoid overheating.
(Nov 28, 2014) Anonymous said:
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.
I think this might be wrong (top right photo).
The lower scale is Celsius (96 C. to 102 C.).
So in Fahrenheit (upper scale) it's 205 F. to 215 F.
Please correct me if I'm wrong. I'm trying to figure out why there are two temperature scales. I'm also trying to figure out why someone would keep their boiler at 102 F. I'm also trying to figure out why some one would keep their boiler at 215 F.
Indeed on the limit switch control dials in one of our photos above you can see both F (Farenheit) and C (Celsius) temperature scales - the manufacturer intends the control to be used in marketplaces that use either of those temperature reading scales.
You can convince yourself that you understand the scale by noting that at 200F on the Farenheit scale you'll see the Centigrade scale reading about 93C.
Keep in mind that these instruments are not lag grade in precision. A service tech may have adjusted a setting slightly up or down from common levels to obtain equipment performance that was wanted.
Typically we won't see a residential heating boiler operating at over 200F.
We might see domestic hot water temperatures (water heaters, or boiler water when only a tankless coil is in use) at much lower temperatures, typically under 120F.
(Dec 4, 2014) Jim said:
We just bought a house with an old oil-fired boiler with a single zone and single circulator pump. It has OLD strap-on limit controls, both on the hot side just above the boiler. One limit control is the high temperature shut-off for the burner, the other controls the circulation pump. The boiler is currently set to 180, which seems right to me. My question is, what temperature should the circulation pump temperature sensor be set for?
Jim, typically the high limit control is set 180 to 200. I like 200 as more efficient. A low limit on your boiler may be used with a tankless coil -
is one In Use on your system?
If so it may want to lock out the ccirculator to give priority to the coil at temps 120 or below.
(Dec 4, 2014) Jim said:
Thanks for the response. This is an old stand-alone oil fired heating system with a multi-section cast iron boiler with no tankless coils. I'm trying to get the best efficiency out of it, so I will test the actual temperature with an infrared thermometer and set the high limit to 200. With regard to the circulation pump, I'm trying to determine the correct temperature to start/stop the pump to obtain the best efficiency. My instinct tells me that the pump should run whenever the water temp is heating the house (within reason...for example, when heating the house to 70, shut off the pump at 90-100), but that may not be the most efficient thing to do. What do you think?
Watch out: one of the down sides of those old strap-on controls is an inaccuracy in sensing temperature - the control is depending on good thermal contact between the pipe surface and the device sensor.
I'm unclear on how the lower limit is being used on your system, but most likely it's just as you say - intended to operate the circulator and includes two ranges - a cut-in and a cut-out. Send me photos and brand, and part number and we can dig up the specs and instructions. Use the email at our CONTACT US link found at page bottom or top.
There is argument about circulating water temperature limits. IN some areas (including much of Canada) circulators run as long as the boiler is switched on, and the TT just turns the burner on and off. Benefits are more even heat distribution and reduced risk of freezing.
(Jan 7, 2015) Mike said:
I have a syste 2000 Ek-2 f I stupidly moved the seting on the white - Rodgers aqua stat. It has two permanent tabs and a screw with two gears which rotate. Can anyone tell me where to put these back to factory settings? I guess meaning first, the main dial should be pointed at the arrow at what temperature. Then my next question would be the arrow with the screw on it needs to be set at what number? Help please.
Mike if you are discussing a single-purpose limit control most likely the large dial is the upper limit and a smaller dial on the side of the unit is a DIFF setting. But I'm not sure - maybe if we saw a sharp photo of the control (use the email at our CONTACT link at page top or bottom) or if you tell me the aquastat model number we can be more specific. There should be a label inside the aquastat cover that gives brand, model and maybe even setting information. With that data you or we can find the installation and operation manual for the control.
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