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Guide to aquastat controls on heating equipment:
Beginning by using the Honeywell R8182D series aquastats, this article series explains how aquastats work and what the different aquastat controls are, what they do, and how they are set. We define the HI LO and DIFF controls on heating boiler aquastats and explains what they do and how they work. We explain the location and use of the heating boiler reset button found on aquastats.
We discuss relationship among HI, LO, DIFF, and heating burner cut-in, cut-out, and circulator lockout that are provided on the combination control. We explain how the Aquastat controls hot water production via a tankless coil on the heating boiler.
We explain how to disable the LO control when a tankless coil is not in use on a boiler. After explaining the operation of an aquastat's Hi, Lo, and Diff controls we review the newer universal replacement aquastat from Honeywell, the L7224U Universal Aquastat, followed by a discussion of the simpler single-limit control switches.
But with the cover on you can't see much. Here at left is part of an inside view of a typical aquastat showing the three controls that need to be correctly set.
[Click to enlarge any image]
If you just want to set the HI LO and DIFF controls and don't care how they work or what they do, see our separate article
On AQUASTAT HI LO DIFF SETTINGS
This article series answers most questions about Heating System Boiler Controls on central heating systems to aid in troubleshooting, inspection, diagnosis, and repairs.
Here we explain how to set and adjust the heating system controls to preserve this cold marriage.
Our photo 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.
Depending on wiring hookup choices, this control operates the heating system oil burner, circulator, and zone circulators. It can also be configured (factory default) to keep the heating boiler hot to provide domestic hot water through a tankless coil even when the building room thermostats are not calling for heat.
The aquastat controller often also includes connections to a flame sensing device, typically a cad-cell
(CAD CELL RELAY SWITCH) or on older heating systems a stack relay
(STACK RELAY SWITCH) to shut down the boiler if the burner is not operating properly.
There is a "reset" button to re-start the heating system if it has shut down for safety. We discuss all of these controls in detail below.
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, poking through the upper-center of the cover of this heating system control 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:
Watch out: sometimes the red plastic button on older heating equipment primary controls can break off.
At RESET SWITCH, HEATER REPAIR we show how to reset the equipment safely if this happens to you.
Where are the reset buttons? If you are looking for the a reset button that may have tripped off heating equipment see:
(Nov 23, 2015) Jackie said:
We have to hit the reset button on our furnace for it to run.
It runs for 4-5 heating/hot water cycles,then reset button again.
We took the cover off the Aquastat(L8124A) and saw a semi exposed wire and the circuit board looks like it been arced. Not sure if arcing mark is from close proximity to exposed wire or damage to board. Should I replace connector(to correct exposed wire) or aquastat or control box?
(6 March 2016) Kev said:
My overheat button knocks off boiler its a honeywall.is there a suggestion what might cause this
Watch out: repeated use of the re-set button risks a dangerous puffback explosion. That's because the burner that is shutting off on "safety" may leave un-burned fuel in the combustion chamber; it can accumulate until we have an ugly experience.
If you see arcing burns on the circuit board I suspect the problem may be more than the wire - perhaps overheating. I would ask for an inspection by your heating service tech to decide:
1. what's causing the system to go off on re-set
2. what caused the arcing you found
3. based on those steps, repair or replace what's needed.
I understand the appeal of just plugging in a new part - certainly I do that myself. But just swapping in a new control or circuit board without having an idea of what was happening risks having to repeat the repair.
If you are referring to the reset button on an oil fired heating system most likely the problem is with the oil burner: an inadequate or faltering flae. DO NOT keep pressing the reset button as you could cause a dangerous puffback explosion. I'd call for service.
Here we explain how to find and use the Aquastat reset button and how to find the HI LO and DIFF controls on aquastats. We explain how the HI LO and DIFF controls function to turn the burner on and off for both heating and for systems where a tankless coil is in use, also for making domestic hot water.
This article series answers most questions about Heating System Boiler Controls on central heating systems to aid in troubleshooting, inspection, diagnosis, and repairs.
At night we turn down the thermostat to 55°F to try to save on our heating cost.
But our hot water is made with a tankless coil. This morning my wife got out of bed and stepped into the shower for a nice hot wake up. She turned on the water, pretty hot, and stepped in to its comforting stream.
I walked over to the thermostat and turned it up to 90 - thinking (in error) that the thermostat is like an accelerator, and the higher I set it the faster the house would reach a comfortable 72 degrees (wrong). I figured my wife would be able to step out of the shower into a nice warm house.
Moments later I heard screaming from the shower as my wife got hit with freezing cold water. What happened? -- anonymous.
On a call for heat from the thermostat, the zone circulator starts when water temperature is above Low Limit setting (if applicable - that is, if this feature is enabled - the default).
The heating boiler temperature is checked. The burner starts when the water temperature is below High Limit setting.
When the boiler temperature reaches or exceeds the High Limit, the burner is turned off. The burner restarts when the
water temperature drops back below the High Limit setting minus the differential.
As long as the circulator is continuing to call for heat and provided the control has not "locked out" the circulator (as we discussed above) the zone circulator will continue to circulate hot water through the heating zone.
When the thermostat is satisfied - that is room temperature has reached the thermostat setting, the circulator and burner are turned off. Stated with a bit of simplification, on most common boiler control set-ups in the U.S. the thermostat turns the circulator on or off, and the temperature of water in the boiler turns the burner on and off.
(Heating systems in Canada operate differently: there most techs set up the circulator to run continuously if the boiler is "on" and a call for more heat at the thermostat turns the boiler's burner on until the thermostat is satisfied.)
Combination control or primary control on heating boilers: this control, such as a Honeywell R8182D combine High Limit and "Low Limit" boiler controls
(The dial marked "low limit" on a combination control may or may not be in use depending on presence of a tankless coil).
This primary control or aquastat is the most common type on modern heating boilers in North America. It
controls the oil burner operation, turning the burner on or off as the boiler low limit or high limit temperatures are reached respectively.
This control may
switch on and off a single circulator pump, and if a tankless coil is installed on the boiler, it may also turn the oil burner on and off as needed to maintain
temperature in the boiler to provide domestic hot water as well.
This control can also be used to control the burner and the first circulator of a multi-circulator zoned hot water heating system. The other circulators are controlled by individual circulator relays and thermostats.
Heating Control Operation Sequence: With some simplifications, the room thermostat calls for heat. The circulators start running, hot water leaves the boiler and heads for radiators or baseboards, cooler water returns from the building into the boiler. Boiler temperature drops to the cut-on or cut-in temperature.
The burner turns on and re-heats the heating water inside the boiler until boiler temperature reaches the cut-off or cut-out temperature. The circulators keep running, and the boiler burner may cycle on and off in this manner, until the room thermostat is satisfied. Then the thermostat turns off the circulators and the burner, if running, will shut off when the boiler reaches the HI limit.
In Canada boilers operate a bit differently: the circulator pump may be set to run continuously, and the thermostat just turns the burner on and off. This gives more even heat and helps avoid a frozen heating zone pipe in very cold climates. Our aquastat function explanation below is bases on the "American" model or as the Canadians say, "the Yanks".
[Click to enlarge any image]
HI Limit: Combination Control High Limit:
The HI cut off temperature for the burner: on a typical aquastat or combination control, the "Hi Limit" setting specifies the cut-off temperature for the heating boiler's burner on a call for heat.
That means that when the burner is running it will heat water inside the heating boiler until that water temperature reaches the HI or cut-off point. Then the burner is turned off.
The HI cut-on temperature for the burner is hard wired in this HI Limit control at 10 °F below the "Hi Limit" setting.
Example: Hi set to 200. Thermostat calls for heat. Circulators turn on. Hot water leaves the boiler, cooler water returns to the boiler, boiler temperature drops to 190, burner turns on and stays on until boiler temperature reaches 200 F.
The internal view of the Honeywell R8182D (left) can be found along with additional details about this control in the Honeywell R8182D,H Combination Protectorelay™ Primary Control and Aquastat® Controller Installation Instructions (link to copy below at REFERENCES).
Following wiring details in the instruction manual, the R8182D,H can replace other aquastat controls such as the Tradeline R8182B, R8182E, R8182F, R8182H, R8182J, or White Rogers 6C92.
A newer combination control from Honeywell, the
HONEYWEELL L7224U Oil Electronic Aquastat Controller [Web article] can replace more than 40 older hydronic controls.
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, as we describe here.
What the LO limit actually does: The LO limit 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 and when the room thermostat's calls for heat are not already keeping the boiler nice and hot.
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 limitOn boiler temperature that applies out of the heating season or when the thermostat is not calling for heat.
On typical aquastat heating control the LO or "Low Limit" is NOT the "cut on" point for heat (we explained the heat cut-on just above).
It is helpful in understanding the LO limit to call this the "circulator" controller. Provided that DIFF is set to 10 (its smallest value), then
when boiler temperature falls 10 °F below the LO setting, the burner turns on and the circulator is forced to turn off - we call this "circulator lockout" - we are re-heating water in the boiler and we are preventing boiler water from being sent out to the baseboards or radiators - giving priority to heating the tankless coil.
when boiler temperature rises back up to the LO setting, the burner turns off and the circulator is allowed to turn on - we are unlocking the circulator and allowing the building to be heated again - and taking priority away from giving heat to the tankless coil.
We say the circulator is allowed to turn on because if the room thermostat is not calling for heat, the circulator may have permission to turn on, but the thermostat is leaving the circulator(s) turned off (in the U.S., not on many Canadian heating systems).
And note that we say that the burner turns off as the boiler temp rises up through the LO setting. But if the boiler is being turned on from a fully cooled-down "cold" condition and the room thermostat is calling for heat, the burner will turn on and keep running right up through the LO and on to the HI. Why? Because the thermostat was calling for heat.
Circulator Lockout: The LO limit works in concert with the DIFF setting to control the operating temperatures of the boiler when it is being asked to heat the tankless coil (used for making domestic hot water for washing and bathing), and together they also control when the circulator pump should be locked out so that priority is given to keeping the boiler itself hot - presumably because you're in the shower and the tankless coil is in use.
At AVOID CIRCULATOR LOCKOUTwe provide more detail about this function.
The DIFF or "differential" setting on an aquastat adjusts the LO range temperature cut-off point when boiler temperature is rising (the burner is on). The DIFF ONLY talks to the LO control, it has nothing to say to the HI control.
Even though the LO and DIFF settings will allow the circulator to run at certain temperatures, the circulator pump(s) still won't run unless the room thermostat is calling for heat.
When boiler temperature is falling: DIFF settings at values other than 10 do not affect the temperature at which the burner turns on and the circulator locks out when boiler temperature is falling.
When boiler temperature is rising: on this control, the "DIFF" or differential control dial specifies the amount above the "Low Limit" to which boiler temperature must rise before the burner will turn off and the circulator will be allowed to run.
We said the circulator pump is allowed to run because if the thermostat is not calling for heat, even though the DIFF + LO are allowing it, the circulators will not run (in the U.S.).
Watch out: as we explain below, realizing that 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, 10, thinking that means the burner will come on as soon as possible when it's needed to make hot water out of the heating season.
As Damian [image] pointed out in email, that's a mistake if you want as much hot water as possible out of your tankless coil.
At this "lowest" DIFF setting of 10, on a boiler temperature rise, the burner turns off (R-B breaks) and the circulator turns on (R-W makes, to deliver heat) at the LO limit (and "circulator setting" setpoint. So if the LO is set to 120 and the DIFF is set to 10, when the boiler temperature rises to 120 the burner turns off and the circulator is allowed to run - that is to circulate water out of the boiler to the baseboards or radiators.
We call the LO the the "circulator lockout" control or the "tankless coil control" to try to make this more clear. In other words, when the LO is satisfied (we have hot water for the tankless coil), then we can go back to delivering heat to the building IF the building room thermostat is asking for heat.
At any DIFF setting above 10, the (R-B make - burner on) and (R-W break - circulator off) temperatures remain the same as we just described above. LO control setting minus ten degrees.
But the (R-B break - burner off) and the (R-W make - circulator on) temperature will now change to be the LO set point temperature plus the difference between the DIFF set point number (for example 20) and ten degrees.
Honeywell gives a helpful example: (refer to the yellow area in the sketch to help understand this feature) [we edited the original text slightly for clarity]:
If the LO is set to 140 °F, and DIFF is set to 25 °F, on a temperature rise, the oil burner turns off (R-B breaks) and the circulator is allowed to run if the room thermostat is asking for heat (R-W makes) at 155 °F (25 °F - 10 deg = 15 degrees; 140 + 15 = 155).
And when the boiler temperature falls, the burner turns on (R-B makes) and the circulator will not be allowed to run (R-W breaks) at 130 °F (LO of 140 - 10 °F).
Adjusting the DIFF (differential) to numbers higher than 10 does not change the boiler turn-on temperature ("R-B make") nor the circulator turn-off temperature ("R-W break"). But it does affect the boiler turn-off temperature (R-B break) and the circulator turn-on temperature (R-W make) as follows: the new boiler-off and circulator-on temperatures will be equal to the LO setting plus (DIFF-10).
LO = 120 F, DIFF = 10 F: when the boiler temperature drops to 110 the burner turns on and the circulator is turned off. As the burner re-heats the boiler and the boiler temperature rises back up to 120, the burner turns off and the circulator is allowed to turn on.
LO = 120 F, DIFF = 25 F: when the boiler temperature drops to 110 the burner turns on and the circulator is turned off, just as before. But as the burner re-heats the boiler and the boiler temperature rises back up to 135 F, the burner turns off and the circulator is allowed to turn on. We calculated the 135F as follows: LO setpoint of 120 is added to (DIFF minus 10) or 120 + (25-10) = 135.
LO= 140 F, DIFF = 25 F: when the boiler temperature drops to 130 F the burner turns on and the circulator is turned off, because the burner-on temperature is always fixed at 10 below the LO, just as before.
But as the burner re-heats the boiler and the boiler temperature rises back up to 155 F, the burner turns off and the circulator is allowed to turn on. We calculated the 155F as follows: LO setpoint of 140 is added to (DIFF minus 10) or 140 + (25-10) = 155.
The effect of setting the DIFF up from 10 to 25 is that when the burner is re-heating the boiler (for example while the tankless coil is in use and you're in the shower), the burner heats the boiler temperature up to a higher level before the burner is turned off and the circulator is allowed to turn back on.
This gives more heat to TANKLESS COILS and therefore more domestic hot water to the building occupants.
Even many heating service technicians (those who failed to read the installation instructions for the control they are servicing) are confused about the relationship among the three adjustable controls on the aquastat: the High Limit Setting, Low Limit Setting, and Differential or "Circulator Setting".
Our text (below) and our adaptation of Honeywell's sketch (left) explain these functions.
[Click to enlarge any image]
Here we reiterate the explanation above, trying a little different approach that may help some 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 (High Limit) 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.
For most aquastats such as in the Honeywell R8182D,E,F,H,J Combination ProtectoRelay™ hydronic heating controls, the High Limit works the same: the high limit "opens" (disconnects) a switch to turn off the burner when water temperature reaches the HI set-point.
As water temperature in the boiler later drops a fixed value below the HI set-point the control automatically re-sets and turns the burner back ok provided that also a thermostat is calling for heat.
In sum: 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 10 degF below wherever the HI is set. (Green in our edited version of Honeywell's drawing.) On some controls this hard-wired fixed gap may be 15 °F.
So if HI is set to 200 °F 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 - 10) = 190 degf (or 185 °F on controls with a 15 degree fixed gap.)
See details about the HI, LO and DIFF optimal settings
Watch out: the details of how the Aquastat HI, LO and DIFF function as described here pertain to the R8182-type aquastats. Some newer programmable aquastats such as Honeywell's
AQUASTAT L7224U UNIVERSAL provider additional features such as a programmable HI-DIFF control.
On combination controls such as the Honeywell R8182-series Protectorelay™ aquastat, the LO and DIFF are used to control the boiler when it must provide heat for a tankless coil used to produce domestic hot water (for washing & bathing).
These two controls work in a way that has confused generations of homeowners and even some heating service technicians. We'll give a couple of explanations of how these controls work.
When discussing the LO and DIFF settings, the of the following actions occur in the yellow colored range on our illustration and all of them occur with respect to the LOW LIMIT setting on the aquastat:
With the DIFF setting set at its lowest position of 10 degF [for example]
with the LOW LIMIT setting at 140 degF [to use Honeywell's example]
1. On boiler temperature rise up to the LOW LIMIT SETTING (e.g. 140F )the circulator is turned on (or is allowed to turn on) and the burner is turned off.
[This "burner-off" may be over-ridden and the burner may be turned "on" IF at the same time the room thermostat is calling for heat.]
On our illustration this is indicated by the horizontal line in the middle of the yellow-area and labeled on the left as "Low Limit and Circulator Setting" and labeled also on the right as "Switch Makes R-W" and Breaks R-B on Temperature Rise.
2. On boiler temperature further rise up to 165 (140F + 25F = 165F where that 25F is hard-wired into the control) the circulator is turned on (or allowed on) AND the burner is turned OFF.
[As in case 1 above, this "burner-off" may be over-ridden and the burner may be turned "on" IF at the same time the room thermostat is calling for heat.]
This is equivalent to saying that the boiler is hot enough to both supply domestic hot water thorough a tankless coil AND to permit heat to be delivered to the heating baseboards or radiators if the temperature is in this range (140-165F).
This point is at the top of the yellow in our illustration.
Honeywell points out in their triangular note 1 that when water temperature reaches 165 degF (LO + 25F) the burner is turned off OR IF there is a call for heat the circulator pump will run.
Note: If heat is being called-for the circulator will continue to run, dropping the boiler temperature as cooler water returns from the chilly building back into the boiler. When the boiler temperature drops the burner will be turned back on as discussed in these notes.
3. On boiler temperature fall to 130F (140F - 10F set on the DIFF) the aquastat will turn ON the burner ("Makes RB"), and will turn OFF the circulator ("Breaks RW").
The purpose of this point is to stop circulating hot boiler water through the heating baseboards or radiators in order to give my family member, standing in the shower with a head full of shampoo, priority for receiving hot water.
What about other DIFF settings. Here the Honeywell text can be a little confusing.
Using the same LO setting of 140F let's set the DIFF up to 25F.
Honeywell says the Burner ON and Circulator OFF temperatures remain the same - LO setting MINUS 10 degF where that 10 is hard-wired in the control.
Honeywell explains that the Burner OFF and Circulator ON (or allowed-on) are the LO setting PLUS the new DIFF setting MINUS the hard wired 10F.
On boiler temperature rise up to 155F the circulator is turned ON (or is allowed to turn on) and the burner is turned OFF.
The HIGHER DIFF setting is actually giving a BIGGER heat-up range for the LO limit control operating range. So the boiler heats up to the LO (140degF) + [net] DIFF 15 degF = 155 degF.
The [net] DIFF is calculated as DIFF Setting (25 degF) minus the hard-wired 10 degF differential in the control. (25-10=15 = net DIFF.
This net diff secret explains why the HI must be set at least 20F above the LO to avoid locking out the circulator.
If a company wanted to design a control and explain it to cause confusion Honeywell deserves a gold star. We've had countless conversations with both readers and service technicians about these settings. In my experience quite a few people, including heating service techs are confused about what the LO and DIFF do and how they work.
The LO must be set at least 20 deg F. below the "HI" limit on a combination control. If we set the "LO" any closer top "HI" 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.
DIFF (Differential or Circulator): the Cut-OFF for the burner when the system is operating in the LO range (i.e. the room thermostat is not calling for heat) is adjustable by the DIFF or differential dial. (Yellow on our edited version of Honeywell's drawing, above).
DIFF specifies the number of degrees above the LO setting at which the burner will turn OFF and the circulator will be allowed to turn ON when the burner has been on and water temperature in the boiler is rising.
Thanks to readers powderfinger5 and Stan S. for careful reading of this text.
We explain more about the DIFF function in detail
For boilers that do not use a tankless coil to make domestic hot water,
at AQUASTAT LO & DIFF DISABLED we explain in detail how and why you might want to disable these controls.
AQUASTAT L6006A HONEYWELL GUIDE [Web article]
Here is a link to detailed for the HONEYWELL L6006A AQUASTAT Installation & Operating Instructions [PDF]
This topic has moved to a separate article.
Please see AQUASTAT L8148 HONEYWELL
Watch out: although it may be marked as "option" in some company illustrations, the company also provides and recommends use of a heat-conducting grease when installing the L606A high limit controller into its immersion well on the heating appliance.
Without good thermal contact between the temperature sensing bulb [ INSERTION ELEMENT in the illustration above and shown in detail just below] the control may not operate reliably.
[Click to enlarge any image]
You will read in just about all of the installation instructions for all aquastats that use a sensor in an immersion well that proper placement of the sensor in the well and use of a thermal or heat conductive grease are essential for safe, consistent, reliable aquastat operation.
At AQUASTAT TROUBLESHOOTING you'll read more about the immersion well and how it affects aquastat operation.
Continue reading at AQUASTAT HI LO DIFF SETTINGS that explains the best settings to use & how & when to disable the LOW & DIFF controls or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
Or see AQUASTAT CONTROL FAQs - questions and answers about boiler control aquastats posted originally on this page.
Or see MANUALS for HEATING SYSTEM CONTROLS for a list of aquastat installation & repair guides
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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.