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Guide to aquastat controls on heating equipment: this article 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. We discuss aquastat problems such as improper adjustment, improper wiring, and relay buzzing or failures in a separate article at AQUASTAT TROUBLESHOOTING.
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Aquastat Guide to the Reset Button & HI LO & DIFF Temperature Settings on Heating Boiler Controls - Aquastats & Primary Controllers
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]
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. CONTACT us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.
How to Find and Use the Reset Button on the Aquastat Combination Control on a Boiler
Summary of the Functions of a Typical Combination Protectorelay™ Primary Control and Aquastat® Controller
Our photo (left) 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, and a "reset" button (Reset Switch - Primary Control) 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: CAD CELL RELAY SWITCH.
Watch out: sometimes the red plastic button on older heating equipment primary controls can break off. At Reset Switch Broken - Quick Repair we show how to reset the equipment safely if this happens to you.
Where are all the heating system reset buttons? If you are looking for the main reset button on heating equipment you'll want to see: AQUASTAT CONTROL Functions and CAD CELL RELAY SWITCH (hot water boilers and some water heaters), Stack Relay Switch on older oil fired boilers and furnaces, SPILL SWITCHES (gas fired equipment), and also Low Water Cutoff Controls on steam heating systems. At ELECTRIC MOTOR OVERLOAD RESET SWITCH we discuss the thermal overload switch and reset button that is found on many electric motors including those operating air conditioning fans, heating system oil burners, and furnace blowers and motors.
Guide to Function & Settings on Combination or Multi Function Primary Controls (Aquastats) for Heating Boilers
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.
Basic Boiler Operation & Control Sequence using an Aquastat
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
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.)
What are the Three HI LO DIFF Controls & Dials We See Inside the Cover of the Honeywell R8182D Aquastat
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".
How the Aquastat HI Limit Settings and Functions Work to Control Building Heat
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 Honeywell L7224U Oil Electronic Aquastat Controller can replace more than 40 older hydronic controls.
How the Aquastat LO Limit Settings and Functions Work to Provide Domestic Hot Water & Circulator Lockout
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 limit on 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 Avoiding Circulator Lockout we 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 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]:
Adjusting the DIFF 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).
More HI LO DIFF Setting Examples:
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.
This Honeywell Aquastat Chart Further Explains of How the HI & LO Limit & DIFF Controls Work on an Aquastat
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 Honeywell's sketch (left) explain these functions.
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.
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 deg.F. 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 deg.F. (or 185 °F on controls with a 15 degree fixed gap.)
See details at Aquastat control HI LO settings
The "LO" is 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). Thanks to reader powderfinger5 for careful reading of this text. 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.
We explain more about the DIFF function in detail at Aquastat control DIFF settings.
Abandon or disable the LO setting if a tankless coil is removed from use?
For boilers that do not use a tankless coil to make domestic hot water, at Disabling the LO and DIFF settings we explain in detail how and why you might want to disable these controls.
But first review the details at AQUASTAT HI LO DIFF SETTINGS
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Frequently Asked Questions (FAQs)
Question: What are the Optimal LO & DIFF settings for moderate hot water use in summer?
What would be the optimal setting for the LO & DIFF settings for fuel savings with low and with moderate hot water usage in the summer months when the boiler functioning for heat does not come into play. If you would please identify the low usage from the moderate usage settings it would be greatly appreciated. - Mark
This is an excellent article on the DIFF setting as it pertains to using a Tankless Coil. During the summer in North Carolina when outside temperatures are in the 90's and my system never calls for heat, I am only running my Burnham boiler to provide hot water. It is not unusual to burn 1 gal of oil/day (30 gals / month) just for residential hot water. With oil prices well above $3.50/gal, $100/ month for hot water is a little extreme.
I am curious if some combination of LO Limit / DIFF might cause the burner to run less while still maintaining adequate heat to provide hot water. For example, if my current settings are LO 160/ HI 180/ DIFF 10, might adjusting the LO/DIFF settings either up or down lower oil consumption? I suppose another option could be to use an electric hot water heater with a tank during the summer months. - Tony
I have a VERY old system. Have two separate aquastats, one used for LO and one used for High. No Diff. I have a separate hot water heater. Because of the old system the oil company technician feels that I should keep the furnace on during the summer so that there isn't a "shock" to the system when it is turned on in the fall. Question: What are the best HI-LO settings in the summer and in the heating season.? - Steve
Reply: Summer Aquastat Setting Recommendations
Mark and Tony and Steve: For summer use, if you set the LO to its lowest temperature setting (typically 102 F) you will be keeping minimal heat in the boiler to support the tankless coil in summer and will lower your heating fuel consumption by as much as 30%.
For summer use, if you set the DIFF to its lowest setting you will also lower your fuel consumption.
The result of these settings will also be a reduction in the quantity and also the maximum temperature of hot water supplied to the home - which may be OK for cases when hot water use is only moderate.
If you have an automatic tempering valve or mixing valve installed on your hot water supply that will increase the total hot water quantity available by drawing heated hot water from the boiler only at the rate needed.
You can leave the HI setting on your Aquastat alone in summer; since the system will never be calling for heat, the HI will be asleep all summer.
Question: What are the best Hi Lo DIFF settings on an aquastat?
Right now I have baseboard heat and I have the boiler set at HI-170 LO-150 and DIFF at 10. Is that a good Setting. I'm trying to save money on oil. thank you - Matt
I'm still not 100% sure on what my settings should be. I have a Weil Mclain boiler that's about 6 years old with a DHW coil installed. I had a 30 gallon electric hot water heater installed after the coil, and during the summer I would shut off the boiler and use only the hot water heater. During heating season the water would be pre-warmed by the coil and the electric heater wouldn't be doing much.
I live in Maine and the winter season is here. What would be a good Hi and Low setting for my aquastat? Its is starting to dip below 10 degrees at night. Thanks. - Nick
Nick and Matt:
Question: What are Normal Boiler Temperatures When the Aquastat has Turned off the System?
Our aquastat shuts off burner at 190 degrees but old and new temp gauge continues to rise to 215 degrees. is this normal? - Warren G.
Reply: It is normal for boiler temperatures to sometimes rise a bit after the burner has shut down
Warren: it is normal for temperatures inside a heating boiler to continue to rise a bit after the Aquastat has turned off the oil burner at the HI or when the thermostat has stopped calling for heat.
This temperature rise may occur because:
As long as boiler temperature does not rise high enough to cause the pressure/temperature relief valve to spill you should be fine.
Question: When Abandoning a Tankless Coil: Should We Change the Aquastat Primary Boiler Control Settings
I am currently installing a hot water heater to replace a tankless coil in my boiler. Do I need a different aquastat to keep my boiler from trying to heat the coil? Can I replace my present aquastat #l8148a with #l8124a??? - John Haines
We installed an electric hot water heater, do we have to change anything to make the boiler function just for heat and not hot water (for bathing, dishes etc) ? - Ernie
I have replaced the tankless coil in the boiler with a gas water heater thus eliminating the need for the boiler to heat water for bathing,washing etc. question does the aquastat still needs to be connected for my boiler to run to heat the house? - jorge gonzalez
Reply: Disable the LO & DIFF settings OR set them to their lowest setting when abandoning a tankless coil
Take a look at the instructions that come with the aquastat and you'll see a list of devices it can replace. But most likely you do NOT have to replace your aquastat with a different model just because you are ceasing to use the tankless coil for domestic hot water.
On the aquastats discussed here, the instructions (at Disabling LO & DIFF) describe a simple process for disconnecting a wire that will disable the low-limit control, thus eliminating that function when a tankless coil is no longer in use.
If you leave the LO/DIFF working, the boiler will keep heating itself even in summer when there is no call for building heat - because it thinks it's staying hot to support use of the (now abandoned) tankless coil.
Jorge, you still need the aquastat control, since it has to manage the boiler on and off temperatures when the wall thermostat(s) call for heat in the home itself. But as we explain above, the aquastat controls can be changed to stop keeping heat in the boiler to support the tankless coil that is no longer in use.
The procedure for disabling the tankless coil heating system is in our Aquastat Settings article at Disabling LO & DIFF
Question: How Much Will I Save on Heating Cost by Lowering the LO/DIFF Settings on the Aquastat?
Dan, thanks for answering my questions on my recent hot water heater installation. I disconnected the differential wire (see Disabling LO & DIFF) on my aquastat and now my system works perfectly. - Tony
Hi Dan. ..Thanks for answering my question from June 28, 2011. I did, in fact, reduce the LO limit setting this summer from 160 to 120 and it reduced my oil consumption for heating hot water by about 30 percent. I left the DIFF setting unchanged at 10. To be honest, I was a little unclear from your response about the effect raising/lowering the DIFF setting would have on overall oil consumption during the summer months. However, adjusting the LO setting definitely had the desired effect. It was not necessary to adjust my manual mixing valve because the shower water was at a comfortable level. Thanks for taking the time to answer my question and for the valuable information contained on this website! - Tony
Reply: Oil Consumption Savings Reported to be 30% When LO set Back
Thanks for the feedback that lowering the LO setting will save heating fuel during summer months. We have edited the HI, LO, and DIFF functions above to make that text more clear.
Basically, by lowering the LO to its lowest point (120) you reduced that amount of heat kept in the boiler during the summer (when the only function of the boiler is to stay hot in order to heat the tankless coil used to make domestic hot water). Like you I'd have also set the DIFF to 10 as well.
If you are continuing to use a tankless coil for your home hot water supply, you might want to set the LO back up to 20 degrees below the HI in winter because in cold weather the tankless coil is receiving colder incoming water from the building public or private water supply.
Reader Comment: heating cost savings from aquastat settings
Hi Dan. ..I just wanted to clear up that the 30% savings you mentioned in the article were those that I reported in my post as a follow up to the June 28, 2011 post. ...
I can confirm the excellent point you made about setting the LO back up to 20 degrees below the HI in winter because of colder incoming water from the public water supply. It is now late October in NC and I still have the LO limit setting from this summer at 120 with the DIFF setting unchanged at 10. I have not begun to use the system for heat yet and am only running the boiler for hot water.
I have noticed a definite decrease in shower water temperature and will shortly need to reset the LO back up to 20 degrees below the HI. Again, thanks for all the valuable information you provide on your website! - Tony [October 2011]
Question: sorting out HI LO and DIFF - what setting does what?
There seems to be some conflicting info on the site. In some articles, you say that the DIFF is the temperature rise over the LO setting. For example, DIFF 15 and LO 120 would mean you'd have a range of 120-135. In this article, it's explained that there always a 10 degree drop below LO and a rise above LO of DIFF-10, meaning DIFF 15 and LO 120 would give you a range of 110-125. Which is true? And what about aquastats that have a 5 degree DIFF setting? - Brian
Reply: summary of HI LO DIFF definitions and functions on an aquastat
Thank you for the clarification request, Brian. We've reviewed our HI LO DIFF setting articles to be sure that the text is now self-consistent.
Summarizing the definitions of HI, LO, & DIFF controls on an aquastat control
HI Limit Definition: The HI is the cut off point on a call for heat. You can think of the HI dial as the "heat control" in this regard: as long as the room thermostat is calling for heat, when the boiler temperature falls 10 degrees below the HI the burner will (normally) turn on. That 10 degrees is hard-wired into the control - it's not adjustable.
LO Limit Definition: the LO is a control function added to keep heat in the boiler when the room thermostat is not calling for heat. This feature is important on heating boilers that use a tankless coil immersed in the boiler to produce domestic hot water.
The DIFF Definition: the DIFF is an added control that defines the cut-off temperature (above the LO) during a burner-on cycle initiated by the LO control - that is, we are not calling for heat, and we're keeping heat in the boiler to make domestic hot water. It works as you described.
Circulator lockout: There is an interaction between the LO and DIFF settings and the delivery of heat to the building however. That is that the LO operating range on an aquastat also will lock out the heating circulator pump - stopping delivery of hot water out of the boiler and into baseboards or radiators - if the boiler temperature is too low. (That's why aquastat instructions specify that the LO should always be at least 20 °F below the HI setting).
The intent is to give priority to the tankless coil and thus the person in the shower. For heating boilers at homes that do not use a tankless coil to make domestic hot water, it is sometimes (not always) reasonable (and a savings on fuel during summer months) to disable the LO/DIFF controls - as we explain at Disabling LO & DIFF
See Aquastat control HI LO settings for details about how the HI and LO settings work.
See Aquastat control DIFF settings for details about the DIFF setting and how it works
See Optimal LO/DIFF settings save fuel for some recommended settings to save on heating fuel cost
Question/Comment: Rusty crud found on the bottom of the heating boiler firebox
As Tony said in his comment above, I also was using about 1 gal of oil per day during the non-heating months. In Connecticut, we are paying close to $4.00 per gal now. I installed an electric water heater in parallel (NOT series) with the boiler's tankless heater. Either the tankless or the electric can be used since the water does not go through one and then the other. (Glad I still had the tankless piped up since I could still have hot water during the recent 10 day power outage in Connecticut. The 4000 watt generator could run the boiler but not the 4500 watt electric heater.) I found that we are using about 11 kwh/day for domestic hot water by timing how long the 4500 watt heater elements were on each day - a 240 volt relay activating a battery powered clock.
The cost at $0.17/kwh is about $1.90/day - better than 1 gal/day at $4.00. I turned off the boiler when I turned on the electric water heater. My concern is that when I went to tune-up the boiler for the 2012 heating season, I found about one cup of rust (some of the material was magnetic) on the bottom of the firebox below the cast iron boiler sections. I open the firebox for cleaning each year and have not seen this much rust before. I have brush cleaned the flue passageways twice since new and don't recall seeing this type of material. The boiler is a Dunkirk that was new in 2001. Water leakage does not appear to be a problem. Should I be worried about damaging the boiler (corrosion) by letting it go stone cold during the non-heating season? - Buddy 1/12/12
Buddy, thanks for the important comments & question. Indeed on many boilers, if you leave the unit shut down for months there can be problems with rust and also with solidification of soot and crud deposits that make seasonal cleaning more difficult (and perhaps costly).
Question: retrofitting a 15 year old boiler with a new Honeywell Electronic Aquastat
I am thinking about retrofitting my 15 years old oil burner with new Honeywell electronic aquastat which includes Outside Temperature Reset module. Has anybody done it ? This would allow me to adjust HI water temperature as a function of outside air temperature. They say this would result in 10-20% fuel savings. Has anybody done it ? Is it worth doing it ? Thank you. - Pavel 2/13/12
The retrofit aquastat control you describe will function properly if properly connected. the actual heating fuel savings ... well that depends on quite a few variables. "Up to" claims, in advertising law, need be true only about 10% of the time.
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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.