Best settings for a heating boiler aquastat control:
Here we explain how to choose the best settings for a heating boiler aquastat - the combination control that sets boiler temperature and may also control hot water production via a tankless coil on the heating boiler.
We also give advice on how to set the aquastat controls if heating with a woodstove.
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After explaining the 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.
At AQUASTAT CONTROLS we explain how aquastats work, defining the functions and dials of the aquastat HI LO and DIFF control along with the reset button often found on these heating boiler controls.
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 the "HIGH" or "HI" setting on a Honeywell R8124A 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.
[Click to enlarge any image]
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.
Lots of people, even some service technicians are confused about the actual function of combination controls and aquastats like the Honeywell R8182D. Some don't know how to set the "HI", "LO" and "DIFF" dials on a combination control.
Or more commonly, we were taught to just use a "rule of thumb" which set the
These were "Safe" settings for this control, and it's in with similar settings on millions of heating boilers. But with a little thought, we can adjust these HI LO DIFF settings and thus set the heating aquastat control so as to save a bit more on heating cost, and/or we can set the control to give us a bit more hot water where a tankless coil is installed.
The "HI" on a combination heating control like this aquastat is usually set by the heating service technician to a spot between 180 °F and 200 °F. Or the "rule of thumb" fellows just park it at about "180" °F as you can see in our photo.
Setting the HI: save money on heating costs by running the boiler at a higher temperature
Generally a heating boiler is more efficient if we operate it at a higher temperature.
The thermal conductivity of heating water inside of finned copper tubing baseboards or through radiator surfaces is exponentially greater at higher temperatures.
In other words, hotter water actually transfers heat into the occupied space more efficiently than cooler water.
Since this is not lab-grade equipment these settings are not precisely accurate. For this reason we like to set our HI limit at around 200 °F.
We let the boiler run through a few heating cycles, watching the temperature and pressure gauges on the boiler to see what temperature we're actually reaching. And we watch the boiler relief valve to be sure we're not causing leakage there.
Watch out: Setting the "HI" too high on the aquastat can result in over-temperature in the heating boiler and can result in dumping water and temperature and pressure at the relief valve. If your TP valve was not leaking before and it begins to drip when you set up the HI to 200 F, step the HI back to 195 and try that setting.
If your TP valve is leaking at 195 or less it's likely that you need a heating service call - something's probably wrong with the relief valve, with system operating pressures, or with the controls.
In our photo at left you'll see where the heating service tech left this "LO" limit control after cleaning and tuning the heating boiler.
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" it is possible that 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.
Your heat will work, but slowly, and more expensively, as we explain next at CIRCULATOR LOCKOUT.
We like to keep our LO set at 20 F below the HI. That keeps the LO temperature operating range as high as possible in the boiler and that in turn gives us the most heat stored in the boiler for making hot water through the tankless coil.
What happens if you "cross the controls" and set "LO" above "HI" or if you set "LO" too close to "HI"? The circulator will not run.
Watch out: don't set the LO above or higher than the HI temperature limit. If you make that mistake, you will lock out the circulator pump and your heating system will not work properly. We call this "lock out" - the circulator will be locked out of running on a call for heat. Others call this LO higher than HI "crossed controls".
We've inspected homes at which the owner, for decades, observed that the heat in the building was very slow to come up in response to the thermostat. The problem was that the LO was set close to or even above the HI - the circulator pump never ran and hot heating water circulated but only very slowly by convection.
We found a home where the owner had set the "HI" to 120 and the "LO" to 180. Luckily for them, because their flow-control valve was either absent or not working, their home would indeed receive heat - but very slowly: their circulator pump had never run.
On seeing this setting we asked the owner about it. "Well you know", he said, "we have noticed that the house was always very slow to heat up in winter."
Our photo (left) shows the DIFF adjustment on a Honeywell aquastat. This DIFF is set to its lowest value: 10 °F.
At the setting shown, if LO were set to 120, when the burner is re-heating the boiler water and water temperature rises to 130 F the burner will turn off and the heating circulator pumps will be allowed to turn on (and they will actually turn on if the room thermostat is calling for heat).
If you re-set the DIFF dial up to its maximum of 25 and LO remained at 120 F, then when the burner is re-heating the boiler water and water temperature rises to 145 F the burner will turn off and the heating circulator pumps will be allowed to turn on (and they will actually turn on if the room thermostat is calling for heat).
In short, DIFF = 25 should give you hotter boiler temperature and thus more domestic hot water than DIFF = 10.
Still not enough hot water from your tankless coil?
First see HOT WATER IMPROVEMENTS where we discuss working with a tankless coil to get as much hot water as we can.
Then when your bath tub still doesn't have but a few inches of hot water, see HOT WATER QUANTITY IMPROVEMENT
Watch out: be sure that a mixing valve or anti-scalding valve is installed to avoid hot water burns as well as to make effective use of this higher boiler temperature.
For details about the HI, LO, and DIFF actually work, AQUASTAT CONTROL FUNCTIONSrefer to our text and to the yellow-colored area in our colored version of the HI LO functions explained in detail
in that article's section that gives more details about HOW THE HI AND LO LIMIT CONTROLS FUNCTION ON AN AQUASTATt above. From that article, this quote helps explain what the DIFF control is doing on an aquastat:
When boiler temperature is rising: on an aquastat control, the "DIFF" or differential control dial specifies the amount above the LO or "Low Limit" to which boiler temperature must rise before the burner will turn off and the circulator will be allowed to run.
It's not hard to understand why heating service techs and homeowners are confused about LO and DIFF settings on a heating boiler, but Honeywell got it right: if we are making hot water with a tankless coil, by locking out the circulator at lower boiler temperatures, the LO and DIFF make sure that heating priority is given to the woman in the shower, not to the building radiators.
[Click to enlarge any image]
The diagram above is explained at AQUASTAT CONTROL FUNCTIONS
If a tankless coil is not installed on a boiler where this combination aquastat control is installed, this combination control may still be in use.
And that's not always desirable. It depends. In this case, the "LO" has almost no use whatsoever and, if you read the instructions provided by the control manufacturer you'll probably see that the manufacturer calls for the "LO" to be electrically disconnected entirely.
Details about how to make this change to a typical aquastat, along with problems to watch-out for have been moved to a new article found at AQUASTAT LO & DIFF DISABLED [Live link is given just below]
Keep in mind that the LO and DIFF control settings only make sense if the heating boiler has a tankless coil installed to make domestic hot water (for washing and bathing).
If your heating system does NOT include a tankless coil, the LO and DIFF are keeping heat in the boiler for a tankless coil that is not present and you could consider either disabling this circuit entirely or setting both LO and DIFF to their lowest settings.
Details on how to turn off the LO / DIFF when you're not using a tankless coil on your boiler are at AQUASTAT LO & DIFF DISABLED - live link given just below.
Continue reading at AQUASTAT LO & DIFF DISABLED or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
Or see MANUALS for HEATING SYSTEM CONTROLS for a list of aquastat installation & repair guides
Or see see HOT WATER IMPROVEMENTS using a tankless coil and additional controls to get as much hot water as we can.
Or see these
Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia
For speed we've moved nearly all of the FAQs for this article to
Please see posted questions & replies there.
(Feb 28, 2016) Bill said:
I have an Oil Fired Boiler forced hot water system (Hydronic) with typical baseboard radiators. Almost standard fare with 3 zones (1st for finished basement, 2nd for first floor and 3rd for second floor) in my typical New England Colonel. Honeywell L8124A Aquastat on a York AP-590 Boiler with tankless domestic hot water.
Almost normal no. 1 because the former owner installed a SuperStor domestic hot water heater. This means that there are now 4 zones, 4th being the SuperStor.
This works exactly like another zone with the ‘thermoset’ that call for heat as part of the SuperStor. And while the original tankless is still plumbed up it, it was done properly such that it is cut out of the domestic hot water circuit and vented to atmosphere. Works just fine.
Almost normal no. 2 because I installed an Intellidyne IntelliCon HW+ Hot Water Hearing System Economizer some years ago. It has a feature that allows for using a SuperStor and it is all set up per the manual. Works just fine and has saved me a noticeable percentage of oil during a heating season (~15%).
In the past I had not studied nor modified my Aquastat settings. Until now. I have now read the various excellent articles on Hi, Low, Diff and disabling the Low that can be found on this site.
So ‘posting’ what I have set the Aquastat to in hopes to get comments that I did it correct or wrong.
And I think these settings apply regardless of the Intellidyne Economizer. Of note is that since I have a SuperStor, while I could conceptually disable the Low Limit, I did not want to because
(1) the concern over boiler cooling down to ambient (condensation, etc.) and then being called upon as the SuperStor called for heat and
(2) it seems that the SuperStor will produce domestic hot water sooner, as in longer to run out, with the family of 5 folks feeding off it (think consecutive showers).
Note: with the SuperStor we have never ‘run out’ of domestic hot water, even in the summer.
I have also crafted a modified version of Honeywell’s ‘set point’ diagram. It occurred to me that a lot of my confusion over the Low Limit is simply due to their diagram showing an “up arrow” for the Differential Setting. This implies the bottom ‘line’ (switch makes R-B and breaks R-W…) is the low limit and the differential takes it to the line above. In fact it is opposite.
Differential ‘drops’ the Low Limit. Add to that they show another “differential” for the High Limit that I think adds to some of the confusion, when I fact that is hard wired.
So onto my setting and justification. Looking for advice that “makes sense, this is good” or “Bill, you need to go back to school”
High Limit Setting: set to highest possible WITHOUT exceeding 200 on Boiler. In my case this is ~ 185, after observing a number of cycles. Note: I have not only the Boiler temp gauge but also the Intellidyne has a senor on the boiler output. And when the circulator is running these closely track one another. Logic from reading on this site is that the hotter the boiler water the better the efficiency and heat transfer to the air. See above in the article for this justification.
Low Limit Setting: Set to lowest possible. In my case this is ~ 110. Logic is that (1) no tankless but do have SuperStor (see my thinking on this above) and (2) do not want to deal with issues of boiler going to ambient.
Differential Setting: Set to lowest possible. In my case this is ~ 10. Logic is that do not want the boiler to drop too low in temp. Not sure what temp things will start to condense but I figure that is the boiler only gets down to 110 – 10 = 100 that is still hot enough to prevent. If NOT, then I need to raise the Low Limit NOT the Differential.
Thanks for reading and if you have some other ideas would appreciate your comments.
Almost right in all respects, Bill.
I used to agree with your DIFF settings until I studied Honeywell's explanation of the aquastat control more closely.
But please take a look at the explanation of HI LO and especially DIFF settings found in this companion article:
inspectapedia.com/heat/Aquastats.php AQUASTAT CONTROL - home
An excerpt from that, given below, forms an argument for setting the DIFF up rather than down.
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.
(Feb 28, 2016) Bill said:
Got it. I missed the 10 degree delta part on the low side. Ok so temp will drop (with no call) to LO - 10 degrees and heat back up to (LO - 10) + DIFF. So in my case with the numbers listed and no call:
* Will drop to 110 - 10 = 100 (the 10 has nothing to due with the DIFF :)
* Will rise to (110 - 10) + 10 = 110
And since I do not have a Tankless, is this temp too low?
Any concerns about condensation building up, particularly in the summer where the only calls will be the SuperStor?
I suspect the boiler will drop to its lowest frequently in summer time...
(Feb 29, 2016) (mod) said:
With no tankless coil what you're gaining by keeping the LO hooked up at all is a slightly faster supply of heat to re-heat the indirect fired water heater.
My opinion is that that gain is not much, since when we are only heating the water inside the boiler itself (that is not also all of the water in the hot water piping and rads or baseboards) the boiler comes up to heat quickly.
There are other reasons to keep heat in the boiler such as avoiding leaks in some older cast iron boiler models, and avoiding condensation as you mention. That's no worry IMO when the boiler runs from time to time to make hot water.
Your Superstor indirect water heater operates as its own separate zone - it looks like a heating zone to the boiler, so the boiler's HI will take effect. I imagine the only summer difference you may see is a slightly-longer time to re-heat the indirect water heater. I guesstimate less than a 10 minute difference from winter, since most of the re-heat time will be heating the larger volume of water in the indirect water heater.
But it would be interesting for you to conduct experiments to actually measure the re-heat time at different starting temperatures in both the boiler and in the indirect water heater. To make comparisons one would need to not run hot water during the tests.
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