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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 CONTROL Functions 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.
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.
Guide to Best Settings for the HI Limit on a Honeywell R8182D Heating Boiler Aquastat & Similar Controls:
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.
Guide to Best Settings for the "LO" Limit on a Honeywell R8182D Heating Boiler Aquastat & Similar 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 below 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." Thanks to reader L. Clark for pointing out that we had inconsistent language in this explanation. It's been fixed, and fixed again thanks to reader Damian for suggesting further clarifications.
Guide to Setting the "DIFF" (differential) on a Honeywell R8182D heating boiler aquastat & Similar Controls:
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.
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 are at Disabling the LO and DIFF settings
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, refer to our text and to the yellow-colored area in our colored version of the HI LO functions explained in detail at AQUASTAT CONTROL Functions in that article's section More about How the HI and LO Limit Controls Function on an Aquastat above. From that article, this quote helps explain what the DIFF control is doing on an aquastat:
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.
Do You Want to Disable the LO and DIFF settings on a Boiler that Does Not Use a Tankless Coil for Domestic Hot Water?
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.
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