Best High, Low, & DIFF Settings on a Heating Boiler Aquastat?

Rule of Thumb for Setting HI LO DIFF on an Aquastat

Or more commonly, we were taught to just use a "rule of thumb" which set the

• HI to 180 °F (should be 200F or lower)
  • a HI-DIFF value is hard-wired in the R8182 series type aquastats, is not adjustable, and no control dial is provided.
    
    A programmable HI-DIFF is available on on some programmable aquastats such as
    
    the AQUASTAT L7224U UNIVERSAL and can be adjusted.
• LO to 160 °F (must be 20 below HI)
  • DIFF (or LO-DIFF) on most aquastats such works with the LO limit only and is often set to 10, or is just set randomly (not recommended - proper numbers are below) .

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.

Our photo above shows that someone has set this aquastat HIGH LIMIT to 175°F - in oujr opinion this is too low and it will increase heating cost and reduce heating system performance.

In a the "rule of thumb" many heating service technicians just park the HI at about "180" °F.

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 drawing above and in the detailed photo below you'll see where the heating service tech left this "LO" limit control after cleaning and tuning the heating boiler.

This LO limit dial was set to about 167 degF.

The "LO" limit must always be set to at least 20°F. below the "HI" limit on a combination aquastat control.

Watch out: if the LO is set to 163 degF on this boiler's aquastat then if the boiler HI was set to any temperature less than 183 degF (20 degF above the LO) the circulator will be locked out - it won't run.

More generally, if we set the "LO" too close to the "HI" limit, the control, trying to give priority to making domestic hot water for someone in the shower, will simply lock out the circulator pump entirely - this boiler's circulator pump won't ever run and the delivery of heat will be at a snail's pace, leaving grouchy building occupants and a higher cost.

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.

Avoid Circulator Lockout: Always Set LO 20°F or more below HI

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:

Where a tankless coil is installed on a heating boiler to make domestic hot water (for washing and bathing) we prefer to set the differential (DIFF) to its highest number (usually 25 degrees) so that people using hot water in the building will have as much hot water supply as possible.

Our photos above and below show 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.

See ANTI SCALD VALVES & TEMPERATURE CONTROL / MIXING VALVES

For details about the HI, LO, and DIFF actually work, AQUASTAT CONTROL FUNCTIONS refer 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 AQUASTAT 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

Disable the LO and DIFF settings on a Boiler Aquastat that Does Not Use a Tankless Coil for Domestic Hot Water

The DIFF feature on a boiler aquastat is there to provide control over boiler temperature when a tankless coil is in use to provide domestic hot water (for washing and bathing).

Out of the box a typical aquastat model that contains a DIFF circuit will come wired up or "active".

If your heating system does NOT include a tankless coil, or if a tankless coil is installed but is not connected to the building hot water piping, then 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.

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 when you're not using a tankless coil on your boiler, along with problems to watch-out for have been moved to

AQUASTAT LO & DIFF DISABLED

where you'll see it's essentially a step to dis-connect a wire in the aquastat.

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).