Guide to the Honeywell now Resideo L8124-series aquastat controls on heating equipment:

Features & applications for the L8124 Aquastat and links to product features & wiring or installation instructions for this and other Honeywell aquastats in current use for oil or gas fired heating equipment as well as some electric boiler equipment.

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.

Honeywell L8124 Series Aquastats

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Excerpting from Honeywell's manual for this controller,

These immersion type controllers provide high limit, low limit, and circulator control in forced hydronic heating systems that include domestic hot water. Table 1 [above on this page] lists the various TRADELINE models and applications.

These Aquastat Relays have a diaphragm powerhead and Micro Switch Assembly that respond to temperature changes in the boiler water.

L8124B has contacts rated for millivoltage loads.

L8124E and G have large transformers and extra terminals for supplying power to low voltage valves.

L8124L includes ZC-ZR terminals for multizone control, using a separate circulator and R845 Relay for each zone, and a large transformer and extra terminals for low voltage zone valves.

All models have a case that allows vertical or horizontal mounting and include a tube of heat-conductive compound and limit stops.

Do not use these Aquastat Relays where the pressure can exceed 100 psi (689.5 kPa) on the bulb when inserted directly or 200 psi (1379 kPa) when using an immersion well, or where the ambient temperature can exceed 150°F (66°C) at the case or 265°F (129°C) at the sensing element .

Note: In 2018 Honeywell announced that Resideo would be the corporate name of the Honeywell Homes product portfolio and ADI global distribution businesses when they became a stand-alone, publicly traded company following completion of a spin that was expected by the end of 2018.

L8124 Aquastat HI, LO & Differental Setting Ranges

• High Limit or "HI" control: 110°F to 220°F (43°C to 104°C), adjustable
• High Limit Differntial: fixed or hard wired in the control at 10°F (6°C), fixed.
• Low Limit or "LO" control: 130°F to 240°F (54°C to 116°C), adjustable.
  
  Watch out: always set the LO to at least 20°F lower than the HI setting - else you will lock out the circulator relay and may not get heat into the building
• Low Limit Differential or "DIFF" control: 10°F to 25°F (6°C to 14°C), adjustable.

L8124 Aquastat Cross-Reference & Substitution

The Honeywell L8124 B-series (AQS8124B1039 or the Honeywell Multi Function Aquastat L8124B1039) can substitute for many of the controllers in the L8024 and L8124 aquastat lines.

We regularly see that controller advertised as replacing the Aquastat models with part numbers L8124B1039, L8124B1021, L8024B, and L8024D.

Also see the Honeywell, the L7224U Universal Aquastat controller.

Diagnostics for the Honeywell L8124 series Aquastat Relays

Steps in Troubleshooting the Honeywell L8124 Aquastat Relay & Similar Controls

If you shorted the power wires to any heating control including the Honeywell L8148 Aquastat then it’s quite possible that the control has been damaged or destroyed and will need replacement.

What this control does

In the U.S. the way heating boilers are wired, the thermostat turns on the circulator pump and it is the temperature in the boiler that separately, by cooling down, tells the Aquastat to Turn on the oil or gas burner to re-heat the boiler.

So our general approach in diagnosing an aquastat is to see if first the thermostat successfully causes the aquastat to turn on the circulator(s) and then to see if second when the boiler is cool or cold whether the aquastat turns on the oil or gas burner.

Watch out: in Canada many heating systems are wired so that the circulator runs all the time during the heating season and the thermostat (and thence the aquastat) turn the boiler on and off. (The reason for this is that in a colder climate house temperature will be kept more even and the risk of frozen heating zone or other pipes is reduced.)

In any event if the L8148 Isn’t turning on the circulator (s) and thus is never causing the boiler to run, here are some diagnostic steps, starting with what a homeowner can do and continuing with what a service technician might do.

Homeowner Checks at the Aquastat

1 .Look at the Aquastat for Obvious Damage

Remove the gray cover on the L8124 Aquastat at the boiler. You may need to loosen two screws at two of the cover edges to remove it from the aquastat body. Don’t fully remove the screws, just loosen them.

Now DO NOT TOUCH anything inside the aquastat; there is live voltage here including 120VAC that could shock or kill you.

JUST LOOK at the aquastat internals; if you see brown or black burn marks on the circuit board or on any other components the aquastat is probably damaged beyond economical repair.

Above we show brown and overheating marks on the green circuit board of a Honeywell heating system control. This controller was shot and needed to be replaced. If you see burn marks like this don't bother with costly diagnostics.

If there is no obvious visual evidence of a burn-up nor smell of burned electrical components, the heating system and possibly even the control still needs diagnostic service and repair.

2. Check that the thermostats are calling for heat

While you listen at and look at the aquastat, have an accomplice turn all of your thermostats all the way up.

You should hear a "click" at the aquastat and if you were watching closely, the aquastat relay switch may have "closed" by clicking "down" to turn on the circulator pump(s).

If not the call for heat isn't being relayed to the circulator(s).

3. Check that the Aquastat electrical circuit power is on

Check at your electrical panel for the fuse or circuit breaker powering your heating system - be sure that electrical power is on.

If in doubt, switch the breaker fully to OFF position then turn it back on, or replace the fuse (if it's a fuse pane) with a new one.

Watch out: do not remove the panel or fuse box cover and don't stick a finger or any tool inside the electrical panel as you could be shocked or killed.

Heating Service Technician L8124 Aquastat Diagnosis & Repair Steps

Call your heating service company. Most heating service techs will simply replace a bad aquastat even though a new Honeywell L8148 is costly - about $175. U.S. - because their time to run detailed diagnostics is expensive too.

However a service technician might elect to try some of the following steps to be sure that her diagnosis of a bad Aquastat L8148 is the right guess.

In the text below “you” refers to “the service technician, not the homeowner.

At the L8148 Aquastat the technician may check for supply voltage and may check the thermostat circuit voltage using a DMM or VOM as follows:

Watch out: Safety details are

at DMMs VOMs SAFE USE OF 

1. Check that the thermostat(s) are calling for heat

And check that and that room temperature is below the set temp in the thermostat. (I turn the TTs all the way up - and leave my truck keys atop one of them so I don't forget to set them back to the homeowner setting before I leave.)

2. Check for line voltage at the power terminals in the Aquastat

There should be 120VAC at terminals L1 and L2 where by convention L1 is the “hot” or “black” wire and L2 the neutral or “white” wire.

If there is no voltage supplied to the aquastat then the problem begins at the electrical panel - blown fuse, tripped breaker, or in a wiring short or disconnect between the electrical panel and the electrical box powering the aquastat and heating boiler or furnace.

[Click to enlarge any image]

If there is voltage then let's be sure the thermostat's call for heat shows up at the Aquastat

3. Check the thermostat wires

On the Honeywell L8124A Aquastat relay

• Call for heat, no voltage:
  
  Turn the thermostat(s) all the way UP and confirm that room temperature is below the thermostat SET temperature; the thermostat should call for heat.
  
  If there is NO voltage at the TT wires at the Aquastat and the Wall Thermostat IS calling for heat then it is possible that there is a break in the thermostat wiring.

• No call for heat, Voltage detected:
  
  Turn the thermostat(s) all the way DOWN and confirm that room temperature is above the thermostat SET temperature; the thermostat should not be calling for heat.
  
  If there IS voltage at the TT wires at the Aquastat when the thermostat IS NOT calling for heat then the wires are shorted together somewhere in their route.

If you need to rule out the thermostat itself as the problem, simply remove the wall thermostat and jump or tie together the two heat wires - usually red and white - to call for heat. This is the same as turning the thermostat all the way up.

Details of troubleshooting thermostats and thermostat wiring are at

• THERMOSTAT WON'T TURN OFF
• THERMOSTAT WON'T TURN ON
• LOW VOLTAGE TRANSFORMER TEST

Watch out: Before leaving the job don't forget to restore the thermostat wires to the terminals wherever you've disconnected them.

If you've ruled out a problem at the thermostat itself and at the thermostat wires, continue below.

4. Check the Aquastat Relay Switch & Voltage at Circulator Terminals C1 - C2

On a call for heat the relay switch should click and should "close" or be in the "down" position on the circuit board.

At the Aquastat, at terminals C1 and C2 check for line voltage (120VAC) supplied to the circulator pump.

Watch out: I have seen techs just push down the relay to confirm that voltage is then produced and the circulator pump runs, but if the thermostat is calling for heat and the relay doesn't close on its own then either the control board or the relay is bad.

Watch out: there is live voltage here. You can be shocked or killed.

If the relay closes as it should and there is voltage at the circulator terminals then the problem is downstream from the aquastat.

5. Check thermostat wires at zone valves or zone circulator relays

If the heating system uses multiple zones each will be controlled by a zone valve (that opens or closes a loop of heating water pipe and that turns a common circulator pump on)

OR each zone may be controlled by individual heating zone circulator pumps that are in turn switched on or off by a separate heating zone circulator relay control.

When a thermostat controlling an individual heating zone calls for heat in this design either an end switch on the zone valve OR a relay switch in the circulator relay turns on the circulator for that zone.

At the zone valve or at the circulator relay control check the thermostat terminals for 24VAC at that control and check line voltage at the common circulator or at the line voltage terminals in the circulator relay if one is used.

On a zone valve system

if the zone valve doesn't operate when there is voltage across the thermostat wire terminals (24VAC) at the zone valve, then the problem is at the valve - the control head itself, or the actual hot water line valve (could be jammed).

If there's no voltage at the TT terminals on the zone valve then we're back to diagnosing the thermostat itself or the thermostat wires or the low voltage transformer - topics we listed earlier.

On a multiple circulator system

if the circulator relay doesn't close when there is voltage across the thermostat wire terminals (24VAC) at the circulator relay, then the problem is a bad relay or relay control board.

If the circulator relay closes and produces line voltage at the circulator terminals C1 C2 in the relay, but the circulator doesn't run then the problem is in the circulator or circulator power circuit.

If the relay has closed and there is voltage on the C1 C2 terminals but the circulator(s) won’t run then

see CIRCULATOR PUMP WON'T START 

6. Canadian systems: circulators always on:

At the L8184 Aquastat check for line voltage to the burner - Terminals B1 B2 (or between L1 and B2 on some models.)

When the thermostat is calling for heat the burner will nevertheless remain OFF until the boiler temperature falls to or below the CUT-IN temperature on the limit control on the Aquastat (typically 15 deg. below the HI LIMIT) so you may need to run heat in the building long enough to cool down the boiler to see the burner turn on.

Once the boiler temperature drops below the cut-in the burner should run.

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At AQUASTAT CONTROLS, our home page for this control, we explain in more detail, from which I excerpt:

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.

On some controls this hard-wired fixed gap may be 15 °F.

IF voltage is being supplied to the burner (to turn it on) and the burner never runs, the the problem is probably not the aquastat but the burner itself, such as a burner that has tripped off a cad cell relay or other safety control.

See details

at OIL BURNER WONT RUN 

or on an LP or natural gas burner system the burner is turned on by a low voltage circuit

Gas burners on a heating boiler typically used a Honeywell L814B like the L8148E1257/B that has an isolated burner circuit typically used with powerpile systems. That aquastat is replaced by the L8148J1009/U Aquastat Relay, a universal replacement offering 120 – 240 F range with 8 degree F fixed differential, low volt/millivolt

See gas fired boiler troubleshooting details starting at

DIAGNOSE & FIX HEATING PROBLEMS-BOILER  at https://inspectapedia.com/heat/Heating-Boiler-Diagnosis.php

Same diagnostics and same electrocution shock warnings as above.

Manuals for the Honeywell L8124 series Aquastat Relays

• HONEYWELL L8124B,E,G,L AQUASTAT RELAYS INSTALLATION INSTRUCTIONS [PDF] (2001) Honewell, retrieved anew 2022/12/01, original source: https://www.sidharvey.com/documents/UNI-New_11740_Inst.pdf
  
  Excerpts:
  
  These immersion type controllers provide high limit, low limit, and circulator control in forced hydronic heating systems that include domestic hot water. Table 1 [above on this page] lists the various TRADELINE models and applications.
  
  
• AQUASTAT L8124A,B,C,E,G,L,M INSTALLATION & OPERATION MANUAL [PDF] (2001) Honeywell, Inc., 1985 Douglas Drive North, Golden Valley, MN 55422 USA
  or Automation and Control Solutions, Honeywell Limited-HOneywell Limitee, 35 Dynamic Drive, Scarborough, Ontario M1V 4Z9 Canada

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

2022/05/26 (Moderator) - use thermal grease when installing the sensor for an L8124 aquastat?

@Anonymous,

The answer is yes, the thermal or heat-conducting compound is necessary.

Details are at AQUASTAT SENSOR PROBE HEAT CONDUCTING COMPOUND

On 2022-05-26 by Anonymous

Is heat conducting compound needed when installing l8123a aquastat

...

Continue reading at AQUASTAT HI LO DIFF SETTINGS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

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AQUASTAT L8124 AQUASTAT CONTROL INSTALL / REPAIR at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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INDEX to RELATED ARTICLES: ARTICLE INDEX to HEATING BOILERS

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