How to Replace a Boiler Aquastat (L8124, L6006, L4006)
The aquastat is the temperature controller that governs your hot water boiler. It tells the burner when to fire (when water temperature drops below setpoint), when to stop (when water reaches high limit), and in combination models, manages the circulator pump as well. When an aquastat fails, your boiler may not heat at all, run continuously, or deliver inconsistent heat.
Understanding Aquastat Types
Honeywell/Resideo makes several aquastat models, and knowing which one you have determines the replacement:
- L8124: The most common combination aquastat — controls high limit, low limit, and circulator pump all in one unit. Typically found on older oil-fired boilers. Has three numbered terminals and a characteristic rectangular housing with three adjustable dials.
- L6006: High-limit-only aquastat. Simpler — one dial, two terminals. Often used alongside a separate low-limit or as a safety control.
- L4006: High-limit aquastat with a manual reset feature. Common on steam boilers and as an overheat safety on some forced hot water systems.
Look at your existing aquastat — the model number is printed on the front. Write it down before purchasing a replacement.
Tools and Safety
You'll need: a flat and Phillips screwdriver, a multimeter, wire labels or tape, and a camera for photographing existing wiring. Shut off power to the boiler at the circuit breaker before touching any wiring. Confirm power is off with a non-contact voltage tester.
Step 1: Photograph and Label All Wires
Before disconnecting anything, photograph the existing wiring from multiple angles. Label each wire with tape and a marker indicating which terminal it connects to. This is the step most people skip and regret.
Step 2: Drain the Aquastat Pocket (If Wet Type)
The L8124 and most aquastats use an immersion well — a metal tube inserted into the boiler's water jacket. You don't need to drain the boiler to replace the aquastat, but you do need to remove the immersion well carefully. Have a rag ready for the small amount of water that may seep out.
Unscrew the aquastat body from the immersion well (it usually screws on) or unbolt the mounting bracket. The immersion well itself stays in the boiler.
Step 3: Remove the Old Aquastat
Once wires are labeled and photographed, disconnect them from the terminals. Remove the mounting screws. The aquastat body will lift away from the immersion well or mounting surface.
Step 4: Install the New Aquastat
Mount the new aquastat body to the immersion well or mounting surface. Reconnect wires to the matching terminals on the new unit — your labels and photos are your guide. For the L8124, the terminal layout is standardized: terminals 1, 2, 3, B, W, R match the original.
Step 5: Set the Aquastat
Standard settings for an L8124 on a residential oil-fired boiler:
- High limit: 180–200°F (200°F is typical)
- Low limit: 120–140°F (controls minimum water temp for domestic hot water priority if applicable)
- Differential: 10–15°F
Match the settings on your old aquastat unless you know they need adjustment.
Step 6: Restore Power and Test
Restore power at the breaker. Call for heat at the thermostat. Watch the burner fire and confirm the aquastat shuts the burner off when water temperature reaches the high limit setting. Check that the circulator pump activates correctly (on L8124 systems).
New England Supply House stocks Honeywell/Resideo L8124, L6006, L4006, and related aquastats. Same-day shipping from Foxboro, MA.