Resideo L4008E1040/U High Limit Aquastat Controller, Range 130-270 °F, Manual Reset, SPST, Contacts Break on Rise, Remote Bulb, 66 in Capillary Length

The L4008E1040 is a controller from Resideo (Honeywell Home) that senses boiler water temperature and switches the burner, the circulator, or both. It operates over a 130–270 °F range, with SPST contacts.

The aquastat is the control that keeps a hydronic boiler inside its working band. It holds the water hot enough to serve a call for heat and shuts the burner down before the boiler reaches a temperature the relief valve would have to handle.

This is a manual reset high limit. Once it trips, the control stays open until someone presses the button — the boiler does not simply restart when the water cools. That is deliberate: a high limit trip means something went wrong upstream, and a self-resetting limit would hide it.

The 66 in capillary lets the sensing bulb sit in the boiler while the control body mounts where it can be reached and wired.

As a high limit, this control has one job that matters more than the rest: it stops the burner before the boiler gets away. Everything else in the heating system is comfort; the high limit is safety.

When you replace an aquastat, match the temperature range, the switch action and the differential to the control coming out, not to the boiler nameplate. A boiler will happily run on a control with the wrong range — it will simply cycle badly, or trip a limit that was never meant to be the working control.

The sensing bulb belongs in an immersion well, filled with heat conductive compound, threaded into the tapping the boiler manufacturer provided for it. A bulb strapped to the outside of a pipe reads the pipe, not the water, and it reads it late.

• Brand: Resideo (Honeywell Home)
• Manufacturer Part #: L4008E1040/U
• Temperature range: 130–270 °F
• Switch action: SPST
• Reset: Manual
• Capillary length: 66 in
• Category: Aquastats & Wells