Resideo L4006H1004/U Aquastat, Breaks on Rise, 100-240F Range, 240F Factory Stop, Manual Reset Less Well

The L4006H1004 is a control from Resideo (Honeywell Home) that senses boiler water temperature and switches the burner, the circulator, or both. It operates over a 100–240 °F range.

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.

This model ships without an immersion well. If you are replacing a control whose well is still sound, reuse it; if you are starting from a bare tapping, order a well to match.

A factory stop at 240 °F caps the adjustment so the high limit cannot be set past the point the boiler is rated for.

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 #: L4006H1004/U
• Temperature range: 100–240 °F
• Reset: Manual
• Immersion well: Not included
• Shipping weight: 1.1 lb
• Category: Aquastats & Wells