Resideo L4006A1678/U Aquastat, Breaks on Rise, 100-240F Range, 240F Factory Stop, No Well, Adjustable Differential

The L4006A1678 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 with an adjustable differential.

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.

The adjustable differential sets how far the water has to cool before the burner is allowed to fire again. A wide differential means longer, less frequent burns; a narrow one holds temperature tighter at the cost of more cycling.

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 #: L4006A1678/U
• Temperature range: 100–240 °F
• Differential: Adjustable
• Immersion well: Not included
• UPC: 085267048644
• Shipping weight: 1.5 lb
• Category: Aquastats & Wells