Resideo L4008A1130/U High or Low Limit Aquastat, Range 130-270 °F, Adjustable Differential, SPST, Contacts Break on Rise, Remote Bulb, 120 in

The L4008A1130 is a control 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 an adjustable differential and 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.

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.

A control that serves as high or low limit covers both jobs depending on how it is wired and where the bulb sits. As a high limit it shuts the burner down at the top of the range; as a low limit it keeps a minimum boiler temperature so a tankless coil or an indirect tank always has hot water to draw from.

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 #: L4008A1130/U
• Temperature range: 130–270 °F
• Differential: Adjustable
• Switch action: SPST
• UPC: 085267049078
• Category: Aquastats & Wells