Resideo L4008B1013/U Circulator Aquastat, Range 100-240 °F, Adjustable Differential, SPST, Contacts Break on Rise, Remote Bulb, 66 in Capillary Length

The L4008B1013 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 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.

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

A circulator aquastat holds the pump off until the boiler water is hot enough to be worth moving. Without it, a cold start sends cool water through the radiators and the house feels the draft long before it feels the heat.

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 #: L4008B1013/U
• Temperature range: 100–240 °F
• Differential: Adjustable
• Switch action: SPST
• Capillary length: 66 in
• UPC: 085267049160
• Category: Aquastats & Wells