A fuel system for an auxiliary power unit, the fuel system comprising: a mechanical fuel pump for providing fuel flow to the auxiliary power unit, the mechanical fuel pump having an output dependent upon an operational speed of the auxiliary power unit; an electric fuel pump located in series with the mechanical fuel pump; wherein the electric fuel pump is located upstream of the mechanical fuel pump on a common fuel supply conduit; a bypass conduit having a first end and a second end, the first end fluidly connected to the fuel supply conduit upstream of the electric fuel pump and the second end fluidly connected to the fuel supply conduit downstream of the electric fuel pump; a bypass check valve located on the bypass conduit, the bypass check valve having an open configuration for allowing fuel to flow from the first end to the second end of the bypass conduit; a relief fuel conduit having a first end and a second end, the first end fluidly connected to the fuel supply conduit downstream of the electric fuel pump and the second end fluidly connected to the fuel supply conduit upstream of the electric fuel pump; and a relief check valve located on the relief fuel conduit, the relief check valve having an open configuration for allowing fuel to flow from the first end to the second end of the relief fuel conduit; a fuel shut off valve located on the fuel supply conduit downstream of the mechanical fuel pump and upstream of the auxiliary power unit, the fuel shut off valve having an open configuration for allowing fuel to flow to the auxiliary power unit; and a controller configured to close the fuel shut off valve causing the electric fuel pump to circulate and thereby warm fuel prior to start-up of the auxiliary power unit. a mechanical fuel pump for providing fuel flow to the auxiliary power unit, the mechanical fuel pump having an output dependent upon an operational speed of the auxiliary power unit; an electric fuel pump located in series with the mechanical fuel pump; wherein the electric fuel pump is located upstream of the mechanical fuel pump on a common fuel supply conduit; a bypass conduit having a first end and a second end, the first end fluidly connected to the fuel supply conduit upstream of the electric fuel pump and the second end fluidly connected to the fuel supply conduit downstream of the electric fuel pump; a bypass check valve located on the bypass conduit, the bypass check valve having an open configuration for allowing fuel to flow from the first end to the second end of the bypass conduit; a relief fuel conduit having a first end and a second end, the first end fluidly connected to the fuel supply conduit downstream of the electric fuel pump and the second end fluidly connected to the fuel supply conduit upstream of the electric fuel pump; and a relief check valve located on the relief fuel conduit, the relief check valve having an open configuration for allowing fuel to flow from the first end to the second end of the relief fuel conduit; a fuel shut off valve located on the fuel supply conduit downstream of the mechanical fuel pump and upstream of the auxiliary power unit, the fuel shut off valve having an open configuration for allowing fuel to flow to the auxiliary power unit; and a controller configured to close the fuel shut off valve causing the electric fuel pump to circulate and thereby warm fuel prior to start-up of the auxiliary power unit.