@HenryHil said in Big 3D printer (BigFDM):

Also I noticed that BigFDM page recommends to use SSRs for this size bed heaters. With Duet 3 it seems that we would be able to drive these with the four IO pins using four SSR or just connect a single SSR's input to OUT0 and then connect these four bed heaters to the SSR.

when mains powered SSR is the way to go for the beds, you could also probably run 4 SSRs off the single output. that would allow you to later reconfigure to use 4 outputs to control quarters of the bed, as well as lower the load and potential need for cooling on each SSR.

(not sure how thermal expansion affects partially heated bed in practical terms, but there are at least on thread on here that deals with it for a large printer).