A corner restricts the flow, so there are charts of "equivalent added feet of straight pipe" for corners, with different values depending upon the angle/radius of the bend.

The pipe diameter also affects resistance-over-length.

So for a required flow rate (so that the instant-heater doesn't choke), there's a limit on overall length, taking into account all of the bends. Exceeding the limit, means moving up to a larger ($$) pipe diameter.

Or so the gas man said.

It's the same sort of flow calculations I had to do when sizing the dust collection tubing for the workshop.

Cheers