Yes, on our condensing boilers you can fit 90 degree bends onto a vertical flue system.
Note: A 87° bend is equivalent to 1.6 m length of flue. If it is necessary to use a 87° bend in the vertical flue, ensure any horizontal flue lengths is sloped towards the boiler to ensure that all condensate is allowed to drain from the flue. The slope must be a minimum of 25 – 30 mm per meter.