A fully heated 36HP boiler needs 16 heat units per tick. A blaze rod gives 800 HU, means it burns within 2.5 seconds. (Which translates to 24 Blaze Rods per minute to keep the boiler running.) A T5 spawner should be able to drop way more than that.
Another option would be to run the blaze rod through a Magma Crucible and turn it into lava. 4 Blaze Rods give one bucket and it requires 8.000 MJ. This bucket delivers 18.000 MJ in a Magmatic engine. If you build 20 engines, then it means that 9 of the engines are simply overhead to keep the Magma Crucible(s) running. But the advantage is, that it scales well. You can add engines until you are at the limit of what the spawner can generate.