What is the maximum length of exhaust pipe on Velit Gas or Diesel Air heater?
Considerations for running a longer exhaust pipe on Velit air heaters.
There is really no problem with exhaust being 8 feet or longer.
You just want to make sure that it has a slightly downward slope so condensation can drain out. If it has to go upward, you need to drill weep holes in the exhaust low points to allow condensation to drain not build up in exhaust.
In some cases, extreme cold, the end of a long exhaust hose can freeze condensation and block the exhaust.
"Backpressure" concerns are negligible with this, so long as the exhaust is not blocked and can otherwise flow freely.