Please help! Im a plumber and I just installed a used 1990 tempstar high efficiency 5000 furnace in my home. All that I can't figure out is how to lower the speed on this model. Its up to high and its not blowing out hot enough. And its blowing to hard.

Depending upon the situation the furnace might need to run on a higher speed since it is the air temperature over the heat exchanger that is listed on the main name plate that actually controls how fast the motor should be hooked up to run. If it is running to slow it can cause too much heat rise over the heat exchanger and trip the hi limit safety switch or warp out the heat exchanger.