Can anyone help?

We have a Brivis Buffalo 85 that we have just turned on for the winter.
The temperature takes 4 hours to increase 4 degrees.We have no holes or blowouts in the ducts ,the intake air is strong , the output air is luke warm. The fan is working and clean,and the thermostat is good.The pilot and flames are also strong.We have paid for a heating guy who tells me it is just a cold winter.(it usually takes 30min to gain 4 degrees)

Anyone got any other ideas.

I had a case like this before and it wound up being the return air duck sucking cold air into it under the floor. The furnace is designed to circulate inside air, not to heat outside air.