Are your gas lines sized appropriately?
We have found many homes have gas lines that are not sized as they should be. Most times home owners have gas lines put in for one or two appliances. Time goes on and then they decide to add another item run on gas. The line originally put in was sized for 2 items. The whole line will need to be adjusted if it is now expected to provide gas to 3 or 4 items. Having a gas line that is undersized will cause the appliance to malfunction as it is not getting the appropriate amount of gas. When this comes to a gas furnace it leads to build up in the heat exchanger causing your unit to run inefficiently. The build up can also cause a cracked heat exchanger which in turn leads to Carbon Monoxide leaking into your home.
Gas pipe sizing checked at every estimation.