I have a rental gas water heater that will be orphaned with the installation of a high efficiency furnace.

I have been told by one HVAC contractor that I don't need to get a liner for the chimney if it's 'within 1 size'.

Other contractors are recommending a 4" liner.

The outside diameter of the flue pipe is 6" (by my measurement).

What would the inside diameter be? - is it 5" inside?

Is it small enough to be withing 1 size or do I really need to get a liner installed to prevent condensation damage in the chimney?

Thanks for the help.

A wary consumer.