high efficiency gas furnace
Is it a "condensing" gas furnace?
That is, is the flue gas leaving the furnace so cool that condensation forms and has to be directed to a drain?
If so, you should be aware that that condensation is highly corrosive. It's true that it's just water, but when CO2 dissolves in water, it forms something called carbonic acid. It's that carbonic acid that gives carbonated beverages their "bite", and it's also what could cause cast iron drain piping to rust out prematurely.
If your drain piping is plastic, I wouldn't worry. If it's cast iron, I'd run that condensate through a 5 foot long 2 1/2 or 3 inch diameter piece of PVC pipe full of limestone before directing the condensate into any metal drain piping.
You can also use landscaping marble, but beware. There are two different kinds of white marble; calcitic marble and dolomitic marble. You need calcitic marble because dolomitic marble is almost unreactive with carbonic acid. Your best bet, however is a high purity limestone because it'll be more reactive than even calcitic marble.