Would you recommend an elbow be installed on this HE furnace exhaust. It is located under the deck. Sliding glass door to deck is directly above.

is the moisture on the deck boards caused by the exhaust vent ?

How would you know if it is raining?

It was not raining. No snow on top of deck either.

I would “recommend” an elbow, due to the moisture, and the heat from the exhaust may create an issue over time.

agreed needs to be away from wood if it’s causing moisture problems

You need more than an elbow if the door is above.

The exhaust should not terminate under the deck

Nor under the door.

This vent needs to be corrected, to include an elbow facing downward…

This vent should be extended out five to six feet from operable doors or windows depending where you are at. But it should be ran out so its not creating any moisture issues on deck framing.

A vent shall not terminate
(a) directly above a paved sidewalk or paved driveway that is located between two single-family dwellings and serves both dwellings;

(h) underneath a veranda, porch, or deck unless
(i) the veranda, porch, or deck is fully open on a minimum of two sides beneath the floor; and
(ii) the distance between the top of the vent termination and the underside of the veranda, porch, or deck is greater than 1 ft (300 mm).

Check your local requirements.