When I moved in a two years ago this heater wasn't working. Turns out the tree roots crushed the gas line. When I did my patio I had a new gas line put in. The heater fires up and runs good. The prior owner removed the vent when he replaced the roof. I can't find an oval vent to connect to this. It's 3" high by approximately 4 1/2" wide. Do I bend a single wall 4" vent to fit and connect the Class B vent to the single wall or does someone make an oval vent?
A common way is with a single wall oval to round elbow. Then you use a listed adapter for joining single-wall vent to Type B round vent. Do not "squeeze" or "bend" a pipe, fitting, or adapter to make it fit.
Contrary to some things you might hear, gas fired appliance vent pipes and fittings must be fit for purpose and used according to code and manufacturers' instructions or you could have a fire or die from CO poisoning. There are all kinds of restrictions; length and slope of horizontal runs, number and degree of bends, requirements by space/area, passing through floors/walls/roof, height of vent stack, type of cap, etc. All the rules are there for a reason (usually the reason is that someone was injured or killed).
To jimmy Fix-it, that heater is probably older than you are and squeezing the connecting flue pipe was the way it was done when this dinosaur was built. Looking at the motor and heater construction this thing is an antique and I would look for a replacement rather than using it If the Writer really wants to use this heater I would call a licensed HVAC company to check this unit carefully for chamber integrity. The fitting you show may work but you have to squeeze the round end to get it installed. According to the name plate this thing was made in Oct. 1967 – d.george 11 mins ago
Side note: The heater looks very old. Please check to make sure the exchanger is still in good shape. Thanks.