I would like to know the different between interior and exterior tankless water heater.
And can I use the interior tankless outside under a carport by using cap for the vent to keep dust out of system

The only difference is the venting...

Interior tankless water heater has to utilize dedicated, double wall vent which is basically smaller pipe inserted inside larger diameter pipe. Smaller pipe discharges burnt fumes with fan while larger pipe sucks in oxygen. It is basically "Power Vented" or "Direct Vented" water heater.

Exterior tankless hot water has no vent pipes attached to it. As with - for example Rinnai water heater, it has only elongated slot on the heater itself through which it discharges burnt fumes. There are some clearances though you will have to maintain from windows, doors, eves and corners.

Yes, you can use interior tankless heater on the exterior. Here, you will be using some extra vent piece that is not needed - in your case the cap. However, you CANNOT use exterior tankless water heater on the inside.

Hope this answers your question... Milo.

Hi benlea and welcome to The Plumbing Page on AMHD.
You piggybacked on a 2 year old dead thread so I gave you one of your own. Before you post look in the upper left hand corner for the date.
Milo's assuming this will be a gas tankless. Are you planing gas or electric? Back to you, Tom

Where do you live, exterior heaters can not be used everywhere

Thank you, Tom. Yes, I assumed it was gas tankless hot water heater.

Let me know if you are considering electric. Back to you. Milo