What services are you offering? I would never use hot water on houses for house washing, too many chances of damaging property with hot water and I have never needed hot water to clean any house or building yet.
Do you really even need hot water? Bubble gum removal, degreasing, etc...?
Are you cleaning large areas of concrete so that you have a real need for 8gpm?
On the other side, 8gpm will shoot water up higher than 5gpm or 5.5gpm for house or building washing, usually 10-20 feet higher that I remember. You can rinse faster with 8gpm, you can run larger surface cleaners with 8gpm like the 24" up to 36" surface cleaners where 5.5 or 5gpm stay under 24" surface cleaners or you will walk so slow you will be aggravated each time you use it, been there and done that.
If you are only house washing, working by yourself and water supply is questionable at some houses, I would probably stay with 5.5gpm, that is how I started out and had no problem, just took forever on huge driveways, drive-thru lanes, shopping center sidewalks and parking lots (I started out with hot water as I already had a couple commercial accounts).
You can always buy another machine like an 8gpm cold that you can put on your trailer later on if needed, they are usually about $3000 to $3500 new and don't take up that much room when needed or if you have the place you are purchasing from build the hot water skid (if needed), spend some more money and have them put a larger engine so that you can change out the 5.5gpm pump for an 8gpm pump later on and then have them adjust the burner (make sure you get the right coil and burner to do this later on if you decide to go this route), this is just another option, or just get an 8gpm hot water skid.
There are many choices available, just depends on what you are doing now and want to do soon.
Good luck.
