I am converting a van into a tiny home and want to mount my greywater tank in the undercarriage behind the rear axle. I'm worried about radiant heat from the exhaust pipe melting a plastic water tank or a PVC tank. How far should the tank be from the exhaust pipe? Or should I heat shield the tank and the plumbing a must?
Depending on the type of plastic/PVC, if the pipes are too close to an exhaust manifold then yes it's likely that it will melt or at least suffer. However if the pipes are a good few inches away from say a rear exhaust box or pipe then you may be ok, as the rear of an exhaust system is not as hot as the manifold end.
You could place a small piece of the plastic that your going to use in the area you need it and see how it fairs.
Also how about manufacturing a light steel/aluminium plate & bracket to place between the exhaust and plastic pipe, in areas that you suspect will see the most heat.
You can also get clamp-on or wrap on exhaust coverings that help prevent the radiant heat from exhaust systems.
Your exhaust manifold is around 700 to 900 degrees, your catalytic converter is even hotter. PVC starts to deform around 130F. CPVC will tolerate a little bit more heat but it will still melt within the vicinity. You would probably want it at least a foot away. I would look for alternative piping.