Is there a kind of hot water pipe that doesn't conduct much heat, so it feels cool on the outside?

Question

In other words, with hot water running through the pipe, the heat of the hot water stays inside the pipe, because the pipe material doesn't conduct much heat. So the pipe would not even feel warm on the outside. What kind of pipe would that be?

Answer 1

Look at PEX tubing. it can be used for hot and cold drinkable water. It's a polymer, so it does conduct some heat, but not as much as copper. They do use it for radiant heat applications, so it does emit heat when running hot water through it.

You're not going to find anything that doesn't draw any heat from hot water on the way to the tap. Insulating a copper pipe is fine, it will probably only take a few more seconds to get totally hot.  If you're that worried about heat loss, get a point-of-heating source like a Hot Water Lobster or a Rinnai on-demand unit.

Answer 2

Go with CPVC (plastic) or PEX.  They do not 'heat up' as fast as copper.  Add insulation to CPVC for best performance for your given conditions.  Also, adding insulation to copper (if that is what you have) will help.  Copper pipe is quick to transfer heat to the air.  So you will heat the pipe, but adding insulation will help the pipe keep that heat.

As for why CPVC is not allowed in some areas, you can blame the plumbing union(s).  Sweating pipes is much more labor (and skill) intensive than gluing plastic.   There were problems with some plastic piping a while back, but those have been resolved.

Answer 3

A pipe with insulation on the outside.

You can get ready-made insulation to clamp around a copper pipe to provide insulation.

Using PVC pipe will provide some insulation, but that is not allowed in most locations. Consult your building codes. Adding insulation is better anyway.

Answer 4

Conventional PVC is not allowed for hot water because it loses strength at hot water temperatures, CPCV has higher temperature resistance is used in some areas. Cross linked polyethylene (PEX) pipe is gaining popularity, for both hot and cold water.

Copper pipe is usually quite thin, if you compare the volume of water to the volume of copper, and consider that volumetric heat capacity of copper is fairly close to that of water (actually a little less) , I don't think that the heat absorbed by the pipe is really all that significant.

If you have existing copper pipe, adding insulation to it would probably be a better option than replacing it with another material. For a new installation you might consider plastic, if allowed by local codes. Though it you want "the pipe would not even feel warm on the outside" you will still need external insulation.