Oven Preheating

It's funny- the new behind the scenes video format has generated a ton more questions than I usually get about the process on this boiler. I take that to mean a lot more people are watching the bonus videos, which is great!

This is another question that has come up so often that it'll be more efficient to answer it here in a post. The question is, why not pre-heat the boiler in an oven before silver soldering?

I'll start by saying I tried that on the very first boiler I ever built many years ago, but it didn't actually help at all. Since then, I've learned why. There are a bunch of reasons why this doesn't get you anything.

The main one is temperature. A household oven only goes to 500ºF, which is less than half of the way to silver soldering temperature. Furthermore, building up heat in a mass is an exponential process, not linear. So skipping the first half of that curve doesn't buy you much. It's the last third of the curve where all the energy is.

Second, it's slow. It takes about an hour to get 15 pounds of copper up to 500 degrees in an oven. That's a lot of time and electricity. Propane does the same thing in a couple of minutes with about 25 cents of gas. If you've ever wondered why gas clothes dryers are so much cheaper to run than electric ones, this is why.

It might be worthwhile to preheat if you have a big heat-treat oven that can reach thousands of degrees, but a heat treat oven big enough to hold even a 3.5" gauge boiler would run you about $10,000.

A third reason is that you don't really want to put stuff like this in your household oven where you cook your food. There are cutting oils, flux residues, cadmium particles, pickling acid, etc on it. All that stuff vaporizes and goes on your oven and in your kitchen. No thanks! You could get an old oven and put it in your shop, but they are huge and take a lot of valuable floor space.

So this is why we do it with a torch and why everyone has always done it with a torch. Torches are cheap, easy, and fast.