Re: Why does water in 250 degree oven not boil?

I am not sure how helpful my thoughts will be on your creme brulee because I only use the oven for the brulee part but I'll try.

First, you are right about water boiling at a lower temperature in Denver. Because you are located roughly a mile (1600m) above sea level, the atmospheric pressure on the water vapor is lower than at sea level so water will boil at ~200F (95C). More information is available through the International Association for the Properties of Water and Steam.

Now, for creme brulee (note: I've only heard of cooking the custard on the stove, not in an oven), a double-boiler or water bath is used to slow down or diffuse the heating of the custard so it doesn't overcook and curdle. It's OK if the water boils but the heat source should be set low enough to only lightly boil the water so as to not scald the cream and egg mixture. I've only used an ice cold water bath in the oven to keep the custard from cooking while carmelizing the sugar on top for the brulee part. And using my method, the brulee will be finished before the water even starts to heat up.

But let's talk quickly about trying to boil water in an oven vs. on a stove top. On a stove, a direct electric or gas heat source is used to transfer heat through a (usually) metallic pot wall and into the water, but using temperatures in excess of 2500F (1400C), roughly the temperature of a candle flame. Metal conducts heat very quickly, so the water will boil in only a few minutes. In an oven at 250F (121C), heat must transfer by conduction from the electric or gas heat source to the air (gas), through the pot or baking tray walls (solid), and finally into the water (liquid). Air is a terrible conductor or good insulator depending on how you want to look at it. And if you are using a glass or ceramic tray for the water bath, the heat transfer will be slowed down yet again because, again, both materials are great insulators. Without more details about the tray, amount of water, water starting temperature, etc, I can't estimate how long it would take for the water to boil, but it will be much longer than on a stove top.

So, I hope I've helped answer your question. I know all this talk about creme brulee has made me hungry! :P

Kieran