I thought I could knock off 20-30 minutes of my brewing process by boiling 2 gallons of water in my lid covered kettle while steeping the grains in another pot with about 3 quarts of water. After removing the grains I brought the 3 quart water to a boil and added it to the already boiling larger kettle, now with the lid removed. I was expecting both boiling mixtures to mix and form a rapid boil. Man was I wrong. The mixture finally came to a boil about 30 minutes later. Anybody know the reason why it would take so long for the combination of equal 212F temperatures to boil?
There is some heat loss due to pouring between containers. Also there can be quite a difference between a slight boil / beginnings of a boil and a rolling boil. To get from an early boil to a rolling boil, while the temperature remains constant at 212 F, you need to add the “heat of vaporization”, which is very significant. So make sure you get your smaller volume to a vigorous rolling boil before moving it over for minimum effect.
The 2 gallons of plain water in your kettle will boil at 212 F at standard pressure (1 bar). But since it was covered, the temp should be slightly higher due to some small pressure build up (that’s the idea behind pressure cookers … water will boil at a higher temp under pressure).
The 3 quarts in your brew pot will boil at a temperature higher than 212 F since it contains sugars … what that happens to be is unknown … you would have to actually measure it unless you knew exactly what was in the water … and knew the exact atmospheric pressure. But in general, the higher the gravity, the higher BP. [I believe Brew Cat mentioned the higher temps in another thread when referring to his work with syrup].
When the two are mixed together, even if they’re both boiling, they will be at different temps prior to mixing because of the different boiling points. The resulting temp should be somewhere between the two. And when they’re mixed together, that 2 gallons of water that used to boil at 212 will now have a higher boiling point (the 3 quarts will have a lower boiling point) … and again the boiling point of the mixture as a whole will be somewhere between the two.
So the temp of the new mixture must reach the new boiling point.
The time it took to bring the mixture to boiling would be lower if you switched things around since the initial temp of the mixture would be higher. But that would require a longer time to get your wort up to temp. It would probably be fun figuring this out by experimenting … since doing calculations doesn’t sound very appealing. 
[quote=“Snipe”]The 2 gallons of plain water in your kettle will boil at 212 F at standard pressure (1 bar). But since it was covered, the temp should be slightly higher due to some small pressure build up (that’s the idea behind pressure cookers … water will boil at a higher temp under pressure).[/quote]There is no pressure build up in a pot with a lid. The reason is that there is a large surface area where heat loss was occurring being covered up and hence not being allowed to lose heat as fast.
I’m not sure if I understand what you’re saying.
A pressure cooker is little more than a pot with a lid … as is a whistling tea kettle … or steam engine. 
And the temperature of water in the liquid phase will never exceed 212 F at standard pressure, period, regardless of much energy you put into it.
[quote=“Snipe”]I’m not sure if I understand what you’re saying.
A pressure cooker is little more than a pot with a lid … as is a whistling tea kettle … or steam engine.
And the temperature of water in the liquid phase will never exceed 212 F at standard pressure, period, regardless of much energy you put into it.[/quote]
A pressure cooker is a pot with a lid that seals in an airtight manner. A standard pot lid still let’s pressure equalize.
That said, a there would be a slight pressure differential within a standard covered pot, but not significant. The major effect a pot cover has is that it blocks the heat loss of the pot’s contents to the surrounding air via convection.
Here’s a biomedical engineering student’s two cents…
I don’t think pressure plays any part in this. Also, despite what you may think, water holds its temp pretty darn well (as can be seen in how long it takes to cool wort!). So, not much temp would be lost in the transferring of the water from pot to pot.
What you noticed is called boiling point elevation.
Pure water will boil at 212 degrees fahrenheit at atmospheric pressure. When you add a solute to water, it makes it “harder” to bring the water to a boil. Therefore, a temp higher than 212 degrees is required.
This is why you can cook pasta more quickly if you add salt to the water. The salt is a solute and dissolves in the water to form a solution. This elevates the boiling point of the water. The water boils at a higher temp, decreasing the necessary cooking time. Without the salt, the highest water temp you can achieve is 212 degrees. With the salt, the water boils at a higher temp that depends on how much salt you add. A common mistake is to think that the salt allows you to boil the water more quickly, when IN FACT it takes longer to boil salt water because you have to reach a higher temp.
Your two gallons of water were boiling at 212 degrees. Your 3 quarts of grain-steeped water contained solute and therefore were boiling at a higher temp. When you added the steeped water to the boiling plain water you created a new solution with a boiling point higher than 212 degrees. Even though your steeped water was already at a higher temp, it wasn’t enough “heat”, per say, to increase the total temp of the 2.75 quarts to its elevated boiling point.
Hope this helps!
P.S. I just realized someone else essentially said what I was talking about, referencing the sugar in the steeped grain as opposed to plain water. I just elaborated a bit.
Just to keep things in perspective:
For a boil volume of 11 quarts and a BG of say, 1.055 … and working backwards, the BPE in the steeping pot would only be about 2 °F. Once everything is mixed together the BPE would only be about 0.4 °F.
Much less than I would have expected
