I'm glad you are not discouraged completely by the negativities here.

What kind of pump do you plan to incorporate for the heat exchanger pipe? You will want to find one that can handle 200F+. Most that I know don't. Hopefully someone more knowledgeable in that can chime in.

There's a basis in our doubt about the tempreature stability, not simply to pour cold water on ya. I personally have used quite many open boiler-ish espresso machine. All of them lose heat too easily, even after multiple rounds of boiling water flushes. So I imagine that'll be a major challenge in design a like this. While your pipe-heating design is sound, most heat exchanger works at temperature above 212F (due to inefficient heat transfer and heat loss) so what you're doing is indeed novel.

Instead of water pipe, you can actually use silicone band heater for heating the group head. I have used one on my commercial lever grouphead and it does work. Basically just stick it on and set the temperature. No need to fuss with hot water and pump, and you can even PID it shall budget allows.

http://www.ebay.com.au/itm/750W-120V-Si ... SwrklVc82T

Also, to move on the idea, can you potentially reduce the machined parts needed, to maybe 2-3 maximum? The less machined part, the more likely your design will succeed. Adding a commercial spring is tempting, but I would suggest for starter let's make that an optional upgrade. Those 9 bar springs are super strong. If you handle one before you will understand (you can't even move them a centimeter with your bare hand), and it can be extremely hazardous and will be a potential problem for a machine not designed robust enough. You can use a home lever spring, those are milder but still very strong. Unfortunately they won't generate enough pressure anywhere close to 9 bar, especially if your piston size is 2".

That silicone band looks interesting. I'll definitely keep that in mind. It looks like it might be a bit less hassle than what I have in mind right now. At the very least, it looks like a viable solution should I run into too many problems with my current approach.

I wasn't aware that pumps had heat limits, although I suppose it makes sense. In any case, that's good information to know, lol. Looks like there are no shortage of pumps capable of working with boiling water tho, so that shouldn't cause any problems.

I want to clear up that I realize flushing with hot water is far from ideal. I know just from messing around with an aeropress how quickly things cool down. One little mistake can cause a huge delay since in the time the mistake happened something important will cool down, so now you've got to heat everything back up, measure temps again, etc... I imagine if one is sufficiently organized though, flushing could work well enough and could be reasonably practical (perhaps while the user saves up for one those temperature controlled kettles). Ultimately, this would only be necessary if the user couldn't capitalize on the intended temperature control mechanism. Flushing is only supposed to be a back-up plan for temperature control not the primary means for controlling temperature, so I don't feel like any shortcomings of controlling temperature in this fashion are a serious issue.

As for the way I intend to tackle the issue of temperature control. I'm thinking of having some kind of heat conducting material in any gaps between the pipe and the cylinder, and the pipe will be tightly wound around the cylinder. I'm pretty sure I should be able to get the whole cylinder within a few degrees of the water that's running through the pipe but I suppose the only way to find out for sure is to try it and see what happens.

At this point, I'm kind of torn.. I'm pretty sure the silicone band is going to work perfectly, and I'm pretty sure my approach will be pretty cheap to test out. So I figure I'll give the pipe a try and see how effective it is, and if I run into problems I'm thinking something like this silicone band may be the answer.

In any case, I appreciate the feedback.

Also, I decided to try messing around with that sketchup program.. holy crap I haven't been this frustrated in a long time. It's so damn hard to do the simplest thing although I probably just need a bit more practice... Here's a work in progress..