While I have zero experience with lambda sensors, I do know tanks a little.
I agree a round tank makes a lot of sense, given the fact that you lose the hole square tank wants to become round stresses, but
a square tank uses less basement real estate. Even if this is a small amount, it does make a difference to the average consumer--though
not necessarily the average Hearth reader. And if designed well, it is not a problem ;^)
Unpressurized tanks do cost less than pressurized ones. The heat exchangers help bump the price up. Once again, we in this forum are mostly DIY'ers.
IF you are paying an installer to put in a tank, an unpressurized tank will win every time, given the labor savings and logistics.
Dick Hill (the inventor of the first gasifier wood boiler) and I NEVER used a heat exchanger to interface a ferrous or non-ferrous boiler with a storage tank. Never! We were too cheap to use the heat exchanger and pumped tank water directly into the heating load. This can be difficult for multi-story buildings, but works fine on one or 2 story homes.
We used a serious corrosion inhibitor when necessary and never had an issue.
When I started working with Tarm/Bioheat, I was slightly surprised to see that they were not interested in running unpressurized. All boilers in Europe are pressurized and use "accumulators". That is fine.
I have just recently built a boiler that is based on work we did at the University of Maine 25 years ago. It pumps tank water through the boiler (non-ferrous) and goes back into the tank. I like simple and will show more of it soon.
We have spent a lot of time and effort to engineer heat exchangers that use a lot less copper than the 3/4"L status quo ones. Feedback from the field has been great. And they are simple.
A common thread for me is that everything should be simple. I used to be fearful of complex electronics in heating and cars. The Prius made me comfortable with electronics in cars. I can see the merit in boilers like the Effecta and Froling, just struggle with the cost/benefits.
Sorry to ramble on. Whenever someone mentions tanks, I get going.
Tom
www.americansolartechics.com