I use a so-called simmercat a lot, as opposed to the standard two-row version, because my testing showed it delivered more total heat per unit of fuel. IOW I find it more efficient though somewhat slower.

But I don't understand the "simmer" notion. Maybe at the very start, but once the fuel starts boiling furiously the thing is like a blowtorch, you can't simmer anything.

The only way a simmercat can be used for simmering is if you load it up with max fuel, get your couple of minutes of simmering, and then be able to snuff the flame and recover the fuel.