We have a oil-fired boiler that generates both the hot water and the heat
for the house. My wife is opening a daycare, and per state regulations, we
need to have the hot water at the tap be 120 degrees or less. While I'm
sure it's possible to regulate the temp of the entire system, I can see
problems with simply turning the whole thing down. (e.g. 20 below zero
nights)

So, I'm thinking there must be some kind of valve that does this sorta thing
automagically. Turns out, I'm right - I did a little research, and lo and
behold, I found "Thermostatic Mixing Valves". The perfect thing for my
application!

Then, I saw the price - they're pretty dear! The cheapest I could find was
right around $100, which seems exorbitant for what it is. Which got me to
thinking - why can't I simply throw in a couple of check valves, and a ball
valve that controls how much cold gets mixed with the hot? The sum total of
the parts would be about 20 bucks. What am I missing here?