"Wej Murf" wrote
I have a 25 year old Burnham boiler (hot water system) with 3/4"
piping and Electrozone zone valves (3 valves).
It's my understanding that parts are no longer available for my zone
valve system.

So here's my dilemma:
Do I just leave it alone until it breaks and deal with it then?

And then if I do replace, should I replace only the zone valves or
just replace the whole boiler? I got one estimate (need to get more)
for $2100 to replace all the controls, or $4500 to replace the entire
boiler with brand new everything.

Key word here is the 25 year old boiler. Gas or Oil? What kind of
condition is it in? My 30 year old was replaced two years ago, and I'm
saving 39% on my oil use. At 3.50 a gallon, that is $1200 a year.

Given that you have three zones, if one drops out you have minor
inconvenience, not a catastrophe. Some other zone valve can be retrofitted
with minor down time.

Check what is available for rebates, zero interest financing and it may be a
good time to replace everything. My System 2000 by Energy Kinetics is
paying for itself with fuel savings compared to my older boiler. In a few
years, that money is going to be in my pocket, the some oil baron.