I"m not too knowledgable about boilers, but I will tell you my opinion. If it is still working keep using the existing equipment.
With gas fired equipment, and boilers, your likely have btu's to spare. Typically with boilers your not playing around with setting the t-stat back and trying to recover the temperature.

How big is the house to begin with?
Is the 600 sq. foot addition well insulated? How about the rest of the house? Can you add insulation?

I"m all for replacing old inefficient equipment, but 93% is very efficient. I would keep if possible.

The best thing, in my opinion, is to first do a proper heat loss calculation on the house with the new addition. Taco's free FloPro Designer software, along with many other software programs, and even the old Manual-J method will get you your heat loss calculation, and then you can go from there. The old "rule of thumb" methods for calculating boiler size are no longer the way to go. After tightening up the building envelope, installed new windows, caulking gaps, etc. your old boiler might even be oversized...

Your boiler is very efficient, and if it shows no signs of failure, and you have it properly maintained, I wouldn't worry about it.

I have a Weil-McLain Gold Oil Steam boiler (from 1995) which was converted to hot water when I converted my 3-family from one steam system to three hot water systems. While it is WAY oversized for my 1st floor apartment, I just couldn't part with it.

PexSupply.com is a great place to research boilers and find prices and specs. If you decide to have a plumber install the system, insist that they perform a heat loss calculation before selecting a boiler for you. They will usually have a "favorite" brand that they like to install, which might not be the one your looking at.

Good Luck,
Nick

thanks for the good data: I have been out of pocket for the last 10 days-- just got back on line. Our two-story house was 3600 sq ft. With the new addition it is 4200+. Our original house is well insulated: R-19 between floors and crawl space; R-22 walls and R-38 in the attics. We have a combo of Marvin and Pella double pane windows with low-E glass. The addition is similar with a little more insulation in the attics.

I think that I'll watch it this winter-- hopefully it won't be as cold as last winter when we hit 30 below and had a ten day period with temperatures averaging below zero!. If it keeps us warm on the coldest days, I will let sleeping dogs lie.

thanks again,

Will

I think you may very well be ok with your existing system. The size increase is only about a 20% increase for your home. Considering the good insulation of your home I think your existing system might do well (unless you get the extremes). If it doesn't keep up during the cold weather this season you might consider replacing next year.

In your situation I'd go with a wait-and-see tactic. A well-running boiler is a happy one and if it was installed in the 90's using the same old calculation methods for load that are known to oversize heating systems then you'll probably be safe.

You might also find it less expensive to add even more insulation to your home. I just increased my attic insulation from roughly what you have to R60. (Alaska.) I also added R10 rigid foam panels to the knee wall that separates my bedroom from the garage attic. That was cheaper and easier than a boiler replacement. This 1982 home had a 140K BTU boiler when built and we downsized it to 70K four years ago. With the additional insulation in those two areas I already sense the ability to downsize again based on our calculations.