How to choose a boiler
No one should specify any boiler before doing a thorough heat load analysis; this is something I do after I win a contract or receive a consulting fee. Your prospective contractor should produce a sample of his heat load calculations. There is little difference in cost between the various boilers available, but an oversized boiler (modulation notwithstanding) can affect over-all system efficiency by as much as 50% and can lead to comfort and reliability issues.
All modern high efficiency gas-fired boilers condense AND modulate flame. Condensing boilers recover the latent heat found in the water vapor (smoke up the chimney) to usable hot water used to drive radiant floors, radiators and even fin-tube baseboard. This act of recovering latent heat typically reducing stack temperature by several hundred degrees and allows for a Class IV vent (PVC).
It would seem a boiler that modulates flame (output) would not require such careful sizing. However, just the opposite is true. It is at low fire that much of the fuel savings occur, and in most ModCon boilers that low-fire is determined by the high-fire output, i.e. the "bigger" the boiler the bigger the minimum output. This condition can lead to short cycling (death to system AND thermal efficiency).
As I am a consultant, AND a contractor, people all over North America often ask me; which boiler is best? Most contractors are married to one supplier and advocate one boiler brand (usually a non-condensing, low efficiency cast iron model). As you can see from my website, I have personally installed nearly all ModCons currently available in the USA. The reason for this admittedly odd practice is twofold; first I like to compare ease of installation, maintenance, performance and factory support. Second, no, one, boiler, fits every application and many of the boiler companies have short lines. This is particularly true of the European boiler manufacturers' N. American offerings.
I would also like to determine the type and size of indirect water heater you will need. Since you are considering one of the most efficient and lowest polluting fossil fuel appliances on the planet, it makes sense to put it to good use satisfying what is most likely your second highest fuel bill, domestic hot water.
With a Mod/Con and indirect or "companion water heater" you will be able to permanently close the old fashioned chimney and the open combustion air pipe, saving even more fuel!
If you have an off-the-shelf programmable thermostat it will lend nothing to comfort and little to efficiency when applied to the typical hydronic heating system. They are made for forced air furnaces. You, on the other hand, will benefit both in comfort and efficiency by use of the onboard microprocessor that resets boiler water temperature to outdoor temperature (at least one ModCon has an onboard setback feature with boost mode to catch-up after a cold night). Some ModCons even feature water temperature setback for more fuel saving.
Properly applied, sized, installed and programmed; all of the Mod/Cons will condense and operate at their full potential efficiency (95%+ A.F.U.E). Warranties are very similar and the last thing I consider; as labor will double or triple any replacement part you may need. Certain models have factory extended parts and labor warranties. Extended warranties are available for all models but are a form of insurance. Unfortunately warranties are not reflective of quality, reliability, serviceability or efficient system design.
Annual maintenance by a factory certified technician (such as me) is key to the safe, reliable and efficient performance of any Mod/Con driven heating system. This is not just a matter of knowledge and experience but requires a combustion analyzer among other expensive tools. So ask your contractor about factory training on the model he advocates.
A final word on "Big Box" boilers. Your local hardware/department store or "Web" supplier may "sell" boilers, but they are seldom qualified to design, install or service any of them, much less a modern ModCon. If your local gas supplier sells boilers keep in mind installation may be done by sub-contractors who are - by definition - the low bidder.
See the Resources Page for more information on high efficiency condensing boilers.