The very first Vitodens 222-F installed in the State of Minnesota - 2013
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. Oversizing a boiler 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.
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! Next time you replace the shingles the old brick chimney be removed and the energy sucking draft it creates with it.
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 meant for forced air furnaces. You, on the other hand, will benefit, both in comfort and efficiency, by the use of the onboard microprocessor that resets boiler water temperature to outdoor temperature, outdoor reset (ODR). Some ModCons even feature water temperature setback for more fuel saving.
Nest, Ecobee, and other such WiFi thermostats must be installed and programmed by a technician familiar with their proper application and suitability for hydronic heating and condensing boilers in particular.
Properly applied, sized, installed and programmed; all of the Mod/Cons will condense and operate at their full potential efficiency (96% thermal). Warranties are very similar and the last thing I consider, as a trip charge, diagnostics and 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 nor should they be considered a substitute for a masterful system design.
Annual maintenance by a factory certified technician 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.
Copper-Cast Iron Boiler on a Budget
If you are on a tight budget, or you are considering a near-future move, you may want to consider the economy of a copper boiler*. The copper boiler has similar combustion efficiency to its cast iron cousin, but generally can operate at lower minimum return water temperatures, lending itself perfectly to low-temperature radiation and radiant floor heating. It's not a ModCon but a less expensive alternative, conventionally (atmospheric) vented and at a lower initial cost. Serviceable life; 25 years.
*Nearly every boiler, and pool heater, west of Denver, CO, is a copper boiler.
The final word on "Big Box" boilers. Your local hardware/department store or "Web" supplier may "sell" boilers, but the folks on the other end of the line 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.