Modern boilers, have moved from cast-iron beasts with low efficiencies, to high efficiency appliances that take up half the space, wall mount, and can save you serious $$$ on heating costs. If you are considering new construction, a remodel, or a heating system retrofit, you should know that the options have changed, as has technology, and you can benefit directly from a little knowledge on the subject.
In the past, most heating (baseboard and radiant anyways) relied on the large thermal mass of the boiler to keep things warm. The idea was get it hot and keep it hot. Boilers of the cast-iron variety fire at full rate, all or nothing. Keeping them hot was necessary, if the return temp from the system cooled the boiler off too much, they could condense, and ruin their own exhaust stack sending harmful Carbon Monoxide into you home. To use them with radiant systems involved elaborate piping schemes, pumps, and or mixing valves. In addition to this, they are inefficient and only operate in the 80% range of AFUE ratings, sending 20% of its heat up the stack. Simpler? Yes, efficient, not exactly.
With the first modulating condensing boilers, or mod-cons, they fought to tackle these efficiency issues with better heat exchangers, and an output that closer-matches the heating output demand. This enabled much tighter boiler control, and allows the mod-cons to take advantage of another great energy saving feature, Outdoor Reset.
What is outdoor reset? Simply put, you match the heat load of the system to the heat loss of the system. This applies not only to the modulation of the boiler firing rate itself, but also to the heat loss of the building as it fluctuates with the temperature outside. For example. A Cast iron boiler gets a call for heat and its 20 below zero, it runs at 100%. If its sized right for the house, the house will be comfortable. If its 40 ABOVE zero and the boiler gets a call for heat, it fires at 100%. No change.
Now a mod-con with outdoor reset, behaves this way. A sensor is installed outside, if the boiler gets a call for heat, it checks with the sensor, at 20 below zero it runs @ 100% and you are comfortable. At 40 ABOVE zero, the boiler gets a call for heat, it checks with the sensor, and calculates the firing rate percentage based on a programmed outdoor reset curve matching output to heat loss, and you are comfortable. The boiler knows from its outdoor sensor, that at 40 above zero, the house will not need as many btus to keep the space warm that it does at -20. So the output temperature of the boiler, feeding the heating loops, can be less as it gets warmer outside. So, in the off seasons like spring and fall, heat is replaced in the building ideally at the rate that it looses it.
I have one client who we swapped out a cast iron boiler for a triangle tube boiler, upsized it by 100,000 btu, installed 1000 foot of snowmelt, and the cost for heat in the winter is still less than the prior year with the old boiler. It works, it's efficient and saves you money.