A hybrid boiler system combines efficiency with longevity.
One of the choices to be made when tasked with a boiler replacement project is whether to use a condensing or standard efficiency boiler. Several considerations should go into that decision.
The first consideration is the type of mechanical system in the building. While condensing boilers are ideal for snow-melt, radiant floor-heating and water-source heat pump systems, there are some applications where a condensing boiler simply does not condense. It is like opening a pistachio nut and finding it empty. If the water temperature inside the boiler is greater than 140° F, a condensing boiler will not condense.
This benchmark temperature - 140° - separates the two boiler types. Standard efficiency boilers will condense when the water temperature is at or below 140°. Operation below 140° in a standard efficiency boiler will result in damage to the boiler flue and chimney. Condensing boilers will stop condensing when the water is above 140°. They actually lose efficiency the warmer the water is for the system.
On boiler brochures, right after the huge font proclaiming the efficiency of the uber-efficient boiler, is usually an asterisk. When you find the asterisk later in the brochure, the sentence following it typically will state that the boiler will reach those unbelievable efficiencies with 60° water. If you have a building filled with cast-iron radiators, 60° supply water will not raise the building temperature 1° on a 10° day.
In older applications, the original designer typically specified a system that would heat the building using 180° supply water at the design outside air temperature. At that supply temperature, the condensing boiler will not be condensing. It will be in the 84% to 88% efficiency range. This is slightly more than the mid-efficiency boiler range.
In Europe, where condensing boilers are more common, the designers oversize the heat emitters to allow lower water temperatures. Let us assume our existing heating system was designed using 180° water with fin-tube radiation. We now want to use 120° water to heat our building. To do so, we would have to have almost three times as much radiation as we originally had. We could have a wall filled with copper tubes and fins. In Europe, they are proud of their radiators and proudly display them. Over here, we hide them.
Another factor to consider is when the condensing boiler is used to heat either the domestic hot water or a swimming pool via a water-to-water heat exchanger. Commercial buildings require 140° water for the domestic water loop. If you require 140° water on the outlet of the heat exchanger, the water temperature has to be at least 10 to 20° higher in the boiler. That would make it 150°-160°. At that temperature, the condensing boiler is not condensing.