The owner asked for a bid to replace her 35-year-old cast iron boiler, which was still operating. 

She had two requirements: Redundancy in the event of a boiler malfunction and lower fuel costs for the building. You will be competing against the usual cast of contractors, including “Low Bid Mike,” who always comes in 10% to 15% lower than everyone else. You could use the work, so how do you show value to the customer and maintain a healthy margin? Perhaps consider a hybrid heating plant. A hybrid heating system consists of one or more condensing and one or more non-condensing boilers. Why would you consider this option?


Does a condensing boiler really condense?

I realize it sounds like some sort of philosophical query, but think about this; sometimes a condensing boiler does not condense. Some believe condensing boilers function like a condensing furnace, and they do not. The condensing furnace operates in the condensing mode all the time because the air temperature flowing over the heat exchanger is below the dew point temperature of the flue gases. 

The dew point temperature is the temperature where the flue gases start to condense. The dew point temperature for a boiler is between 128° and 133° F water temperature, based on 3% to 6% oxygen in the flue gases. This means the boiler will not condense until the water temperature is between 133° and 128°, or lower. When the boiler water temperature is above the dew point temperature, the boiler efficiency is around 85% to 88%, or slightly higher than a non-condensing boiler. 

To achieve the really high efficiencies, the condensing boiler water has to be very low. While reviewing the efficiency curve for a boiler, I noticed the boiler reached 98% efficiency at low fire with a return water temperature of 65°. You won’t be able to raise the room temperature from 68° to 70° with water that cool. Does that make the boiler bad? Absolutely not. It is simply operating the boiler outside of the sweet spot, like using a driver to putt a golf ball. If you were able to replace the heating system and emitters with a system designed for 130°, the boiler would condense all the time. 

Since 1899, most hydronic systems were designed to provide 180° supply temperature at the outdoor design temperature. The outdoor design temperature is the temperature used by engineers and designers to size a heating system. It is close to the coldest temperatures in a typical heating season in an area. I have enclosed a chart, which shows the bin temperatures for Charleston, West Virginia. The graph shows the design temperature for the city is 12°. This means the heating system has to provide 180° water when the outdoor design temperature is at or below 12°.


Reset control 

For almost as long as boilers were installed, control companies sold boiler reset controls. The idea is the warmer the outside temperature is, the lower the supply water temperature required to heat the building. While the building needs 180° at the design temperature, the building can be heated using a much lower temperature at 32° than at 12° outside temperature. 

The graph for Charleston, West Virginia shows the water will reset between 180° and 120°, depending on the outside air temperature. The reset ratio on this project is 1.2° to 1°, which means the boiler water temperature changed 1.2° for every degree change in the outdoor temperature. If you are using a reset control, the condensing boiler may not condense until the outdoor temperature is 50° or warmer. If the boiler is operating at 160° to 180°, the condensing boiler efficiency is hovering around 85% to 88% efficient.


Lower equipment cost

A condensing boiler is typically about 15% to 20% more expensive than a non-condensing boiler. By using one condensing and one non-condensing boiler instead of two condensing boilers, your equipment costs are already lower than Low Bid Mike’s.


Lower installed cost

Most condensing boilers have to be vented to the outside with separate flues. When using a hybrid system, the non-condensing boiler may be able to reuse the existing flue and chimney while having to cut only one hole through the outside wall for the condensing boiler flue.


Building aesthetics

Some owners find the look of flue vents through the sidewall of their building unappealing. With a hybrid system, you would limit the outside wall penetrations as the non-condensing boiler may be able to use the existing chimney.



Another consideration is it may be easier to safely sidewall vent one boiler rather than two or three through the wall in an existing building and still meet the code requirements in the International Mechanical Code.


Longer life 

According to CIBSE, the average life of a condensing boiler is about 10 to 15 years, while a standard boiler has a life expectancy of almost double, according to ASHRAE. To extend the life of the condensing boiler, I like to use the condensing boiler during the shoulder parts of the heating season when the outdoor temperature is warmer. This limits the operating time and extends the life of the condensing boiler. During the colder weather, I let the non-condensing boiler, which lasts 30 years, be the workhorse. 


Lower maintenance costs

A condensing boiler requires more maintenance than a standard boiler because of the acids formed by the condensing flue gases. The condensing boiler has to be opened and cleaned every year or two.  The conventional boiler will not require the same amount of maintenance as the condensing boiler. If you only have one condensing boiler rather than two, the owner’s maintenance cost is lower.


Controlling the hybrid system

Most off-the-shelf reset controls will not properly sequence a hybrid system without some programming help. To control the hybrid system, I use this control sequence. When the outside temperature is below 45°, I have the standard efficiency boiler as the lead boiler and the condensing boiler as the lag or backup boiler. When the outside air temperature is above 45°, I have the condensing boiler as the lead boiler and the standard efficiency boiler as the lag or backup boiler. 

In closing, a hybrid system allows you to have the efficiency of a condensing heating plant with the longevity of a standard boiler plant all at a lower installed cost than Low Bid Mike.