Hybrid Boiler Systems – Definition, Sizing, and Efficiency

A “Hybrid Condensing/Non-Condensing Boiler System” is a combination of condensing and non-condensing boilers working together to heat a building. The predominant reasons to use a hybrid system are lower installed cost and optimization of system efficiency. Here we discuss why a hybrid boiler makes sense and how to size and design a hybrid boiler system.

A Slant/Fin hybrid boiler system consists of a Jaguar or CHS modulating, condensing boilers installed with Caravan modular cast iron boilers, or even an existing boiler system. Since modular boilers inherently have built-in back up, a boiler system can be sized closely to the calculated heat load. This also helps control the initial cost of the boiler system.

A Slant/Fin hybrid boiler system is controlled so the system control modulates the input of the CHS or Jaguar boilers and then step fires the Caravan modular boiler while resetting system water temperature based on outdoor temperature. The result is a simple system that delivers high efficiency while improving system control and dependability. One control for a hybrid boiler system is the Heat Timer CNC Control, CNC stands for “condensing and non-condensing”.

The water piping of a Slant/Fin hybrid boiler system is simple. Slant/Fin recommends “injecting” the condensing boiler water with primary/secondary piping into either the supply or the return water piping of the Caravan modular boiler, please see figure 22 in the Jaguar Caravan Application Guide. Publication No. CJ-10-HWG. This drawing shows 1 J-390C Jaguar boiler and 2 each GGT modules used in one hybrid boiler system. As mentioned, Jaguar boilers can also be added to existing boiler systems provided there is room to do so.

Condensation of the water vapor in the products of combustion for natural gas in a high efficiency boiler occurs when the flue gasses are cooled by the heat exchanger to less than 130 F. As the water temperature falls further below 130 F the amount of water vapor condensed increases thereby driving efficiency up further. It could go as high as 96.7% with low water temperature and therefore low flue gas temperatures. With higher water temperatures in the heat exchanger the boiler efficiency will decrease, depending on water temperature to a low of approximately 86- 87%.

A properly controlled “hybrid condensing/non-condensing boiler system” operates the condensing boilers when the water temperature is low enough for condensing to occur and when the water temperature is above that point the non-condensing boilers are added to handle the additional load. With return water temperatures at or below 130F, condensing boilers operate and when the water temperature required is above 130F the non-condensing boilers are fired when additional btu’s are required.

The system designer determines the heat load of the heat emitters when the supply water temperature is at 140F and the return at 120F, an average water temp of 130F. Typically in the New York City area the heating system would require this temperature when it is 38F outdoors. This load is used to size the condensing boiler(s) used in the hybrid system, which is usually about 47% of the load in NYC. The non-condensing boiler(s) of a hybrid boiler system can be sized to handle the remaining heating load – typically 40-60%, again in New York City. When the return water temperature is above 130F, the non-condensing boiler(s) will handle part of the load because the water temperature required in a heating system needs to rise as the outdoor temperature drops and the load increases. As evidenced by the heat loss and the outdoor reset curve.

A properly applied hybrid boiler system will optimize efficiency and be cost effective. There will be a faster payback for the boiler plant and a higher return on investment. In the example above, and generally speaking*, the High Efficiency boilers would be operating at least 50% of the time by themselves with average efficiencies of say 87- 92%. When it is more than 38F outside, the cast iron boilers would run at 82% and the Jaguars about 87% with a combined average efficiency of 84.5%. Therefore half the time we are at let’s say 92% (between 91.7 to 96.7% depending on boiler output) and half the time at 84.5%. But at the 92% efficiency the building only needs half of the Btu’s. When it is running at 84.5% we are at full load, needing all the output of all the boilers. This means that 25% of the Btu’s are consumed at 92% and for 75% of the time at 84.5% efficiency. This will give us a yearly average efficiency of 86.4%. (In the example, an all high efficiency system would run at 92% efficiency 25% of the time and 87% for 75% of the time for an average of 88.25%. Interestingly however, according to Brookhaven National Laboratory testing, in a residence near New York City, they found that a residential high efficiency boiler when installed in a typical, older designed home, it would operate 85 – 95% of the time in the condensing mode because of the low water temperatures required due to the fact that it is frequently over 38F outside. Using that figure here would result in a high yearly combined average efficiency.

A new heating system can also be designed to satisfy the full heating load under design conditions with a water temperature at or below 140F. In this case, a boiler plant using only condensing boilers makes sense. Slant/Fin publishes residential baseboard and commercial radiation output ratings down to 110F water temperature. Many existing buildings are over designed with much more radiation than needed. This should be taken into consideration when replacing the boiler plant. With extra radiation, lower water temperatures can be utilized thereby changing the ratio of condensing to non-condensing boilers.

Sizing a Hybrid system example:

Job has 1250 feet of H-1 element in HD-850 cover. Heat output 720 BTUH’s per foot at 180F for a total of 900,000 BTUH’s. Design conditions: 10F outside, 70 F inside. At 38 F outside, heat loss is 47% of total load or 423,000 Btu’s. At a reset curve of 1.8:1, the boiler will provide 140F supply water temp with 340 BTUH’s per foot for a total of 425,000 BTUH’s. Performing a heat loss at 38 F reveals that the output at 140 F would meet it. This means that two Jaguar 390’s (612,000 net IBR) and one GG 399 HEC (284,000 net IBR) module would be used. Some designers would size the job with all high efficiency boilers but that would mean extra equipment costs with a small incremental increase in overall efficiency over the hybrid.

Other design conditions for other climates using different reset slopes could be calculated similarly.

*This whole discussion is very general in nature. More scientific data (such as temperature BIN Data) needs to be calculated to determine the actual energy savings. Suffice it to say, higher efficiency equipment saves energy and needs to be weighed against equipment costs to determine payback and the efficacy of these types of systems. The calculations above may not reflect actual savings of the project you are considering.