Jaguar Caravan Multiple and Cascading Boilers
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A “Hybrid Condensing/Non-Condensing Boiler System” is
a combination of condensing and non-condensing boilers
working together to heat a building. The predominant rea-
sons to use a hybrid system are lower installed cost and
optimizing system efficiency. In this chapter we discuss
why a hybrid boiler makes sense and how to design a
hybrid boiler system.
Within the ASHRAE Equipment Handbook the subject of
condensing or non-condensing boiler is covered. Figure 1
of this chapter in the Handbook (Effect of Inlet Water
Temperature on Efficiency of Condensing and Non-
Condensing Boilers) is a graph illustrating the relationship
between inlet boiler water temperature and boiler efficien-
cy. The dew point or start of condensation of the water
vapor in the products of combustion for natural gas starts
around a 130 F return water temperature. Above that tem-
perature condensation does not occur and as the water
temperature decreases below the 130 F the amount of
water vapor condensed increases. As the amount of con-
densate increases the boiler efficiency increases. Typically
a 90% boiler efficiency is achieved when the water temper-
ature is 120F or lower. The lower the water temperature,
the higher the boiler efficiency and conversely as the water
temperature increases the boiler efficiency decreases. With
a 180F water temperature the boiler efficiency is around
86%. This is just plain physics.
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 handle the load. With water tem-
peratures below 130F condensing boilers operate and
when the water temperature is above 130F the non-con-
densing boilers take over.
The system designer determines the heat load for the heat
emitters when the water temperature is at 130F. This load
is used to size the condensing boiler used in the hybrid
system. Slant/Fin does publish residential baseboard and
commercial radiation output ratings down to 110F water
temperature. If the heating system is designed to satisfy
the full heating load under design conditions with a water
temperature at or below 130F, then a boiler plant using
only condensing boilers makes sense.
The non-condensing boiler of a hybrid boiler system is
sized to handle 100% of the heating load. When the water
temperature is above the 130F, the non-condensing boiler
does handle 100% of the load. After all, water tempera-
tures required in a heating system increase as the outdoor
temperature drops and the load increases.
A properly applied hybrid boiler system will optimize effi-
ciency and be cost effective. There will be a faster payback
for the boiler plant and higher return on investment.
A Slant/Fin hybrid boiler system consists of Jaguar modu-
lating/condensing boilers installed with Caravan modular
cast iron boilers. Since modular boilers inherently have
built-in back up, a boiler system can be sized closely to the
calculated heat load. This helps decrease the initial cost of
the boiler system further.
A Slant/Fin hybrid boiler system is controlled so the system
control modulates the input of the Jaguar boilers and then
step fires the Carvan modular boiler while resetting system
water temperature. The result is a simple system that deliv-
ers high efficiency while improving system control and
dependably. One control for a hybrid boiler system is the
Heat Timer CNC Control, CNC stands for “condensing and
non-condensing”. Heat Timer can be reached at
201-575-4052.
The water piping of a Slant/Fin hybrid boiler system is sim-
ple. Slant/Fin recommends “injecting” the condensing boiler
water into either the supply or the return water piping of
the Caravan modular boiler, please see figure 22. This
drawing shows 2 each J-390C Jaguar boilers and 3 each
GGT modules used in one hybrid boiler system. Jaguar
boilers can also be added to existing boiler systems
provided there is room to do so.
12. Hybrid Condensing/Non-Condensing Boiler Systems
