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Combi-boilers 101: How Do Combi-boilers Work?

Combi-boilers are a common residential heating system used to heat your home and provide endless hot water. This video explains how combo-boilers work, and why they're such a popular choice among homeowners.

Learn More About Navien Combi-boilers

Navien’s high-efficiency combi-boilers are AFUE 95% Energy Star certified and backed by the industry’s strongest warranty. Learn more about the benefits of Navien residential combi-boilers.

Navien’s NCB-H series condensing combi-boilers generate maximum heat and the highest flow rate for domestic hot water.

Navien’s NFC-H series condensing fire tube combi-boilers have even more space heating capacity for larger homes.

What's a Combi-boiler

Combi-boiler Heating System

A combi-boiler is a home heating system that can be used for both hydronic space heating and domestic hot water production.  


This dual-function boiler uses dual heat exchangers to heat the hydronic water, and a third heat exchanger to indirectly heat the domestic hot water. 


Combi-boiler Applications 

When a Combi-boiler is used for space heating, the heated water flows from the boiler to radiators, baseboard heaters or in-floor radiant systems that distribute heat throughout the home.  

Separately, water travels through a domestic hot water heat exchanger delivering endless hot water to the home’s faucets, showers and appliances. 

Combi-boilers can replace the need for having both a heating boiler and traditional tank water heater as your hot water needs can be supplied by a single compact appliance.  


Combi-boiler External Connections

Straightforward on the outside – you’ll see connections for combustion air, exhaust, cold water, hot water, boiler supply, boiler return, gas, and condensate drainage.


Combi-boiler Internal Components

On the inside of the units, the key components include the dual heat exchangers, intuitive controls, circuit board, fan, burner, circulator pump, gas valve, control wiring, and internal piping, as well as a built-in domestic hot water module with a separate stainless steel DHW heat exchanger. 


How a Combi-boiler Works  

Similar to a forced-air heating system, your home’s thermostat senses a drop in temperature and signals the boiler to turn on.  

Using natural gas or propane, the boiler heats the water in the system. This heated water then flows through your house via radiators, hydronic baseboards or in-floor radiant heating systems to warm your home.

As the water cools in the system, it travels back to the boiler to be reheated and recirculated throughout the house.

This process continues until your home reaches the set temperature and your thermostat calls for the boiler to turn off. Your boiler remains in this standby mode until the thermostat calls again for heat.

When there is a demand for domestic hot water, the system prioritizes this water heating capability and space heating is paused. 


Selecting a Navien Combi-boiler 

When selecting the right Combi-boiler for your home, there are several factors that need to be taken into consideration, including space heating capacity, domestic hot water usage, fuel type, and heating type.



Navien offers seven different sizes of combi-boilers that are designed to meet the needs and comfort level of any homeowner.

Navien requires that every combi-boiler unit sizing and installation be performed by a qualified contractor and conform to the combi-boiler installation and operation manual included with the unit.


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