Combi-Boilers FAQs

Frequently Asked Questions

A combi (combination) boiler is designed to heat a building, using hot water (similar to a boiler), but also includes the components and controls to internally heat the domestic hot water (DHW) required in the application. By having both heating and hot water in one unit, the combi will save space in the mechanical room and also save on the initial installation compared to separate appliances.

A combi (combination) boiler provides both space heating and on demand domestic hot water. In heating mode, water from the space heating system will flow through the heat exchanger and out to the system similar to a standard boiler. When a domestic hot water demand (open faucet) occurs, water flows through a separate heat exchanger in the combi, and a valve inside the unit diverts heating water over to the DHW heat exchanger. Space heating water is used to heat the domestic hot water until the demand ends. At the end of the domestic demand, the combi will switch back to supply space heating.

Inside the combi-boiler is a heat exchanger that separates the space heating flow from the domestic water flow, so the water in the heating system is isolated from the domestic hot water. The only thing that happens in the heat exchanger is that the heat from the higher temperature space heating water is transferred to the lower temperature domestic water.

Since the combi boiler provides both space heating and domestic hot water all in one unit, it can provide space savings over two or more units. Combi boilers also can provide endless hot water in DHW mode, similar to a tankless water heater. Combi boilers may also reduce the installation cost by reducing the number of appliances, possibly reducing the number of wall or roof penetrations required for exhaust and intake piping, and may reduce some of the gas piping requirements.

Combi-boilers do not require any additional requirements, compared to a condensing boiler and water heater. Compared to installing two different appliances, the installation may even have fewer requirements.

When a DHW (hot water) demand happens in a Navien combi boiler, the unit switches from space heating mode over to DHW mode. Referred to as DHW Priority, the unit will utilize the full capacity to heat the domestic hot water until the flow stops, providing endless hot water. While in DHW priority mode, the space heating is disabled. Even with a long shower or extended DHW run time, the home may lose a few degrees, but the chances of the home freezing during that time are extremely low.

The heating size of the boiler is determined by calculating the heat loss of the building, which is based on the size of the structure, the insulation value of the surfaces, and the door and window sizes. Since combi boilers also provide domestic hot water, the flow demand of the hot water fixtures also needs to be calculated to determine the maximum DHW flow required. A combi boiler size is chosen by ensuring that the unit can satisfy both demands. Typically with a combi boiler, the BTU input will be higher for the DHW demand than for the heating load. A qualified contractor can perform a heat loss and help recommend a proper boiler size.

Please see the following attachment on where you can locate your serial number.

1. Locating Serial Number

The applicable period for this labor allowance for all water heater models is One (1) year from the effective date.

The payment and amount of any payment are subject to approval at Navien’s sole discretion.

The labor allowance will be paid based on Navien’s schedule of labor allowances.

Because our product involves water, electricity, and gas, there is only a limited amount of troubleshooting that can be done with a homeowner/end-user.

We recommend contacting your original installer or a Navien Service Specialist (NSS) to troubleshoot the unit.

NOTE: Charges will apply for service calls outside of 1 year labor period and/or non-warranty issues. Please see terms of our labor warranty.

Because our product involves water, electricity, and gas, there is only a limited amount of troubleshooting that can be done with a homeowner/end-user. We recommend contacting your original installer or a Navien Service Specialist (NSS) to troubleshoot the unit. NOTE: Charges will apply for service calls outside of 1 year labor period and/or non-warranty issues. Please see terms of our labor warranty.

If you feel like there is an issue with your unit, please have someone contact Navien Technical Support while in front of the unit with your Serial Number ready. We recommend that a license contractor, original installer, or NSS contact is the person who calls us, so we may do in depth troubleshooting to determine if there is a part failure.

Navien Technical Support will be unable to send out parts without registration of product (Serial Number, Date of Installation, Homeowner Information, and Installer Information).

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