Combi-Boilers: How to Choose the Right Size

In order to properly size a combi-boiler for a home or business, you need to consider both heating and hot water demand, since a combi-boiler supplies both. With seven models across two series for both residential and commercial use, Navien has the right size combi-boiler for your application—and this article will help you find it!

How Combi-Boiler Size Is Measured

Combi-boiler “size” (heating output) is measured using BTU/h (British thermal units per hour). BTU/h represents the heat energy a combi-boiler can generate. A British thermal unit is how much heat it takes to raise the temperature of a pound of water by one degree Fahrenheit. So the more heating and hot water you need, the higher BTU/h your combi-boiler will need.

Heating Considerations

Consider the following factors that affect the required heating output for a combi-boiler:

• Regional climate
• Building insulation value
• Doors and windows (quantity & sizes)
• Skylights
• Number of radiators
• Desired indoor temperature

The best way to find out the required heat load for your home or business is to have an HVAC professional do an assessment. They’ll run the numbers using various measurements including gas meter readings in cold months, R-values (wall, ceiling & floor insulation ratings), U-values (window heat loss), radiator output specs and more to calculate the maximum heat load, which will determine what size combi-boiler you need.

Hot Water Considerations

Consider the following factors that affect the required DHW (domestic hot water) output of a combi-boiler:

• Incoming groundwater temperature
• Number & flow rate of hot water fixtures (taps, showers, dishwashers, etc)
• Number of people in household

The groundwater temperature in your region determines the temperature rise required: how much energy it will take to raise the inlet temperature to the desired temperature. The flow rate of fixtures is measured in GPM (gallons per minute) and should be calculated based on how many fixtures will be used at the same time.

A gas plumber or HVAC company that works with combi-boilers can evaluate your DWH needs, factor in heating demand, and recommend the right size combi-boiler.

Pipe Sizing

Pipe sizing can also affect what size combi-boiler you need, since your home heating and hot water system will need to be compatible with the gauge of hydronic heating pipes and hot water pipes you have in your home.

Small-gauge piping carries less water at a higher pressure, while larger-gauge piping carries more water at lower pressure. If there’s too much pressure in the system it can cause unnecessary wear and tear on radiator valves as well as the boiler part of your combi-boiler.

When a professional evaluates your home’s required heat load they’ll factor in pipe sizing to make sure it meets the pressure requirements of a high-performance heating and DWH system like a Navien combi-boiler.

Should I oversize my combi-boiler?

No, for two reasons: first, because it’s a waste of fuel and energy. Second, because if you oversize your combi-boiler it will cycle (keep running and shutting off repeatedly) instead of running continuously. Navien combi-boilers are designed to modulate, meaning they adjust the amount of heat produced to match the amount of heat needed. Running consistently and continuously provides the highest level of efficiency and energy savings. The exception to this is if you plan to add additional heating zones in the future.  Always let your contractor know your immediate and long-term plans for your heating system so they can determine which Navien combi-boiler best fits your needs.

Explore Navien’s NCB-H Series combi-boilers (residential only) and NFC-H Series combi-boilers (commercial or residential) to learn more about your options, or find a combi-boiler installer to get a quote for your project.

This size guide is for combination boilers that provide home heating and endless hot water. If you're interested in a boiler just for home heating, see our condensing boiler size guide.