Updated Energy Standard Requires Condensing Boilers

The 2019 edition of ASHRAE Standard 90.1 incorporates a change that significantly impacts the design of high-capacity space heating gas boilers. The standard mandates that gas boilers, ranging from 1 to 10 million Btu/h, must be at least 90% efficient. This effectively makes condensing boilers a requirement on all new construction projects requiring this boiler capacity level.

Why Only Condensing Gas Boilers?

Condensing boilers are the only natural gas-fired boilers designed to sustain 90% or greater efficiency safely. Their condensing design enables energy extraction from the water vapor produced during combustion. Often referred to as the “latent heat of combustion,” this energy comprises up to 10% of a condensing boiler’s heat production, while the remaining 90% comes from sensible heat transfer.

Return Water Temperature Requirements

Most engineers, but not necessarily owners, understand that condensation on a boiler heat exchanger is a function of return water temperature. Return water temperature must be 130°F or lower for condensing to occur. The lower the temperature, the more energy the boiler can extract from the condensed flue gases. The new standard accounts for this operational requirement, stating that:

And,

The latter addresses the fact that bypassing the hot supply water into the return water increases the return water temperature. This increase in temperature reduces—and may even prevent—condensing, thus reducing the boiler efficiency.

Are There Exceptions to Compliance?

There are a few exceptions to these new boiler efficiency standards, which are contained within “Section 6.5.4.8 Buildings with High-Capacity Space-Heating Gas Boiler Systems” of the ANSI/ASHRAE/IES Standard 90.1-2019. These include systems where:

- On-site energy, site-recovered energy, or heat recovery chillers provide 25% of the annual space heating.

- Space heating boilers are installed in individual dwelling units.

- Perimeter convective heating and/or radiant ceiling panels serve 50% or more of the design heating load.

- Individual gas boilers have less than 300,000 Btu/h input capacity. These boilers do not need to be included in the total system input or efficiency calculations.

Prepare Now

Many states, including Florida, Louisiana, Maryland, and Virginia, have incorporated these standards into their current commercial energy code. It is a matter of time before other states follow suit. Click here to check the status of your state's energy code adoption. Be sure to click on the commercial portal tab.

If you still need to incorporate these new standards into your boiler design practice, we recommend familiarizing yourself with them soon. Click here to download a copy of The ASHRAE 90.2 - 2019 Energy Standard for Buildings Except for Low-Rise Residential Buildings.