An Energy Standard That Also Limits Carbon

ASHRAE Standard 90.2 now sets carbon emission limits for housing.

Mid-rise housing covered by ASHRAE 90.2; note air conditioners on roof.


 Copyright (c) David B. Goldstein 2023

Energy standards have been one of the most effective clean energy implementation tools over the past 50 years. These standards reduce the amount of energy that new buildings can consume when operated normally.

But today many cities and states care more about reducing carbon emissions than energy. This concern can now be turned into action with newly published revisions to ASHRAE Standard 90.2.

For decades we assumed that energy limitations result in nearly proportional carbon emission limitations. This proportionality was more or less true in the past, when electric grids were dominated almost exclusively by fossil fueled generators. 

But by 2023 the world had changed in ways that make this connection between energy and emissions weaker, as emissions-free renewables start to dominate over polluting fossil fuels. 

Now a new standard, ASHRAE 90.2, recognizes this change in newly published Addendum f , by setting a limit on the amount of greenhouse gas (usually referred to as “carbon”) a building can emit. Standard 90.2 retains its limit on building energy use as well.

The carbon limit adopted by 90.2 is based on high levels of energy efficiency including the use of heat pumps for space and water heating. If other fuels are used, the carbon emissions can be no higher than they would have been for the same house with heat pumps. Thus the standard encourages electrification in an individualized, data-based way. The encouragement is larger for grids where the next unit of electric generation is mostly clean energy and lower for grids where the next unit is a mix of fossil fuels and renewables. 

The method for calculating emissions is described here. This method is new: it incorporates innovations based on the rapid change toward renewable power sources in the United States and most everywhere else as well. The three most significant changes are:

  1. Renewable energy has grown rapidly due to state policies and improved economics. Where in the past a new kilowatt-hour (kWh) of energy consumption meant almost one new kWh worth of power from a coal or gas power plant, now most of the NEW demand for electricity comes increasingly from more solar and wind generation, and also causes utilities to construct or acquire more renewables. Thus fuel switching from gas or oil furnaces to electric heat pumps is cleaner than it has ever been, and will continue to make progress toward zero emissions in the future.
  2. New emissions calculation methods now account for the greenhouse gas emissions from both combustion and leakage of fossil gas, when it is used as a building heating fuel and as a power plant fuel. This change makes a difference: it shows that heat pumps are lower in emissions than we had thought. 
  3. The emissions of a renewables-heavy grid now depend dramatically on time of use: both hour of the day and month of the year. As a result of new models that account for this dependence, we now find lots of technologies and design choices that can cut carbon emissions but that increase energy consumption, such as heat storage and batteries. A building that heats its water when the grid is clean and turns off the heating system when the grid is dirty can supply hot water when you need it by limiting its electric use to those times when emissions are low. This raises energy consumption a little but cuts emissions by a lot. The same issue arises for batteries: they improve carbon performance but worsen energy performance due to the round trip energy losses of the battery storage system.

So designing buildings for low carbon in addition to low energy can cut emissions far more than climate models had anticipated. ASHRAE 90.2 has taken a crucial step by setting a carbon limit using a calculation that accounts for renewables properly.

ASHRAE standards on energy are intended to be used as energy codes for jurisdictions that want to take action to meet global climate goals, or as the basis for financial incentives offered by utilities or governments, or as voluntary targets for companies with climate protection goals. Cities and states should make use of this new revision to ASHRAE 90.2 and update their energy codes to limit carbon emissions.

Related Issues

Related Blogs