(Originally published in the April 2009 Housing Journal)

Programmable Thermostats Requirement Results in Unintended Consequences

NM Builders have reported a conflict between the 2008 NM Mechanical Code and the performance-based concepts of LEED-H and Build Green New Mexico (BGNM) regarding highly efficient home heating systems. NMHBA believes this is a result of CID failing to effectively coordinate “green” code changes which were rushed into publication in 2007.


Programmable Thermostats and Modulating Furnaces

The latest highly efficient climate control systems work on providing low airflow volume of moderate heat on a frequent basis instead of high-volume hot air whenever the house cools below the thermostat setting. These “modulating” furnaces save electricity and natural gas by not requiring large blower motors or blast-furnace type gas supplies. They also provide more comfort for homeowners because they make less noise and keep the house at a more constant temperature, assuming the systems can run more or less continuously. Some of the modulating furnaces have “matched” thermostats that allow the homeowner to set the units to a slightly lower temperature during the day while the inhabitants are at work. Others have a sensor on the heating unit itself. Ultimately, modulating furnace systems are simply not intended to let a home cool by the eight or 10 degrees a setback thermostat mandates. These systems may take three or four hours to accomplish the re-heating demand and, if the homeowner doesn’t know what is going on, the builder may get an angry phone call about an inadequate heat system.

The unintended consequences of the new mechanical code are two-fold:

    1. Those homes with underfloor radiant heat (which take about 8 hours to respond to request from thermostatic changes in temperature) will have to install programmable thermostats. Anyone who has ever owned or installed underfloor heating knows setback thermostats have never worked nor are they ever expected to work on these systems. Yet the Mechanical Code now requires their installation in every remodel, room addition or new home.

    2. Certain highly-engineered green homes with modulating furnaces work best when left to operate on their internal computer instead of adding a programmable thermostat in front of the furnace computer. The difference in cost for these thermostats is not the main issue, however wasted money is always an issue for those builders trying to produce affordable housing. The performance of the home’s green features as engineered is the issue.


Unintended Consequences

NMHBA leadership believes the programmable thermostat is just the first of many “unintended consequences” of rushing to compile “green” codes under political pressure without adequate research and coordination. The particular problem here is that the new idea of whole building integration performance-based building codes has by accident been sabotaged by prescriptively-based green programs. We predict it won’t be the last time this happens, and this mismatch problem is a serious bump in the road to the brave new world of “green” building codes.

NMHBA is waiting for a decision from the Mechanical TAC on whether they will schedule this issue on their Mechanical TAC agenda for additional review. Interestingly there has been reluctance on the part of some Mechanical TAC members because of fear of political retribution for seeming to be “ungreen”. Our codes should be products of thorough research and careful coordination and not the result of bumper sticker campaigns and political pressure.