An efficient home means a smaller, less expensive conditioning system.
Synopsis: Although manufacturers have designed complex HVAC equipment to increase energy efficiency and give homeowners more control, it’s much easier to combine a simple HVAC system with a tight building envelope. A properly air-sealed and insulated house can itself be the primary heating and cooling system, requiring only modest-size mechanicals. In addition to explaining the benefits of this approach, engineer Jordan Goldman profiles four mechanical options for a tight house: ductless air-source heat pump, ducted air-source heat pump, combination heat-and-hot-water tank, and condensing boiler with indirect hot water.
Today’s HVAC systems are increasingly complex. Manufacturers have designed them this way to improve energy performance and to provide better control over the indoor temperature throughout a house. But it’s actually much easier to couple a simple HVAC system with a tight building envelope. Improving the envelope first and then designing an HVAC system to match leads to substantially smaller and simpler heating and cooling systems, while providing improved comfort and energy efficiency.
Clients are increasingly demanding energy-efficient homes and buildings. The threat of climate change has led to stricter energy codes as well as a desire from homeowners to reduce their impact on the environment, and for most of the country, heating represents the largest end use of home energy.
Energy efficiency is a relatively new concept
Insulation has been used for a couple of thousand years in an effort to make homes more comfortable. Insulation materials have evolved from wattle and daub to fibers and foam, but comfort always was and still is a primary objective. To satisfy that objective, we also have designed central heating systems that directly combat heat loss through the walls, floors, and roof by incorporating a tight building envelope in their design.
In the late 19th century, it became standard practice to install radiators in front of windows, and a large radiator in entry vestibules, to combat discomfort in these cold and drafty spots. Forced-air systems then came along in the mid-1930s. But the idea of decreasing the operating-energy costs and carbon footprints of our homes and buildings is relatively new. It’s now common practice to insulate homes beyond the levels that are required to maintain a minimum acceptable level of comfort. That has created new opportunities for HVAC systems.
Treat the envelope as the primary heating and cooling system
While many in the home-building industry view efficient, high-performance mechanical systems as peripheral components to a house, they’re actually integral to the building envelope. In a superinsulated home, the envelope does most of the heavy lifting, keeping the house warm in the winter and cool in the summer. As a result, much less is required of the mechanical systems.
To start with, mechanical systems in a superinsulated house can be smaller and simpler than those for conventional houses. There is no longer a need to provide heat at the windows and exterior walls. Ducted distribution systems can be scaled back and confined to the interior core of the house, or eliminated altogether in favor of ductless options. These changes not only mean lower up-front investments, fewer components to maintain, and lower ongoing maintenance costs, but they also require less square footage, leaving more area for living space.
In the event of a power outage or heating-system failure, a superinsulated house has passive survivability — that is, the ability to maintain an acceptable indoor temperature. A recent house of ours lost heat during the polar vortex of 2014, and it took three days for the homeowner to notice. In that time, the indoor temperature dropped only 8°F.
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