If not installed or maintained properly, forced-air heating and cooling systems can perform poorly, creating air quality problems and unnecessary expense. To optimize a forced-air system, you need to design or refine it to prevent leaks and pressure imbalances, protect it properly during construction (or make sure the ductwork is clean and sound), and use the right filter.
The performance spec of the heat (or cooling) source is also important, but it is critical to address the distribution system first before investing in better furnace and air handler performance. This article is the THIRD of four focused on optimizing forced-air systems on the topics of Distribution, Filtration, Heat Source (and Controls), and Ventilation.
The most effective upgrades to the heat source in a forced air system are typically: 1) Improve the thermal envelope of the home; and 2) Seal and insulate the ductwork. Once those things are done, investments in improving the furnace or heat pump are likely to be returned to the homeowner rather than being lost. If you’ve dealt with those, then consider the following.
The simplest way to improve performance of an existing system is to add a programmable thermostat, if it doesn’t already have one. EPA claims you can reduce energy consumption by 5% to 15% with a properly programmed thermostat. But there’s the rub. Just installing it doesn’t help; it has to be properly programmed. Make sure it’s set up and the homeowner knows how to use and reprogram it. Heat pumps need a heat pump thermostat. In cold climates, many say it doesn’t pay to set back the night time temperature with a heat pump system, because the system will switch to electric strip heat during the early morning warm up cycle. However, the better heat pump thermostats include algorithms in their programming to lock out or minimize the use of strip heat at such times, which can significantly reduce overall energy consumption.
Zoning can be an energy-saving upgrade. This can be achieved in a forced air system by including motorized dampers to control air flow to each zone, controlled by multiple thermostats connected to a system controller that operates the heat source and air-handler. Once again, it is important to have a reasonably air-tight, thermally efficient building envelope before you think about this upgrade, or it may lead to unanticipated comfort and control issues. Zoning can be difficult to retrofit depending on the duct design and accessibility. Since turning off a zone reduces the effective size of the system it can result in too much back pressure which may damage the air handler. For this reason, zoned systems generally use oversized ductwork, though there are other ways to address this.
What if it is in the budget to think about upgrading the heat source? I’m only going to address a few big picture issues here, since this is a huge topic with a lot of great resources available on the web and elsewhere.
First - Make sure the person who designs the HVAC system does a proper room-by-room load calculation, using a reputable tool (ask for ACCA Manual J or equivalent) the actual design specs for the home (floor, wall and ceiling insulation values, glazing area and u-factors, envelope infiltration rates). This way, the heat source will be properly sized for the house and sufficient heat will be delivered to each room. If the loads are significantly different from the original system design, the ductwork may also be oversized and may require some redesign, too.
Second – If you have the option, locate the heat source and all the distribution ductwork inside the conditioned space, eliminating almost all the heat loss to the outside. This can be challenging in smaller homes or retrofit situations, but it can be done. If you are retaining the existing ductwork, this likely means relocating the thermal and pressure envelope to enclose the mechanical space and should only be done if you are using a closed combustion appliance.
Third - Consider a multi-stage heat source (gas furnace or high-efficiency electric heat pump). Since a heating system is designed to provide enough heat to keep the home comfortable on the coldest day of the year, it spends most of the heating season running at less than full capacity, which is generally less efficient. In a combustion system (furnace), this means the burner/gas valve has more than one heating level, so it can modulate how much gas is burned according to the amount of heat that is needed. A dual stage furnace might have a peak capacity of 60 kBTUh and a first-stage capacity of 30kBTUh. In a house with a peak heating load (the heat required to keep the inside of the house comfortable on the theoretical coldest day of the year) of 24 kBTUh (not uncommon for a typical, quality-built 3- or 4 bedroom house), such a furnace might NEVER use the full 60 kBTUh capacity.
In a heat-pump system, performance depends on the outdoor compressor unit’s ability to extract heat from the outside air – the colder the outside air, the harder it is to get heat from it. So on the cold days when more heat is needed inside, there is less heat available outside. In older heat pump systems, this situation triggers the “Emergency Heat” source, electric resistance heat strips in the heat exchanger. This is expensive and inefficient. There are a few cold climate forced air heat pumps that use multiple, staged compressors to address this. They seem to perform well but availability and technical support is currently limited and these systems do need specialist attention. To address this, some HVAC installers recommend hybrid systems with a heat pump AND a combustion furnace – the furnace replaces the electric emergency heat with a more efficient gas combustion supply. This is more efficient, but it adds a lot of cost and equipment up front.
The latest in heat pump technology features multi-stage or variable speed compressors. With this technology, heat pumps can deliver satisfactory heating performance even at outdoor temperatures well below freezing. The heat-extracting capacity of the outdoor units is matched to the outdoor temperature and the need for heat indoors, so that the system runs at its most efficient whatever the conditions.
While this technology is commonly available in Ductless Mini-split heat pumps, which are a great solution for smaller homes with small heating and cooling loads (typically less than 1-ton per unit), it is not available in ducted forced air systems – at least not yet. In a smaller home with a thermally efficient envelope, however, abandoning the forced air system altogether and installing one or two ductless mini-split heat pumps may yield great, energy efficient and comfortable results. But that’s for another column.(Just to whet your appetite, we understand that several mini-split heat pumps, most notably the Fujitsu Halcyon system pictured in the photo, perform quite well in our Northwest climate). We are waiting with baited breath to get the results from a two-winter study of heat pump technology being conducted by the Northwest Energy Efficiency Alliance. For a great overview of heat pumps, check out Green Building Advisor.
Fourth – Use a variable speed fan. The air handler – the fan that moves air around the building – provides another opportunity to improve performance. Most air handler fans are single-speed units – they blow at the same speed whenever they are switched on, using the same amount of electricity. Variable speed fans (which typically use Variable Frequency Drives or VFDs, or Electrically Commutated Motors or ECMs) are an optional upgrade, often standard in higher-efficiency furnace packages. When a house is at a steady temperature, it takes relatively little heated air to keep it comfortable. A variable speed fan will run at a much lower speed in these conditions – reducing energy consumption and noise. (See my fourth and last installment of this series on Ventilation, next week).
Climate – In moderate heating climates, where it rarely stays much below freezing, and in cooling dominated climates, electric air-to-air heat pumps are a good choice. In cold climates, a combustion-based heat source is generally preferred - natural gas if it is available, propane or fuel oil if it is not. But be aware that oil and propane prices fluctuate significantly with supply and demand, sometimes making them more expensive than electricity in the heating season. If gas is not available, and the budget allows, it might be worth considering a ground-source heat pump. Very high efficiencies are achievable but unless you are dealing with large heating and/or cooling loads, the simple payback on such a system will be very long. Other benefits, such as quietness and reduced carbon footprint may be important to the homeowner (see fuel source).
Fuel Source - If your homeowner is concerned about carbon emissions and your electricity comes from gas, oil or coal, an electric-based heating system is probably not the greenest choice. Because of distribution inefficiencies in the electricity grid, you get less than half the energy value of the source fuel delivered to your house as electricity. Better to burn that fuel in a high efficiency appliance in the house and get 90%+ of the fuel value. A heat pump is much more efficient than electric resistance heating, but only the most efficient air-to-air heat pumps for forced air systems offer enough efficiency to offset the grid energy losses (look for HSPF of 9.5 and above, or a COP of 2.5 or better). In the Pacific Northwest, we are less carbon-dependent because of the large portion of hydroelectric power in our grid energy. Also, in the area west of the Cascades and Coast Ranges, we have mild climates, making heat pumps a great choice. In all cases, if you choose an electric system, consider a green power purchase to effectively neutralize those carbon emissions
Alistair Jackson, LEED AP, CSBA is Principal in Charge of O’Brien & Company’s Residential Technical Services, and is a LEED for Homes Rater, LEED for Homes Faculty, Energy Star Verifier and Performance Tester, ARSCA Accredited Professional, and Built Green Verifier. He is a regular contributor to the Building Capacity Blog, and was a major contributor to the Northwest Green Home Primer on the subject of home operating systems and the energy picture. He is a regular
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