Category Archives: Ask A Builder

The “Ask a Builder” series is dedicated to answering some of the many questions Alaska residents have about building, energy and the many other aspects of home life. Here you’ll find responses from CCHRC experts to commonly asked questions in Fairbanks, Alaska and beyond.

Should I consider replacing my heating system?

If you’re thinking about replacing your heating system, here are some questions to ask yourself. A “yes” to any of them may warrant a call to an energy rater or heating contractor.

Have you recently upgraded the thermal envelope of your house?

The thermal envelope of your house is everything that separates the living space from the outside, including walls, doors, windows, insulation and the roof. If you’ve been sealing leaks, eliminating drafts, replacing old windows with double-pane or triple-pane models, or adding insulation, you’ve been making your home more energy efficient.

With thermal envelope upgrades, the home will lose less heat in the winter, and therefore it’s likely the heating appliance won’t need to provide as much energy. Depending on the reduction in energy use, it is entirely possible that your heating appliance could be come oversized, and a smaller system may operate more efficiently

Is your current heating appliance more than 20 years old?

Technology marches on. Appliances made today are far more efficient than older models. Not only do they use less fuel, they are also safer and come with more advanced controls to improve efficiency. Also, the methods to size a heating system are better and can be tailored to individual homes.

Is your house uncomfortable?

Do you have rooms that are always too hot or too cold? This can be the result of air leaks, inadequate insulation, an improperly sized heating appliance, or lack of zoning in your heating system. Start with a call to an energy rater to find out which improvements you can make to solve this problem. If you need to add insulation or seal leaks, take care of that before upgrading your heating system so that the heating system will be sized properly for your home.

Who to call?

Energy raters will examine your entire house, measuring doors and windows, checking insulation levels, assessing your heating system and testing air leakage rates. The rater will input the data collected from the home inspection into AKWarm — a software program maintained by the Alaska Housing Finance Corporation. The AKWarm program is then able to calculate a home’s theoretical energy requirements, which it prints out as part of an energy rating certificate. This rating describes how efficient different components of your home are on a point scale. Included in the rating are suggestions to improve performance, which may or may not include the heating system. The rating will help prioritize upgrades, show the energy benefits of each one and may qualify you for the Alaska Home Energy Rebate Program.

Heating contractors will focus specifically on your heating system, evaluating its current efficiency and whether it is sized properly. A contractor can sometimes test for the efficiency of the distribution system (depending on the type). The contractor will provide you with information on improving your current system and purchasing new appliances.

How can I keep moisture and ice from forming on my windows in the winter?

Windows can be a barometer for how much humidity is inside the home.

On really cold days, you might notice condensation forming on the inside of your windows. This can be caused by one or a combination of factors: excess humidity, inadequate ventilation, or poor windows. To understand and correct a particular issue in your home, you need to know some basic properties of moisture.

Condensation occurs when water vapor (a gas) turns into water droplets as it comes into contact with a cold surface. The point at which this happens depends on the temperature and humidity of the inside air. The warmer the indoor air, the more water vapor it can “hold,” and moisture can better remain in the vapor state. When air moves next to a cold window, the temperature drops and it can’t “hold” as much vapor. That’s when you start to see condensation forming.

For example, if the indoor temperature is 70 degrees and the outdoor temperature is 0, then moisture will begin to condense on a single-pane window when there is roughly 15 percent relative humidity in the house. A double-pane window will cause condensation at around 25-40 percent relative humidity, and a triple-pane window at between 30-50 percent. These are rough numbers are based on average window insulation values.

The recommended indoor humidity levels for occupant health and comfort range from 30-50 percent. The general rule in a cold climate, however, is to target the lower end of this spectrum due to the risk of condensation within walls and ceilings. If your house has adequate mechanical ventilation, humidity is less of a concern. In Fairbanks, it’s tough to maintain anything close to 50 percent humidity in a properly ventilated house, because the winter air is so cold and dry. Because of its low moisture content, the inherent dryness of Fairbanks winter air is good for homes but not always the occupants, since discomfort related to the dryness can be problematic.

What can I do about it?

Three things: make sure your home is properly ventilated, aim for less than 40 percent relative humidity to keep both you and your home healthy, and consider replacing your windows or adding moveable window insulation during cold months.

If you already use mechanical ventilation and have low interior humidity, but are still having problems, you may need to examine your ventilation setting. If you have a heat recovery ventilator (HRV), it may be recirculating too often, which can contribute to increased moisture build up in the air. Recirculation mode closes the connection to the outside and brings exhaust air back into the rooms. Recirculation mode keeps the HRV core defrosted and saves energy, but it is also possible for it to run for too long. Some experimentation with the HRV settings may be necessary. For example, in 20/40 mode the HRV brings in fresh air for 20 minutes and then recirculates for 40 minutes, and likewise for 30/30. If you’re getting condensation in your current mode, try decreasing the amount of time the unit recirculates.

Also make sure air is allowed to circulate—either passively or mechanically—throughout the entire house. If you close the door to the bedroom, the air can become cold and moist enough to condense on windows.

Older, poorer performing windows can create problems no matter what you do to your interior air. Bad seals around operable windows, metal spacers between the panes, and inadequate insulating value can cause the window surface to get cold enough for condensation to occur. If you’re not ready to invest in new windows, consider some type of moveable window insulation like foam board (on the outside) or well-sealed plastic film (on the inside). A CCHRC guide to different types of window insulation can be found at http://www.cchrc.org/evaluating-window-insulation.

How does the recirculation mode on an HRV work, and is it safe in a cold climate?

We often stress proper ventilation as the key to maintaining a healthy indoor environment in a home, and promote heat recovery ventilators (or HRVs) as the best option for energy efficient ventilation in a cold climate.

HRVs exchange stale indoor air with fresh outdoor air, capturing heat from the outgoing air to pre-heat incoming air. They exhaust excess humidity, carbon dioxide, and indoor pollutants from pet dander, cleaning supplies, offgassing furniture, and other sources. The role of the HRV becomes increasingly important as homes are built tighter to save energy, which cuts down on passive air exchange.

To maximize the benefits of having an HRV, it helps to understand the different operation modes. One of the often-debated modes included in most HRVs in the United States is the recirculation mode. This mode is not often used in Europe because it is believed that the health risks outweigh the energy benefits. This article provides a description of the recirculation mode and gives pros and cons for the house and its occupants.

Under normal operation, the HRV replaces moist indoor air with fresh outdoor air. While HRVs recover much of the energy from the heated air during winter months, a considerable amount of heat is still lost due to the frigid temperatures in the Interior Alaska. In addition, extremely cold outdoor air contains virtually no moisture, which can result in very low humidity levels indoors—a negative for some homeowners.

In recirculation mode, the unit closes the connection to the outside and brings the exhaust air back into the rooms. This saves a lot of energy, since there is no cold air coming in from outside. On the other hand, moisture and indoor pollutants are no longer being flushed out of the home, and their concentration will continue to rise and can eventually reach harmful levels. Recirculation can also spread unwanted smells from more to less polluted areas, such as from the bathroom to the living room.

In order to maintain sufficient air exchange, HRVs offer modes where these two strategies can be combined. For example, 20/40, 30/30, or Smart Mode. In 20/40, the HRV will bring in fresh air for 20 minutes and then recirculate for 40 minutes (likewise for 30/30). Smart modes usually require some kind of sensor (humidity or carbon dioxide) to dictate when to ventilate and when to recirculate, based on which measurements the HRV controller decides is more relevant at any given time.

The major advantage of recirculation mode is that it saves energy and redistributes heat throughout the house, particularly helpful if you have a localized heat source like a woodstove. On the flip side, it can potentially transfer pollution from one room to another rather than expelling it altogether. While Smart Mode seeks a happy medium between the two, there are still times when recirculation mode should not be used at all—if someone is cooking, smoking, or during times of high occupancy. One way to override the Smart Mode during these situations is with a push-button timer, a common feature of HRV installations that temporarily ventilates the HRV during such events.

If you do use recirculation mode, here are some best practices to maintain good air quality:

–High quality filters (High Efficiency Particulate Filters, HEPA, in combination with activated carbon filters) should be added to supply duct to mitigates odor or pollution from spreading

— Constant recirculation should only be used when the building is unoccupied

–If recirculation is used during occupied periods, make sure the HRV is exchanging indoor and outdoor air for at least part of every hour

While recirculation offers the perk of saving energy, if you rely on it too much, you can undermine the benefit of having an HRV—to maintain indoor air quality that is healthy for both humans and structures.

What are HRVs and how do they work?

Heat recovery ventilation (HRV) systems are becoming increasingly common in cold climate construction and are almost indispensible in today’s super-insulated, airtight homes. As older homes are receiving energy retrofits and becoming tighter and more insulated, they are facing the same indoor air quality issues you find in new construction. HRVs improve the indoor air quality of your home and save more energy than other types of ventilation. This article provides an overview of the basic purpose and advantages of HRVs.

The main job of the HRV is to supply fresh outdoor air to the house while expelling stale indoor air — which can contain things like moisture, animal dander and gases from combustion appliances and carpets. This is especially important in a home that is too tight to rely on passive air exchange.

At the heart of the HRV is a heat exchanger (often called a “core”) where exhaust air flows next to, but separate from, supply air. Here the cold incoming air is warmed by the heated outgoing air, recovering heat that would otherwise be lost. Most HRVs recover 70 to 90 percent of the heat, depending on the unit and controls, making it much more efficient in a cold climate than a simple exhaust fan that blows warm air directly outside. One of the newest and most advanced models is capable of recovering more than 90 percent of the heat from exhaust air.

The ducting of an HRV system typically supplies fresh air to bedrooms and living areas while exhausting humid air from bathrooms, kitchens, laundry rooms and crawlspaces. The HRV does not eliminate the need for a cooking fan, so a range hood still should be the main outlet for grease and smoke above the cook stove.

The HRV is designed to be balanced, meaning it takes in as much air as it exhausts, maintaining close to neutral pressure inside the home. It should not create a negative pressure in the home, like an unregulated exhaust fan might, which can cause appliances to back draft (suck in air from an exhaust flue and expose you to dangerous gases). It’s also important to remember that HRVs are not meant to supply air to combustion appliances.

Efficient residential units use about as much power as a 60-watt light bulb when running, and are getting more and more efficient. As with any appliance, an HRV requires some maintenance, such as checking the built-in filters every fall to see if they need to be cleaned or replaced.

In addition to providing reliable ventilation in a home, HRV systems can serve several other roles. You can install an in-line filter system on the warm-side supply air port that will filter particles and odors from the incoming air. For example, in the winter this can help keep particulate pollution (from wood-burning and other sources) out of your home.

With the right controls, an HRV can also work in recirculation mode, which distributes heat to hard-to-reach areas in the house (a big help for occupants using a woodstove). Although be aware that recirculation reduces overall fresh air exchange and can redistribute odors from unwanted areas in the house. If you own an older unit, a control upgrade may be a beneficial and cost-effective option.

While it may seem expensive up front, you should look at an HRV system as an investment in a healthy home and peace of mind. In this climate, indoor moisture can cause problems not just for the structure, potentially condensing in the walls and leading to mold and rot, but also for occupant health. An HRV will protect the occupants as well as the structure by removing excess moisture before it has a harmful effect.

If you’re thinking about purchasing a system, make sure you learn about the specifics and find an installer who is willing to educate you and stand behind their work.

What is reflective insulation and does it work in a cold climate?

Reflective insulation is typically made of aluminum foil on a backing like rigid foam (pictured here), plastic film, polyethylene bubbles or cardboard.

Reflective insulation is a type of thermal insulation with at least one reflective surface that is installed so that the surface faces an air gap. It is usually made of an aluminum foil installed on a variety of backings, such as rigid foam, plastic film, polyethylene bubbles or cardboard.

CCHRC recently researched the use of reflective insulation in cold climate construction, reviewing other studies and testing two foam insulations with reflective facers. Researchers found that the use of reflective insulation has very little to offer cold climate construction.

To understand how it works, you need to understand the three types of heat transfer: convection is heat transfer through air movement; conduction is heat transfer through solid materials that are touching; and thermal radiation is when heat travels in electromagnetic waves, like energy from the sun.

Reflective insulations are designed to reduce heat transfer through radiation by placing a surface that reflects thermal radiation in combination with an air gap. The reflective surface reflects most of the thermal radiation toward the air space, preventing it from being absorbed by the material. If you don’t have an air space, then the heat is lost by conduction through the reflective surface. In real life, all these forms of heat transfer occur simultaneously. (Unless you travel to space to remove the atmosphere (air) from the equation. This partially explains why NASA took an interest in reflective insulations, as they faced very different conditions than we do in Alaska.)

In warmer climates, it is common to add reflective insulation in the attic to reduce heat transfer from the roof decking to the underlying insulation, reducing overall solar heat gain within a building. But in cold climates, we have different concerns. For example, homes lose heat primarily from air leaking through the attic and walls and conduction through all components of the house. Because most heat loss occurs this way, reflective insulation would not make much difference in reducing the overall heat loss of your home.

To illustrate this point, let’s examine part of a house where a reflective insulation system is added. If you created a 1-inch air gap into the wall or ceiling with a reflective surface on one side, you could expect to gain around R-2. But walls and ceilings are typically insulated in the range of R-20 to R-60, and reflective insulation faces sharp diminishing returns if multiple layers are installed. Also remember that the air gap needs to prevent airflow and the reflective surface needs to stay clean from dust and moisture.

In addition, many reflective insulations can increase the potential for moisture problems in your home if not placed properly, as they often act as strong vapor retarders. So if you’re using these products, you need to consider not just how they affect heat loss, but also moisture flow.

Watch out for claims about reflective insulations providing benefits that go beyond R-value. All of the product’s insulation value is captured by the R-value, just like fiberglass batts, foam board, and other insulation products. If there are additional benefits, such as reducing air leakage, then those benefits can be measured and compared to other air barrier systems.

In essence, reflective insulation may help in warmer climates but is not a great fit for a cold place like Alaska.

What to look for in an energy efficient house

Shopping for a home in Fairbanks can be difficult, especially if energy efficiency is a priority. With heating oil prices volatile and resale value at stake, finding the most fuel-efficient home makes sense.

Following are just a few of the things to look for in an efficient home.

Site Location

South-facing slopes that are exposed to sunlight will be warmer in the winter and require less heating than comparable homes on north-facing slopes or obscured by dense tree canopies. Deciduous trees, such as Alaska birch, are desirable because they lose their leaves in winter and allow sunlight to shine through.

Ideally, homes should be situated lengthwise east to west in order to take advantage of the sun.

Protection from wind, provided by trees or hills, can help to conserve heat in winter. Low-lying evergreens or shrubs placed on the sides of a house that are exposed to wind will also help conserve heat.

Design

Houses that share common walls with other structures, such as townhomes, lose less heat than standalone homes.
The overall shape of the house will affect heat loss due to the amount of wall space exposed to the elements. L-shaped, H-shaped, or U-shaped homes, for example, will tend to lose more heat than rectangular homes.

Arctic entryways that are sealed from the outside and the inside living areas by separate doors can help retain heat.

South-facing windows are preferable to windows on any other axis because they can collect sunlight and minimize heat loss.

Plumbing should be run inside heated or indirectly heated areas and consolidated as much as is practical. Sinks, baths, and laundry should be close to the water heater to minimize standby heat loss or, alternatively, on-demand water heaters can be used.

Insulation

There’s a saying among energy raters in Alaska – “You can’t over-insulate, you can only under-ventilate.” When inspecting a house, ask how much and what type of insulation is in the floor, walls, and attic. Other than airtight construction, no other single factor will affect a home’s energy use more than insulation. But insulation without adequate ventilation will invite moisture problems.

All gaps and cracks in the house should be well sealed or caulked.

Doors and windows need effective weather-stripping.

Mechanical Systems

The performance of heating appliances such as boilers can vary widely and replacing an aging existing system can be expensive. It’s not uncommon for heating systems to be oversized in relation to a homes energy needs, which can also contribute to efficiency losses. Consider having the heating system professionally inspected to assess reliability and performance.
Doors and windows need effective weather-stripping.

Previous years’ fuel bills can help gauge heating costs, but be aware that the presence of a woodstove, pellet stove, or other heating appliance other than the boiler can make heating oil usage numbers misleading.

Home Inspections

Check to see if the home has already had an energy audit done. An energy audit will provide a detailed assessment of the home’s energy performance and will help identify problem areas. If energy efficiency is a priority, an audit/home inspection by a state certified energy rater can provide valuable insight into a home’s real world performance.

How long does it take to cure firewood in the Interior?

Firewood can dry in a single summer if split and stored properly.

While we won’t mention the dreaded “W” word, it’s never too early to start thinking about the heating season, when many Interior Alaska residents burn wood for heat.

While wood burning is a cheaper and more renewable alternative to heating oil, it also contributes to the air quality problem in the Fairbanks North Star Borough. Burning wet wood produces excess smoke and PM 2.5-sized particles, which disperse into the air and can be harmful to health. These emissions can be lessened by burning dry firewood. Fully cured wood — moisture content of 20 percent or less — is not only cleaner but also produces more heat.

How long does that take in this climate? It depends on the species of wood, when you harvest it, how you cut it and how you store it. A study at the Cold Climate Housing Research Center shows that wood can dry rapidly during a single summer — no matter when it’s harvested — but takes quite a bit longer over the shoulder seasons or winter. No matter what wood or method you use, firewood harvested in the fall won’t be fully cured by winter.

In our study, split wood harvested in the spring took anywhere from six weeks to three months to dry during the summer, depending on the storage method. Split birch and split spruce, for example, dried in one and a half months when stored in a simulated wood shed or left uncovered. In general, the fastest way to dry split wood was by storing it in a wood shed or leaving it uncovered, although uncovered wood is at the mercy of the weather and could be wet again by fall. When stored under a tarp, the wood took three months to cure.

Unsplit wood, on the other hand, didn’t cure during the summer in any storage scenario. Though it neared 20 percent moisture content by the end of the summer, it required another summer to reach a full cure.

Firewood harvested in the fall didn’t cure by springtime no matter how it was cut or stored. While it dried out somewhat in a wood shed (to between 30 and 40 percent moisture content), some samples got wetter under a tarp during the winter.

Several other factors should be considered when seasoning your wood. Spruce and birch tend to dry more quickly than aspen. Your drying times also will vary based on exposure to sun and air circulation (the more, the better).

The good news is that it’s possible to harvest firewood in the spring and cure it during a single summer — so you can stay cozy and burn cleanly during the winter. Just make sure to split it early and store it so it can dry.

The “Ask a Builder” series is dedicated to answering some of the many questions Fairbanks residents have about building, energy and the many other parts of home life.

Read more: Fairbanks Daily News-Miner – Ask a Builder How long does it take for wood to season

I’ve seen more solar panels around Fairbanks lately. How do they work and what are the different types?

CCHRC has a 12 kW photovoltaic array that is tied to the utility grid.

Solar is a growing resource in Fairbanks, and there are two different types of panels you may be seeing around town: solar photovoltaic, which generate electricity, and solar thermal, which generate heat for space heating or domestic hot water. CCHRC uses both types of panels at the research center. While they both turn sunlight into energy for your home, they have very different applications. Considering your site conditions and heating, plumbing, and electric systems will help determine if one (or both) technologies would work for you.

What’s the difference?
Solar thermal, or solar hot water, collectors absorb heat from the sun and transfer it to water or glycol to provide space heating or domestic hot water.

The two most common types are flat-plate collectors and evacuated tubes. Flat-plate collectors are the oldest and most dominant type of solar thermal. They generally consist of a 4×8-foot glass-encased panel that contains a thin metal sheet, with a dark coating to absorb energy. Beneath the sheet are coils filled with the heat-transfer fluid. Insulation lines the back of the panel to maximize heat transfer to the fluid. Fluid circulates through the tubing, absorbing heat and then transferring it to a storage tank. A typical residential system used to supplement domestic water heating includes two panels.

An evacuated tube collector contains several rows of glass tubes connected to a header pipe. Each tube is a vacuum, which acts like a sealed thermos and eliminates heat loss through convection (due to wind). Because of this, evacuated tube collectors lose less heat to the environment than flat-plate collectors.

A small copper pipe filled with fluid (glycol, water, or some other antifreeze) runs through the center of the glass tube. The fluid heats up, vaporizes, rises into the header pipe, and transfers heat (through a heat exchanger) to another pipe filled with fluid. This fluid carries heat to the storage tank. From here, water can be used for hydronic heating and domestic hot water or converted for other uses.

Solar power
Solar photovoltaic (PV) panels convert sunlight into electricity. They have a silicon sheet that is made up of semiconductors. When light strikes the sheet, part of the energy is transferred to the semiconductors, which knocks electrons loose and allows them to flow freely through connected wires. This flow of electrons is called direct current (or DC). The current then flows into an inverter, which changes it into AC (alternating current), the power used by your appliances. This current can either be used to power appliances (if there is demand), stored in a battery, or returned to the electric grid.

Cold Climate Specifics
Fairbanks is a unique place for solar energy because of the excessive summer sun and the virtual darkness in winter months, which means a few months a year where solar doesn’t contribute much. For example, the 12-kilowatt photovoltaic array at CCHRC produces more than 10,000 kWh from March-September (about 30 percent of the building’s electric demand) but only 1,833 kWh during the rest of the year.

Most households with solar thermal systems use them to offset their primary heating sources. If you want to use solar thermal as a main source, you need some type of seasonal thermal storage system to bridge winter months. PV systems simply offset electricity purchased from the grid in most cases.

With PV, you can produce more power from your panels year-round if you keep them free of snow and change the tilt angle twice a year. The most productive months for CCHRC’s panels are April and May, when they enjoy long daylight hours and also capture reflected solar gain off the snow cover.

Different types of solar thermal panels perform better at different times of the year. For instance, evacuated tube collectors produce more BTUs during the spring and fall shoulder seasons, while flat plate collectors produce more heat during the summer.

Which ones are better to install?
A 1,000 watt PV array will produce about 1,000 kWh a year in the Interior, offsetting $210 in electricity at today’s rates. A two-panel solar thermal system could produce roughly 7 million BTUs a year, offsetting either 54 gallons of oil (saving $215) or 2,050 kWh of electricity (saving $410). In other words, homeowners with electric water heaters stand to save more from solar thermal than those heating with other fuel types.

The actual cost of solar thermal in Interior Alaska (roughly $4-$5 per installed kWh) is lower than solar photovoltaic (approximately $8-$10 per installed kWh). Yet PV panels are still more common in Fairbanks largely because they are easier to install and retrofit, don’t require plumbing, don’t have to be integrated into existing mechanical systems, and have no moving parts (whereas solar thermal systems have fluid and pumps that must be replaced over time).

The actual output and cost of your system will depend on many factors, like the solar exposure of your particular site, the type of heating or hot water system, the type and number of heat exchangers required, and others.

With the cost of conventional energy on the rise, solar is becoming an increasingly attractive long-term investment. Anyone with good solar accessibility may be wise to consider these systems as an option.

Decoding Boiler Terms

It’s easy to get lost in the jargon when shopping for a boiler or other home heating appliance. This article covers some of the common terms you may encounter when shopping for a combustion boiler. If you have questions about what type of boiler is best for you, be sure to talk with a heating professional.

Most oil boilers are mechanical draft boilers, which use a fan to draw in combustion air. There are two main methods of mechanical draft that are common in residential models.
–Induced draft uses a fan to remove flue gases from the furnace and force exhaust gas up the stack (and usually operate at a slightly negative pressure).
–Forced draft uses a fan and ductwork to force air into the furnace, and usually operates at a slight positive pressure.

In mechanical draft boilers, the fan also creates turbulence in the combustion chamber, allowing for a more complete burn. These are typically more efficient than natural draft boilers.

Natural draft boilers rely on the buoyancy of hot combustion exhaust. The exhaust is hot, so it rises passively out of the flue. As the hot exhaust gases exit upwards, the draft causes fresh air to enter the combustion chamber. Because natural draft boilers consume a large amount of air in this process, they are less efficient than mechanical draft boilers. If the air pressure inside the house is less than the air pressure outside, a natural draft boiler can backdraft and poisonous gases such as carbon monoxide could potentially enter the home.

Examples of natural draft heaters are propane water heaters and drip-oil stove heaters.

Sealed combustion boilers use a duct to bring in outside air directly to the combustion unit and not from inside the house. The combustion chamber (where burning occurs) is sealed off from the inside of the home. These boilers are safest, because they are unlikely to backdraft poisonous exhaust gases such as carbon monoxide (CO) into the home.

Condensing boilers are more efficient than standard combustion boilers. A condensing boiler is able to reclaim additional heat from the exhaust gas by cooling it to a point where water vapor from combustion condenses out. The condensation releases the latent heat from the gas, and this heat is captured by a second heat exchanger. The condensate water is acidic (it has the same acidity as some vinegars), so corrosion-resistant materials like stainless steel or PVC pipe must be used for the heat exchanger and pipes. Condensing boilers must have a drain that allows the water to enter the wastewater plumbing system. In older homes with pipes that could corrode, a neutralizing filter can be added to the drain line. These boilers also have a fan to blow the cooler exhaust gas, which is not buoyant enough to exit the flue on its own, outside the building.

Non-condensing boilers are less efficient because they have to operate at higher temperatures to prevent condensation. However, they do not require a drain and can be made of materials such as iron, steel or copper that would eventually corrode in a condensing boiler.

High mass boilers are very heavy, as the name implies. The mass comes from a large heat exchanger, which contains heavy metal, often cast iron, and large diameter pipes that contain a high water volume. The high mass design helps the boilers maintain steady state efficiency. These boilers take longer to heat up when they are started, so they should not be short-cycled, or turned on and off frequently, as this will lower their efficiency.

Low mass boilers have a smaller heat exchanger that does not contain a large mass of metal or iron. While short-cycling a boiler (or turning it on and off frequently) is never ideal, a low-mass boiler will generally respond better than a higher mass design, as it takes less time to heat up. These boilers also have less standby loss when they cool down, because they do not have the mass to retain a lot of heat while firing.

The paybacks of energy efficiency investments

The state of Alaska invested an estimated $110 million from 2008 to 2011 on extra insulation, new boilers, air sealing, and other retrofits for roughly 16,500 homeowners—about 10 percent of all homeowners in Alaska.

The Home Energy Rebate Program provides funding to help homeowners make their houses more energy efficient. CCHRC recently worked with the Institute of Social and Economic Research to look at the economic impacts of the program. The study, funded by the Alaska Housing Finance Corporation, showed homeowner investment, fuel savings, payback periods, job creation and more. Here are some highlights:

· Total spending for energy efficiency improvements was about $185 million, with state rebates covering 60 percent and homeowners 40 percent. Homeowners should recoup their investment in roughly 3.5 years. State and private spending will be returned in homeowner savings in less than 9 years.

· Annual fuel use dropped an estimated 33 percent for households who participated. The average homeowner will save an estimated $1,300 a year on fuel (or 26 percent).

· Every $1 million in state spending generated 12 Alaska jobs—7 direct retrofitting jobs and 5 indirect jobs—amounting to about 1,330 jobs.

· Overall, participants are saving an estimated $22 million annually. If they spend those savings locally, every $1 million in new household spending generates 11 jobs throughout the state economy—an annual average of about 240 jobs.

· The biggest money savers were more efficient boilers or furnaces (constituting 50 percent of energy savings). Adding extra insulation to walls, doors, and ceilings made up 25 percent of savings; sealing air leaks accounted for nearly 15 percent of savings; replacing windows and water heaters comprised 10 percent of savings.

· Anchorage homes made up 49 percent of retrofits; other Southcentral communities 27 percent; Fairbanks 14 percent; and Juneau 6 percent.

The full snapshot is available here.

*Changes in fuel costs and savings are estimates from AHFC’s energy-rating software as actual household heating bills aren’t currently available.