Features
Energy Use
This aspect of the house is all about reducing energy use and thereby reducing carbon dioxide emission, whether produced from a gas fired hot water heater or an electric company power plant. Our house has solar panels and a geo-thermal heating an cooling system, both of which reduce our use of fossil fuels and production of “green house” gases. But much of the energy savings will come from things that are less obvious: design features, choice of insulation and appliances.
Insulation
We used a combination of spray on foam and dry packed rockwool.
We are trying to accomplish two things with the insulation, reduce heat transfer and seal openings that would allow heat transfer and drafts. Foam was used in the roof, the joist that rest on the foundation and in second floor overhangs. Rockwool was used in the exterior walls.
Spray on foam has three features that we like: 1. it is less susceptable to poor installation, 2. it fills in nooks and crannies better than batts, and 3. it provides higher insulation factors per square inch. We constructed the house with 2×6 studs in the walls and 2×12 in the roof. This gives us a lot of room to work with. I have been told by my HVAC engineer that given the difficulties of installing fiberglass batts, that they only provide about 50% of their manufacturer rated R-factor. (The industry measures the ability of an insulation to stop the transfer of heat or cold in R-factors. Higher R-factors mean less transfer of heat and cold.) If we were to fill-up our 2×6 walls with fiberglass, the manufacturer states we would get an R-factor of 22, but this will effectively be 11. With blown-in foam insulation, you have two choices, open cell and closed cell. Open cell is softer, less rigid, has a lower R-factor and is water permeable. Closed cell dries to a rigid and hard finish, has a higher R-factor and is water impermeable. To achieve an R-factor of 22, we would need 6” of open cell and 4” of closed cell. To get the effective R-factor of fiberglass, 11, we would need 3” of open cell and 2” of closed. We chose open cell foam.
Heating and Cooling
A geo-thermal system is used to heat and cool the house. Heat is also assisted by solar generated hot water. A geo-thermal system works like the freezer in your kitchen, transferring heat rather than actually cooling air. In this case, transferring heat from the earth to the house in the heating season and transferring heat from the house to the earth when cooling.
The process relies on two facts: 1. heat flows from hot matter to cold, and 2. a few feet below the surface, the earth stays at a constant year around temperature of 50 to 55 degrees. By running a series of pipes through the ground (ground loop), heat can be drawn from or transfered to the earth through fluid running in the pipes.
During heating season, a cooled liquid is pumped through the ground loop and absorbs the earth’s heat. The heated ground loop liquid is then channeled past pipes that contain a refrigerant. The heat from the ground loop causes the refrigerant to move from a liquid to a gaseous. This phase change requires heat, which is drawn from the ground loop. The now cooled ground loop fluid is ready to go back into the ground to absorb more heat, the gaseous (hot) refrigerant moves on to provide heating for the house.
The gasified refrigerant is compressed in order to increase its temperature to home heating levels. This super heated gas then travels through a heat exchanger transferring heat to air or water (for radiant heat) that is used to heat the house. This exchange causes the refrigerant to cool and makes it ready, once again, to receive heat from the ground loop.
For cooling, the process reverses. Heat is drawn out of the house and brought into contact through the heat exchanger with a cool, liquid refrigerant. Heat transfers to the refrigerant causing it to gasify and taking heat out of the air. The cooled air is then vented back into the house. The hot, gaseous refrigerant is brought into contact with the cool ground loop and heat is transfered to the ground loop liquid, causing the refrigerant to cool and making it available to, once again, receive heat from the air being drawn out of the house. The warmed ground loop travels into the earth, transferring its heat into the ground. It then comes back up cooled and ready to start the process again.
By drawing on the stored energy in the earth in the Winter and using it to deposit heat in the Summer, the geo-thermal system has a head start on other heating and cooling alternatives. The system does, however, require electricity to operate the pumps and compressor. So, while we are not relying on burning natural gas in the house, we are relying on an external power source, the electric company. While power companies are using more sustainable sources of power such as wind, most of our electricity still comes from the burning of natural gas or coal. On most days the geo-thermal system will provide enough heat for the home. It is possible, however, on very cold days that the system will need to be supplemented. For this supplemental heat, we will be using a small gas furnace.
In order to be comfortable with our choice of the geo-thermal system, we compared its energy use to an efficient gas fired furnace and a high efficientcy air conditioner. We compared these systems in two areas, cost and pollution generated. The geo-thermal system and the traditional air conditioning system rely on electricity which is measured in kilowatt hours (kwh). The gas fired furnace burns natural gas, which is measured in therms. Based on our utility’s (ComEd) source of power each kwh they produce generates 1.3 pounds of greenhouse gases. Most of this is CO2. A therm of gas generates 11.7 pounds of CO2.
Our geo-thermal system will produce, on average, 12,000 btus per kwh. A natural gas furnace produces 95,000 btus per therm. Therefore, it will take 7.9 kwh (95,000/12,000) to produce the same amount of heat as a therm of natural gas. For each 95,000 btus in our house, CO2 emissions will be 10.3 lbs (1.3 lbs times 7.9 kwh) or 1.4 lbs less than a house heated with natural gas.
We will also save money, as the 7.9 kwh will cost $.87 versus $1.30 per therm.
The geo system performs even better in the summer. Cooling efficientcy is measured by a SEER rating. The best A/C on the market today have a SEER rating of 16. The geo system is expected to perform at a SEER rating of 18 to 20.
We have modeled the total heating and cooling needs of the house and with this estimated the money and CO2 savings the geo-thermal system will provide. These numbers include all circulating pumps and compressors.
Power Use
Heating Cooling
kwh Therms kwh
Geo-thremal 10,540 474 5,643
Gas Furnace/AC N/A 1,670 7,720
Total Cost and CO2 Output
Cost CO2
Geo-thermal $2,219 27,069 lbs
Gas Furnace/AC $2,942 29,069 lbs
Difference $814 2,729 lbs
Our CO2 emissions will be further reduced by the use of solar panels.
Solar Hot Water
We have six solar hot water panels on the roof. Two of these will meet our needs for domestic hot water and four will produce hot water that will supplement the heating system.
With 4 to 5 hours of sun, each panel will produce 33,300 BTUs a day. The amount of sun we get depends on the time of year. The domestic hot water panels will be used all year, and we estimate that they will be 70% effective. The panels used to supplement the heating system will be used 8 months a year (mid-September to mid-May) and we estimate that these will be 60% effective. With these assumptions, each of our two domestic hot water panels will produce (.7 x 365days x 33,300btus) 8.5 million BTUs per year. Each of our four heat assist panels will provide (.6 x 244days x 33,300btus) 4.88 million BTUs per year. This totals 36.5 million BTUs per year. If this hot water were produced by a gas furnace instead, we would need 385 therms. A therm currently costs $1.30, so, at current gas rates, we are saving $500 per year. Assuming gas prices increase at 5% a year, this system will pay for itself in 20 years. Not bad considering the life of home. But this system also reduces our annual C02 output. Each therm produces 11.7 pounds of C02, therefore, we are reducing our C02 output by 4,505 pounds per year.
Electric Solar Panels
We have 6 photo voltaic (electricity generating) panels on the roof. It is estimated that these will gernerate 1,869 kwh per year. This will only save us about $225 per year in electrical bills, so it is impossible to justify these panels on economic terms. However, from a green house gas perspective, we will be eliminating 2,486 lbs of C02 per year.
Appliances
Carbon Dioxide
The factors listed above are estimated to reduce our carbon footprint by 10,000 lbs a year. Cars produce 25 lbs of C02 for every gallon of gas, therefore, assuming 20 mpg, our energy saving features is equivilent to 8,000 miles driven in a car. Almost the average driven by the typical American in one year.
John Miller said,
March 16, 2009 @ 2:50 am
Does the stimulus package address conservation efforts in home building? I have heard a lot of talk about finally doing something about energy, but have not heard of actual programs.
It sounds like your project ROI will be fairly long-term for the payback. Are you not sharing the cost analysis you are doing because you do not want to scare us?
Are you doing an ROI calculation on the alternatives your are evaluating?
Am I asking too many questions?
matt said,
March 16, 2009 @ 3:09 am
John,
Thanks for your comments. Not sure yet what the stimulus package does for this type of home. I do know that tax credits for the solar panels and the geo thermal system were made more favorable.
I plan to share all the numbers, just have not had the time. The pay back is long on some of the stuff we are doing. Of course I will know more once the house is running. Also, I hope people will view the CO2 savings as having value beyond the dollars spent.