In this current series of articles we are chronicling the steps to build an energy efficient home based on a current project here in Flagstaff. We are building a house for a professor just north of NAU who is prioritizing energy efficiency. The first articles covered the basics of getting out of the ground. The next few topics dive deeper into the details.
Now that we are sealed up tight we need to address getting the house heated and ventilated correctly. Building tight houses requires special care to make sure there is plenty of fresh air and that combustion gases and humidity are not allowed to build up in the house.
In our house we are using a Heat Recovery Ventilator (HRV) to make sure we are reaching our fresh air requirements. The HRV takes cold fresh air from outside and tempers it with the stale outgoing air to provide fresh air 24/7 in the home. Our unit recovers 70% of the heat off the outgoing air stream before expelling it from the home. There are many different models to choose from including units that have very efficient electric motors and HEPA filtration.
For the most part here in Flagstaff we are concerned with heat. Cooling load is minimal and in most tight houses that have good air flow cooling is not necessary. We mainly use in-floor radiant heat, or gas forced air. The budget usually becomes the determining factor, with the in-floor systems costing as much as 5 times what the forced air can cost. Tight house construction will also play a roll in what type and size of heating equipment will be needed. The computerized energy model that you will generate when designing the home will help you to decide what size unit to employ. When you build an ENERGY STAR home you will be surprised at how small of a furnace or boiler you will need. As a rule we always use sealed combustion appliances. This means that there is a pipe that brings the air needed for combustion into a combustion chamber in the unit and another pipe that brings the byproducts of combustion out of the house without mixing with the air in the home. The old technology uses the draft of the heated gases to expel the dangerous CO out of the house. This method does not work well in a tight house and can lead to serious problems and health implications.
Hot Water Heating
There are two mainstream technologies for heating water in a house today. A heated tanked unit that stores heated water until it is called for, and a tank-less unit that waits until there is a demand to fire the heating element. In the tanked unit water is heated that may not be needed and the “standby” losses can be quite large. That being said there can also be problems here in Flagstaff with installing tank-less units on water systems that have high hardness levels or mineral content. We like to use a supper high efficiency tanked unit called the AO Smith Vertex 100; it uses 96% of the gas to make water hot, sealed combustion to keep CO out of the house, and a large foam shell to keep the water hot when not needed. By comparison the water heater in the typical house only uses about 60% of the gas to make water hot, the rest is lost out the roof. Make sure you weigh all the factors with you contractor and rater before deciding what type of unit to use.
It is important to remember that all the techniques and equipment that we have been discussing over the past few months are meant to work in concert with each other. In the next article we will wrap things up and get ready to move in! You can follow the progress of the home on our blog at: http://hopeconstructionaz.blogspot.com/.
By David Carpenter, Flagstaff’s Green Builder
David Carpenter is the owner of Hope Construction, a general contracting and construction management firm with an emphasis on sustainable building. He can be reached at 928-527-3159