Air Supply

When I was a young teenager in general science class, my teacher tried to explain that color exists only when there is light. At the time I had a pink bedroom that in my mind stayed the same color all the time, day or night. Since every time I peeked at it in the night with a flashlight, it WAS pink, then obviously just because I couldn’t see in the dark didn’t mean the room was not pink. Right? Wrong! Sigh. I still struggle with that concept, but now I do understand it. I don’t like it, but I accept it. Comprehending what Vince has planned for what could literally be called “air conditioning” has been a similar struggle. My experience with heating systems has been more in relationship to the location of the thermostat than in understanding exactly how the warm or cool air is produced, and my habit is to simply open a door or window when I am uncomfortable. See the problem?

There are three big goals for the Heating, Ventilation, and Air Conditioning (HVAC) systems in the house: Climate control, efficiency, and air quality. This sounds simple, but there are a lot of factors to be addressed in each category. Temperature control is quite manageable most of the time, since the Outer Banks enjoys an average annual temperature of about 63 degrees and a comfortable indoor temperature would be around 72 degrees. There are fewer than 10 winter days of with temperatures below freezing and only a few weeks of weather that is 90+ degrees in the late summer. Controlling humidity is more challenging, with a high average annual humidity at about 75% and the comfort level for humans in a house between 40% and 60%. We humans make a habit of increasing that level in the house just because we breathe, move around, cook, do laundry, play, have pets, sit by the fireplace, on and on. As for efficiency, our only choice for continual power is electricity (with a back-up generator), because there are no sources for fossil fuels or wood in our area of the beach. Protecting the equipment is important as well, because being on the oceanfront exposes anything outside to a lot of variables, like heat from the sun, changing winds, exposure to salt air and a huge body of water that occasionally comes for an uninvited visit. In general the Outer Banks has very clean outside air, but maintaining good indoor air quality is a different challenge. Often, people don’t think about what they are breathing, but with the lack of air infiltration in a tightly closed structure and the amount of Volatile Organic Compounds (VOCs) and other emissions that are a part of modern life, indoor pollution control is an essential idea. As with the siding, solving one issue impacts another, so that the whole house is really an integrated sum of its systems, rather than of separate parts…like a person, in a way. Given these constraints, Vince decided to use a closed loop geothermal heat pump system aided by a whole house dehumidifier. He added energy recovery ventilators (ERVs) to assure an efficient system for maintaining good indoor air quality.

Before this project, the only thing I knew about a heat pump was that it doesn’t produce its own heat. In a nutshell, a heat pump moves heat from one source to another, and that source can be the outside air, water, or the earth. An air-to-air heat pump system is the most common, but often the difference between the outside air and the desired household temperatures overwork the heat pump and cause it to be less effective and inefficient. In an air-to-air system, the work
that the heat pump must do is greatly variable depending on the vagaries of climate, which makes it difficult to correctly size the heat pump unit. Water-source heat pumps are much more efficient than air-source units and are often used in large commercial buildings, but they require a fossil fuel fired boiler system to produce heat in the winter. For our use, the geothermal heat pump makes the most sense, especially since the earth where we are maintains a constant
temperature of about 61 degrees Fahrenheit starting ten feet below the surface. Another benefit of a geothermal system is that the compressor and heat exchanger sit inside the house, where they will not corrode. Very important for a salty area. This article and illustration from the Consumer Energy Center has a very clear explanation of a closed loop system:

http://www.consumerenergycenter.org/home/heating_cooling/geothermal.html

Here's a video that illustrates how a heat pump works:



and another about the geothermal system:

Our choice for the HVAC sub-contractor was R.A.Hoy, one of the oldest HVAC contractors on the Outer Banks. (http://www.rahoy.com) In addition to being knowledgeable and experienced regarding the geothermal system, the guys are great to have around the work-site. They put in long days of continual work, but they are laughing and fun and happy to explain all they are doing in the moment. The installation supervisor for our job is Fred Marklin, with Toby and his apprentice, David, putting the ductwork together for the rough-in. It has been interesting to watch how something that looks like big Legos when it arrives in the truck comes out being neat and orderly installed. We had planned for the placement of the ductwork when we designed the house, but there still were challenges associated with feeding the hard ducting through the chase in the middle of the house and allowing space for insulating the roof deck on the top floor.

The ducting is rigid galvanized steel except for the last few feet of the branch runs, where the ducts are flexible aluminum. The rigid duct is harder to handle, but it prevents the trapping of dust in the system because it is smooth. To prevent air leakage and air infiltration, mastic was used where two pieces of duct come together, and in some places aluminum tape was used instead. The tightness of the duct lines also contributes to the indoor air quality, because the system remains uncompromised. In our house, all of the ductwork is internal to the house, so there is less opportunity for the growth of molds and mildew. Each duct is insulated with aluminum faced fiberglass batts.

The house will have two complete HVAC systems, one for the top floor and one for the bottom floor. Steve Jenkins, our mechanical engineer, determined the sizing of the heating and cooling systems using Air Conditioning Contractors of America (ACCA) Manual J-compliant software. Although the Manual J is a complicated calculation, it prevents the incorrect sizing of the system and lessens the possibility of an uncomfortable living environment and wasted energy. Steve also performed the Manual D calculations, which establish the correct sizing of the ducts. (These Manuals are actual books, which are alphabetically named technical instructions for many different HVAC calculations, both commercial and residential.) A temperature control system is generally sized to the maximum expected thermal load of the house, so that the system can handle the worst temperature conditions. Unfortunately, since the usual operation would not be running at peak capacity, the system can become sub-optimal in terms of function. Our response to this is to have a two-stage heat pump which can also operate as a smaller system that is a sub-section of the actual heat pump (about 50% capacity). In this case, the system runs longer but is better able to remove the humidity from the air. It is important to realize that an HVAC system in a temperate climate is cooling dominated. Vince decided to use the WaterFurnace Envision series geothermal heat pump (www.waterfurnace.com) based on his research and Hoy’s recommendation. Florida Heat Pump (http://www.fhp-mfg.com) and Carrier (www.Carrier.com) are also good choices.

Contrary to his Sicilian heritage, my husband has an avid dislike for humidity and decided to use a whole house dehumidifier. Steve Jenkins provided Vince with psychometric charts which graph the physical properties of moist air at a constant pressure so that he could properly size the dehumidifying units. The units of choice are made by Honeywell:

http://www.forwardthinking.honeywell.com/products/dehumidification/dehumidification_products.html

Basically, the dehumidifier is a small, self-contained HVAC system that takes moisture laden air from the return duct, removes the moisture from the air and then sends it back either to the return duct or to the supply duct. It functions like an air conditioning unit by compressing the working fluid which runs through the cooling coils. The moisture laden air flows over the cooling coils, and the humidity condenses on the coils and is drained off as water. Then, the newly cooled and dehumidified air is run over the heat exchanger from the compressor and rewarmed. As air is cooled, its moisture carrying capacity decreases, so the moisture can be extracted. Vince explains that the process is like that of thermal fog, when warm air flows over a cool body of water and can no longer hold the physical droplets of water. In the house, the humidity will be measured and adjusted by a humidistat, which mounts on a wall and looks like a thermostat but has a humidity sensing element.

To improve and maintain good indoor air quality, Energy Recovery Ventilators in two zones will be used to actively manage the intake of air from the outside and the exhausting of contaminants and humidity from the inside. An Energy Recovery Ventilator (ERV) is a type of air-to-air heat exchanger that not only can transfer sensible heat (temperature) but also latent heat (in water vapor). Since both temperature and moisture is transferred, ERVs can be considered total enthalpic devices. An ERV takes the heat from the current air and transfers it to the incoming air stream, moving heat between the two air streams whichever way it needs to go. These mechanical ventilation systems use fans to maintain a low-velocity flow of fresh outdoor air into the house (incoming air stream) while exhausting out an equal amount of stale indoor air (exhaust air stream). Fresh air is supplied to all levels of the house while stale air is removed from areas with high levels of pollutants and moisture. Models with heat recovery and moisture recovery transfer heat and moisture from the exhaust airstream to the incoming air stream during the heating season, and transfer heat and water vapor from the incoming air stream to the exhaust air stream during the air conditioning season. An ERV runs on a proportional timer that is set up and monitored by the person (in this case Vince) who determines the appropriate need for air quality control.

Here's a great video explaining how ERV's work:

In the next couple of weeks, the ERVs will be roughed in by Hoy, and the dehumidifiers will be added after the heat pumps are installed. The ground loops, obviously integral to the system, are schedule to be installed by Steve Van Horn at Chesapeake Wells later this month. We're getting there!