Air Conditioning for Humid Climates, continued
A device called the "user interface" is the brain center of the zone system: It continually sense temperatures in the home, checks the outdoor temperatures, and tells the control boards in the air handler and the condenser unit what to do. It will also talk to the zone dampers, opening and closing them as needed to condition each of the home's various zones.
The "user interface" circuit board is the brain of the comfort system.
It continually adjusts the air handler, outdoor heat pump, and zone dampers based on sensor input and its programmed instructions.
The board shown, a Bryant Evolution control, is capable of managing a wide variety of mechanical systems and can serve as many as eight independent zones.
The Bryant board I use can handle any combination of residential heating and cooling. It can control heat pumps, gas furnaces coupled with central air conditioning, or gas furnaces with piggyback heat pumps. It can run variable-speed systems or two-speed systems and is capable of independently managing as many as eight zones.
The Bryant controller will work with any brand of equipment, from any manufacturer. I use it with virtually every system I install. I am qualified to sell other brands of control unit, but this is the most flexible and capable zone-control product I have ever used.
The thermostat for this device has a seven-day independent control program. You can program it for four different periods per day, per zone. When I install a system, I program the boards on the equipment, I talk to the homeowners about what conditions they want in the home, and I program the user interface for the customer. Once everything is set up, no one has to touch it again.
This component is one of the keys to the Bryant system's effective humidity control. A typical thermostat senses room temperature and initiates a call for cooling when the temperature rises. But the Bryant Thermidistat, matched with an air handler equipped with a variable-speed blower, offers an option for those times when it is cool in the home but the humidity is high, causing that "sticky" feeling.
This control can be set to maintain humidity levels in the cooling mode as low as 50% RH. If the humidity level rises in the home, the control initiates a call for cooling. During the dehumidification mode, the indoor blower speed is reduced approximately 40%. This allows the evaporator coil to become much colder than normal; meanwhile, lower airflow velocity allows the air to maintain contact with the coil longer, allowing the cold coil to wring the humidity from the air. This drastically increases the latent capacity of the comfort system without overcooling the structure.
A Case Study
To monitor how the system works, I installed data-logging devices in one home where conditions were particularly severe. This house was built on totally saturated soil, downhill from a pond. To provide accessibility, the floors had been set level with the outdoor grade. During construction, the excavation had to be continuously pumped in order to stay dry enough for the foundation stemwalls to be built and for concrete to be placed for a crawlspace slab.
Unfortunately, no vapor barrier was installed under the crawlspace slab for this house, and the concrete constantly wicks moisture into the house from the saturated soil beneath it. My system was designed according to Manual J standards, with no allowance made for this extreme and unusual moisture source. But the data I collected show that the system I installed, which is much like the one I've described in this story, has been able to maintain the exact design conditions I set it up for: 75°F temperature and 50% RH, with variation of no more than one degree or 5% RH.
I was hoping that the data I collected would also verify the energy performance of this system. But the energy used to pull all that excess humidity out of this home threw those numbers out of whack. The electrical bills for running the system were not outrageous, but they were somewhat higher than I would expect if the house didn't have that particular moisture problem. In houses that have mainly atmospheric humidity to contend with, my systems are typically very economical to operate.
Workmanship Crucial to Performance
Installation is the critical part of any comfort system. You can go out and buy the best system on the market, but if it's not properly installed, you've got a piece of junk. I can't cover every aspect of workmanship in one short article, but let's take a look at a few important details.
I'm brazing a joint in a refrigerant line as I attach a refrigerant dryer to the line. The dryer's purpose is to clear oxidation and moisture out of the refrigerant. These systems use a lubricant that is particularly vulnerable to moisture; if the line isn't kept dry, there's likely to be corrosion and excessive wear.
Careful installation work is key to any system's success.
Here, the author makes a brazed joint in a copper refrigerant line as he installs a refrigerant dryer that will protect the system against moisture and corrosion.
The refrigerant dryer is a mandated standard item that has to be installed on every new system. Any time the line is opened for repair or modification, the dryer should also be replaced. What's not standard, however, is the solder I'm using to braze the tubing joints. I like to use a 15% silver formula, because it is the most compatible formula with the copper in the lines. Less expensive phosphorus-based formulas are more commonly used, but they don't provide the strength and durability of the 15% silver joints. It's not critical in this particular case, but when we place copper lines underground, as I do for some of my geothermal installations, it's very important.
When I braze a joint, I clamp a set of tubing cutters onto the line to hold the plastic insulation back from the area where I'm applying heat. That keeps the insulation from melting. I've also wrapped the valve in a wet rag, to keep it cool while I apply my torch to the joint and melt the brazing compound. When the joint has cooled, I'll take the cutter and the rag off, and put the insulation back in place.
Below shows us flowing nitrogen through our refrigerant loops to clear out any oxygen or water vapor before we load the system with refrigerant. Not all contractors do this, but it's a good idea — it eliminates the moisture and oxidation that can cause corrosion, the same contaminants that the refrigerant dryer is supposed to help protect against.
Before filling the system with refrigerant, the author purges the loops with nitrogen to eliminate moisture and oxygen that could corrode refrigerant lines and mechanical components.
Proper duct installation is key. We use only metal ductwork — no fiberglass duct board or flexible duct. We also seal every duct joint with mastic. When ducts are leaky, the losses don't occur just when the system is running. Air never stops moving — you have leaks when the system is on and leaks when it's off. It's a continuous loss, not a momentary one. Air and moisture getting into the duct system cost money, stress the system, and contribute to all kinds of problems. Careful duct-sealing is well worth the trouble.
Thorough sealing of all duct joints is a critical quality-control step. Leaks in the ductwork allow moisture into the airstream, which will decrease system efficiency and may lead to condensation and mold growth inside the ducts. Applying sealant mastic with brushes or gloved hands is the preferred method for achieving airtight ductwork.
Duct sizing and layout are also critical to system performance, but that topic deserves its own article. Here are a few key points:
• ducts should be well supported
• runs should be kept as straight as possible
• sweeping curves are better than sharp bends
• ducts should be insulated to R-6 or more
All of these quality details help the system perform effectively and reliably. But providing first-rate comfort along with high efficiency is about more than workmanship — it starts with good design. If your hvac contractor makes a careful, room-by-room estimate of the sensible and latent loads for the particular house, and if he chooses equipment of the right capacity that can adjust to a changing balance between those loads, you're well on your way to meeting the homeowner's needs.
Dwayne Akersowns and operates Akers Custom Comfort, Inc. (www.acchvac.com), a second-generation residential and commercial hvac installation and service contractor based in Stokesdale, N.C. Akers Custom Comfort specializes in state-of-the-art energy equipment, including direct-exchange geothermal heat pump systems.