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HeatPump
When a heat pump is operating in the heating mode or heat cycle, the outdoor air is relatively cool and the outdoor coil acts as an evaporator. Under certain conditions of temperature and relative humidity, frost might form on the surface of the outdoor coil. The layer of frost will interfere with the operation of the heat pump by making the pump work harder and, therefore, inefficiently. The frost must be removed. A heat pump has a cycle called a defrost cycle, which removes the frost from the outdoor coil.
A heat pump unit will defrost regularly when frost conditions occur. The defrost cycle should be long enough to melt the ice, and short enough to be energy-efficient.
In the defrost cycle, the heat pump is automatically operated in reverse, for a moment, in the cooling cycle. This action temporarily warms up the outdoor coil and melts the frost from the coil. In this defrost cycle, the outdoor fan is prevented from turning on when the heat pump switches over, and the temperature rise of the outdoor coil is accelerated and increased.
The heat pump will operate in the defrost cycle until the outdoor coil temperature reaches around 57° F. The time it takes to melt and remove accumulated frost from an outdoor coil will vary, depending on the amount of frost and the internal timing device of the system.
Interior Heating Element
During this defrost cycle with older heat pumps, the indoor unit might be operating with the fan blowing cool air. To prevent cool air from being produced and distributed inside the house, an electric heating element can be installed and engaged at the same time as the defrost cycle. In defrost mode, this heating element will automatically turn on, or the interior blower fan will turn off. The heating component is wired up to the second stage of a two-stage thermostat.
The Typical Cycle
The components that make up the defrost cycle system includes a thermostat, timer and a relay. There is a special thermostat or sensor of the defrost cycle system, often referred to as the frost thermostat. It is located on the bottom of the outdoor coil where it can detect the temperature of the coil.
When the outdoor coil temperature drops to around 32° F, the thermostat closes the circuit and makes the system respond. This causes an internal timer to start. Many heat pumps have a generic timer that energizes the defrost relays at certain intervals of time. Some generic timers will energize the defrost cycle every 30, 60 and 90 minutes.
The defrost relays turn on the compressor, switch the reversing valve of the heat pump, turn on the interior electric heating element, and stop the fan at the outdoor coil from spinning. The unit is now in the defrost cycle.
The unit remains in the defrost cycle (or cooling cycle) until the thermostat on the bottom of the outdoor coil senses that the outdoor coil temperature has reached about 57° F. At that temperature, the outdoor coil should be free of frost. The frost thermostat opens the circuit, stops the timer, then the defrost cycle stops, the internal heater turns off, the valve reverses, and the unit returns to the heating cycle. A typical defrost cycle might run from 30 seconds to a few minutes. The defrost cycles should repeat regularly at timed intervals.
SEER 13 AIR CONDITIONERS
The standard for newly installed air conditioners has changed from SEER 10 to SEER 13: a 30% increase in efficiency. However, for many with older homes (pre-1992), the increase in efficiency can be even greater than 30%, due to the older units much lower SEER ratings--usually around 6 or 7. Thus the “payback” will be even bigger, and faster, and the reduction in electricity costs will be even nicer!
Initially, the up front costs for the new SEER 13 units are going to be higher than the SEER 10 units. Talking with a well respected Atlanta HVAC firm who represents several well known brands, the representative noted the price difference between a SEER 10 and a SEER 13 two-and-a-half ton unit, including the cost of a matching evaporator coil (if needed) would range about $600 higher on average.
There may be additional costs for sheet-metal work around the new, larger sized evaporator coil at the furnace, possibly new copper tubing from the compressor to the evaporator. The new units require very clean plumbing, so the current plumbing may need to be cleaned or replaced. The new units required 40% more “freon”.
There has been much speculation about how much larger the new outside units will be. Actually some manufacturers like Amana, Goodman and Bryant (and perhaps others) new units will be the same or smaller than their current SEER 10 units.
A new digital thermostat is recommended if your unit is an older, say 15 year old analog thermostat, for more efficient operation.
And, just like car a/c systems where the old R-12 was changed, in 2010 the current R-22 air conditioner coolant will be changed to the R-410A. At least one manufacturer, Carrier, already includes the new freon, so you‘ll already have an a/c that meets the SEER 13 requirements with the new components.
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