Heat Pump Technology
HOW MUCH EFFICIENCY?
The common question of how much energy an air source heat pump is using is a hard one to answer. What size is the system? How many zones are there? Is there more than one system? How many compressors are running and what capacity are they running at? The specifications of each individual heat pump and the behaviors in which that heat pump is set play a big roll in how much energy it used to change or maintain a room’s temperature.
RAMP IT UP
These systems are programmed with an incredible amount of code to evaluate each heating and cooling scenario and adjust the compressor and blower accordingly. Most systems have, in the range of 5 stages that the system uses to ramp itself up or down depending on the difference in temperate its trying to make up. So even if the system is on stage 3 it may only be near half its capacity. If you adjust the remote temperature 1 or 2 degrees the system may only go into the first stage to produce the heating or cooling capacity it needs to adjust the room temperature by those few degrees. Whereas, if you adjust the remote temperature by 8 or 10 degrees the system will ramp up several stages to make up this difference faster. The higher the stage, the harder the compressor is working and therefore the more energy it uses. Keep in mind, the unit is not only heating up the air in the space but also all the objects like furniture, walls, flooring, etc. This is why it’s recommended to “set it and forget it”. The heat pump will use less energy to maintain a temperature over the course of a day/night than it will to ramp up during certain hours of the day. This methodology is different than what we all learned with traditional heating and cooling systems. We went out and bought programmable thermostats to adjust temperatures throughout the home or building when we were sleeping or away at work. But a heat pump is so efficient and designed to maximize its efficiency it’s better to set one consistent temperature throughout all hours of the day. If your building is well insulated then there isn’t a lot of temperature loss and the unit can easily keep a consistent temperature. Most traditional systems have a 100% on/100% off functionality meaning that when they’re running, they’re running at 100% capacity. So they can’t run on a low setting to save energy like a heat pump can. Another thing to note is that all air conditioning systems, regardless of the technology, removes humidity first and then cools the air. So if you’re in the habit of turning your system off, the system takes longer for it to satisfy the thermostat than if it’s left on maintaining a temperature. Since heat pumps are so efficient it’s more beneficial to leave it on regularly. There’s also a dry mode that can remove humidity without cooling the space.
You may hear the term “deep setback”. This refers to the temperature difference between the room and the unit’s set temperature. Anything over 2-4 degrees is considered a deep setback. As stated above, the larger the temperature different the system has to makeup the higher the stage and harder the system will work to achieve the desired temperature, thereby reducing it’s efficiency. So it’s best to set a consistent temperature throughout the day rather than having the system make up large deltas.
OTHER EFFICIENCIES
Another difference between conventional systems and air source heat pumps is that each indoor unit is it’s own zone. This gives you maximum control over which spaces you heat or cool and which ones you don’t. You can close and open doors to section off areas of the home or building to maximize or reduce airflow. Traditional systems are often one or two zones for a whole house so you may be heating or cooling spaces you don’t often use.
It’s also debatable that if you don’t run the compressor harder than it needs to, the system will last longer. If the system only has to run at a low capacity on a regular basis you aren’t taxing the equipment unnecessarily. Also, since the system runs in both heating and cooling mode it gets more constant use throughout the year. For example, a central air conditioning system sits unused for an entire season.
AIR SAMPLING
Another difference between an air source heat pump and a traditional central system is how it measures temperature. A traditional system uses a thermostat generally mounted on a wall in the room. The thermostat is the device that measures the air temperature and calls for heat or air conditioning to change the room’s temperature. An air source heat pump measures the temperature of the air in the indoor unit itself. Every few minutes it turns the fan on to the lowest setting to pass air across the coil and measure the air’s temperature. The airflow during this phase is virtually undetectable. You won’t feel it unless you hold your hand up directly against the vent. A heat pump uses a remote to control set the desired temperature and settings. The remote doesn’t measure air temperature so keep in mind the air temperature up at the unit may be different than wherever the remote itself is located in the space. This is why it’s recommended to adjust the temperature on the remote to where you’re comfortable, noting it may be different than the temperature you used to set your traditional system to. There are upgraded thermostats and remote thermostats you can add to your system to help read and control temperature from other points in the space.
CYCLES
When a system is first turned on, it’s smart enough to know that the indoor coil isn’t warm yet so any air passed over the coil and into the room would also be cool. The opposite is true in the summer. The coil may be too warm to start cooling the room. Refrigerant cycles between the indoor and outdoor unit until the coil is ready. At that point, the indoor blower fan turns on to start circulating air into the space. You may notice the louvers changing position during different points of the cycle. This is in an effort to direct air away from occupants when the coil isn’t at the desired temperature yet. If the system has a significant amount of temperature difference to make up sometimes the system will make a quiet ticking, gurgling, or popping noise. This is just the coil heating up and expanding and contracting. It’s similar to the sounds a water baseboard heating system makes when it’s heating up.
These systems also have a defrost mode. This usually only happens during the extreme cold days of the year. The fan motor in the outdoor unit moves air across the coil. When the outdoor temperature is extremely low the coil may collect frost. When this happens, the outdoor fan motor stops running to allow the coil to heat up and melt the frost. The indoor fan motor may continue to run at a very low speed and the air will feel slightly cooler than when the unit is in heating mode. This cycle lasts about five minutes.
UNIT PLACEMENT
If you recall to back to your early years in science class you’ll remember warm air rises. A properly positioned indoor unit should take this into account. Many homeowners want to mount their unit high and out of sight or in a corner of the room where it’s less noticeable. Indoor wall units are generally mounted near ceiling height in a space with a standard 8’ ceiling. If you have higher than average ceilings, the unit should not be mounted proportionally. It should be in the same position as if you had an 8’ ceiling. This will reduce the heat rising above the room occupants. Likewise, the unit should be centrally located in a room so it can move air from left to right as well as floor to ceiling. The blower speed and angle of the louvers can be adjusted to circulate air more productively in certain space layouts. Each space is unique and may require slight modifications to the remote settings.
LOW TEMP MODELS
Traditional heat pumps could operate at their advertised efficiency until the outdoor temperature dropped to around 30 degrees, depending on the model. As technology has evolved, there are now heat pump models that operate at their rated efficiency to -15 degrees outside. This has allowed air source heat pumps to become the primary source of heating for a building in the Northwestern region of the country. If the system is sized properly, they can provide adequate heating during even the coldest days. When the outdoor temperature drops below -15 degrees the system doesn’t shut off. It will just need to work a little harder to capture the heat it needs to heat the space. So the efficiency will start to drop but the system will continue to run.