Atlanta Series Geothermal HVAC Control Board
One common complaint about heat pumps is that even though they are heating the house, the air coming out of the ducts often times feels cool or even cold to the home owner. This is especially common when the home owner has replaced a gas fired furnace with an efficient geothermal heat pump.
Georgia Controls solves this problem through the use of our trademarked ComfortMode. Through the use of temperature sensors along with a variable speed fan motor, the Atlanta series control board varies the fan speed to regulate the duct temperature. Internally, a PID loop is employed to control duct temperature when comfort mode is enabled. One of the great features of ComfortMode is that the target temperature is programmable in the field and can be tailored to the home owner's needs without sacrificing heat pump efficiency unnecessarily. Certain Georgia Controls sensors and a variable speed fan motor are required to enable ComfortMode.
- PID controlled
- Programmable target temperature
- Head temperature is monitored to avoid high pressure (return to summary page)
Another great advantage to ComfortMode, which is well worth mention, is its usefulness in forced air systems that also have one or two domestic hot water systems. In a conventional heat pump of this nature, when the heat pump is heating the house, if a priority demand call comes in, the heat all gets "sunk" in a cold surge from the domestic hot water exchanger. This causes an extremely cold "burst" of air from the ducts making occupants extremely uncomfortable during the priority demand call. NOT SO with Atlanta series control boards using ComfortMode! When the cold rush of water sinks the heat away from the heating coil, the PID loop quickly reacts and slows the fan down to ensure warm air from the ducts. The volume of air goes down, but the occupants are not subjected to the cold blast seen in other systems. In fact, the initial users of the Atlanta series control board reported not being able to tell in the house when priority demand called or was satisfied. OEMs specifying ComfortMode have happy customers in the winter regardless of hot water demands.
Whenever the number of compressor stages on a controller exceeds the thermostat stage count (3 stages on a 2 stage thermostat or 2 or 3 stages on a single stage thermostat) it is up to the controller to determine if and when to upstage. The Atlanta series control board has two methods of doing this:
- Low tech method - Atlanta series control boards can use a programmable timer. For example the control board could be set up to upstage to stage 3 after 45 minutes of continuous run time in stage 2.
- High tech method - Atlanta series control boards can be set up to use intelligent upstaging. Intelligent upstaging uses an air temperature sensor in the return air duct or plenum. It tracks the temperature changes over time using what we call DCDT which stands for delta temperature over delta time and compares that to the programmed values in the controller. Certain Georgia Controls sensors are required to enable intelligent upstaging.
High Tech Intelligent Upstaging Examples
For the following examples, let's consider a 3 stage system where the installer has set the DCDT heat setting to 1.00F/Hr and cooling settings to 1.25F/Hr:
- During the summer on a mild day the compressors start in stage 1, and after some run time a thermostat requests stage 2. The Atlanta control board measures the DCDT to be 1.37F/Hr, so it stays in stage 2 and the calls are quickly satisfied without upstaging to stage 3.
- On a hot summer day the power has been out. The system comes on and is immediately called to stage 2. At first the low outdoor differential combined with the high temperature differential across the coil makes the system very efficient and the temperature comes down at 1.5F/Hr, so the system stays in stage 2. With time, as the house cools and the day gets hotter, the Atlanta control board determines stage 2 cooling is only dropping the house temperature 1.21F/Hr so it changes to stage 3.
- In a terrible blizzard, the system has been requesting heating and has been in stage 3 for several hours. As the geothermal loop cools and the outside temperature dips, even stage 3 is no longer heating the house at a sufficient rate. As the heating DCDT reading gets lower, eventually it gets below the 1.00F/Hr threshold. Luckily, this system has strip assist enabled, so the Atlanta control board turns the heat strips on as a booster for stage 3. This gives the system the extra BTUs it needs to satisfy the heating request which gives the loop some time to heat up and recover somewhat for the next cycle. Notice however, that the Atlanta control board would not ever turn the expensive strips on as long as the compressor system was able to meet the DCDT heating target set by the installer. (return to summary page)
Georgia Controls Atlanta series control boards can help to regulate relative humidity. Certain Georgia Controls sensors and a variable speed fan motor are required for automated humidity control. There are several advantages in having the heat pump control system handle humidity management:
- Less wiring
- Fewer installation mistakes
- Internal humidity systems are aware of what is going on internally, avoiding scenarios like humidifying and dehumidifying at the same time
- Less individual components to purchase and install
Humidity Control During Heating
A humidity target for heating can be set at the factory or by the installer and the Atlanta control board will control an external humidifier to increase the humidity. When heating, if the Atlanta control board determines the humidity is too low, it can close a relay to drive a humidifier. The relay used can be any of the up to 4 auxiliary relays.
Humidity Control During Cooling
We all know the 350 CFM/ton guideline, but that is an industry average across many types of heat pumps and air conditioners of various designs. Its main purpose is to compromise between efficiency, dehumidification, and freezing the coil. Georgia Controls testing has yielded a better method for determining CFM during cooling. The Atlanta control board uses a 350 CFM/ton based PID-like algorithm to lower fan speed during dehumidification without damaging the heat pump. During cooling, when the Atlanta control board determines the relative humidity is too high, using its control algorithm, it slows the fan down while monitoring system temperatures. As the relative humidity approaches the set point, the Atlanta board speeds up the fan toward the normal fan speed. Should the coil start to get close to freezing, the fan is increased to its normal speed. The algorithm has built in hysteresis, so the fan speed does not hunt, which would annoy the customer.
But there is more! if the relative humidity target is met, and the humidity is too low, the algorithm works to increase efficiency instead! When working with heat pumps, the ducts must be sized for heating, which means they must be able to pass at least 400 CFM/ton of air. Georgia Controls takes advantage of this caveat and allows the Atlanta control board to increase cooling CFM to as much as the heating CFM rating. This causes the heat pump to have more effective tonnage, and efficiency increases during cooling when the humidity is below set point.
We at Georgia Controls have seen this in action, and it works very well if we do say so ourselves. In a house with uncomfortable humidity levels, when humidity control is turned on, the Atlanta board first works to lower the humidity in the house. After a while, as the humidity gets under control and the comfort level goes up, the fan speed slowly increases and eventually runs in overdrive mode giving the house effectively more cooling without upstaging. We love it, and your customers will too!
Humidity control parameters are all programmable and easily changed in the field:
- Heating fan speeds
- Heating relative humidity target
- Cooling fan speeds
- Cooling relative humidity target
- Coil low temperature threshold
- Coil dehumidification hysteresis temperature (return to summary page)
The Atlanta series control board offers a fine mix of capabilities that are well suited for geothermal heat pump applications.
- Priority Demand - the ability to turn on the heat pump to create domestic hot water as a far more efficient alternative to strip or gas heating the water
- AutoDHW TM - during cooling takes 100% of the unwanted house heat and puts it into the hot water tank
- Temperature Based Desuperheating - uses sensors to determine when there is excess heat which can be captured and stored in the hot water tank
- Standard Desuperheating - turns on the desuperheating pump any time a multi stage heat pump is in 2nd or 3rd stage
In a fully sensored system you can create any combination of Priority Demand, AutoDHW, and Desuperheating.
In all but standard desuperheating the head temperature and freon pressure inputs are monitored to avoid stressing the heat pump. As the tank water warms to near the set point, the Atlanta control board may opt to turn the geothermal loop pump on to avoid extremely high pressures and temperatures in the freon system.
Enabling priority demand with an Atlanta series control board allows a properly equipped heat pump to make all domestic hot water for the hot water tank without the use of gas or electric strips. With priority demand, the hot water tank being cool can actually turn on the heat pump and heat the water without turning on the forced air system. Priority demand can be activated by an external control unit or using a Georgia Controls temperature sensor. Priority demand can be used in combination with AutoDHW and desuperheating.
This option allows the heat pump to take the heat from the home's air during the summer, and put that heat into the hot water tank without overheating the water. The Atlanta series control board does this by shutting off the geothermal loop pump, turning on the domestic hot water pump, and diverting the unwanted heat from the building into the hot water tank instead of into the earth.
Temperature Based Desuperheating
Additionally, year round, the Atlanta series control board can use thoughtfully placed temperature sensors to work as a desuperheater. This option by itself will not normally be able to produce all the domestic hot water in most cases. It is able to use a small amount of heat from the running compressor(s) to keep the hot water tank "topped off", without running the heat pump specifically to heat the water. Without overheating the water, this option is performed when heating and/or cooling, and will take the water tank to a hotter temperature than priority demand or AutoDHW.
This is a low budget option that is offered mostly with retrofit application in mind. Using this function does not require any sensors or external inputs and runs the desuperheater pump whenever the heat pump is in the second or third stage. This option is only available in multiple stage heat pumps and has the drawback that it may not always be heating the water when the desuperheater pump is running.
Dual hot water systems(!)
To address some very special cases, Georgia Controls made the Atlanta series control board capable of heating two separate hot water systems in addition to forced air! The two hot water systems support combinations of, priority demand, AutoDHW, and temperature based desuperheating. There is a built in algorithm to satisfy both hot water systems in a logical, specific order to avoid either system going cold. This very powerful capability opens up many options for OEMs and installers to use their systems for applications they could not address previously. For example, in addition for forced air, now a geothermal heat pump with an Atlanta series control board can also create hot water for:
- In Floor Heating
- Domestic Hot Water
- even Pools! (return to summary page)
A top priority for Georgia Controls when designing the Atlanta series control board was for it to turn on silently. When power comes on, the relays are already off, and they do not allow the connected contactors to close until they are supposed to. This means when the power is turned on, there is no "BOOM" from all motors trying to start simultaneously, and then shutting off all together a few milliseconds later like competitive control boards. Atlanta starts the heat pump silently and does not close any contactors before they are supposed to be closed. (return to summary page)
The Atlanta series control board can be set up at the factory or in the field to have heating, cooling, or zone priorities. These different schemes determine whether heating or cooling is addressed first, and can be a blanket choice, or it can be chosen to be satisfied by zone. Here is how they work:
- Heating priority - the zone(s) calling for heating will be satisfied before the zones calling for cooling
- Cooling priority - the zone(s) calling for cooling will be satisfied before the zones calling for heating
- Zone priority - the closer to 1 the zone is, the more priority it has. In a nutshell, when zone 1 calls for heat or cool, then it and any other zone with the same call will be addressed. When zone 1 is not calling, and zone 2 calls for heat or cool, then it and any other zone with the same call will be addressed. If zone 1 and 2 are not calling, and zone 3 calls for heat or cool, then it will be addressed and zone 4 will also be addressed if it is the same type call as zone 3. If zones 1, 2, and 3 are not calling and zone 4 calls for heat or cool, then it will be addressed by itself.
Here is a zone priority example: Zone 1 is not calling for heating or cooling. Zone 2 calls for heating, so the heat pump turns on and starts putting heat into zone 2. A few minutes later, zone 4 calls for heat, so the Atlanta control board opens the damper for zone 4, and begins putting heat into that zone as well. Then, zone 1 calls for cooling. The damper for zone 1 opens, the freon reversing valve changes direction, the fan changes speeds, and the heat pump starts making cooling. A few seconds later, the dampers for zones 2 and 4 close. After a while, when the cooling call in zone 1 is satisfied, everything reverses back to making heating for zones 2 and 4 because they are still calling. (return to summary page)
Because of the physics of the the inertia of blower fans, when we turn them off, they continue to spin, moving air volume as they spin to a stop. Because of this, in a system with zone dampers, when dampers are closed, the fan may still be spinning, creating destructive pressure. Georgia Controls designed the Atlanta series control board to include adjustable damper close delays for just this reason. When a zone is to be closed, it waits to close the damper several seconds to give the fan time to spool down, or to allow another damper to open up to protect the plenums and ducts from damage. (return to summary page)
For a geothermal heat pump to work, there has to be an ample supply of liquid to transfer heat to or from. To help assure sufficient flow to transfer the heat, the Atlanta series control board starts the pump prior to starting the compressor(s). For closed loop systems, a loop pre-flow of as little as 1 second can be selected. However, the loop pre-flow can be extended to as much as 180 seconds for systems (especially open loop ones) which need a long pre-flow time to get slow turning valves wide open. (return to summary page)
You no longer need to size your ducts so that every zone is big enough to take every stage!
(read that statement again!)
One of the biggest "claims to fame" for the Atlanta series control board is its ability to mix differently sized zones on one multi-stage heat pump. This easy to set up ZoneSum TM system is a real marvel in this industry because zones only get the maximum tonnage and air flow that they are capable of handling. This amount of control allows installers to make small duct runs so small zones can be conditioned by ducts which cannot take the maximum air output of the heat pump. Zones with larger ducting can be set up to call for higher stages, and smaller zones only for lower stages, or even no stage for parasitic micro-zones! Here are some examples of how an installer might set up a heat pump using powerful ZoneSum TM technology:
- Mixing big and small zones - Zone 1 is a large zone and can accept 4 tons, and zone 2 is capable of 2 tons. When zone 2 calls with Y1 it gets stage 1 (2 tons). Then zone 1 calls with Y1 and both zones share air flow on stage 1. Later zone 2 calls with Y2, and the heat pump changes to stage 2 (4 tons). Since zone 1 and zone 2 combined tonnage is capable of 4 tons, this is not too much air for zone 2. Later zone 1 is satisfied, and zone 2 is still calling for Y2, but the heat pump changes back to stage 1 because zone 2 is only capable of 2 tons of air flow.
- Equal small zones - The ducts in zones 1 and 2 are each sized equally and are each able to flow 2 tons worth of air. A Y1 from either zone will result in a 2 ton, stage 1 response. A Y2 from either zone by itself results in a 2 ton, stage 1 response. A Y2 call from either zone in combination with a Y1 or Y2 from the other zone will result in a 4 ton, stage 2 response.
- Tiny zones - Occasionally, a small zone is needed that has duct work that is too small to support the smallest stage in the heat pump. The micro-zone has a thermostat but cannot take any stage of heating or cooling by itself. If any other zone is calling, the micro-zone can open its damper and get some heating or cooling too. (return to summary page)