How Can I Tell 1 or 2 Transformers for Heating/Cooling System? I get this question a lot and I also see this question used in my stats on how people find the site but I do not believe I have directly addressed or dedicated an article to this subject yet but there are residential HVAC systems out their that use 2 transformers and it befuddles some people. In commercial systems you can have several step-down transformers in a single unit that will break down the voltage from 277 volts all the way down to 24 volts and everything in between but in residential systems there is usually only a need for one step down transformer to provide control voltage for the entire system. So, How Can I Tell 1 or 2 Transformers for Heating/Cooling System? Read on for more explanations.
NEC and other Code Considerations for Control Wiring
That control voltage in residential HVAC systems is (typically) 24 volts. 24 volts is standard in all residential systems and a lot of commercial systems partially because the control wiring needs to be ran to each piece of equipment for control and 24 volts is not applicable to the NEC which states anything over 50 volts is regulated by the NEC. So drop the control voltage down to 24 volts and it makes it easier for the contractor or HVAC installer to run new wire each unit in the HVAC System. The only requirement which I am aware of that would be applicable to the NEC would be running the wire in a plenum if the wiring is not plenum rated wire no matter the voltage.
One other additional note about using a single transformer in residential versus sometimes using multiple transformers in commercial units: Every transformer has a VA rating on it. The VA rating is important to understand because you should never exceed the VA rating on the transformer. When an engineer specs a new system they select the proper transformer that will handle the load based on the number of components in the control circuit that the transformer will be required to power. Each coil or component which is powered by the transformer will have a VA rating on it. It is a simple act to add up the components and their VA rating and make sure the transformer is large enough to handle the load based on the VA rating. When sizing transformers for control circuits I always leave a little room and over size the transformer a little taking into consideration the fact that in the future someone may add some things into the circuit and I want a little cushion there to ensure the transformer I am installing will handle the load.
2 Transformer Heating and Cooling Systems
There are some situations where a new air conditioner is installed where there is an existing boiler system. It is not uncommon to find these combinations of HVAC systems with two transformers. The boiler system utilizes its very own control wiring and circuits while the air conditioner does the same and each separate system has its own transformer. In that case each system can have its own thermostat or if a single thermostat is used for both systems there must be RC and RH terminations in the thermostat to keep the two control/transformers circuit(s) apart. I have observed both types where there is a single thermostat and where there are two thermostats. It is important to note in the systems that two thermostat to make the occupants aware that with two thermostats it is possible to have both systems on and fighting each other in which they are going to use a lot of energy which means their bill will be high.
A good indicator of determining that the system has two transformers is to look at the thermostat terminations. If you see the RH and the RC have their own dedicated wiring this means you will have two separate control circuits and two separate transformers for those circuits. This is not typical but it is also not rare especially when someone installed an air conditioner and left the existing heating system in place without integrating the control circuits for both systems. These two systems can be integrated but my personal rule is to leave well enough alone so if the two transformer system is working with no problems I would leave it alone.
2 transformer systems can be wired for a single transformer however this will require the skills of a good HVAC tech to design the new control circuit that will work properly for both systems and to make sure the new circuit does not exceed the VA rating on the transformer. The rule of thumb in some electrical calculations is plus or minus ten percent. When sizing a transformer for a new circuit I always go way over. Additionally, transformers rated by NEMA or the National Electrical Manufacturers Association are the better transformers. It is not uncommon for a 40 VA transformer to be in a circuit that only has <10 VA load. It seems like overkill but the transformer is not consuming a lot of power and it is better to stay under the rating on the transformer than it is to go over and blow the circuit.
How Can I Tell 1 or 2 Transformers for Heating/Cooling System
Finally in conclusion, if your system was installed as an integrated heating and cooling system even if it is a split system air conditioner or heat pump then you should only have one transformer. If not then something is really weird about the installation. If you truly do not understand electrical circuits including control circuits then the best advice for you is hire an HVAC technician to assist you in determining the type of heating and cooling system you have and how it is controlled. that is How Can I Tell 1 or 2 Transformers for Heating/Cooling System.