To operate furnaces that have a two stage gas valve but the furnace is being controlled from a thermostat that has only a single stage heating terminal, there are a few options. One method is to utilize a second stage heat relay such as the ICM 104 Delay on Make Relay model HMVR24A2K1000. We found this relay controlling the second stage heat feature on an older Tempstar furnace (model NTP6100KJA1 built in 1999).
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The photo above shows an ICM 104 Delay on Make Relay model HMVR24A2K1000
The delay on make relay (this means that when the power is applied to the relay coil, there is a delay in the “making” or “closing” of the second stage heat circuit) was utilized in a two stage gas furnace that was being controlled by a home thermostat that only had a single heating stage. This means that the thermostat wiring terminals had a single W terminal and NOT a W1 terminal for first stage heat and a W2 terminal for second stage heat. We would normally see the use of this type of second stage heat relay when a home has an existing single stage heat thermostat and a single stage heat furnace but the homeowner wants to upgrade to a two stage gas furnace for improved energy efficiency but they don’t want to go the added expense of running new thermostat wire if their existing thermostat wire does not have an extra unused wire that can be used for the second stage heat.
The photo of the drawing above shows how the delay on make relay is connected in the furnace.
The relay coil is connected between the T2 and T1 terminal where T2 connects to the W1 terminal on the thermostat terminals on the furnace control board. The T1 terminal on the relay connects to the common terminal on the furnace control board. When there is no call for heat from the thermostat, then the W1 terminal on the furnace control board is de-energized. This means that the relay coil is also de-energized. The 24V power from the R terminal of the furnace control board connects to the COM terminal on the relay and this terminal always has power. When the relay coil is de-energized, then the NO contacts (which stand for normally open) are open thus, there is no power supplying the wire connected to the W2 terminal on the furnace control board. This configuration would occur when the thermostat is not calling for heat.
The thermostat is calling for heat when the thermostat is in heating mode (as opposed to being switched to off, or being switched to cool) and the heat set point temperature is above the current room temperature. When the thermostat is calling for heat then the W terminal in the thermostat is energized with 24V power. This 24V power from the thermostat W terminal travels down the (usually) white thermostat wire from the thermostat to the W1 terminal on the thermostat terminals inside the furnace. This means that the W1 terminal on the furnace control board is now energized with 24V power.
The above photo is of the lower furnace compartment on a Tempstar two stage gas furnace built in 1999.
The thermostat wire from the thermostat contains the small red wire, the small green wire, the small white wire, and the small yellow wire that enter the photo from the bottom center of the photo. These four wires compose the thermostat wire that connects the thermostat on the wall on the main floor of the house with the furnace control board in the basement of this home.
The little red thermostat wire connects to the R terminal (for 24V power) on the furnace control board. The other end of the little red wire connects to the R terminal on the thermostat.
The little green thermostat wire connects to the G terminal (for fan) on the furnace control board. The other end of the little green wire connects to the G terminal on the thermostat.
The little white thermostat wire connects to the W1 terminal (for stage 1 heat) on the furnace control board. The other end of the little white wire connects to the W terminal on the thermostat (for heat).
The little yellow thermostat wire connects to the Y terminal (for cooling) on the furnace control board. The other end of the little yellow wire connects to the Y terminal (for cooling) on the thermostat.
The photo above shows a better view of the wiring connections to the relay.
So when the thermostat is calling for heat in this case with a single stage heat thermostat, then there is 24V power to the W1 terminal at the furnace control board. The furnace control board sends a 24V signal to the first stage of the gas valve to operate the furnace in first stage heating. Having a furnace with two stages of heating improves overall heating season furnace efficiency. The furnace runs in the lower first stage heat when there is not a lot of heat load on the furnace. This saves on natural gas usage. When there is more of a heat load on the furnace, such as when the winter temperatures plummet and the heat leaves the home at a faster rate, then the furnace can operate on the second stage heat, which operates the furnace at a higher heat rate. For the second stage heat to kick in, the W2 terminal on the furnace control board must be energized. In the configuration here with the single heat stage thermostat and the two stage heating furnace, we must be able to somehow energize the W2 terminal for the second stage has valve to energize.
The delay on make relay works in the following way. When the thermostat first calls for heat, the W1 terminal is energized. The W1 terminal is also connected to the T2 terminal on the relay which activates the relay coil. This actuates the timer feature of the relay. The timer can be adjusted for the number of seconds that the normally open contacts (NO) stay open and then are repositioned to closed once the timer has timed out. So let’s say the relay timer is set for 500 seconds or (500 seconds divided by 60 equals) 8.3 minutes. After this 8.3 minute time delay, the NO contacts close. When the NO contact closes then the wire connected between the NO terminal on the relay and the W2 terminal on the furnace control board becomes energized to 24V. Now that the W2 terminal is energized, the furnace control board sends a 24V signal to the second stage terminal on the gas valve thereby placing the furnace in second stage heating.
So the furnace will begin a call for heat in first stage heating and if there is a call for heat for 8.3 minutes, which means the furnace has continued running for 8.3 minutes and has not yet satisfied the set point on the thermostat, then the furnace automatically initiates second stage heating.
When the thermostat heat set point is reached, then the thermostat is no longer calling for heat and the W1 terminal is de-energized. This resets the relay for the next call for heat.
The use of the delay on make relay is one of the less common methods of activating the second stage heat for a two stage gas furnace. The second stage heat is more commonly controlled by using a thermostat that has the option of having second stage heating. Many newer furnaces have dip switches that can be set to automatically initiate stage two heating after first stage heating has run for a preset amount of time and then the furnace assumes that first stage heating is not sufficient to heat the home and second stage automatically kicks in.
For technical material on troubleshooting a low voltage transformer failure, refer to this blog.
For information on how to properly connect the primary side of a low voltage transformer on roof top units, refer to this blog.
For more information about upgrading your current furnace to a more efficient two stage natural gas furnace, just give us Accurate Heating & Cooling a call at 402-238-2425. We provide expert furnace service and we also professionally install furnace equipment.