Now that winter has arrived, your client's heating system is operating to keep their house or business warm. However, a significant number of furnace units may not be operating efficiently due to being oversized and/or are operating with an unoptimized thermostat default configuration. This brief guide provides a brief summary of the issue and specific instructions on how to correct it. Candidates for this thermostat parameter update require the following characteristics:
- Forced air gas furnaces with 80 percent efficiencies or higher;
- Programmable (digital) thermostats, but not a smart thermostats such as those sold by NEST, etc.; and
- Mean furnace run (ON) times of 4 to 8 minutes.
As part of the thermostat optimization, we will be increasing the furnace run time. As a trade-off, the controlled room temperature will increase from +/- 0.5° F to +/- 1.0° F of the setpoint.
When the furnace turns on there is a period of time in which the system takes to heat up and reach steady-state operation. During this phase, the furnace is delivering minimal heat to the designed space/room as it takes time for the furnace, ductwork, and other items to heat up. During this period the furnace has a 50 percent runtime efficient (at best). An actual furnace startup time response profile (delta T versus runtime) is shown below.
For best furnace efficiency it is desirable for the unit to spend as much time as possible in the steady-state phase of operation.
RUN TIME MEASUREMENT
The first step is to measure or estimate the furnace ON runtime. This measurement may be done by changing (increasing) the thermostat set point by 1 degree and measuring the furnace ON time with a watch or smartphone timer.
- The ON start time point (time = zero) is when the user determines that the blower fan has started;
- The ON end time point (time = cycle ended) is when the user determines that the blower fan is stopped; and
- It is best to perform this measurement four to five times over a period of a few days. Some units continue running after the furnace has stopped producing heat. This time should be excluded from the runtime measurement.
Alternatively, many of the HVAC monitoring devices on the market for residential units calculate this metric directly (e.g. www.mhvacmonitor.com).
If the average of your results is above ten minutes your system is optimal and does not need to be adjusted. However, if the results are in the four to eight-minute range your system is a good candidate for adjustment.
It is relatively simple to update programmable thermostat settings, but it does require entering the Installer Setup portion of the thermostat. This article references two major thermostat manufacturers but the general procedure provided here applies to all thermostat manufacturers.
Honeywell Systems: The instructions below apply to the VisioPRO™ 8000 thermostat.
- Press and release the System Key;
- Press and hold the two blank keys on either side of the center blank key for five seconds;
- Release the two bank keys when the Installer setup is displayed;
- Update Settings 0240 to two (2) Cycles per hour;
- Update Setting 0250 to two (2) Cycles per hour if your system includes dual-stage heating; and
- Select the DONE function key when completed.
For a Lennox branded system, you will need to determine if the thermostat is a Honeywell branded thermostat in which the prior instructions apply and/or the thermostat is an actual Lennox thermostat. For a true Lennox thermostat, you should increase the Stage 1 Difference value per the following guideline.
- If the current value is 0.5° F, you should increase it 1.0° or even 1.5°;
- If the current value is 1.0°, you should increase it to 1.5°;
- For units with dual-stage heat capability, you should also increase the value of Stage 2 Delay. On a typical system, it is set to 20 minutes so you should increase it to 25 or even 30 minutes. This update delays the Stage 2 heat cycle allowing the system to run in low heat mode longer.
The following bar chart summarizes the actual results obtained for an oversized furnace in a residential unit. The results before and after adjustments are as follows:
- Before the adjustment, the number of furnace start (ON) events was approximately 47 times per day or two every hour. The mean furnace runtime was five (5) minutes. As configured the furnace was approximately 50 percent efficient as during operation the furnace spent most of its time in startup mode.
- After the adjustment, the number of furnace start (ON) events was reduced by 50 percent to 23 times per day or one every hour. The mean furnace runtime was increased to over 10 minutes. As a result, the actual furnace runtime efficiency was increased to 75 percent. The furnace operates the first 5 minutes in the startup phase at 50 percent runtime efficiency followed by an additional 5 minutes operation at 100 percent runtime efficiency.
By changing the cycles per hour from two (2) to one (1), it is possible to increase the mean furnace runtime to a higher number (15 - 18 minutes) but the room temperature swing will be larger.
Optimizing the thermostat furnace and air conditioning settings can save your customers money in energy costs as well as prevent premature failure of your unit due to short cycle run times.
Publication date: 12/24/2018