Steps in order for furnace commissioning:
- Set gas pressure to manufacture default
- Set input of the unit. Change orifices if needed
- Set gas pressure
- Get the temperature rise in the manufacturer suggested range
- Combustion analysis test
An essential step in commissioning a newly installed gas appliance or even just on routine maintenance for both efficiency and safety includes clocking the meter, running a combustion analysis, checking temperature rise, verifying stack/venting draft, and setting manifold pressures.
A 94% gas furnace installed fresh out of the box could be running at quite a bit less when not set up correctly. You’re not doing your customer any favors when not finishing an install as you could add thousands of dollars in gas usage and repairs that could be avoided over the life of the appliance.
Clocking the meter and setting gas pressure
Clocking the meter is an important first step in setting up a new install and is done to ensure you’re not over or under-firing your appliance which can cause stress on the appliance and possibly premature failure.
When clocking the meter, you’re matching the actual input to what’s printed on the appliance label. Furnace’s ship with the orifices for the highest calorific value in the country being shipped to, so you could have the incorrect orifices in the appliance.
Step 1: Set the fuel pressure to what’s listed on the appliance nameplate. Do not go by what is printed on the gas valve. This valve can be used in multiple different pieces of equipment requiring different pressures.
Step 2: Make sure all other gas appliances are turned off, pilots and all.
Step 3: Have the unit running at the highest output (highest heat stage). If you have a 2 stage unit, get it going in 2nd stage.
Step 4: A stopwatch is needed for this step, so break out your cell phone. Using the smallest test dial on the gas meter, usually 1/2 ft³ (imperial meter) or .01 m³ (metric meter), track how many seconds it takes for one full revolution.
I like to cover half the test dial on an angle using a debit or business card. Be sure to not move your head and change your line of sight.
Take at least 3 readings, add them together and divide by the number of readings taken to get your average. The smallest test dial does not always move smoothly and can give false readings which is why you get an average.
Example:
Revolution 1 – 16.84 seconds
Revolution 2 – 23.23 seconds
Revolution 3 – 20.79 seconds
Revolution 4 – 23.18 seconds
Total = 84.04 seconds / 4 readings = 21 seconds average
Step 5: Complete the calculation to get actual input using either metric (.01 m³) or imperial (1/2 ft³) calculation depending on the type of meter used.
Metric meter:
We will be using the 21 seconds average from above for the seconds of a revolution.
The actual equation to solve for is:
((# seconds per hour x dial size) / time in seconds) x calorific value x 35.31
- seconds per hour is always 3600
- seconds per turn is 21 seconds from the above example
- dial size is .01 m³
- 35.31 is used to convert m³ to ft³
- The calorific value for my area is 1000 btu. You will need to contact your utility for their average. Generally, it’s between 1000 btu to 1075 btu
3600 x .01 / 21 x 1000 x 35.31
= 60,531 btu
Imperial meter:
We will be using the 21 seconds average from above for the seconds of a revolution.
The actual equation to solve for is:
((# seconds per hour x dial size) / time in seconds) x calorific value
- seconds per hour is always 3600
- seconds per turn is 21 seconds from the above example
- dial size is .5 m³
- The calorific value for my area is 1000 btu. You will need to contact your utility for their average. Generally, it’s between 1000 btu to 1075 btu
3600 x .5 / 21 x 1000
= 85,714 btu
Step 6: Now if you have to adjust high fire plus or minus 10% of the nameplate manifold pressure to get it to the appliance input, the orifices will need to be changed. If you need to go above 10%, the orifices are undersized. If you need to go below 10%, the orifices are oversized.
You will need to access the gas output on the gas valve to measure the gas going to the burners. These are the two most common connections in residential and light commercial gas valves.
The 3/32 hex screw only needs to be cracked and not fully removed. From there you can use an adapter to slide over for easy access. I like the fieldpiece RMA316.
A proper setting for manifold pressure is necessary to ensure the correct flame characteristics and appliance operation.
If the gas pressure is too high on the outlet side of the gas valve, you may cause too much heat which can shut the furnace down. It can also cause flame sensor failure or make the flame lift-off the burners or for the flame to actually touch the first pass of the heat exchanger and cause a crack. Too high of pressure on the inlet of the gas valve can cause the gas valve to not even open or even fail.
Too low of gas pressure can cause a no heat or intermittent heat failures, sooting, flame sensor lockout, or delayed ignition.
Temperature Rise
The temperature rise is performed to ensure proper airflow across the appliance heat exchanger. Moving too little airflow across the heat exchanger will cause the high limit safety to stop the call for heat to protect the heat exchanger from permanent damage. In this case of a high limit, the flame would be disabled, and the blower would continue to run to cool off the heat exchanger. After a set amount of time, the high limit would close and the call for heat would start again.
When setting temperature rise, the lowest end of the temperature rise will give the appliance the greatest efficiency, but will not necessarily be as comfortable for the customer. Most appliances have a temperature rise range of around 30 – 60℉.
The heat produced is going to go either to the space being heated or out of the exhaust venting. A 50℉ change in the stack temperature is somewhere around a 1% savings. Setting the appliance temperature rise somewhere mid-range will provide more comfort for the customer as well as leave a little room for the filter to get dirty.
Combustion Test
The next step is to do a combustion test. Please see the combustion testing article here.
It’s important for the condensate trap to be primed with condensate or water prior to completing a combustion test. Either pour water into the trap or allow the furnace to run 10-15 minutes.
If the trap is not primed, air will be pulled backward from the condensate trap instead of through the heat exchanger. Air pulls from the least restive path. We need the water weight in the condensate trap to allow the appliance to pull air from the correct location.
Combustion air supply and ambient CO
Ensure ambient CO is 0 and the combustion air supply is correct. Please see the previous module on combustion air supply for additional information.