Full turndown is crucial for efficiency. Poor turndown increases cycling, and can make it impossible to maintain the steady low steam pressure that steam systems need.
The goal is to get the lowest low-fire possible, low enough that the boiler never shuts off on pressure. This enables the boiler to maintain a steady head of low pressure steam for the entire heat cycle.
Sometimes, reducing high-fire allows low-fire to go even lower. This is worth doing. Almost all boilers are oversized, and derating improves their operation.
Check the burner nameplate to get the boiler’s designed low- and high-firing rates.
It’s a good idea to download and check the burner manual to see if you can get low-fire even lower. You can check this ahead of time if you know the burner model.
Gas Burners: Clock the gas meter while the burner runs on fixed fire. To use the Turndown Calculator, time how many seconds it takes for the meter dial to complete one rotation. Or you can let the meter run for several rotations, then calculate the firing rate using this formula: (Total Cubic Feet) x 3,600 ÷ (Total Seconds) = Mbtu/H.
To get an accurate reading on rotary gas meters, you must keep your timer running for several dial rotations, then do the calculation for the total reading.
Pressure-atomized Oil Burners: Make sure there’s a good liquid-filled pressure gauge reading the oil pressure to the nozzle. Install one if necessary. Read the oil pressures, then use the Turndown Calculator (or a nozzle chart) to determine the firing rates.
Air-atomized oil burners: It is not possible to directly verify the firing rates on these burners without installing an oil meter. Instead, confirm that the oil metering device (pump or valve) is going through its full range of motion. On an IC burner, read the metering pump model and pin size, then use the Turndown Calculator to determine the firing rates.
Tune the burner to achieve the target low-fire rate. If you can’t get low fire to be low enough, check things you normally take for granted. Does the gas regulator have the right size orifice for the actual inlet pressure? Is the oil pressure to the nozzle right? Is the nozzle (or nozzles) the right one? Was the metering pump re-pinned?
If you start to get flameouts at low fire, check the draft. It should be slightly positive at the furnace, and steady. If draft is excessive even after you adjust the dampers, you may need a motorized draft control.
On some burners you can lower the firing rate even more. For instance, on a Carlin 702 you can put in the smallest pair of nozzles that the burner supports. On a Webster JB1 you can adjust the gas firing rate through a wide range.
There’s no downside to derating. Most boilers are much larger than they should be. Derating them improves combustion efficiency, reduces cycling, and helps to dry out the steam. Derating also makes it possible to hold the very low steam pressure that TRV’s need, rather than continually cycling off on pressure.
Now that you've done the work that enables the system to run at very low pressure, it makes sense to install a pressure control that's accurate at low pressure. One option is a Vaporstat. Not only do vaporstats facilitate low-pressure operation, they prevent lazy technicians from boosting the steam pressure.