So your budget is tight and you are pressured to make some energy savings somewhere. Why don’t you try some control adjustments? “Like that will make any major change,” you say. Now, that all depends on how your boilers and processes are set up right now. Usually as long as the boilers work, who really cares? But that attitude is precisely why boiler controls deserve a closer look.
Experts in the energy conservation field usually recommend that the highest efficiency facilities can hope to achieve comes when the boiler produces continuously what the load demands and also changes to accommodate slight variances in the load. This requires flame modulation, a feature many medium- to large-sized hydronic (hot water) and steam boilers have as standard equipment.
Most will have modulation control as well as the on/off controls. Energy conservation begins by noting where the boiler turns on and off and where the modulation controls operate. Simple changes to those controls may enable greater efficiency, reducing energy use and leading to lesser impacts on the environment and the bottom line.
Most steam boiler installations are set up to operate as follows:
- The safety valve is set to blow off at 125 psi and reset at a few pounds below that.
- The high limit control is set to cut the boiler at about 120 psi and requires a manual reset once it has tripped off.
- The boiler regularly shuts down at about 115 psi and restarts at 100 psi.
- The programmer will start and run through the start-up cycle at 100 psi.
- The modulation control will be at “High” fire at 103 psi and be at “Low” fire at 112 psi.
Hydronic boilers operate similarly with the water temperature as the controlling parameter instead of steam pressure.
This is great for the installer because it will show all the controls operate at their minimum and maximum settings and function when demonstrated to the customer at start up. For energy savings, it is regrettable because the owner/operator rarely changes those settings.
Identifying the inefficiencies
There is barely enough time in this cycle to establish internal boiler circulation before the burner is off. Internal boiler circulation, once established, is important as it aids in the removal of energy from the hot flue gas and into the boiler water. That tends to lower the stack temperature.
Boilers set at high-fire, low-fire, and shut-off cycles of the boiler temperature create problems in hydronic systems. This on/off operation interferes with larger zone control loops by creating thermal cycles where there should be a steady state. Such disruption in many systems will create unexplainable no-heat or excessive-heat situations in sub or sub-sub zones. These zones react to disruption like pendulums of varying lengths, swinging with different rates. Some react in an additive cycle under certain conditions, and others in a subtractive cycle. These cycles can range in duration from two to six hours. The best way to defeat this is to have a steady temperature supply so that zones will not experience a mixture of supply and heat load temperature cycles. Rather, they will experience only the heating load and will operate as they are designed, controlling only the actual load conditions.
A high stack temperature of 200˚F hotter than the steam or hot water produced usually indicates considerable energy loss. This energy goes past the heat exchange surfaces and up the stack never ever to be recovered.
Under light or heavy loads, this performance changes very little. Under a light load, the overshoot almost trips the high limit. Under a heavy load, the boiler pressure may get extremely low before the programmer’s lighting sequence releases the main burner and allows modulation. The boiler pressure or hot water temperature curves always have a cyclical action and are far from that steady ideal discussed earlier.
Corrections for greater efficiency
Once a few changes are made to the boiler settings, greater efficiency is immediately realized. Under the new settings:
- The safety valve remains as before at 125 psi (hydronic boilers may be at 30 psi).
- The high limit remains unchanged at 120 psi (hydronic high-limit temperature may be at about 210˚ F).
- The burner “Off” point also remains the same at 115 psi (hydronic service would be at 200˚ F).
Here is where the change takes a grand departure from the original set-up:
- Readjust the burner “On” point to as close to the burner “Off” setting as possible.
- Some boilers may have been fitted with an inexpensive pressure control that may not have a close differential capability. In such cases, the control may need to be replaced with one that has a separate adjustable differential screw with a minimum of 2 or 3 psi setting (or a 3˚ to 5˚F differential for hydronic boilers).
- The modulation control may have to be readjusted for it to function in the proper place.
Where has the pressure (or hydronic temperature) dropped with these new settings? On extremely light loads, one will see the damper at low fire, and the boiler will be shutting down and starting up because the load is below the low-firing rate. On light loads, expect the modulation to be on low or quarter fire with a continuous flame. On heavy loads, the pressure (or temperature) would have dropped faster to a half or three-quarter fire position initially and still operate with a continuous flame. Only on cold start-up will modulation at high fire occur for very long.
With this continuous flame the modulation and the burner damper will stay in one position for several minutes and jerk slightly to match any small change in load. A large change in load will see the burner damper move in a series of tiny jerks.
The boiler now has achieved the steady state of load matching that is the most economical fuel use as possible. Internal boiler circulation is established and remains steady, which lowers the stack temperature by extracting the most heat from the flue gas. Operating at the lowest firing rate possible makes the heat exchange surfaces seem larger to the flame. In other words, the boiler operated at the lowest BTU input per square foot of heating surface possible while still maintaining proper operating steam pressure or hydronic temperature.
Stack temperatures, however, should be maintained high enough to prevent condensation of the flue gas on its way up the chimney and prevent precipitate downwind from the top of the stack. Most boilers will have no problem doing that, but there may be some that create this problem.
The burners of older boilers were only capable of a 3:1 turndown ratio. At full or “High” fire, the boiler would operate at the nameplate rating. A 3:1 turndown ratio simply means the boiler is capable of operating at a third of the nameplate rating. Put another way, a 6 million BTU per hour boiler will operate down as low as 2 million BTU per hour on low fire. Below that load it will inefficiently cycle on and off.
Late model burners have been designed to operate at a 5:1 turndown ratio. A higher turndown ratio allows the boiler to operate with a continuous flame down to a fifth of the nameplate rating. Our 6 million BTU per hour boiler can now operate down to the 1.2 million BTU per hour level on low fire with a continuous flame before it has to cycle on and off. There are some new burner designs on the market that have a 10:1 turndown ratio in which case that 6 million BTU per hour unit will operate down to 600,000 BTU per hour on full low fire.
Many boiler manufactures offer an upgrade package to give an older boiler a higher turndown ratio. However, in any case, resetting the controls enables a facility to achieve the most out of its fired equipment. Depending on the way a hot water or steam boiler is set up, the bottom line is major fuel savings that can significantly reduce a facility’s carbon footprint on the environment.
New control settings