By George Lanthier
Last time we looked at setting the damper motors and the servicing of the linkage on commercial burners equipped with a damper. In this article, we’ll look at the controls that make these burners operate. When used to their full abilities, they can save a customer 20 percent or more on their oil bill. It’s better that you come up with this for an existing customer, rather than the competition, or you lose an account and lots of credibility.
I’m going to start with steam since there are still plenty of these jobs out there, and finish with a hot-water boiler and a warm-air furnace, or as they prefer to call them when they get this big, a make-up heater.
I’m going to use the simplest burner to understand for most of us and that will be one with two separate oil valves. Although most of the OEMs out there have produced burners with damper motors, things are changing and Beckett’s CF1400 (see Figure 1) now uses a mini-servo style motor. Although both Carlin and Beckett produce only one burner each using Suntec’s two-step pump, in my opinion a burner like Carlin’s 702CRD with the two-step pump (Figure 2) is a lot easier to work on and setup once you know how everything works.
OK, first things first. If the job was put in by a professional heating man you’ll probably find the pressure controls sitting on the top of the boiler like they are shown in Figure 2. The reason for doing this is just ‘good industry practice,” and when I walk in, I can tell what the control is just by its height. It’s easy when we all follow the same rules.
On the left are my normal boiler and burner operating controls. The ‘low-fire switch” is what is actually going to make my burner cycle, and the ‘operating limit” is my high-limit operating control. On the right is my ‘safety limit.” This is a manual reset device and on a separate siphon because all good steam control men know that stuff happens and without this control I’m on my way to building a bomb. That separate siphon and location may also be required by the OEM or code.
The safety limit is a backup control for my operating limit. If the normal high-limit setting is reached and, in fact, the boiler pressure goes beyond this control, it prevents my boiler from becoming unsafe. Whenever one of these controls is properly set and has to be reset, it normally means the operating limit has failed. You should always investigate and never leave the boiler without all safety controls operating properly.
OK, so let’s put some pressure readings on these controls and firing rates on the burner. My burner pump is set to 150 psi and I have two nozzles. My low-fire nozzle is a 6.50-gph nozzle that will actually put out 7.80 gph. My high-fire nozzle is an 8.50-gph nozzle. With both nozzles pressurized and on high-fire, the burner actually puts out a high-fire rate of 18.00 gph.
I’ve also determined that to heat the building I need about 6 psi of steam at my outside design temperature, but I only need that 6 psi and the 18.00 gph firing rate when it’s 0(F in Boston, what about the rest of the winter? Well, that’s where modulation in any form whips the pants off of single-stage firing. I know, I’ve been able to do step-firing in my own home and it saved me 15 percent over an exact equal burner.
OK, so for about 75 percent to 80 percent of the winter I may be able to heat this building with the low-fire rate of 7.80 gph and about 4-psi steam. So, on to my control settings. I’m going to set my controls as follows:
The safety limit will be set to shut the burner off at 10 psi.
The operating limit will be set to shut off the burner at 7 psi and turn it back on at 6 psi.
The low-fire switch will be set to go to low fire at 4 psi and go back to high fire at 3 psi.
With the controls set like this, let’s see what happens. On a normal startup, the burner will fire off and immediately go to high fire to kick the system up and get things moving. As soon as the burner is able to drive the boiler to 4 psi, the burner cycles down to low fire and continues to heat the building at the lower rate, or until the thermostat is satisfied. If the demand stays on and the pressure drops to 3 psi, the burner will kick back to high fire and this will continue until the building is heated.
But, it’s very cold out, there is a big demand and, in addition, there are traps and valves and all kinds of other things at work here and the pressure in the boiler exceeds 7 psi. No problem, the operating limit does what it is supposed to do and the burner goes off. As soon as the pressure drops below 6 psi the burner can start again if it has to. If we go over 7 psi and the operating limit fails, the safety limit will shut down the burner at 10 psi and a service call is on the way.
With a high-pressure system you can change those numbers to hundreds and that will save your dry cleaner, laundry, bakery or processing plant a ton of dough; well maybe just the baker. I would set a high-pressure plant at an example like this.
The safety limit will be set to shut the burner off at 170 psi.
The operating limit will be set to shut off the burner at 160 psi and turn it back on at 150 psi.
The low-fire switch will be set to go to low fire at 140 psi and go back to high fire at 130 psi.
For a water or air system you can pretty much do the same thing, and here are some examples of how I would set a water boiler and the make-up heater.
The safety limit will be set to shut the burner off at 205F.
The operating limit will be set to shut off the burner at 190F and turn it back on at 180F.
The low-fire switch will be set to go to low fire at 175F and go back to high fire at 160F.
In the case of a warm-air application, the controls could be set as follows.
The safety limit will be set to shut the burner off at 200F.
A standard warm-air fan and limit will be set to shut off the burner at 190F and turn it back on at 165F, and that’s a set factory differential.
The low-fire switch will be set to go to low fire at 170F and go back to high fire at 150F.
In all cases you should consult the manufacturer of the boiler or furnace and never do anything outside of their recommendations.
It’s also important for me to mention steam controls and Honeywell pretty much dominates this market. Although I’m not crazy about their PA404-type control for residential steam and would prefer to use the L408 and L608 type, the PAs work pretty well at commercial levels. If you do need sensitive controls, though, use the L404 or L604 types and check out their L4079 and BCS7700 controls if you do a lot of commercial work. One other thing, too, don’t forget that the limits must both be ‘break on rise” and that the low-fire switch must be a ‘make on rise” type control.
George Lanthier is the owner of Firedragon Enterprises, a teaching, publishing and consulting firm. He is a proctor and trainer for the industry’s certification programs and is the author of nine books on oilheating and HVAC subjects. His company specializes in programs for both OEMs and in-house training of service technicians for service firms. He can be reached at 132 Lowell Street, Arlington, MA, 02474-2756. His phone is (781) 646-2584, fax (781) 641-7099 or e-mail