Modulation mode on Steam Boiler not properly keeping desired pressure

jgordon

So glad to have come upon this website, and appreacite all the good information that comes of it.

Having an issue the modulation feature on a Weil McClain Delta 88 hooked up to a 3hp Power Flame burner that were installed in 2016. The Honeywell Modulation Control (see photo) has been recently replaced as well as the brain for the burner and while the system often hums along at the designated 2lbs, out of the blue it calls for more gas and the pressure jumps to 4lbs where it shuts off usually on low water or high pressure safety.
As a mere property manager I’m scratching my head, as we replaced the only two items that would affect this, and hoping from some guidance from you with much more experience.
Thanks in advance.

ethicalpaul

Honeywell Modulation Control? Is that a new name for a Pressuretrol?

Jamie Hall

At least the one which i can enlarge enough to see is a Pressuretrol. Interestingly, it is hooked up with both the make on rise terminal and the break on rise terminals connected. I think we need to know a lot more about this setup.

Are both controls pressuretrols? Are they both wired with all terminals connected? What are they set at (the one i can enlarge has a truly odd setting)? Is this a two stage power burner they are controlling? How is everything wired together?

JUGHNE

Is this truly modulating or simply hi-low?

What does the control panel look like?

SlamDunk

If it is truly a modulating burner with new brains, it may be your PID loop needs tweeking. You are overshooting your setpoint. Try easing up on proportional gain. Maybe ease up on integral gain too.

EBEBRATT-Ed

That is an L91B or equal Honeywell modulating pressure control. It does not cycle the burner but operates the burner mod motor which controls lo-high fire. Called 'full modulation" the burner can be at low fire, high fire or anywhere in between. The burner controls are set up so that when the burner starts it is always on lo fire. After the main fuel valve opens and the burner flame is established it "modulates" to adjust the size of the flame based on the steam pressure. The control operates on 24 volt 3 wire series 90 Honeywell control which is based on the "wheatstone bridge" balancing with resistors.

@jgordon

Piping to the L91 clear of debris and gunk

Mod motor and linkage not sticking, binding

Check all wiring for loose connections

Sometimes if the differential on the L91 is set low the control will "hunt" almost constantly moving the mod motor back and forth. This causes a hole or a bad spot on the resistor in the L91 control which can cause erratic operation. Used to be able to buy the resistors and change them without changing the whole control. Might be something to check for. Then try to widen the differential if you can

Jamie Hall

Odds are that if it is some form of variable resistor or potentiometer that DigiKey will have something which can be used, if it turns out that's the problem.

And thanks for the description, @EBEBRATT-Ed . Learn something new and useful every day here!