How to Diagnose Your Boiler Fill Issue
/By Michael Teahan
Funny Story . . .
While getting this article together, I had a bit of a delay that was quite ironic.
I am a partner in a small neighborhood coffee bar in Southern California with a three-group lever machine that was, to be fair, in need of a serious overhaul. I had acquired an older two-group machine and decided to overhaul it completely and throw lots of modifications at it in the process: LED lights above the drip tray and on the back, a complete rework of the color scheme, dual PID temperature controls, and group head temperature sensors.
With a week’s worth of work yet to do, the lever machine was down to one group, and I had to get this replacement machine installed immediately. Unfortunately, the float and magnet autofill wasn’t working quite as it should. I didn’t have the factory magnet but had several same-sized magnets I used for other projects and thought it would be a perfect fit.
Instead of getting this article in on time, I spent the better part of three days sorting out why the float switch worked perfectly on the bench and failed every time when actually installed on the machine.
Let’s just say that this article is ironically timed.
Breaking things down
Like most problems with espresso equipment, the causes are ridiculously logical. The problem is knowing where to start and the connections between the components. When boilers overfill or fail to fill at all, there are a few observations you can make at the outset that will point you in the right direction.
Unless you are using rare earth magnets, but that’s another issue . . .
For this exercise, I’m going to break it down into a couple of sections and a couple of subsections each.
Logical ones.
It doesn’t fill: which could be mechanical, electrical, or Homer Simpson stupid.
It fills too much: same scenarios but the Homer Simpson “Doh!” is less likely.
The boiler isn’t filling as it should
Electrical issues:
When there is an electrical issue with a boiler fill, it's usually all or nothing: it either works or doesn’t. The control board simply sends a signal to the fill solenoid and the pump at the same time. It’s never on the same leg, which means that separate sets of points connect the pump and valve. This is because the fill valve would open every time you pulled a shot. This also helps diagnosis.
If the pump comes on when calling for water but the solenoid doesn’t open, there are three possible issues.
The relay on the board has a burned set of points. Since many fill circuits use the same double pole relay, it’s possible that one of the contacts has failed. However, since the pump draws much more current than a coil, this is a long shot.
The solenoid coil has failed or shorted. A closed short coil can fry a relay so that it could be related. The bottom line is that the solenoid is the first place to look if the pump comes on.
The third likely possibility is a sticking relay. While it’s technically a mechanical issue, it is an electric component to hedge a little. Fill valves typically use Viton and not ruby seals and are a little less likely to stick in a closed position, but the scale can prevent a valve from opening. Sometimes a good whack on the valve while the machine asks for water can temporarily fix the problem.
Checking the valve requires a decent voltmeter. You should get about 1100-1400 ohms on the coil, but the numbers will vary with voltage. If the coil is stamped with wattage and voltage, use Ohm’s law to find the proper resistance. No matter what, it is bad if the reading is “0” or in double digits. Checking the leads to the coil is a bit different. The voltage will vary by machine, but one of the legs will always have voltage to ground on North American 220 volt machines or have continuity with the standard wire connecting all the other valves. It’s just the one leg that comes on to fire the coil. If you aren’t getting voltage to the coil when asking for water, the problem goes back to the circuit board.
If the machine doesn’t call for water at all
If removing the probe wire doesn’t initiate a fill cycle at all—no pump nor solenoid coming on—the problem is rooted on the board.
If an automatic machine still doses water from the groups, the fill circuit on the board may have failed.
Since most fill circuits aren’t replaceable, a new board is going to be necessary. However, if the board is ridiculously expensive (aren’t they all?), you may be able to fit a dedicated autofill box to the machine and separate the fill circuit from the mainboard.
Most semi-automatic machines will still brew coffee even with inadequate water level control since those functions bypass the autofill board completely. The bottom line is that if the machine doesn’t ask for water when it should, the customer will buy something.
Important tip: when checking the probe wire, make sure you aren’t holding it in your hand. Set it on a clean, dry towel or let it go not to touch anything. Your fingers alone can create enough of a ground circuit that the autofill sensor thinks it has grounded to water. The machines are designed to sense distilled water, and your body is enough to fool the machine.
Second important tip: you may want to remove the probe wire at the box to ensure that it hasn’t shorted to the frame on its path back to the box.
If removing the probe wire causes the filling cycle to start, the board is probably OK, and the issue is the probe. Water in the boiler isn’t the only thing that can close a circuit. The Teflon insulator around the probe can, over time, develop a film that can short the probe to the boiler enough to fool the autofill sensing circuit. While you could replace just the insulator, replacing the whole probe makes more sense.
Mechanical Issues:
The Homer Simpson diagnosis is that someone turned the water off. If this is the case, you likely got called out for something else.
Many manufacturers will put a mesh screen before the fill valve to keep crud from sticking to the valve, particularly in machines that use a smaller orifice to regulate fill rate rather than the solenoid itself. The screen that protects the valve is also prone to clogging. If you can feel the solenoid click when the machine calls for water, you are looking for an obstruction somewhere.
Calcium likes to form in two conditions: a change in temperature and/or a change in pressure. This is why scale forms in group valves, jets, and cold-water injectors.
Cold water entering a hot boiler is another as the cold water enters the boiler, calcium forms around the fitting. Often, this fitting includes a small pipe to preheat the water as it enters, so look for a blockage at the end of the pipe.
The boiler fills too much
Electrical issues
If the filling cycle is coming on, one of two things are happening
The board thinks you need water. If there is a short delay after turning on the machine before the auto-fill circuit kicks in, the board is working correctly but thinks the level is low.
The board needs to pass enough current to ground to close the circuit, and if the resistance is too high, it will ask for water. This can result from carbon or calcium accumulating on the probe creating resistance or even a heavily scaled boiler. Since the boiler's metal is what the water is using to pass current to the ground, a layer of calcium is enough to fool the fill circuit into thinking the machine needs water.
If the filling cycle starts immediately when the power is turned on, the problem is likely the relay on the board. The reason is that the board needs a couple of seconds to think about whether it needs to fill the boiler. A slight delay is built-in to prevent the board from rapidly coming on and off from waves in the boiler, even during a normal fill cycle. The water's surface in the boiler is never smooth as glass, it’s always boiling away or moving as water fills the system, and the delay is to keep it from rapidly cycling. If the fill circuit is immediate, the relay on the board is likely welded in the on position. You will have to check the coil to make sure this isn’t the source of the weld.
If only the valve opens immediately without activating the pump, the coil may have shorted to ground in such a way as to activate the coil. More often, this kind of short keeps the valve open after the board ends the filling cycle.
Mechanical issues:
If the boiler fills even though the board isn’t calling for water—which means that neither the solenoid nor pump is activated—water is entering the boiler from somewhere else.
A leaking solenoid valve can result from a bad seal or some crud stuck between the seal and the seat. Since most valves are Viton, the soft surface can hold onto that crud for a while. You may be able to avoid pulling the valve apart if you can drain the boiler and force an autofill cycle. I have been lucky more than once where the water flow through the valve clears the crud and sends it on to the boiler.
A leaking manual fill valve can also overfill the boiler. Since these are rarely used, it’s one of the less likely sources, but it’s a possibility on older machines.
The worst-case scenario is a breach in a heat exchanger. If you drain the boiler and disconnect the lines from the manual fill and solenoid valves, water will come from somewhere with the water on and machine off. If it comes from either of the fill valves, you have found an easy fix. If it starts coming from the boiler, the heat exchangers are breached either by a crack or a pinhole. If the exchanger is breached and isn’t replaceable—you have to replace the boiler.
My more complicated issue
The machine I was working on this weekend uses a float and magnet to trigger a reed switch to call for water. A rare-earth magnet is a bit more powerful than most, apparently. When installed, it was passing enough of the magnetic field to the low carbon stainless steel studs holding the float assembly in place to prevent the reed switch from releasing when the magnet moved away from the switch. The studs weren’t there when testing off the machine—which is why it worked perfectly.
Installed, however, it was a disaster.
The solution was positioning a different reed switch in a slightly different position, less vulnerable to the field effect of the magnet on the studs but close enough for the magnet to work.
It only took me six hours over two days to figure out.