This is a write up of a problem I had with the Gaggia TS.
Each morning, I turn on the machine, and allow it to heat up for half an hour or so. The pressure gauge would rise to the normal 1.5bar level, and happily stay there.
I would then draw a shot to make a cappuccino.
Next, I would purge the steam wand – and very little steam would come out. At the same time the pressure gauge would immediately drop to zero and the water in the boiler would start heating. Once it was back up to pressure it would then work completely normally all day.
This puzzled me for a while. My first thought were that it could be the Pressure Stat having dirty contacts. This would mean that it might make initial contact to heat the boiler, but then refuse to make a good connection to maintain the heat. This was dismissed, as the pressure gauge showed pressure, so there must be pressure in the boiler, right ? Wrong.
There WAS pressure in the boiler, it just wasn’t there anymore. My thoughts were that, in the morning I would I switch the machine on and the pressure switch would see no pressure in the boiler and close its contacts to bring on the heater. The pressure would then slowly rise and the gauge needle would register this correctly.
When the boiler was at the correct pressure (1.5bar on my machine) the Pressure Stat contacts would open and turn the element off. For some reason, as the boiler naturally cooled, the pressure reading did not fall and the Pressure Stat did not cut back in to maintain the boiler pressure and hence the temperature.
The fact that the gauge showed 1.5bar, but opening the steam valve caused it to immediately drop, and that the boiler heats normally when the pressure gauge shows less than 1.5bar, and stops at 1.5bar, shows that the gauge, pressure switch and steam wand are operating normally – they just aren’t reliably connected to the boiler.
A scale plug in the boiler outlet would allow pressure to pass from the boiler into the pipework feeding the gauge, pressure switch and steam wand, but would prevent the high flow required to steam the first milk in the morning.
But wait, it works fine for the rest of the day? The scale must be a moveable plug, which once moved by opening the steam valve in the morning then allows the machine to work fine all day. When the machine cools at night, the plug moves to block the system again.
Below is a picture of the connection from the boiler to the steam wand, pressure gauge and pressure switch.
Far fetched? A descale cured the problem. And I have had question from a reader of my blog with the same problem. Hopefully a descale will cure the issue for them.
Update: I have the same problem on another machine. My Bezzera Ellisse is exhibiting exactly the same symptoms, though this machine has separate connections to the boiler for the steam wand, pressure switch and pressure gauge.
It looks like the vacuum breaker has been the problem all along with both these machines.
When the machine is turned off, the pressure in the boiler falls as the water cools. The vacuum breaker is designed to let atmospheric air into the boiler as this happens, meaning that when the boiler is cold, it is at atmospheric pressure. The boiler contains cold water, with a layer of air above it.
Heating the boiler creates steam, which is what we want for our steam wands. As the steam pressure builds, it eventually forces the vacuum breaker closed. You should be able to hear a sudden rush of steam, before the valve abruptly clicks shut. The boiler now contains hot water with a steam blanket above it. The pressure in the boiler then rises until the Pressure Switch opens.
The problem is, if the vacuum breaker sticks shut, when the water cools, air is not admitted via the vacuum breaker. It will enter slowly via the steam taps and perhaps pipework joints. As the water heats, we get hot water with a blanket of hot air above it. Opening the steam valve releases this hot air and the boiler pressure rapidly drops to zero. The hot water is not at boiling point, so the steam is not replenished.
The cure is to descale or replace the vacuum breaker (seen in the centre of the photo below of the internal pipework of the Gaggia TS).
On the Bezzera Ellisse, the vacuum breaker is part of a combined vacuum breaker and pressure release valve assembly. This unscrews (obviously after releasing all pressure), using a 32mm spanner. An adjustable spanner just fits if the rear panels are removed.
Inside the valve is a stainless steel ball bearing which is lifted by the steam pressure to seal the valve. It should be heard to rattle around when the valve is shaken. If not, shock it loose by banging the valve down on a hard surface.
I chose to descale the valve to remove any possible scale causing the valve to stick. Replace using PTFE tape to seal the thread. Do not over-tighten.