Vendors typically recommend periodic draining or flushing of the steam boiler to address this. Doing a little back-of-napkin calculating about that, consider a home machine that makes two large lattes per day, pulling maybe 100 ml per day of pure water from the steam wand. In a steam boiler with 1.5 liters of water, at the end of the week you will have built up a mineral content equivalent to 2.2 liter of feed water inside the boiler, a concentration factor of about 1.5. So if your feed water had a hardness of 50 mg/l, the hardness in the boiler should have risen to 75 mg/L by the time you drain/flush it. So I think for most of us a weekly drain/flush is probably reasonable.
For machines that have a conventional hot water tap there is another way. It's easy to do and also fairly easy to calculate and adjust for your particular situation. The key is to use the hot water tap to manage your 'steam ratio'. The formula for steam ratio and boiler water concentration was provided years ago in Jim Schulman's Insanely Long Water FAQ:
Steam Ratio here is the amount of water that is removed from the boiler as pure steam divided by the total amount removed from the boiler. In the case of an HX or DB machine it would be approximately the amount that is removed via the steam wand divided by the total amount removed by the wand plus the hot water tap. (Assuming a conventional tap that simply pulls water from the steam boiler.)Actual mineral concentrations in the boiler, if saturation isn't reached, depend on the ratio of steam to water extraction and is as follows:
Boiler Concentration = Feedwater Concentration/(1 - Steam Ratio)
Knowing that, you can use a routine where you pull a pre-calculated amount of water out of the water tap before each steaming operation. If you're like me you'll want to always draw at least enough to trigger an autofill which may sometimes be a little more than the calculated amount. The calculated amount is based on how much concentrating effect you can live with, but in practice about 2 - 3 ml per each ml of water out the steam wand is good. You get diminishing returns going higher than that (see table below).
You will need to roughly estimate what your steamed water use is for your usual milk drinks. It will vary depending on the starting and ending milk temperatures, and the steam boiler temperature so I think it's worth doing a little measurement on your own machine. First get an idea how many seconds of steaming is required for your drink. Then get your pitcher nearly full of ice water, and tare that on a scale. Purge your steam wand into the pitcher, and then with the wand sunk deep in the water steam the ice water for the same number of seconds as for your drink. Follow that with a blast into the pitcher that simulates your wand cleaning routine. Then measure the water gain, which is your estimate of the steamed water use for that drink. Multiply that by 2 or 3 to get the amount of water you should draw from the tap before making this milk drink in order to keep your concentration factor down in the 1.3 - 1.5 range.
Sometimes an example is clearest:
I typically steam 120 ml of milk for a double small cappuccino, sometimes 240 ml for two split single caps. So I estimated my steam use for that 120 ml:
- I timed how long I typically needed to steam the 120 ml, which was 30 - 40 seconds. (I use a slow steam tip.)
- I filled my tallest pitcher within 1" of top with ice and water and tared that on my scale.
- I purged my wand into the pitcher, then with wand deep gave it full steaming for 35 seconds. Then gave it another few seconds of steam to simulate cleaning the wand.
- I weighed the pitcher and noted that it gained 20 g.