Water deposits in thermostat housing?

So how do I prevent this from happening?

Stop using house water when mixing with antifreeze and use distilled water?

The thermostat was even worse. A replacement has been acquired.

I already stuck a snake camera into the radiator and it looks spotless compared to the housing.

Distilled water will help, but you still have a brass thermostat in an aluminum housing with an elecrolitic solution. It will always try to be a battery.

With dissimilar metals in the t-stat housing, the corrosion inhibitors in coolant are more critical than elsewhere in the system.  So if the coolant is old and the inhibitors are wearing out, that's where you'll see it first. 

Distilled water may help, but changing the coolant every few years is likely to make a bigger difference. 

OK Thanks and sadly the antifreeze doesn't last a season in this car.

I'll pick up some distilled water before the engine goes back in.

There are schools of thought on the distilled vs tap water.  Some people claim that distilled is the only way to go because distilled water doesn't conduct electricity... but then you mix it with antifreeze and it does conduct electricity.  Even if you put straight distilled water in there it will be an electrolyte in 5 minutes as it dissolves all kinds of minerals out of the metals in the block, head, and radiator.

Distilled water is like $1/gallon, and I also spring for name-brand coolant.  Forget electricity--I just don't want to be adding minerals to my cooling system.  I recently did the water pump / thermostat on my 200,000 mile '91 Miata after 10 years without changing it and it looks clean.  My bathtub, on the other hand, looks a lot more like your thermostat housing.

Distilled water may corrode the metal faster.  Water is a really, really good solvent, distilled is "empty" water that sucks up metal in the form of corrosion.

I always use premixed coolant.  No matter what the water is, in theory the corrosion inhibitor package takes it into account.  All of the different coolants exist because water is different in Germany is different in Japan is different in Michigan.

That said, some vehicles seem to have made their aluminum cooling system parts out of some seriously cruddy alloys Jeep.

In reply to Pete. (l33t FS) :

Yeah, not buying all that.  Water is a really good solvent yes, but tap water with the minerals isn't "full".  It's just as good a solvent, but now with extra chlorine!  Yes, all the different coolants are different, but they all say "mix with distilled water".  You're right that the pre-mixed stuff avoids these questions, and that's a perfectly fine way to go.

In reply to Berck :

All that schmutz in the thermostat housing isn't stuff falling out of solution from the water.  Water doesn't just release its prizes willy-nilly, it will stay in solution.  That junk in the thermostat housing is corrosion, because the coolant went acidic.

This is where the voltage comes into play.  If you can dip a voltmeter into the coolant and measure over .250v to ground, that is time to change the coolant, as it is turning into electolyte.

The other thing that causes problems is living with a coolant leak, allowing air into the system.  This also includes things like using worm clamps on the hoses (creates a path for air to get in, full circle spring clamps do not have this problem) and running a system with an overflow bottle.  There will always be a small amount of oxygen in the water, this will corrode things and then stop when the oxygen runs out.  If you keep supplying oxygen by allowing air into the system, it allows corrosion to continue.

I knew someone who did a science experiment with some steel, an aquarium, and a brine solution.  The metal stopped rusting after a couple days because the oxygen in the water was depleted.  He added a little fish tank bubbler and it started rusting again.

I'll grant you all of that!  And I also think all of that argues for distilled water over whatever might come out of your tap.  Antifreeze manufacturers all say to use distilled water, presumably because they've optimized for it.  Guessing that the random minerals, chlorine and pH in your tap water is better is a wild guess that's almost certainly wrong.