OT: Instant hot water heater spatters

enough for the bubbles to remain vapor. The spatter is a relaxation effect where an airspace accumulates until enough back pressure develops to break it loose and thereby cause the spatter effect.

How does this apply to cavitation? Cavitation bubbles are full of nothing, possibly a very low pressure from the water vapor, but the bubbles are not sustained once the very low pressure that created them goes away. You are thinking of bubbles that are created when high pressure is suddenly released. That is not cavitation. Cavitation happens because water is not compressible (or expandable) and under the right conditions there is not enough pressure to keep water on the surface of a propeller or similar object. The vacuum bubbles form and as soon as the pressure returns they collapse. It is the manner in which they collapse that makes them interesting. They simply can't continue to exist for very long, no different than a tin can sealed after heating collapsing when cooled, just faster.

Air is getting into the thing some how. Most of these things don't work under pressure. The way they work is:

The cold water feed (pressurized) runs through the counter top valve and then back down into the tank. The output of the tank runs back up and then out the spout at the counter top. When you open the valve, you are admitting water into the tank, where it displaces heated water, pushing it out the spout. When the valve is closed, the tank is just at atmospheric pressure.

Any leaks downstream of the valve are not under supply pressure and might even draw air in as the water flows. Its all cheap plastic tubing as well, so check for cracks.

If the thermostat is set too high, the water will boil right out the spout since there are no valves downstream of the tank. You should be able to see that when its just sitting there.

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Then there is a problem with the internal gaskets or seals, either way it becomes remove and (re)install. New or repaired is your decision.

?-)

Hard to see how it can leak air into the system without also leaking water out but there might well be a bellows somewhere to allow for thermal expansion when cold water is heated up in a closed system.

Our village hall one sputters a bit when it hasn't been used for a while. But it quickly in a few seconds calms down an behaves normally.

Another possibility is corrosion or something about the surface of the heating element allowing dissolved gas to come out of solution.

If you run a tumbler of water from the cold tap is it milky with dissolved gasses coming out of solution?

No, it's clean. We fill the dog bucket from there and that's around two gallons each time. Got big dogs :-)

Both are fed from the same carbon cartridge filter.

It might be hard to imagine this happening, but the fact is it does. A leak doesn't always mean you have water spraying all over the place.

A vacuum breaker is an example of this. They can "leak" air, but not water when working correctly. They can even do both at the same time. When they're broken they do all sorts of great things.

The heating tank isn't closed in many of these. It is at atmospheric pressure. The counter-top valve admits cold water into the tank. The tank output is always open through a tube to the outlet over the sink.

So its possible that any flow through the heated part of the system will result in lower pressure which will draw air in.

That isn't how the ones I am familiar with in the UK work. There is quite a complex piece of plumbing under the bench along the lines of:

mains water feed [non-return-valve] [pressure reducer] [expansion bellows] [pressure gauge] [waterheater] [emergency overflow pipe and/or bursting disk] [tap]

And fairly stern warnings with the kit that bad things will happen if there isn't sufficient length of pipe and/or bellows. Our mains water pressure is very high (especially so at night) and requires the full monty to work. You can get away with a simpler setup elsewhere.

There are also two main sorts of instant hot water. The ones where there is a reservoir of already very hot water boosted on demand and the other where an insanely high current heater takes cold water right up to the set point continuously. I am no longer sure which Joerg has.

I wish our VH had the latter since users sometimes run the hot tap wasting a tank full of hot water which then takes a while to recover. The sink is only 10' from the main 3 phase high capacity electricity distribution box but then it was installed by a plumber...

I'll disagree with the Wikipedia definition here. The implosion of cavities is IMPORTANT, but the cavity formation alone constitutes cavitation.

When the liquid is recently released from a pressurized water system, it can be so far out of equilibrium with atmospheric pressure that the cavitation bubble fills with dissolved air too fast to allow it to do the implosion.

So, cavitation combined with excess of dissolved gases can make for bubbles far in excess of what a noncavitating flow would produce.

If the element split open and exposed the wire you'd get brown's gas in the output that'd cause sputtering even when cold.

Please tell me what is your source for this information?

BTW, I don't consider Wikipedia to be a primary reference, just like Wikipedia. They won't allow you to quote one wiki page from another... That says something significant.

Ok, but not if it is unplugged, right?

See:

Typical fix is tear it out and replace it. They don't last long.

snipped-for-privacy@gmail.com prodded the keyboard

Its likely that there is a perforation in the heating element. Its essentially scrap now. Check the earth leakage, it should be 10M or more.

If it does NOT show leakage (electrical or water) you could try getting a little vinegar into it, let it sit for half an hour, then flush with water. If crud comes out, repeat until no more crud is produced.

Solubility of various salts decreases as the water gets hot, so salts precipitate out of even soft water in these heated tanks. It builds up over the years if not dissolved out. Vinegar works REALLY well at cleaning heated tanks.

Jon

I agree with the perforated heating element theory. I've had two heating elements in my home water heater do the exact same thing (years apart... not at the same time). Bubbles, steam, etc. When I removed the elements, the outer steel cladding covering the element was split, exposing about 2 nches of the resistance wire inside.

New elements solved the problem.

Dave M

J>

Anyone have information on the "instant" heaters under your sink for the _regular_ hot water, providing hot water long enough for the hot water from the water heater to arrive? ...Jim Thompson

Den torsdag den 12. februar 2015 kl. 23.34.02 UTC+1 skrev Jim Thompson:

I now you can get self regulating heat tape set for 50'C mean to be used to get instant hot water at taps that are far from the heater

I think it is mostly meant for add-ons, isn't most hot water installations made as a circulating loop so the water is almost instantly hot?

-Lasse