Does water boil inside the water heater?

Close....but more like 'Film Boiling

Not like film boiling at all. Film boiling occurs when a surface is so hot that it is not wetted. It is separated from the liquid by a layer, a film, of vapor. Spray water on a very hot skillet and watch the droplets dance on film boiling.

Nucleate boiling is correct. A steam bubble forms in the thin layer of hot water on the heating surface, the bubble is exposed to colder water outside the layer and collapses rapidly, making a sound. Hundreds of bubbles collapsing per second make noise similar to rain on the roof.

A heating element with more surface area of the same wattage would have a lower heat flux and be less noisy. A large 6000 watt 240 volt element operated at 120 volts would produce 1500 watts quietly, but might be hard to fit in a small tank.

The key here is you turn the water pump on and the boiling goes away. If you WH isn't under pressure either from the water pump or city water supply you will likely hear water boiling because of air within the HW tank allows this to occur when not under pressure.

Same Thing. I have a 1,500 watt kettle from around 30 seconds until just before boiling a knocking rattle can be heard. Then just before boil it goes silent.

jplante4 wrote:
It's called nucleate boiling. Small steam bubbles form on the heating element surface. The same thing happens on the fuel rods in a nuclear reactor. You don't worry about it until DNB - departure from nucleate boiling.

The boiling point for water at 40 psi is about 265°.

Close....but more like 'Film Boiling'

THough I can boil water at most any temp (Down to the triple point of water which Well let's not discuss that) at normal pressure or above (Water in the heater is above) water under 200 degrees WILL NOT BOIL Though as someone said minerals can cause some "Hissing" sound.

YOU would not want to open a hot faucet if it boiled.. Look at the Instant Pot Warning photo (That was not by the way an Instant Pot that blew up in the woman's face) That is what would happen. or the TPR would melt and niagra falls. I have seen that happen on a poorly designed gas heater. (Residential).

It's called nucleate boiling. Small steam bubbles form on the heating element surface. The same thing happens on the fuel rods in a nuclear reactor. You don't worry about it until DNB - departure from nucleate boiling.

The boiling point for water at 40 psi is about 265°.

Water boils at 212?F. The water heater relief valve is set at 210?F. If pressure exceeds too much or if the temperature gets too hot to wear it will begin boiling the pressure relief valve will blow. So what you are hearing is not boiling water. It's just water being heated.

The brass colored valve with the little lever on it on your water heater is a "pressure temperature" (PT or T&P or TPR) valve. If the pressure in the tank bet too high of the water temperature get too high (before boiling) the valve will open an let some hot water out thus allowing cold water in.

Yes, it boils. But only near the heat source, not a rolling boil. The water in contact with the heat source boils (changes to gas) and then almost immediately condenses, producing the sound. You can see similar in a pot on a stove, where the same thing happens long before the bulk of the water is anywhere near boiling temperature.

I use an electric tea kettle at home, with a temp indicator. It sounds like water boiling from stone cold.