Air causes the whistling noise. I'm currently on a quest to eliminate the problem in my house (just had new water heater installed about a month ago, sounds like a choir of banshees when the water's running, toilets are flushed, washing machine or dishwasher are running,, etc.) When I figure out how to get the air out, I'll post a follow-up. In the meantime, at least you know what it is!
Not directly.
A TNP valve is a temperature and pressure valve found on a water heater.
if an element is constantly expanding and contracting (if you keep switching the hot water heater off and on again) all the time it causes cracks and over time the water leaks into the element causing it to short which means you have to replace it.
No and yes. No, you won't get more water pressure. To do that you'll need to find out why the water pressure is low. Perhaps flow restriction, perhaps a poorly adjusted water pressure regulator, perhaps low pressure from the well/city water. But an additional water heater will certainly give you more hot water. It would be like putting in a LARGER water heater. In most cities the water pressure required is about 20 PSI. Small city's do not add pressure pumps to their system but use a gravity supply. That is why most places there is a pressure problem either build their water tanks taller or erect them on a hill.
Install a expansion tank to absorb the pressure caused by expanding water as it is heated.
It's a sign that the blower motor is going to give up. It could sound like that for a whole year before it does completely.
A high pressure will be able to push the coolants into the heater cores.
You prob have to replace the heater core
The heater motor is failing.
Thermostat is not allowing the engine to warm up enough to produce heated coolant to the heater core. Heater control valve is faulty or not opening. Low coolant level; coolant level is insufficient to pump into the heater core. Blocked or damaged heater core. Start with the thermostat, that's very common. While you're at it, make sure the coolant level is up. After that, if you suspect a blocked heater core, try backflushing the heater core. Turn the heat on full to open the valve, then with the heater hoses removed, run water BACKWARDS through the heater core. As you remove the heater hoses. Don't use much water pressure, if you go over 15 PSI pressure you could blow the heater core and then you have real problems. Typical water pressure at a house is 50 to 120 PSI, so if you use your water hose make sure you control the pressure.
no pressure release valve
Older style tubs have a pressure switch usually mounted directly on the heater manifold, or attached to the plumbing. The pressure switch is connected to the spa pack/circuit board with 2 wires and shuts the heater off if there is not enough pressure in the heater tube/manifold to protect the heater from becoming damaged or even exploding or causing a fire in a dry heater condition. Newer spas that use Balboa controls have sensors built right into the heater so a pressure switch is no longer needed as the safety function of monitoring the heater for constant water pressure is now all performed within the heater on newer quality spas.
The Thermostat and Temp and Press Valve is faulty and needs to be replaced. or the PRV (pressure reducing valve) is dirty and must be replaced
The sun.
Corrosion.
My guess is that the hot water heater is installed downstream from a pressure reducer to limit the maximum water pressure that the water heater is exposed to.
You would need to specify the size of the heater