Started humming periodically when using hot water and tankis filling up with cold water

Unless you are using a tankless hot water heater, there is a limited amount of hot water available in the tank. Electric and gas water tanks need time to recover after use, and if you are using heavily, it can exhaust the available supply. That said, there are a couple reasons why you may have less hot water than seems normal. In both electric and gas water heaters, silt can form in the bottom of the tank. This will build up over time and displace water. It also reduces the heating efficiency of gas units. There is a drain valve on the bottom of water heater tanks to allow for purging of this silt periodically, but few ever do it. You could also have, in the case of an electric hot water heater, a bad element or thermostat. Most electric heaters have an upper and lower element and thermostat. If one fails the other will still work, but the water takes longer to get hot and will deplete faster.

that adjuster is there to reduce the water temperature so that scalding from boiling water doesn't occur
they can be adjust but if this problem has come in after a period of time then the sensor wax inside the adjuster is not as it should be.
they do fail and need replacing
most plumbers put the unit on the hot water outlet because they are lazy as the standard says that it is primarily for use on the shower, bath, basin systems and wash machines and kitchen sink are not required to be temp controlled.

You could start by seeing if the thermostat is wide open. Sounds like it is on low or vacay mode. How far is the run between your heater and the bathing area. Might need to insualet some piping.

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Had this trouble with a condensing boiler,water pressure was too low, also if thereis an adjustable stat try turning this up ?

Hi this is not a problem with any equipment you are using its just a case of you living in a hard water area, their are filters you can buy but can be expensive to maintain, as the cold water feed is the same as the hot water feed then it may be worth getting a filter fitted to the cold tap as this is the water you will be drinking you can either buy one that fits on the tap or you can buy a brita free standing filter that you just keep filling up with water and use it when you need to.

Their is no problem with the water and it wont harm you and if you say run a bath or fill the sink up with hot water and it looks milky just wait 5 minutes then it should be perfectly clear.

You mainly get this type of hard water when you live near the coast, it can upset your stomach if your not used to this water so boiling it before drinking will give you chance to get your body use to the different water conditions.

How big is your bathtub? Is your water heater set on the hottest setting? What is the temperature of the hot water at the tap?

Keep in mind that hot water can cause severe burns. Children and those with diminished mental capacity can be seriously harmed by hot water.

The "40 Gallon" rating on the water heater is usually the amount of hot water the unit can produce in one hour. So the tank, and the amount of immediately available hot water, might be smaller than 40 gallons.

My bathtub is 2 feet wide by 1 foot deep and 4 feet long. This is 8 cubic feet or almost 60 gallons. So a 40 gallons of water at 120 degrees, mixed with 20 gallons of cold water (60 degrees), fills the tub with 100 degree water. My January cold water is 42 degrees, so it will take 40 gallons at 129 degrees to fill the tub. If the tub is cold to start it will take even hotter water.

So the water heater might be just fine.

Possible solutions:
1. turn the temperature up on your water heater- or the temperature control could be malfunctioning and not getting the water hot enough.
2. insulate your hot water pipes so that the water loses less heat on the way to the tub.
3. bigger water heater.
4. smaller bathtub, or less full bathtub.

SOUNDS LIKE THE CARTRIDGE ASSEMBLY IS BUGGERED UP. PART #46463.
MAYBE THIS WILL HELP:
All parts and finishes of the Delta faucet are warranted to the original consumer purchaser to be free from defects in material and workmanship for as long as the original consumer purchaser owns their home. Delta recommends using a professional plumber for all installation and repair.
Delta will replace, FREE OF CHARGE, during the warranty period, any part or finish that proves defective in material and/or workmanship under normal installation, use and service.
Replacement parts may be obtained by calling 1‑800‑345‑DELTA (3358) or by writing:
Delta Faucet Company Product Service 55 E. 111th StreetIndianapolis, IN 46280

sounds like you have a hot water leak un der your foundation.

What type of pipes are install on this heater?