Bath tub water doesn't stay hot

recently installed?

The water inlet to the heater must be plugged.

1) Cheapest and most reliable solution: Install tank-type electric water heater from local box store for $250. Get store coupons, offer 10% off store price. Repairs are made same day with generic off-the-shelf parts.
http://waterheatertimer.org/pdf/How-to-install-electric-water-heater.pdf

2) Install grundfo water pressure boosting pump.
http://www.ronhazelton.com/projects/how_to_increase_water_pressure_in_your_home
http://waterheatertimer.org/How-to-solder-pipes.html

3) Remove areators and low flow shower heads, Maintain unit as shown in product manual: Delime tankless unit yearly, Clean water filter monthly.

4) Call city and report low pressure and report issue as fire hazard, move to different apartment. Call apartment complex and ask why they didn't read flow requirements for tankless before installing tankless.

5) Call Bosch 866-330-2729 and complain that nobody read the flow requirement or tested local water flow before installing tankless.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

If the heater is that old and you have not changed the anode rod over the years then the chemicals in your water are eating at the tank metal and it will soon fail. A twenty year old tank is so inefficient compared to the new ones that it would pay you to replace it now anyway.

In short YES to your question.

Hello, Your 20 year old water heater is nearing the end of it's useful life. Most water heaters last only 20 years before they start to leak. The reason most likely it has intermittent heat spurts is most likely due to two things, 1. The unit has a lot of rust in the water (you don't normally notice it during a shower) If you were to bleed off the rust by attaching a hose or bucket to the bottom of the heater's water outlet, and /or the over pressure tube located off of the top of the unit that has a copper pipe on it pointing toward the ground, that you would see a lot of rust come out of it. You could do this and get a bit more life out of it and better heat to boot, unless it decides to leak.
2. The water heater has a bad electric heating element (if it's electric that is). Replacing it may solve the problem but most likely the heater still isn't going to last too much longer anyways, so that would be a judgement call on your side.
You would in all likely-hood be better off replacing the entire water heater now while it is in a controlled environment before it leaks at some unfortunate time causing water damage and other damage at the same time when you are not around to control it.
Good Luck! Rich L.

Check for an air switch at your local hot tub store

take the old one in so they can match it up to a new one

Tankless water heaters normally take a little longer than a regular heater to provide hot water because they have to start and get warmed up. I'm guessing the heater uses a pilot light and that the short amount of hot water when you turn on the tap is hot because when the water is sitting still in the heater the heat from the pilot light is enough to get it hot. The tub faucet will get hot a lot faster than the shower because it lets a lot more water through and empties out the pipes faster than the shower. You could probably speed things up by letting the tub faucet run on completely hot for like 10-15 seconds and then turning on the shower.

It could be back flowing into the hot water heater tank which is not full because of the leak. Even though the valve is closed on the cold water entry side, there is probably no valve on the exit side. That is how it is returning to the tank.

I assume you are talking about an electric water heater. The normal life span of an electric water heater is up to 11 years. Before you replace it, try turning off the unit, turning off the water supply, and draining the tank of any built up rust and sediment. Refill the tank, and turn the power back on and see how it performs. If it is still acting up, look at a replacement. On a positive note, the new water heater should pay for itself in pretty short order due to better insulation saving energy (and replacing with a higher efficiency unit may qualify for a tax credit).

It is most likely the water valve stuck open, telling the controls to heat water, that it thinks is flowing through it, but without actual flow it overheats. Depending on how comfortable you are with taking it apart yourself, you could open the valve to see if it just needs cleaned out, or if you have a ruptured diaphragm or some other cause.