Our ecosmart 18 turns on and heats the water when we turn the faucet on. But when we turn the faucet off the heater stays on. Help

SOURCE: takes too long for hot water.

You need a recirculating line. You will have to hire a plumber to run a water line from the farthest fixture or to the fixture you are concerned about on the hot side and return it back to the water heater. this will cause hot water to constantly run throught the hot water line (via the recirculating line) and when you want to draw water from the fixture, you will already have hot water right at the line.
I know this works with standard tank water heaters, but it is a lil trickier with rinnai units(but very possible) if your plumber pleads the 5th and tries to sell you another unit call the Rinnai manufacturerand ask them how.

SOURCE: I have a tankles hot water heater. The problem is

If this ia a Chronomite Tankless Electric water heater we suggest you call our technical service department at 800/447-4962 for assistance.

SOURCE: Installed an ARISTON point of use hot water

After installation check the water temperature. If adjustment is necessary, remove front

cover to adjust thermostat. Clockwise to increase the temperature and

counter clockwise to decrease it.

To reach the thermostat:

1. Pry off the round cover plate from its right hand edge with

a small flat-head screwdriver.
2. Remove the Phillips screw revealed beneath the round cover plate.

3. The cover can now be removed by pulling out its left-hand edge.

When reassembling, work in the opposite way being careful to insert the tongue
of the cover into the slot.

SOURCE: Hi, I have the 1600 NG

http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html#flow
1) Tankless require minimum flow rate before burner will activate.
2) Sounds like the water pressure drops quickly when faucet turns on. This implies combination of problems including sediment build up inside lines, crossover at single-handled faucet, or sediment filter clogged or water softener clogged.
3) Low flow faucets can reduce needed flow rate.
4) Other problems can contribute: sensor failure, loose wires, PC board failure, high temperature setting, cold incoming water temperature during winter months.

SOURCE: I have a recently installed richmond water heater. When the shower is running and someone turns the faucet on in the kitchen the water gets drastically colder in the shower. We didnt have that problem

Which type Richmond water heater?
Is this a tankless water heater or tank-type?

Tankless are notorious for 'cold water sandwich' under exact conditions you describe. You might need tankless with larger BTU burner.
Don't install a tankless.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

If you have tank-type water heater, then I suspect a clog in the hot water line, or single-handle faucet cartridge in show is going bad.
Same problems can contribute to tankless problems.
http://waterheatertimer.org/Low-hot-water-pressure.html

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

Instant Repair Help over the Phone

6ya.com

Save Time & Money by instantly connecting to...