I hooked my water heater under the sink. It works great, but I hooked it into the hot water supply. Is that okay? Also had a slight leak - rust around pressure relief sensor...cleaned, all okay.

Hi -

If your water heater is working (ie: making hot water) then most likely your heater isn't leaking. Rather, the leak is coming from the cold water supply and running down the tank where you see it at the bottom of the tank. A working tank that is leaking would produce a leak of hot water - not cold. You should carefully check the cold water piping for a leak, drip or stream of water and make the required repairs.

The water inlet to the heater must be plugged.

Insinkerator stainless tank does not have anode rod to protect from rusting. So these water heaters rust out at welds. And must be replaced. And this is frequent problem.
Copy following link for installing under-counter water heater and see manuals for various models available. Non-insinkerator water heaters have anode rod to protect from rusting:
http://waterheatertimer.org/How-to-install-under-counter-water-heater.html
Gene

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

Since you have hot water at all the other fixtures, the problem lies in the sink, not the water heater. It could be that the sink is farther away from the water heater than the other fixtures. It would take longer to get hot water.

If pipes are leaking, then that can be repaired.
If tank is rusted out and water escaping from the tank, then turn off the water plus power to the heater for safety, and then replace water heater.
Rusted water heater can be welded for repair, but remember that hot water is under pressure and tank can rupture any time.
Safest course if to replace heater.

Open following link for leaking water heater checklist:
http://waterheatertimer.org/Leaking-water-heater.html

Your tank is rusted out and needs to be replaced.
Link below is a troubleshoot for leaking water heater.
http://waterheatertimer.org/Leaking-water-heater.html

Protect yourself first:
If your tank is rusted, there is danger of rupture.
My friend welded his leaking tank, but he assumes the risk.
Ruptured tank can cause massive damage and water runs until it is shut off.

Hot water tank is under pressure because hot water expands and pushes against tank wall.
Turn off water to tank.
Turn off power supply:
Electric: turn off circuit breaker.
Gas: rotate knob to pilot so pilot stays lit but burner will not fire. To turn back on, rotate knob to ON.
Open hot side bathtub valve and relieve pressure on tank.

You can limp by with rusted tank by turning water heater on when hot water is needed.
Shower uses 5-9 gallons hot water
Bath uses 12-15 gallon hot water
Ordinary gas heater makes 41 gallons hot water per hour
Ordinary electric heater makes 20 gallons hot water per hour.

I buy water heaters at Lowes, Home Depot, Ace.
Look for scratch and dent models.
Buy drip pan for water heater.
Wrap electric heater totally with insulation except TP valve.
Buy new flexible connectors for water lines.
Offer 15% less than marked price on tank, because the store makes money on additional items.

> Most people can install their own electric heater since you just connect water pipes and electric wire, and it's done. Read link below since there are tips that help you buy and install electric.
> Generally people are nervous about installing gas water heater, so step-by-step instructions are shown on link below. Remember gas line is under less pressure than water line, so if you can install a water pipe, you can install gas line to water heater with crescent wrench, and soapy water to check for leaks.
How to install gas water heater:
http://waterheatertimer.org/How-to-install-gas-water-heater.html

Add a comment for more free help.
Also take advantage of fixya phone service.
For a price, fixya expert speaks with you over phone while you work on water heater, or any do-it-yourself project.

I wrote a page on leaking water heaters with resources and information:
Page lists things to check.
http://waterheatertimer.org/Water-at-bottom-of-water-heater.html

Also run hand over tank to find warm spot that says where leak is coming from
A rusted tank cannot be repaired.

If your tank is rusted out, it might burst open and flood the area.
Hot water inside tank is under pressure.
Turn off heat source to water heater.
Turn off water valve located above tank.
Open tub faucet on hot side and relieve pressure from tank.

You can limp along with a leaking water heater by turning it on when hot water is needed.
For example a shower uses 5-9 gallons hot water.
Turn gas water heater on for 15-20 minutes.
Turn electric on for 20-30 minutes.

To conserve resources with next heater:
http://waterheatertimer.org/9-ways-to-save-with-water-heater.html

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.

basically the principle of the heater is water flows into a internal tank and is heated by an element. You open a hot tap and the mains cold water forces the hot out and replaces it inside the heater.As you have a flow problem i can only suggest you may have some kind of restriction or blockage in the cold water supply at the water heater or from the hot supply from the heater.If you live in an area where lime content is high then this can calcify when heated.If you can, isolate the water supply and electrical supply and check the pipework for blockage.Also investigate your hot tap as this may be the fault. Good Luck