An undersink instant hot water dispenser (like my InSinkErator SST) is one of those kitchen luxuries you may not think you need until you get one… after which, you’ll never know what you did without it. But after a few years, I noticed that it was taking longer and longer to fill mugs, pots, or ramen bowls with near-boiling hot water from my InSinkErator faucet, as the once-steady flow was now reduced to a sad trickle. 🙁 After thinking back to my DIY hot water heater flush project with my whole-house hot water tanks, I figured I’d try a similar approach to fix the low flow on my InSinkErator SST tank.

InSinkErator SST Instant Hot Water Dispenser

Of course, depending on the brand and vintage of your instant hot water dispenser, your model’s connections and details may vary, but the principles will be the same. Basically, you need to disconnect your tank, drain it, flush it, reconnect it, refill it… and party with hot water. The entire process (except for letting the hot water cool before you start) should take less than 30 minutes.

Unplug and Wait

First, I unplugged the electrical cord from my instant hot tank and waited. In fact, I unplugged it at night right before going to bed, and then did the rest of this procedure the following morning. The important part is that you unplug the unit and allow the water in the tank to cool before doing anything else. Boiling hot water and DIY projects don’t mix.

Clean and Prep

With all under-sink plumbing projects, I removed everything from under the sink (which is always a good excuse to purge stuff you’ve stashed under there and no longer need) and placed a towel under the area where I was working. If you want to keep your wife happy, I recommend not using your “good” bathroom towels from the guest room. I used one of our beach towels instead.

Disconnect and Remove the Tank

After allowing the hot water to cool completely, I found the shut-off valve that connected cold water to the tank and turned it all the way closed. Although, in my case, the shutoff valve doesn’t actually connect directly to the tank. Instead, it connects to the InSinkErator faucet above, which feeds cold water to the tank only when the InSinkErator faucet is turned on. Therefore, technically, I could have got away with this procedure without shutting off the valve at all.

Next, I looked at the three connections on the top of my InSinkErator SST tank. Here’s what they look like:

From left to right: cold water inlet, hot water outlet, air vent connector.

The blue tube on the left connects to the tank’s cold water inlet, the silicone tube in the middle connects to the hot water outlet, and the clear plastic tube on the right connects to the air vent. Removing all three tubes required no tools: a thumb press released the quick-connector on the cold water tube, and some gentle tugging and twisting removed the other two.

My tank was mounted to the side of my under-sink cabinet by two screws. I loosened them, but didn’t need to remove them all the way, since the plastic mounting bracket had large holes (see picture above) that allowed me to lift the tank and guide it off the screws. Tank removed!

Check the Inlet First

Shining a flashlight down into the cold water inlet on my tank, I could see that the opening was almost completely blocked by hard water deposits. Using a small jeweler’s screwdriver, I pressed down gently and unblocked a tiny hole in the middle of the inlet, which is probably only a millimeter or two in width. I was surprised that only a small part of the inlet was actually open to allow water into the tank, but I’m thinking that maybe it’s designed this way to act as some sort of flow restrictor, which allows the cold water to come into the tank in a tight jet and send cold water to the bottom of the tank more efficiently — and maybe helps with faucet pressure.

Drain and Flush

After unclogging the inlet, I held the tank over my sink and flipped it upside-down to allow it to drain. That’s when I saw the stainless steel drain screw on the bottom of the unit. 🙂 Always be careful any time you’re removing screws over any type of drain. Drains are like black holes to screws: they suck them in, and you’ll never get them out!

I unscrewed the drain screw (being careful not to lose the rubber washer or drop the screw down the drain), and flipped the tank upright to let it finish draining. I had to shake the tank a couple of times when the flow slowed down, meaning there was probably lots of hard water deposits and sediment inside that were contributing to the tank’s reduced flow. After the first draining, here’s what was left on the bottom of my sink:

Small Particles from the First Draining of the Tank

You can see a few larger chunks of calcium, but the majority of what came out was fine sand-like particles. I re-installed the drain screw, hung the tank back on its mounting screws under the sink, reconnected all three tubes, and turned the shut off valve back on. I didn’t, however, plug in the electrical cord. No need to do that until the very end.

I opened the faucet and could hear cold water flowing into the tank, as well as air flowing out of the faucet as the tank filled. Rather than holding the faucet open, I used a screwdriver handle to force the faucet open until water started to flow, like this:

Lazy Man’s Method of Holding Open the Faucet

Already, I could see a massive improvement in the water flow from the faucet. I let the water run for a minute or two, then removed the screwdriver to shut off the faucet. I disconnected everything again (without closing the cold water shut off valve under the sink this time), brought the tank back up to the sink, removed the drain screw, and drained it a second time.

During the second draining, I shook the tank occasionally to try and agitate some of the hard water deposits inside, and kept flipping the tank over to let it alternate between draining from the top and the bottom. The first draining had removed most of the sand-like particles, so I got lots of bigger chunks out of the tank the second time:

Hard Water Deposits from the Second Tank Draining

I repeated the fill and drain process one more time, and got clear water out on the final drain.

You might be tempted to pour some mineral remover (such as CLR or LimeAway) into the tank to help dissolve the deposits, but I don’t recommend it. That stuff is crazy toxic, and even if you let the unit flush for 15 minutes or more after reconnecting everything, you just can’t be sure that you got it all out. Plus, it tastes super nasty. How do I know? Don’t ask…

Final Re-Assembly and Test

After the final draining, I reconnected everything one last time and used my screwdriver trick to refill the tank and let flush out for about 10 minutes. During that time, I removed the nozzle on the end of the faucet and used my jeweler’s screwdriver to help remove hard water deposits that had collected in the screen.

After a good final flush, I turned off the faucet, screwed the nozzle back on, then plugged in the electrical cord to let the tank start heating again. Fifteen minutes later (which was slightly longer than it took me to put all the cleaning supplies, dishwasher detergent, and spare garbage bags back under the sink), I had piping hot water flowing briskly from my InSinkErator instant hot water dispenser!

This DIY project only cost me 30 minutes of time and no money in parts. That’s way better than $188 for a replacement tank from Amazon, or a spendy service call from a plumber with the associated visual trauma — when a plumber is bent over working under your sink, there are just some things you can’t un-see…

Did This Work For You? Tell Me About It!

If this procedure helped you get your instant hot water flowing again, make yourself a celebratory batch of instant hot chocolate, then tell me about it in the comments below!