Flush a Tankless Water Heater Yourself With Vinegar and a Pump
The Problem
If you have hard water, a tankless water heater scales up inside. Mineral buildup coats the heat exchanger, which drops efficiency, weakens hot-water flow, and shortens the unit's life. Manufacturers want the unit descaled about once a year on hard water. Plumbers charge for the visit, but with a few cheap parts it's a doable DIY job.
What You'll Need
A pair of pliers, a small submersible pump, a bucket, two washing-machine hoses, and a gallon of white vinegar. Your unit also needs isolation/service valves on the bottom — most modern tankless heaters have them.
The Technique
This is a checklist to use alongside the source video — watch that first, then follow these steps to jog your memory.
Set up
- Unplug the unit and turn off the gas.
- Close the cold-water inlet valve and the hot-water outlet valve to isolate the heater from the house plumbing.
- Connect a hose to each flush fitting on the service valves (they use garden-hose thread).
- Connect the cold-side hose to your submersible pump's outlet and set the pump in the bucket.
- Run the hot-side hose from the hot flush fitting back into the same bucket.
Flush
- Open the service valves so liquid can flow through the flush fittings.
- Pour the gallon of vinegar into the bucket.
- Turn on the pump. If nothing flows, you may need to plug the heater back in — it will probably beep that it has no gas, which is fine for this.
- Let the vinegar circulate for about 45 minutes to dissolve the scale.
- Shut off the pump.
Rinse and reassemble
- Close the cold service valve and disconnect the cold hose.
- Open the regular cold-water feed to run fresh water through the unit and out the hot hose into the bucket, flushing the vinegar out. Leave the hot side to the house closed for now.
- Flush the pump and hoses with about 10 gallons of clean water — vinegar left sitting in them will ruin them.
- Disconnect the hoses, recap the flush fittings, return the service/bypass valves to normal, open the hot-water valve to the house, then turn the gas back on and plug the unit in.
Pro tip: Restore things in order on the way out — caps back on the flush fittings, service valves to their normal run position, and gas and power last, once everything else is buttoned up.
Safety note: You're working around both gas and electricity. Turn off the gas and unplug the unit before you start, and don't restore either until everything is reconnected. Don't leave vinegar sitting in the pump or hoses, and check your unit's manual — some manufacturers specify a descaling solution or flush procedure to keep the warranty valid.