A tankless RV water heater can lose water pressure and heat less long before it quits. In many cases, the cause is scale, the mineral buildup left behind when water heats fast inside the heat exchanger.
Any RV owner with a tankless unit will deal with this sooner or later. In the video at the end of this post, Ross from RV Tips and Travels shows the simple process on a common Suburban setup.
Why an RV tankless water heater needs descaling
When water heats quickly, dissolved minerals such as calcium can turn solid and stick to the inside of the heat exchanger. Over time, that buildup cuts water pressure and makes the heater less efficient. Every tankless owner will run into it because it’s basic water chemistry.
A good schedule depends on use. Most RVs need descaling once a year. Full-time use or hard water bumps that to twice a year, and full-time use with hard water can mean three to four cleanings a year. A water softener can slow the buildup.
Scale buildup doesn’t care how new the RV is. If a tankless heater gets used, it will need to be descaled on a regular schedule.
First, prep the heater and drain the water
Ross says the owner’s manual comes first because drain steps can vary by model. Before anything else, the propane service valves should be closed, the remote display turned off, and the water heater powered off.
The descaling solution should never be heated inside the water heater!
If the RV is on city water, the water should be shut off at the hose bib and the hose disconnected at the RV. If the fresh tank is in use, the water pump should be turned off.
Two common ways to drain
For a gravity drain, open the low-point drains and the T&P (temperature and pressure) valve, then close them after the water is out. For a blow-out, open the low points and T&P valve, let it drain, then close only the low points. With the wet bay switch set to city water, connect compressed air through a blow-out adapter at the pressure listed by the manufacturer, usually 30 psi max. Once no more water comes from the T&P valve, disconnect the air and close the valve. A tiny bit of water left inside is fine.
Set up the vinegar loop
A water heater descaling kit helps, but the included 3-gallon bucket is too small for the job. The actual cleaning setup needs a 5-gallon bucket, a submersible pump, 4 gallons of 5% acidity distilled white vinegar, towels, and often GHT (garden hose thread) to NPT (national pipe thread) adapters, because the kit hoses use 3/4-inch garden hose threads while RV fittings are NPT.
Some water heaters have a service valve body that isolates the unit. If not, the hot and cold hoses need to come off the heater. First, mark the lines if they are not already labeled. Then place towels down, disconnect the hoses, and check the inlet for a screen or debris filter. If one is there, rinse it with warm water and reinstall it.
Next, connect one hose to the cold side and to the pump. Connect the second hose to the hot side and place the other end in the bucket. Add the vinegar, turn on the pump, check for leaks, and let it recirculate for 45 to 60 minutes. The vinegar loop runs only through the heater, not through the RV plumbing.
Flush it out and put everything back
After the vinegar cycle, move the pump to four gallons of fresh water and recirculate for 10 to 15 minutes. Then turn off the pump and drain the heater at the outside T&P valve before removing the hoses, or water can spill inside.
Once that is done, reconnect the RV hoses and use plumber’s tape if a fitting needs help sealing. Finally, let the pump and hoses air-dry for a couple of days before storage. A few holes drilled in the bucket lid can help stop mold. We hope this step-by-step list makes the next service easier.
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