If you’re planning your first tankless flush, look under the heater before anything else. The pair of valves on the water pipes, or the lack of them, decides how the flush goes. With isolation valves, a flush is a routine job. Without them, it starts with plumbing work.
Isolation valves are also called service valves or flush valves. They are not part of the heater itself. An installer adds them to the cold water line coming in and the hot water line going out.
What isolation valves do
Each valve does two jobs.
- It shuts off the water. Turning the handle closes the pipe between the heater and the rest of your house. The heater is now a closed loop, cut off from your plumbing.
- It gives you a port. Each valve has a small side outlet with a cap, often called a service port or purge port. With the main valve closed, a hose can be screwed onto the port.
During a flush, a technician closes both valves and connects two hoses. One runs from a small pump sitting in a bucket of descaler to the cold-side port. The other runs from the hot-side port back to the bucket. The pump pushes the solution through the heat exchanger, the narrow tubes where the water is heated, and back to the bucket, over and over. Then the solution is drained, and fresh water rinses the heater out through the same ports.
Without the valves, there is no way to close the heater off from your house and nowhere to attach the hoses. Descaler could run into your pipes, and the heater can’t be rinsed properly.
What each manufacturer says
| Brand | What the flushing instructions say | Document |
|---|---|---|
| Noritz | States that isolation valves are necessary | Noritz procedure for flushing the heat exchanger |
| Rinnai | Says isolation valves are strongly recommended so the heat exchanger can be flushed | SENSEI installation and operation manual |
| Navien | The procedure begins: turn off and unplug the unit, then close the hot and cold water isolation valves before connecting the pump hoses | NPE-2 user’s manual |
| Bosch | The procedure begins: shut off water with isolation valves, which Bosch notes are installer-supplied | Therm descaling bulletin |
| Rheem | The documents we used describe a pump kit, a submersible utility pump (Tankless Flush Kit RTG20124), but don’t address valves directly | RTGH troubleshooting manual |
The wording ranges from “recommended” to “necessary”, but in practice every procedure we read depends on them. Noritz is the plainest, and Rinnai’s “strongly recommends” leaves little room. See the Rinnai and Noritz pages for each brand’s other maintenance notes.
How to tell if your heater has them
Stand in front of the heater and look at the pipes coming out of the bottom. You are looking for:
- Two water valves close to the heater, one on the cold inlet and one on the hot outlet. Each has a handle, usually a lever.
- A small capped port on each valve, pointing sideways or down, often with its own small handle. This is the service port.
- Markings. Many valve sets mark the hot and cold sides with color or a label.
Things that are not isolation valves:
- The gas valve. It is on the gas line, often with a yellow handle. Never touch it as part of a flush; a technician handles the gas side.
- A plain shutoff with no port. Some heaters have a basic shutoff on the cold line only. That stops the water but gives no way to connect a pump.
- The main water shutoff for the house. Closing it doesn’t create a loop through the heater.
If you aren’t sure what you’re looking at, take a clear photo of the pipes under the heater, including the brand label. A technician can tell from the photo whether you have usable valves.
When the valves are missing
Some heaters, especially older or quickly done installs, have no isolation valves. Others have valves that are seized or leak at the handle. Either way, the fix is the same: a technician adds or replaces the valves before the first flush.
That is plumbing work. Adding valves means cutting into the hot and cold water lines and fitting new parts. Utah treats work on a plumbing system as licensed work, and its handyman exemption says plumbing-system work must be done by a licensed electrical or plumbing contractor. Utah’s exemption for minor incidental plumbing applies only when no modification is made to the existing water piping, and adding valves is a modification. Our guide to who can work on a water heater in Utah explains the rules and the questions to ask.
Once the valves are in, every future flush is quicker and simpler. It often makes sense to have the valves added and the first flush done in the same visit. Ask about that when you book.
Valves and the inlet filter
Isolation valves help with more than the flush. Many tankless heaters, including Rinnai and Navien models, have a small screen filter where the cold water enters. Rinnai’s procedure says to remove and clean the in-line filter at the cold water inlet, and Navien’s says to remove the cold water inlet filter and clean out any residue. To take that filter out without flooding the floor, the water to the heater has to be off. A closed cold-side isolation valve does that without shutting off water to the whole house.
Looking after the valves
- Don’t force a stiff handle. Valves that sit untouched for years can seize. Forcing one can break the stem or start a leak. Let a technician deal with it.
- Check the caps. The service port caps should be on and snug, with no drips.
- Open them fully after a flush. A valve left partly closed can cut the heater’s flow and cause low hot water or error codes.
- Look for leaks at the handle. A drip at the stem is a sign the valve needs service.
What to tell the technician
When you call, it helps to say:
- the brand and model of your heater,
- whether you can see two valves with service ports,
- whether any valve is stuck or leaking,
- when the heater was installed, if you know.
That lets the technician bring the right parts, so valves and a flush and descale can be done in one trip. If you’re also weighing whether to flush it yourself once valves are in, see DIY vs professional. If the heater is near the end of its life, ask for isolation valves to be included in the new installation.