What scale does inside a tankless heater
Water carries dissolved minerals, and heat makes them drop out as a hard crust. A tankless heat exchanger runs hot and its channels are narrow, so even a thin layer can matter. Scale insulates the metal, makes the unit work harder to reach its setpoint and narrows the passages water has to squeeze through.
The signs build slowly. Water takes longer to warm, flow at a tap seems weaker, the unit cycles on and off, or it shuts down with an error code. None of those prove scale, but all of them are reasons to ask about a flush.
How a tankless flush and descale is done
Most manufacturers sell units with two isolation valves, often with small service ports, because flushing is expected maintenance. Without them, a plumber adds valves first.
- Power down and isolateThe unit is switched off, the gas or power is shut, and the isolation valves on the hot and cold lines are closed.
- Hook up a pump and hosesHoses run from the service ports to a small pump and a bucket of cleaning solution, forming a closed loop through the heat exchanger.
- Circulate the solutionWhite vinegar or a descaler made for water heaters is pumped through the loop for a period set by the manufacturer. The fluid usually runs out cloudy.
- Rinse and clean the filter screenThe valves are opened to flush the loop with fresh water. The inlet filter screen is pulled, rinsed of grit and put back.
- Restore and testGas or power returns, the unit fires, and the plumber checks the temperature and flow at a tap.
How often to flush depends on your water, and we cannot name the interval
The right flush schedule depends on how hard the water is. This site cannot state your hardness. The town's 2025 water quality report gives no hardness figure for the general system, so the report alone will not tell you.
Test your own tap with a hardness test kit, or ask your water provider. For the Middletown Water Department, call 302-378-2211. If your water comes from Artesian Water Company, ask them. Once you have a number, the plumber can pick a flush interval to match, and your unit's manual may suggest a starting point.
Which water system are you on?
Some Middletown homes are on the town's own system, which pumps groundwater from the Upper Potomac and Magothy aquifers and also takes purchased water through interconnections with Artesian Water Company. Other neighborhoods, such as Willow Grove Mill, Parkside and Estates at Dove Run, buy water from Artesian Water Company, which draws on 67 wells in New Castle County.
Ask which system you are on, because it tells you who to call for hardness and which water quality report applies. Your answer also helps the plumber set the flush interval.
Gas or electric: the choice shapes the install
A gas tankless unit heats water faster at higher flow, but it needs venting to the outdoors and often a larger gas line than a tank heater did. A tank might sip gas through a half-inch pipe, while a tankless unit can demand a lot more at full burn. Ask the plumber to check the existing line size and the length of the run before you buy.
An electric tankless unit needs no venting, but it draws heavy current, so the panel and wiring matter. Many electric units are better suited to a single bath or sink than to a whole house. In both cases, Delaware's plumbing regulation (16 Del. Admin. Code 4455) adopts the 2021 International Plumbing Code with state amendments covering water heater piping, relief valves and expansion tanks, so ask how those parts apply to your layout.
Common tankless complaints and where they come from
A few quirks show up again and again with tankless units.
| What you notice | Likely cause | What to do |
|---|---|---|
| A short burst of cold between two hot spells. | Cold-water sandwich: hot water left the pipe before the unit fired again. | Wait a few seconds. A recirculation line or small buffer tank can help. |
| Hot water stops at a low trickle. | Flow is below the unit's minimum, so it never ignites. | Open the tap further or fix the aerator. |
| Lukewarm water at full flow. | Scale, a clogged filter screen or flow above the unit's capacity. | Ask for a flush, or run fewer fixtures at once. |
| Display shows an error code. | A sensor, ignition, vent or flow fault. | Check the manual for the code, then call. |
| Unit cycles on and off. | Low flow, a dirty screen or an exhaust restriction. | Clean the screen and schedule a service. |
Photograph the display when a code appears, because the plumber starts from that number.
Error codes and minimum flow
Error codes are not standardized between brands, so we will not guess what a code means. Look up the exact number in your unit's manual. The manual lists the code, the cause and what can be tried before calling. If a reset does not clear it, a plumber can test the part behind the code.
Minimum flow is the lowest rate at which the burner or element will turn on. A low-flow showerhead or a half-open valve can stay under that rate, and the unit then never starts. If hot water works at one tap but not another, this is the first thing to check.
When tankless is not the better choice
Tankless is not always the smart move. If your gas line is small and the run is long, upgrading it may add more than the unit saves. A household with several showers running at once may overwhelm one unit. A home with very hard water and no flushing routine can shorten the life of the heat exchanger.
A tank unit also costs less to install in a space already set up for it. If your old tank has failed, compare a tank replacement with a tankless swap before choosing. For a failing tankless unit, a water heater repair visit may fix a sensor or valve without replacing it.
What to gather before booking a tankless service
A few details turn a general call into a useful one.
- The brand and model number from the label on the side of the unit.
- Whether it is gas or electric, and whether isolation valves are installed.
- Any error code shown on the display, photographed.
- When the unit was last flushed, if you know.
- A hardness test result for your tap, or the name of your water provider.
- Whether you notice trouble at all taps or only one.
What changes the price of tankless service and installation
A flush and a full install are very different jobs. These factors move the price, and you get a clear price before work starts.
Whether isolation valves exist
If the unit has service ports, a flush is quick. If not, valves are added first.
How much scale has built up
A unit flushed regularly is simpler than one that has been neglected for years.
Fuel and venting
Gas units may need new venting or a larger line, and electric units may need new circuits.
Unit size and flow rating
A whole-house unit costs more than a point-of-use heater.
Access and layout
Wall-mounted units in tight closets take longer to reach.
Insurance rarely covers maintenance such as a flush or a heater that has worn out. It may cover sudden water damage from a failed fitting. Read your policy or ask your insurer.
Call (410) 378-7601 to schedule a flush or talk through whether tankless fits your home.
Getting a tankless unit installed
If tankless sounds right, the install is its own project, covered in more detail on the page about installing a water heater. We also serve Townsend, about 7 miles south on DE-71, so Townsend homeowners can use the same line.
Tankless Water Heater: Common Questions
How often should a tankless water heater be flushed?
It depends on your water hardness, which this site cannot state. Test your tap or ask your water provider. Your manual may suggest a starting interval, and a plumber can adjust it once you know the hardness.
Can I flush a tankless heater myself with vinegar?
Many homeowners do, if the unit has isolation valves and service ports. You need a small pump, hoses and white vinegar or a made-for-purpose descaler. Follow the manual, and call a plumber if the valves are missing or stuck.
What is the cold-water sandwich?
It is a brief burst of cold water between two hot flows. The pipe holds cooled water from the earlier use, and the unit needs a moment to fire again. It is a design trait, not always a fault.
What does an error code on my tankless heater mean?
The code depends on the brand, so check your unit's manual. It will list what the number means and what to try first. If a reset does not clear it, call a plumber.
Is a tankless water heater worth it?
It depends on your fuel, gas line size, number of baths and water hardness. It suits some homes well and others poorly. A plumber can walk through your layout before you decide.
Why is my tankless water only lukewarm?
Scale on the heat exchanger, a clogged filter screen or too many fixtures running at once are the usual causes. A flush often restores performance. If it does not, a sensor may need testing.
Have a tankless unit acting up? Call (410) 378-7601 and tell us what the display shows.
Standards and sources
Delaware adopts the 2021 International Plumbing Code (IPC) through 16 Del. Admin. Code 4455 and amends its water heater piping and relief valve provisions. Manufacturer installation instructions and the unit's own manual remain the first reference for venting, gas sizing and error codes.