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How often should a tankless water heater be flushed or desca
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| plumber | Date: Tu, 07/28/2026, 6:57 AM | Message # 1 |
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| How often should a tankless water heater be flushed or descaled?
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| Guest | Date: Tu, 07/28/2026, 6:58 AM | Message # 2 |
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Group: Guests
| I had a Navien tankless water heater installed about a year ago. Do I really need to flush it annually, and what happens if I skip this maintenance?
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| plumber | Date: Tu, 07/28/2026, 6:58 AM | Message # 3 |
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| Yes, annual flushing (descaling) is essential for tankless water heaters, especially in areas with hard water.
Why it’s necessary:
Limescale Buildup: Mineral deposits (calcium and magnesium) accumulate on the heat exchanger elements. Over time, scale acts as an insulator, forcing the system to work much harder to heat water.
Error Codes & Shutdowns: Excessive scale leads to overheating, triggering error codes (e.g., Code 012 or 016 on Navien units) and causing system shutdowns.
Warranty Protection: Most manufacturers (Navien, Rinnai, Bosch) require documented regular maintenance to honor warranty claims on heat exchangers.
What to do: Flush the system with a non-toxic descaling solution or food-grade white vinegar using a submersible pump for 45–60 minutes once every 12 months.
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| Guest | Date: Mo, 08/10/2026, 3:47 AM | Message # 4 |
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Group: Guests
| Can I flush it myself with a pump and vinegar, or does this need to be a professional service call? Also, our water is pretty hard here — does that mean I should flush more often than once a year, like every 6 months?
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| plumber | Date: Mo, 08/10/2026, 3:47 AM | Message # 5 |
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Soldier
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| DIY vs. professional:
Flushing is one of the more DIY-friendly maintenance tasks on a tankless unit, if your unit has isolation valves (a service valve kit) already installed — most new installs do. The basic process is:
Shut off power/gas and the cold water supply to the unit.
Close the isolation valves and connect a submersible pump to circulate the descaling solution through the hot and cold service ports in a closed loop.
Run the solution for 45–60 minutes, then flush with clean water for 5–10 minutes.
That said, a professional visit is worth it if: you don't already have isolation valves (they'll need to be added), you're not comfortable working around the gas/electrical connections, or you want the inlet filter screen cleaned and the venting inspected at the same time — problems there often get missed on a pure DIY flush.
On hard water and flushing frequency:
Yes — the 12-month interval assumes moderate water hardness. If you're in a hard water area (roughly 10+ grains per gallon), I'd move to every 6 months, or better yet, install a water softener or scale-inhibiting filter ahead of the unit. That single step often does more to protect the heat exchanger than frequent flushing alone, since it reduces how much scale forms between services in the first place.
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| Guest | Date: Th, 08/13/2026, 3:26 AM | Message # 6 |
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Group: Guests
| Why does my tankless water heater take a long time to deliver hot water?
My tankless water heater is working normally, but it seems to take quite a while before hot water reaches the shower and faucets.
The unit is located some distance from the bathrooms, so I'm wondering if the problem is simply the distance of the plumbing. Could a dirty inlet filter, low water flow, or lack of a hot-water recirculation system also cause this?
Would installing a recirculation pump make a noticeable difference in how quickly hot water reaches the faucet?
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| plumber | Date: Th, 08/13/2026, 3:27 AM | Message # 7 |
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Soldier
Group: Administrators
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| Why does my tankless water heater take a long time to deliver hot water?
A tankless water heater can produce hot water very quickly, but that does not necessarily mean hot water will reach the faucet immediately. The biggest factor is often the distance between the water heater and the fixture.
When a faucet is opened, the cold water already sitting in the hot-water pipe must first travel back to the water heater before heated water reaches the fixture. The longer the pipe run, the longer this can take.
A dirty inlet filter or restricted plumbing can also affect flow through the heater. If the problem has recently become worse, checking the inlet filter and having the unit inspected for mineral buildup is worthwhile.
A hot-water recirculation system can significantly reduce waiting time by keeping hot water circulating closer to the fixtures. Some tankless water heaters have a built-in recirculation function, while others can work with an external pump and dedicated or crossover piping.
The system should also be properly sized and configured for the home's water demand. If the heater is frequently starting and stopping because the flow rate is near its minimum activation threshold, that can also affect performance.
If the wait time has always been long, the pipe distance is probably the main reason. If it has suddenly increased, however, it's worth checking the heater, filters, valves and plumbing before installing additional equipment.
For a home with long hot-water runs and frequent use of multiple bathrooms, a properly designed recirculation system can be a worthwhile comfort upgrade.
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| Guest | Date: Fr, 08/14/2026, 8:31 AM | Message # 8 |
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Group: Guests
| Why is my tankless water heater producing less hot water than before?
My tankless water heater used to provide plenty of hot water, but recently the water temperature seems less consistent and the flow of hot water has decreased.
The unit is several years old and has been flushed before, but I'm wondering if mineral buildup could still be restricting the heat exchanger or inlet filter.
What are the most common reasons a tankless water heater starts producing less hot water? Could a dirty inlet filter, scale buildup, low water flow, or a temperature sensor cause this problem?
How can I tell whether the unit needs another service or if there is a more serious problem?
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| plumber | Date: Fr, 08/14/2026, 8:31 AM | Message # 9 |
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Soldier
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| Why is my tankless water heater producing less hot water than before?
A tankless water heater that gradually produces less hot water can have several possible causes. Mineral buildup is one of the most common, particularly in areas with hard water.
Scale can accumulate inside the heat exchanger and reduce heat transfer. A dirty inlet water filter can also restrict the flow entering the unit. If the water flow becomes too low, the heater may have difficulty maintaining the desired temperature.
Start by checking whether the problem affects every hot-water fixture or only one faucet. If the entire house is affected, the water heater itself is more likely to be the cause.
The inlet filter should be inspected and cleaned according to the manufacturer's instructions. If the unit has not been descaled recently, mineral buildup may also need to be addressed.
Other possible causes include a partially closed water valve, incorrect temperature settings, a malfunctioning flow sensor, temperature sensor problems, or insufficient gas supply on a gas-fired unit.
If the heater has been properly maintained but performance continues to decline, it's worth having a technician check the error history, sensors, water flow, heat exchanger and gas or electrical supply.
Regular descaling is important, but maintenance should not be limited to flushing the heat exchanger. The inlet filter, valves, sensors and overall operation should also be checked during service.
If performance has changed suddenly rather than gradually, professional diagnosis is recommended because the problem may not be related to scale at all.
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| Guest | Date: Su, 08/16/2026, 3:18 AM | Message # 10 |
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Group: Guests
| Why does my tankless water heater shut off when I take a low-flow shower?
My tankless water heater works fine when I open a faucet fully, but sometimes the burner shuts off when I'm taking a shower with the water flow turned down.
The water suddenly becomes cooler, then gets hot again when I increase the flow.
Could this be related to the minimum flow rate required to activate the heater? Would cleaning the inlet filter or descaling the heat exchanger help?
Is there a way to prevent the heater from cycling on and off during low-flow use?
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| plumber | Date: Su, 08/16/2026, 3:18 AM | Message # 11 |
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Soldier
Group: Administrators
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| Why does my tankless water heater shut off when I take a low-flow shower?
Yes. A tankless water heater needs to detect a minimum water flow before it activates and continues heating. If the shower flow is reduced below that threshold, the burner may shut off and the water can become cooler.
This can be especially noticeable with low-flow showerheads or when another fixture changes the pressure and flow through the system. The current discussion already notes that a tankless heater operating near its minimum activation flow can start and stop more frequently.
Before assuming the heater is defective, check the inlet filter screen. Mineral deposits or debris can restrict incoming flow and make it easier for the system to fall below its activation threshold. On a unit operating in hard water, scale buildup can also affect performance; the thread recommends more frequent descaling under those conditions.
I'd troubleshoot it in this order:
Check whether the problem occurs only at very low shower flow.
Clean the manufacturer's inlet-water filter.
Check that the isolation and supply valves are fully open.
Check the showerhead for mineral buildup.
Check the heater for scale buildup and error codes.
Compare the actual flow rate with the manufacturer's minimum activation-flow specification.
If increasing the shower flow immediately makes the heater operate normally, that is a strong indication that you're dealing with a minimum-flow issue rather than insufficient heating capacity.
A recirculation system can reduce the wait for hot water, but it generally isn't the first solution for a heater that shuts down because the fixture's flow is below the activation threshold. The heater itself should first be checked for proper flow and operation.
If the problem has appeared recently and the shower flow hasn't changed, I'd have the inlet filter, heat exchanger and flow sensor checked. A previously reliable heater that suddenly begins shutting down at normal shower flow may have a restriction or sensor problem rather than simply being undersized.
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| Guest | Date: Tu, 08/18/2026, 6:50 AM | Message # 12 |
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Group: Guests
| Why does my tankless water heater turn off during a power outage?
I understand that a gas tankless water heater still needs electricity to operate its controls and ignition. During a short power outage, our heater stops producing hot water even though the gas supply is still available.
Would a UPS or battery backup be a practical solution for a tankless water heater? How much power does the unit normally need, and are there any safety concerns with connecting it to a backup power source?
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| plumber | Date: Tu, 08/18/2026, 6:51 AM | Message # 13 |
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Soldier
Group: Administrators
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| Why does my tankless water heater turn off during a power outage?
Yes. Even a gas-fired tankless water heater generally requires electricity for its control board, ignition system, sensors and other components. Having gas available does not necessarily mean the heater can continue operating during a power outage.A properly sized UPS, battery backup or generator can potentially keep the heater operating, but the backup system must be compatible with the manufacturer's electrical requirements.Before choosing a UPS, check the heater's installation manual for:- operating voltage;
- maximum electrical consumption;
- startup requirements;
- whether the manufacturer permits generator/UPS operation;
- grounding and polarity requirements.
A small consumer UPS designed for a computer should not automatically be assumed to be suitable. Some gas appliances are sensitive to the quality of the AC power supplied to their control electronics.If the goal is only to maintain hot water during short outages, a properly sized battery backup may be practical. For longer outages, a standby or portable generator can provide substantially longer runtime, provided the installation complies with electrical and fuel-safety requirements.It's also worth remembering that a tankless heater isn't the only component that may need backup power. If your system includes a hot-water recirculation pump, that pump may also stop during an outage.The safest approach is to determine the exact electrical requirements from the heater's model documentation and then select a backup system with adequate capacity and appropriate waveform/grounding characteristics. Don't modify the heater's wiring or bypass its electrical safety controls.In other words, gas availability alone doesn't guarantee hot water during a blackout. The tankless heater's electrical requirements need to be considered as part of the overall emergency-power plan.
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| Guest | Date: Fr, 08/21/2026, 2:32 AM | Message # 14 |
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Group: Guests
| Why does my tankless water heater fluctuate between hot and cold water?
My tankless water heater doesn't shut off completely, but the shower temperature keeps changing. It starts hot, then becomes lukewarm for a short time, and then gets hot again.
The problem is more noticeable when only one shower is running. Could this be caused by a dirty flow sensor, low water flow, scale buildup or a problem with the mixing valve?
Is there a simple way to determine whether the problem is with the water heater or the plumbing fixture?
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| plumber | Date: Fr, 08/21/2026, 2:33 AM | Message # 15 |
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Soldier
Group: Administrators
Status: Online
| Why does my tankless water heater fluctuate between hot and cold water?
Temperature fluctuations can have several causes, and it's useful to determine whether the problem affects one fixture or the entire house.
If only one shower has the problem, start with the shower valve or cartridge. A worn thermostatic or pressure-balancing cartridge can mix too much cold water and create temperature fluctuations even when the water heater is operating correctly.
If multiple hot-water fixtures experience the same problem, the tankless heater becomes a more likely source.
One of the first things to check is water flow. Tankless heaters need a minimum flow rate to remain activated. If the flow repeatedly drops below that threshold, the burner can cycle off and then restart, producing the hot-cold-hot pattern already discussed earlier in this thread.
Other possible causes include:
- partially blocked inlet filter;
- mineral buildup in the heat exchanger;
- dirty or malfunctioning flow sensor;
- partially closed water-supply valve;
- incorrect temperature setting;
- inadequate gas supply on a gas-fired unit;
- a problem with the mixing valve or plumbing fixture.
Because this thread already establishes that hard-water scale can reduce heat-transfer performance and that the inlet filter should be inspected during maintenance, those are reasonable first checks
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