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Fast Plumbing Answers: No Hot Water From Tankless Heater?
table of contents
table of contents
You rely on your tankless hot water heater for efficient, on-demand hot water delivery. But right now, there’s no hot water at all. Nothing disrupts your day like an unexpected cold shower. So why isn’t your tankless unit producing hot water when you need it?
Don’t worry- we’ll answer all your questions in 2 minutes or less. (Use the Table of Contents to jump right to your specific issue.)
We’re Mother, a refreshingly modern plumbing company in Dallas-Fort Worth. Our tankless water heater repair specialist Dillon Ashton provides the most likely causes for a lack of hot water in this article- and how to solve them.
Live in the DFW Metroplex? If your tankless system isn’t producing hot water, call Mother Modern Plumbing 7 days a week. We’ll be there within 24 hours for a free estimate and tech-driven solutions that restore hot water (and hot showers) to your home.
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Top 5 Reasons Your Tankless Water Heater Produces No Hot Water
“If your tankless unit isn’t putting out hot water, there are a few common suspects,” Dillon explains. We asked Dillon to rank the potential causes for inefficient hot water delivery from on-demand water heaters- here are the top 5, in order:
- Scale buildup in the heat exchanger
- Clogged inlet filter
- Gas supply issues
- Temperature settings
- Failing sensors or control board
Several of these causes are highly reliant on the type of tankless system you own. For example, electric water heater users can cross gas supply issues off their list. Hybrid water heater owners should move faulty control board problems higher in the rankings.
Not sure what some of these terms mean or what some of the listed components are? Don’t worry- let’s explain each of these potential hot water disruptors in simple terms.
1. Scale Buildup in the Heat Exchanger
The heat exchanger is the component in your tankless water heater that transfers heat from the system’s heat source to the water supply. This method of heat transfer depends on your fuel source:
- Gas tankless water heaters use a burner to provide direct heat to a water chamber
- Electric systems use an on-board heating element to heat a series of metal coils
This heat exchange system allows your tankless unit to deliver continuous hot water on demand with increased efficiency.
When hard water travels through the system, it leaves behind mineral deposits of calcium and magnesium. Much of this scaling occurs where your water is heated. Since there’s no conventional tank in your system, deposits form in the heat exchange process.
“Over time, minerals coat the inside of the heat exchanger,” Dillon says. “That prevents efficient heat transfer- your burner or heating element is working fine, but the water isn’t getting hot enough.”
Is scale buildup causing your tankless water heater issues?
Likelihood of scale buildup causing no hot water from your tankless heater: HIGH
Mineral scaling is the most common cause of poor hot water delivery in tankless systems- especially in regions with exceptionally hard water.
Our Texas customers have the 6th hardest water in America. The increased presence of calcium carbonate (limescale) in our water supply leaves increased deposits and scaling on all our fixtures and appliances- water heaters are especially vulnerable.
“Scaling is the #1 cause of no hot water on newer tankless units that haven’t been maintained regularly,” Dillon reports.
The best solution for you
Scheduling tankless descaling service is vital to the health and efficiency of your tankless water heater. The average homeowner should descale their system once per year to eliminate mineral deposits from their heat exchanger.
If your home suffers from hard water, plan to descale your unit every 6-9 months.
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2. Clogged Inlet Filter
Your inlet filter is a small screen that prevents scale, debris and sediment from entering your tankless heater’s internal components. It’s located at the cold water inlet- where your external water supply enters the system.
“A partially blocked inlet filter reduces water flow,” Dillon says. “This can confuse the sensors, which causes weak heating.”
Your system’s sensors interpret reduced water flow as a need for less heat. The total amount of heat exchanged is less than the actual volume of water you need- the result is extremely lukewarm or cold water delivery to your taps.
Is a clogged inlet filter causing your tankless water heater issues?
Likelihood of clogged inlet filter causing no hot water from your tankless heater: ABOVE AVERAGE
A byproduct of internal scaling issues in your on-demand water heater.
The good news? It’s a relatively early indication of hard water problems you can catch before deeper internal scaling issues begin. It’s also a pretty easy fix if you’re handy.
The best solution for you
Handy homeowners can DIY inlet filter cleaning. As with descaling your system, check your filter screen at least once per year for buildup. Here’s how:
- Turn off the power to your tankless water heater, and shut off cold water delivery to the system.
- Close the hot water valve, then flush all existing hot water out of the system.
- Carefully remove the inlet filter, located at your cold water inlet. This filter is either side-mounted or located at the center of the unit.
- Clear the filter screen of all debris. This should be easily achieved with a brush or damp rag. Don’t use chemicals- remember, you’re putting the filter back into your water heater. A simple hot water rinse will suffice.
- Reinstall the filter securely, then close any open panels. Open the hot water valve, turn on cold water delivery to the system, then restore power.
3. Gas Supply Issues
Issues with insufficient gas supply to your tankless water heater system cause your burner to underperform. This leads to cold water delivery.
“If the burner isn’t getting enough gas, it won’t fire at full capacity,” Dillon notes.
Dillon outlines the 3 most common reasons for insufficient gas supply:
- Low gas line pressure
- A failing gas valve
- An undersized gas line
Not sure how to properly size your natural gas line? Colton Staley of Mother Modern Plumbing answers your question in 2 minutes or less.
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Are gas supply issues causing your tankless water heater issues?
Likelihood of insufficient gas causing no hot water from your tankless heater: DEPENDS ON YOUR SYSTEM
If you own an electric tankless water heater, cross this potential suspect off your list entirely.
Hybrid water heater users will still suffer from undersized gas lines and faulty gas valves, but you can generally get by on “electric only” mode until a plumber arrives for repairs.
By contrast, our internal data shows that insufficient gas delivery is the #2 cause of insufficient hot water delivery by natural gas tankless systems.
The best solution for you
Gas tankless water heater users should exercise caution when addressing this issue. We don’t recommend any DIY gas line work- if you suspect an issue, do 3 things immediately:
- Ventilate your home by opening windows, then leave the house.
- Call your local gas supplier’s emergency number to have the gas shut off. (Dallas customers: call the ATMOS emergency line.)
- Call a plumbing expert licensed to perform gas line diagnostics and repairs. (If you’re in DFW, call Mother 7 days a week for immediate scheduling.)
4. Temperature Settings
Improper on-board temperature settings are a sneaky reason for no hot water production.
Think of it like a tripped circuit breaker causing your water heater to not ignite- it’s the simplest solution to the problem, so most homeowners look right past it.
“Sometimes, it’s just as simple as the thermostat being set too low,” Dillon says.
Pro Tip: The usual factory default on tankless water heater systems is 120°F. Temperatures over 120°F greatly increase the risk of scalding and burns.
Are temperature settings causing your tankless water heater issues?
Likelihood of your thermostat causing no hot water from your tankless heater: AVERAGE
Though less common, it’s possible a misset thermostat is causing poor hot water delivery.
The best solution for you
Before you call a plumber, check the settings on your tankless system’s thermostat. Check the system’s current preset temperature in one of 2 ways (depending on your unit):
- The digital display panel
- The temperature dial
Catching a potential issue here means saving money on a diagnostic service call.
(Of course, when you provide free estimates like Mother does, this is never a problem.)
5. Failing Sensors or Control Board
Technical issues with the “smart” elements of your tankless water heater system are a less frequent, but highly problematic cause of no hot water.
“You’re looking at two components for signs of damage or failure: thermistors and your control board,” Dillon says.
Thermistors are on-board sensors that provide precise temperature readings to the control board of your system. These heat-sensitive resistors operate like semiconductors- their electrical resistance changes with the surrounding temperature.
As water flows through the tankless system, thermistors measure its temperature at several locations (like the heat exchanger and inlet valve).
Those readings are sent to your control board- the main computer that manages every operation in the system. It sends signals regarding gas flow, electricity needs, ignition and safety parameters to all other active components in your tankless water heater.
“When your thermistors or control board aren’t reading temperature properly, the heater may throttle itself back,” Dillon explains.
Are sensors or control board issues causing your tankless water heater issues?
Likelihood of temperature components causing no hot water from your tankless heater: LOW
If there’s a problem with your tankless water heater’s control board, you’re not going to be dealing with just inefficient hot water output. You’ll likely notice error codes, resets and other major problems before this occurs.
Thermistor lifespans aren’t “set in stone”- ongoing maintenance, hard water quality and system age all contribute to the health of your temperature sensors. As with your control board, thermistor failure is a far less common cause of no hot water than scaling buildup.
The best solution for you
Call a licensed plumber- ideally, one with specialized tankless water heater knowledge. Issues with a tankless heater’s control board aren’t analogous to conventional water heater tank problems.
Replacing internal components of your on-demand heater- especially ones responsible for communicating with the rest of the system- requires technical expertise.
Our team of licensed water heater technicians receive regular training on replacing the intricate components specific to tankless systems.
Fix Your Tankless Water Heater and Restore Hot Water
No hot water from your tankless system usually means mineral scaling in your heat exchanger or inlet filter, caused by hard water. If you have a gas unit, insufficient natural gas delivery may be to blame. If not, there may be an internal issue with temperature regulation.
Dallas residents upgrade to tankless heaters for on-demand, continuous hot water that increases home efficiency and lowers utility bills. When you suffer through cold showers or “cold water sandwiches”, it’s time for modern plumbing solutions.
Call Mother 7 days a week for innovative tankless water heater repair in the DFW Metroplex. We’ll have a trained tech to your home within 24 hours to diagnose the issue, provide fixed quote service, and get you back to your daily routine.
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