If your gas hot water unit is delivering lukewarm water in winter, you're not alone — it's one of the most common cold-weather complaints from NZ homeowners. The good news is there are several well-understood reasons this happens, and some basic checks you can do yourself before calling a professional.
Why Winter Makes Things Harder for Your Hot Water Unit
Your gas hot water unit — whether it's a continuous flow (instantaneous) unit or a storage cylinder with a gas burner — has to work significantly harder in winter. The main reason is simple physics: cold water coming into your home is much colder in winter, sometimes 5–8°C colder than in summer. The unit has to raise that water temperature by a greater number of degrees to reach your set temperature, and if it's already running near its capacity, it may not quite get there.
NZ's climate varies considerably — homes in Southland, Central Otago, or the Central Plateau can see inlet water temperatures drop very low in winter, putting real strain on smaller or older units.
Common Causes of Lukewarm Water in Winter
1. Cold Inlet Water Temperature
As mentioned, this is the most frequent culprit. Continuous flow units are rated by their temperature rise (e.g., a unit rated at 16–20 litres per minute may only achieve that flow at a certain temperature rise). In winter, the required rise is greater, so effective hot water flow drops. Try reducing your flow rate slightly at the tap — you may find the water gets hotter.
2. Temperature Set Too Low
Check your unit's temperature controller or remote. Many units are factory-set to around 50°C. In winter you may need to nudge this up. However, be aware that Tenancy Services Healthy Homes standards require hot water to be delivered at a safe temperature — and scalding is a real risk above 55°C, especially for children and elderly people. Always balance comfort with safety.
3. Reduced Gas Pressure or Supply Issues
Gas pressure can fluctuate, particularly during peak winter demand periods when many households are running heating and hot water simultaneously. Low gas pressure means the burner cannot fire at full capacity. This is something only a licensed gasfitter can properly diagnose and rectify — do not attempt to adjust gas pressure regulators yourself.
4. Dirty or Partially Blocked Burner or Heat Exchanger
Dust, debris, and mineral deposits can accumulate on the burner or heat exchanger over time, reducing efficiency. This is more noticeable in winter when the unit is working harder. A service and clean by a licensed gasfitter can restore performance significantly.
5. Failing or Dirty Inlet Filter
Many continuous flow units have a small inlet filter screen that can become clogged with sediment, restricting water flow and affecting performance. Checking and cleaning this filter is generally a straightforward homeowner task — consult your unit's manual for guidance.
6. Unit Undersized for Your Household
If your household has grown, or you've added bathrooms, your unit may simply be undersized. A unit that coped in summer may struggle in winter. A gasfitter can assess whether an upgrade or additional unit is warranted.
7. Venting or Flue Issues
For flued indoor units, a blocked or partially obstructed flue can affect combustion and burner performance. Never ignore signs of poor combustion — this is a safety issue. See the safety note below.
What You Can Safely Check Yourself
- Check the temperature setting on the unit's controller or remote.
- Reduce flow rate at the tap to see if water gets hotter.
- Inspect the inlet filter screen (refer to your owner's manual).
- Check that the gas supply is on and other gas appliances are working normally.
- Look for any error codes displayed on the unit and check your manual.
- Ensure vents and air intakes around the unit are not blocked by leaves, cobwebs, or debris.
What You Must NOT Do Yourself
Gas work in New Zealand is restricted work under the Gas Act 1992. This means you must not adjust gas pressure regulators, modify gas pipework, service burners, or carry out any work on the gas components of your unit. This work must be done by a licensed gasfitter registered with the Plumbers, Gasfitters and Drainlayers Board (PGDB). Always ask to see their licence before work begins.
Safety Note
⚠️ Carbon monoxide (CO) is a serious risk with gas appliances. If you notice yellow or orange flames (should be blue), sooty marks around the unit, or anyone in the household experiencing headaches or nausea when the unit runs, turn off the unit, ventilate the area, and call a licensed gasfitter immediately. Do not use the unit until it has been inspected. For more on gas safety, refer to WorkSafe NZ.
When to Call a Professional
Call a licensed gasfitter if:
- Basic checks don't resolve the problem.
- You suspect low gas pressure or a burner issue.
- The unit is displaying fault codes you can't resolve.
- The unit is more than 10 years old and has never been serviced.
- You notice any signs of incomplete combustion (yellow flame, soot, smell).
- You're considering upgrading to a higher-capacity or more efficient unit.
A qualified gasfitter can also advise whether your unit meets current NZ Building Code requirements, particularly if you're renovating or adding bathrooms. You can find licensed local gasfitters on tradedirectory.co.nz.
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