Hot Water Duration Calculator
How Long Will Your Hot Water Last?
Find out how long you'll have hot water after a boiler failure based on your system type.
Boiler System Information
If your boiler breaks down, does that automatically mean you have no hot water? The short answer is: yes, usually. But not always. And knowing why - and what might still work - can save you time, money, and a very cold shower.
How a Boiler Actually Works
A boiler isn’t just a water heater. It’s the heart of your home’s heating system. In most homes in Dunedin and across New Zealand, the boiler heats water for two things: your radiators (central heating) and your taps (domestic hot water). How it does this depends on the type of system you have.
There are three main types:
- Combi boilers - the most common in New Zealand homes. They heat water on demand, directly from the mains. No tank. No storage. When you turn on the tap, cold water rushes in, gets heated instantly, and flows out hot.
- Regular boilers - these work with a hot water cylinder (tank) in the airing cupboard. The boiler heats water and stores it until you need it.
- System boilers - similar to regular boilers but with built-in components like the pump and expansion vessel. They still need a hot water tank.
If your boiler breaks, all three types lose their ability to heat water. But here’s the catch: the hot water tank in regular and system boilers might still have hot water inside - even if the boiler is dead.
When You Still Have Hot Water (Even With a Broken Boiler)
Let’s say your boiler’s control board has fried. Or the pressure dropped and it shut off. Or the pump seized. The boiler isn’t working. But you turn on the kitchen tap… and hot water comes out.
That’s possible - if you have a regular or system boiler with a well-insulated hot water cylinder. That tank holds 100 to 200 litres of heated water. It doesn’t need the boiler running to stay warm. Depending on how long it’s been since the boiler last fired up, you might have anywhere from 30 minutes to 12 hours of hot water left.
One homeowner in Port Chalmers told me she had hot showers for two days after her boiler broke. She didn’t notice until the water started turning lukewarm. She used it wisely - quick showers, no baths - until the repair guy arrived.
But if you have a combi boiler? No tank. No backup. Break the boiler, and hot water stops immediately. There’s nothing to fall back on.
What “Broken” Actually Means
Not all boiler failures are the same. Some are total. Others are partial. Here’s what can go wrong:
- Complete failure - no power, no gas, no ignition. No heat. No hot water.
- Faulty thermostat - the boiler thinks the house is warm enough, so it won’t turn on. But the hot water tank might still be full.
- Low pressure - the boiler shuts down for safety. You can often repressurise it yourself in minutes.
- Pump failure - water can’t circulate. Radiators stay cold, but if you have a tank, the hot water might still be there.
- Heat exchanger crack - this is serious. Water leaks internally. The boiler shuts off. Hot water stops. You need a replacement.
Low pressure is the most common fixable issue. Many people panic when the boiler goes quiet, not realising it’s just a simple 10-minute fix. Check the pressure gauge - it should read between 1 and 1.5 bar. If it’s below 0.5, you can usually top it up using the filling loop (it looks like a small tap or hose under the boiler).
Why You Shouldn’t Assume It’s Just a Boiler Problem
Just because the boiler isn’t working doesn’t mean it’s the boiler’s fault.
One winter in Dunedin, a family called for boiler repair because they had no hot water. The technician found the boiler was fine. The issue? A broken immersion heater in the hot water cylinder. The boiler was working perfectly - it was just not needed. The immersion heater, which is meant to kick in if the boiler fails, had died. So when the boiler went down, so did the hot water.
Other possibilities:
- A faulty diverter valve (in combi boilers) - this directs water to either radiators or taps. If it sticks, you get heating but no hot water - or vice versa.
- Blocked pipes or air in the system - can stop water flow even if the boiler is running.
- Thermostatic mixing valve failure - this controls the temperature at your taps. If it breaks, water might come out cold even if the boiler is heating fine.
That’s why jumping straight to “replace the boiler” is expensive and often wrong. A proper diagnosis takes 30 to 60 minutes. Don’t let anyone rush you into a new unit.
What to Do Right Now
If you have no hot water and suspect the boiler:
- Check the power. Is the boiler plugged in? Is the circuit breaker tripped?
- Check the gas supply. Is the gas valve open? Are other gas appliances (like your stove) working?
- Check the pressure gauge. If it’s below 1 bar, top it up using the filling loop.
- Try the hot water tap. Is it completely cold, or just not as hot as before? If it’s lukewarm, you might still have stored hot water.
- Look for error codes. Most modern boilers show a code (like F28, E13, etc.) on the display. Write it down - it tells the technician exactly where to look.
If none of that works, turn off the boiler and call a registered gas fitter. Don’t try to fix it yourself. Gas appliances are dangerous if handled wrong.
How Long Can You Last Without Hot Water?
It depends on your system.
If you have a tank and it was recently heated, you might have 6-12 hours of warm water. Use it for washing hands, brushing teeth, quick rinses. Save showers for after the repair.
If you have a combi boiler? No buffer. Zero hot water. That’s when you start using kettles to wash dishes, or fill a basin with warm water from the tap - if the boiler still has a bit of residual heat.
Some people in Dunedin keep a 5-litre electric kettle on standby during winter. It’s not ideal, but it’s better than cold showers.
When to Replace vs. Repair
Boilers last 10 to 15 years. If yours is older than 12, and it’s breaking down often, replacement might be smarter than repair.
Here’s a quick guide:
| Condition | Repair? | Replace? |
|---|---|---|
| Boiler under 8 years old, one-off fault | Yes | No |
| Boiler over 12 years old, frequent breakdowns | No | Yes |
| Cracked heat exchanger | No | Yes |
| Cost of repair over 50% of new unit price | No | Yes |
| Boiler is noisy, inefficient, or has high bills | Maybe | Strongly consider |
New boilers in New Zealand are 90%+ efficient. Older ones can be as low as 65%. Replacing a 15-year-old boiler can cut your gas bill by 30% - and that pays for itself in a few years.
Prevent This From Happening Again
The best way to avoid a broken boiler in winter? Annual servicing. A registered gas fitter will clean the burner, check the flue, test the pressure, and spot small issues before they become big ones.
Also, keep the area around the boiler clear. Don’t store boxes or cleaning supplies on top of it. Overheating is a common cause of failure.
And if you live in a cold area like Dunedin, make sure your pipes are insulated. Frozen pipes can burst and flood your boiler - a costly disaster.
A broken boiler doesn’t always mean no hot water - but it usually does. The key is knowing your system, checking the basics, and not panicking. Most problems are fixable. And if it’s time to replace, do it right. A new boiler isn’t just about hot water. It’s about safety, efficiency, and peace of mind through the long, wet winters ahead.
Does a broken boiler always mean no hot water?
Not always. If you have a regular or system boiler with a hot water tank, you might still have hot water for several hours after the boiler breaks - as long as the tank was recently heated. But if you have a combi boiler, which heats water on demand, then yes - no boiler means no hot water.
Can I still use hot water if my boiler is off?
Only if your system has a hot water cylinder and it was heated recently. The water in the tank stays warm for hours, even without the boiler running. You’ll notice the water getting colder over time. If you have a combi boiler, the answer is no - you need the boiler to heat water instantly.
What should I do immediately when my boiler stops working?
First, check the power and gas supply. Then look at the boiler’s pressure gauge - if it’s below 1 bar, you can often fix it yourself by repressurising using the filling loop. Check for error codes on the display. If nothing works, turn off the boiler and call a registered gas fitter. Don’t attempt repairs yourself - gas systems are dangerous.
How long does a boiler last before needing replacement?
Most boilers last 10 to 15 years. If yours is older than 12 and breaking down often, it’s likely more cost-effective to replace it than keep repairing it. New boilers are much more efficient - they can cut your gas bill by 30% or more.
Is it safe to use a boiler that keeps turning off?
No. If your boiler keeps shutting off, it’s likely detecting a safety issue - like low pressure, overheating, or a gas leak. Continuing to use it can cause further damage or create a hazardous situation. Turn it off and get it checked by a professional.