Written by JSW Plumbing | Published on June 2025
An undersink water heater is a compact hot water unit installed directly beneath a sink, delivering hot water almost instantly at that tap without waiting for it to travel through long pipes from a central system. These units are popular across Hornsby, Thornleigh and the Hills District, where many homes have kitchens or bathrooms positioned far from the main hot water system.
In this guide, we cover how undersink heaters work, the different types available, typical installation costs in our area, and how to decide whether one is right for your home.
Understand What an Undersink Water Heater Actually Does
A standard hot water system heats water in one central location, usually the garage, laundry or outside wall. When you turn on a tap at the far end of the house, the cold water sitting in the pipes has to clear before hot water arrives. This can take 30 seconds to over a minute, wasting water and time.
An undersink water heater solves this by heating water right at the point of use. It sits in the cabinet beneath your sink and supplies that single tap or fixture directly. You get hot water within a few seconds of turning the tap on.
These units are also called point-of-use water heaters or mini storage heaters, depending on the type. They are not designed to replace your main hot water system. Instead, they work alongside it to eliminate the wait at specific outlets.
Compare the Two Main Types of Undersink Heaters
There are two primary types of undersink water heaters available in Australia: storage and instantaneous (also called continuous flow). Each works differently and suits different situations.
| Feature | Storage Undersink Heater | Instantaneous Undersink Heater |
|---|---|---|
| How it heats water | Preheats and stores a small tank (4-30 litres) | Heats water on demand as it flows through |
| Hot water delivery | Immediate, up to tank capacity | Continuous, no tank limit |
| Power requirement | Low draw, runs off standard power point | Higher draw, may need dedicated circuit |
| Best suited for | Kitchens, bar sinks, granny flats | Bathrooms, high-use outlets |
| Unit cost (AUD) | $150 – $500 | $300 – $900 |
| Running cost | Higher (always keeping water warm) | Lower (only heats when needed) |
Storage models are the most common choice for kitchens in the Hills District. They are simple, affordable, and run off a standard 10-amp power point. Instantaneous models cost more upfront but can be more efficient long term, especially in high-use situations.
Explore How an Undersink Heater Is Installed
Installation involves both plumbing and electrical work, which means you need licensed tradespeople for both components. In New South Wales, it is illegal to carry out plumbing or electrical work yourself unless you hold the appropriate licence. This applies to undersink heater installations in Thornleigh, Hornsby, and all areas of the Hills District.
Here is the general installation process a plumber like JSW Plumbing would follow:
- Assess the under-sink cabinet space and existing pipe layout.
- Shut off the cold water supply to the sink.
- Fit a cold water inlet valve on the supply line feeding the heater.
- Connect the hot water outlet from the heater to the hot tap on the sink.
- Install a pressure relief valve as required by Australian standards (AS/NZS 3500).
- Connect the unit to a power supply (a licensed electrician handles this step).
- Test for leaks and check temperature output at the tap.
Most installations are completed within 1 to 2 hours when the space is accessible and no extra electrical work is required. If a new power circuit is needed, an electrician visit adds time and cost to the job.
Review Typical Costs for Undersink Heater Installation in Hornsby and the Hills District
Costs vary depending on the unit you choose, the complexity of the install, and whether any electrical upgrades are needed. Here is a general cost breakdown for the Hornsby, Thornleigh and Hills District area as of 2025.
| Cost Item | Estimated Cost (AUD) |
|---|---|
| Storage undersink heater unit (4-15L) | $150 – $400 |
| Instantaneous undersink heater unit | $300 – $900 |
| Plumbing labour (installation) | $150 – $300 |
| Electrical connection (if needed) | $100 – $250 |
| Pressure relief valve and fittings | $30 – $80 |
| Total typical range | $430 – $1,200+ |
These figures are estimates. Your actual cost depends on access, existing pipework, and the unit brand you select. JSW Plumbing can provide a firm quote before any work starts, so there are no surprises on the invoice.
Identify the Right Situations for an Undersink Heater
Undersink heaters are not always the right answer. Knowing when they make sense helps you spend your money wisely.
Good candidates for an undersink water heater include:
- Homes in Thornleigh or the Hills District where the kitchen is at the far end of the house from the main hot water system.
- Granny flats or secondary dwellings where running new hot water pipes to a separate system is expensive.
- Home offices or workshops with a small sink that rarely needs hot water from the main system.
- Older homes where pipe insulation is poor and heat loss is significant along long runs.
- Rental properties where tenants have complained about slow hot water to a specific outlet.
Situations where an undersink heater is not the best fit:
- You need hot water for a shower or bath. These units are sized for sinks, not high-volume fixtures.
- Your main hot water system is already undersized or failing. An undersink unit will not compensate for a system that needs replacing.
- Cabinet space is very limited. Some models require a minimum clearance that may not be available under a compact vanity.
Understand the Maintenance Requirements for Undersink Heaters
Like any hot water appliance, undersink heaters need occasional attention to keep working reliably. The good news is that maintenance is straightforward compared to larger systems.
Key maintenance tasks include checking and replacing the anode rod (in storage models) every 3 to 5 years, flushing sediment from the tank annually if you have hard water, and testing the pressure relief valve (TPR valve) once a year to make sure it opens and closes properly.
Water quality in parts of the Hills District and Hornsby can affect how quickly sediment builds in storage-style units. If you notice reduced hot water output or lukewarm temperatures from the unit, sediment buildup or a failing element is often the cause.
JSW Plumbing recommends booking a check on your undersink heater every two years as part of your general home plumbing maintenance. Catching a small issue early is always cheaper than an emergency callout.
FAQs on Undersink Water Heaters
Can I install an undersink water heater myself in NSW?
No. In New South Wales, all plumbing and drainage work must be carried out by a licensed plumber. Electrical connections also require a licensed electrician. Attempting a DIY installation can void your warranty and may not meet the requirements of AS/NZS 3500, which applies across Hornsby and the Hills District.
How long does an undersink water heater last?
Most storage undersink heaters last 8 to 12 years with proper maintenance. Instantaneous models can last slightly longer, often 10 to 15 years, because they have fewer parts under constant thermal stress. Regular servicing extends lifespan significantly.
Will an undersink heater increase my electricity bill?
A small storage undersink heater adds roughly $30 to $80 per year to your electricity costs, depending on usage and the unit’s energy rating. Instantaneous models use no energy on standby, so they can be cheaper to run if the outlet is used infrequently. Always check the energy star rating before purchasing.
What size undersink heater do I need for a kitchen sink?
A 4 to 10 litre storage unit is usually sufficient for a standard kitchen sink used for handwashing and food prep. If the outlet is also used for filling pots or a dishwasher connection, a 15 to 25 litre unit or an instantaneous model may be more appropriate. A plumber can recommend the right size after assessing your usage.
Is a pressure relief valve required on undersink heaters in Australia?
Yes. Australian Standard AS/NZS 3500 requires a temperature and pressure relief (TPR) valve on all storage water heaters, including undersink models. This valve prevents dangerous pressure buildup inside the tank. Your licensed plumber must fit and test this valve as part of any compliant installation.
Can an undersink heater be used for a bathroom basin?
Yes, undersink heaters work well for bathroom basins, particularly in ensuites or bathrooms located far from the main hot water system. They are sized for basin use, not showers. If you want faster hot water to a shower, a different solution such as a recirculating pump or a larger point-of-use system would be more appropriate.
Wrap Up and Next Steps
An undersink water heater is a practical, affordable solution for eliminating hot water delays at specific outlets in your home. Whether you choose a storage or instantaneous model, the right unit installed correctly will save water, reduce frustration, and add convenience to your daily routine.