First, where and when
Is it every hot tap, or only some? If only the bathroom is lukewarm, the cylinder may be fine and the valve that tempers bathroom water is a suspect. If every tap is lukewarm, look at the cylinder and whatever heats it.
Is it always, or only at certain times? Water that starts hot and goes cool fast suggests the cylinder can't keep up, or that something is drawing hot water you don't know about. Problems that only show up on frosty mornings, cloudy weeks or when the fire's out point at the heat source.
Checks you can do
Try every hot tap
Run the kitchen, laundry, basin and shower in turn, and note which ones fall short.
Hunt for drips and leaks
Check hot taps, the shower, any pipes you can see and the floor around the cylinder. Rheem warns that even a small leak wastes a surprisingly large quantity of hot water and energy.
Check the cylinder's switch
Make sure its switch at the switchboard is on. Reset a tripped one once, and only once.
Check what's meant to heat it
Is the fire lit for the wetback, the solar boost switched on, the heat pump running? With the fire out, the element has to do all the heating.
Think about what's changed
New people in the house, a new shower head or a new bath? More use can outrun a cylinder that used to cope.
What a tradie will look at, by type
Tempering valve. Rheem names a faulty tempering valve as one reason hot water isn't hot enough, and says a local plumber can test the system and replace the valve. See tempering valves.
Element or thermostat, in an electric cylinder. These do the heating and switching. Getting at them means removing covers, which Rheem leaves to a suitably qualified person. See thermostats and elements.
Heat pump. It draws warmth from the outside air, and in very cold weather Rheem's units call on an electric back-up element. Tell the tradie if the problem is worse on cold mornings.
Solar. On cloudy days the gas or electric boost has to carry the load. If the boost isn't working, a dull week can mean cool water.
Wetback. Rheem says where the fire sits relative to the cylinder is crucial to a wetback working. If it has never heated well, or has got worse, a plumber needs to see the layout.
Gas. A heater short of gas can't heat properly. Rheem also says continuous flow needs a larger gas pipe from the meter than most storage heaters, so mention it if a califont struggles when two taps run.
Don't turn the thermostat up yourself
It's tempting, but Rheem's owner's guide says an authorised person must make any temperature adjustment on its electric cylinders. The Building Code also requires water at personal washing fixtures to avoid the likelihood of scalding. If the setting is the issue, an authorised plumber should check it alongside the tempering valve. More on cylinder temperature.
What's yours to check
You can
- Run each hot tap and note which are lukewarm.
- Check the cylinder switch, and reset a tripped one once.
- Look for drips, damp patches and water in the safe tray.
- Confirm the fire, solar boost or heat pump is on and working as usual.
- Photograph the rating label.
Leave to an authorised tradie
- Testing or replacing the tempering valve.
- Anything under the cylinder's covers, such as the element or thermostat.
- Changing the temperature setting.
- Gas supply, burners and gas pipework, which are for an authorised gasfitter.
- Fixing leaking hot water pipework.
Questions to ask the tradie
- Is this the tempering valve, the cylinder or the supply?
- Have you tested the element and the thermostat?
- Is the cylinder big enough for how we use it now?
- Could a leak be wasting hot water somewhere we can't see?
- If you change the setting, what will it be, and why?
What we would do
Write down which taps are lukewarm and when, because that pattern narrows the search quickly. Then leave the covers and the dial alone, and book an authorised tradie who specialises in hot water cylinders. Ask them to rule out parts such as the tempering valve, element and thermostat before anyone talks about a new cylinder.