Hot Water Not Hot Enough? Lukewarm Versus Running Out

Hot water that is not hot enough usually fits one of three patterns, and each points somewhere different. Water that never gets properly hot at any tap points to the heating side: a setting, a supply or a part. Water that runs hot at the kitchen but lukewarm at the shower points to the valve that controls bathroom temperature, not the heater. Water that starts hot and runs out sooner than it used to points to demand, tank size or the time the system needs to reheat.

The type of system shifts the likely cause again. Storage systems, whether electric, gas, heat pump or solar, heat water and keep it in an insulated tank, so all of them can run short. A gas continuous flow unit heats water as it passes through, and Your Home, the Australian Government's guide to sustainable homes, notes that instantaneous systems cannot run out of hot water. A continuous flow unit that turns cold mid-shower has a flow problem instead, covered further down. Start with the pattern, then read the section for your system.

If even the kitchen tap never runs properly hot, the water heater itself is not delivering its usual temperature. Rheem's troubleshooting guide gives heavy hot water use as the first cause. Its owner's guide for its 48 and 49 series electric tanks says the same thing in more detail: heavy use can leave the water cooler than expected because insufficient heating time was allowed.

Some systems are built to run cooler. Rheem notes that with a 50°C temperature limited water heater, hot water to the whole house is limited to 50°C, kitchen included. That is the design, not a fault.

Turning the thermostat up is not an owner job on an electric tank. On those Rheem models the thermostat sits inside the front cover, adjustment requires a licensed electrical worker, and the maximum setting is 70°C. Rheem also warns that hotter water increases the risk of scald injury. If the temperature has dropped with no change in how much hot water you use, a fault is the more likely explanation, and the electric hot water systems page describes the common ones.

When the kitchen runs hot but the bathroom is lukewarm, the heater is working and the cause is usually the valve between it and the bathroom. Rheem's owner's guide explains that a temperature limiting device fitted to the pipe serving the bathroom and ensuite keeps water there below 50°C, while still allowing hotter water to the kitchen and laundry, and says regulations may require one. Some difference between the two taps is therefore normal.

A big drop is not. Rheem's troubleshooting guide names a faulty tempering valve that is excessively cooling the hot water as a cause of water that is not hot enough, and says a plumber can test the system and replace the valve if necessary. The tempering valve guide explains how these valves work, and why a solar or heat pump tank that is not reaching temperature can make a healthy valve look faulty.

Adjusting or replacing the valve is not owner work. The QBCC says most plumbing work in Queensland is regulated and requires a QBCC licensed plumber.

An electric tank that runs out earlier than it used to is usually being asked for more than it holds or can reheat. Rheem's owner's guide says that very often the amount of hot water used is not realised, particularly when showering, and suggests having a plumber fit a flow control valve to each shower outlet. Your Home puts typical use at about 50 litres of hot water per person per day, and warns that a smaller tank may run out when there are visitors.

The tariff matters too. Your Home says electric tanks larger than 250 litres are better suited to controlled load, because a smaller one may run out before the controlled load circuit switches on to recharge it.

A twin element tank has its own check. Rheem explains that the bottom element normally supplies all the hot water, usually on an off-peak or time controlled supply, while the top element runs on a continuous supply and only operates in periods of high demand. Power must be available to the top heating unit circuit at all times for the tank to work as designed, so check that the switchboard circuit marked hot water or water heater is on.

A heat pump that runs short is usually being asked to reheat faster than its settings allow. Rheem's owner's guide for one of its heat pump models says heavy hot water use can leave the water cooler than expected because insufficient heating time was allowed, and that where a timer has been set, it must allow sufficient time to reheat the tank. A timer window that suited a smaller household can leave a bigger one short.

Many heat pumps carry a backup for exactly this. Your Home explains that many heat pump systems include a boost element that heats water directly during very cold weather or periods of high hot water use. Your Home also says air-source heat pumps work best in relatively warm, humid climates. In a warm, humid climate, a heat pump that runs short points first to the timer setting and the household's hot water use rather than the weather.

Fault codes, noise and running costs are covered in the heat pump problems guide.

A solar system that runs lukewarm on cloudy days, in winter or first thing in the morning usually needs its booster to do more of the work. Your Home explains that most solar systems come with a gas or electric booster to provide hot water on cloudy days or when demand exceeds supply, and that boosters can be switched manually or controlled by a thermostat. It also notes that water cools overnight, so a household that uses most of its hot water in the morning may rely on the booster before the sun has had a chance.

Rheem's advice for lukewarm solar water is to check that the booster is being used as the installation instructions for your model say, and that any booster timer allows enough time to heat the water the household needs.

Shading is the slower cause. Your Home says solar collectors should not be shaded by trees or nearby buildings for long periods, particularly in winter when the sun is low in the sky.

A gas continuous flow unit that swings between hot and cold during a shower is usually not getting enough water through it to keep running. Rheem's troubleshooting guide says fluctuating temperature means the hot water flow is not adequate to keep the heater operating, names flow restrictors and water saving low flow taps as causes, and says a plumber can increase the hot water flow rate to a minimum of two to three litres per minute.

Your Home adds that some instantaneous gas systems operate erratically at low flow rates, especially when the incoming water is relatively warm, as in warm climates or when it has been pre-warmed by solar. It advises checking that a water-efficient showerhead is compatible and does not reduce flow excessively. Your Home also notes that standard units deliver adequate hot water to only one or two outlets at a time.

The QBCC lists replacing a shower head as unregulated plumbing work in Queensland. Raising the flow at the unit or its pipework is a job for a licensed plumber.

A storage tank that empties faster than it should may be losing hot water nobody is using. Rheem's owner's guide notes that even a small leak will waste a surprising quantity of hot water and electricity, so a dripping hot tap or a weeping joint on a hot pipe matters more than it looks. Rheem asks owners to have a plumber fix any leaking pipework.

The relief valve is the other place to look. Some discharge while the tank heats is normal, but Rheem says that if the valve discharges more than a bucketful of water in 24 hours, there may be another problem. Rheem's owner's guide lists gently raising and lowering the valve's easing lever every six months as minor maintenance the occupant can do, and the relief valve guide explains how. A valve that keeps running, or one that discharges more than a bucketful in 24 hours, is a job for a licensed plumber.

An owner's checks stop at what you can see and switch. Rheem asks owners to do only the basic maintenance specified in the owner's guide, and says all repairs must be made by an approved plumber. The safe checks are the circuit breaker marked hot water or water heater at the switchboard, the timer and booster settings, any leaking pipe or dripping hot tap, and how much hot water the household has been using lately.

Stop there. Thermostats, elements and wiring are electrical work: Queensland's Electrical Safety Office says to always get a licensed electrician to do any electrical work, and under the Electrical Safety Act 2002 electrical work includes testing, maintaining, repairing and replacing electrical equipment. Valves and the tank are plumbing work for a licensed plumber.

When you call, say which pattern you have, which taps are affected, what kind of system it is (a photo of the data plate helps) and when the change started.