Undersink Water Heater Not Working? Common Faults and Fixes

Which kind of heater do you have?

Small water heaters for one sink or basin come in two very different types, and the faults and fixes depend on which one is under your sink. Look at the label on the front or side of the unit, and at the pipework going into it.

• Vented, or open outlet, heaters are not under pressure. Cold water pushes hot water out, and they must be used with a special tap made for them. You can tell them apart by the tap, which has three pipes going to it, and the lack of any valves or small vessel next to the heater.
• Unvented heaters are small pressurised cylinders. They work with an ordinary tap, and you will see a group of valves on the cold pipe going in, usually a pressure reducing valve and an expansion relief valve, and often a small expansion vessel. Heatrae Sadia's Multipoint and Streamline and Ariston's Andris are names you will see on this kind.

No hot water at all

Start with the power. Undersink heaters are normally wired to a fused spur, a switch with a small fuse and often a red light, on the wall near the sink or in the cupboard. Check it is on, and check whether anything has tripped in the consumer unit. A kitchen spur is easy to knock off when a cupboard is being emptied.

If there is power and still no heat, the overheat cut out is the next suspect. Every one of these heaters has a safety cut out that switches the element off if the water gets far too hot. Some have a reset button behind the cover or on the thermostat. It is designed to trip rarely. If yours has tripped, something made it trip: a thermostat stuck on, scale around the sensor, or an element that runs hot. Resetting it once is reasonable if you are comfortable doing it with the power isolated. If it trips again, leave it off and get it looked at.

If neither of those is it, the element has probably failed, or the thermostat has stopped calling for heat. Both are checked with a meter by an engineer with the power isolated. If the heater also trips the RCD, the element is almost certainly the cause, and our guide for when an immersion heater keeps tripping explains why, since the element in an undersink heater is the same idea on a smaller scale.

Do not take the cover off an undersink heater with the power on. The terminals are live, the cupboard is often damp, and there is water inside the unit.

Water only lukewarm, or runs out fast

A small heater holds a small amount of water, so it is worth checking what it was sized for. One that serves a handbasin will not keep up with a kitchen sink being filled for washing up. If it used to cope and no longer does, something has changed.

Scale is the most common reason in London. The element sits in a small volume of water that is heated and reheated all day, and in hard water it quickly gets coated. A scaled element heats more slowly and the heater can shut off early because the sensor thinks the water is hotter than it is. Our guide to hard water and hot water cylinders in London explains what scale does inside any water heater, large or small.

The other causes are a thermostat turned down, often by accident when someone was in the cupboard, a failing thermostat, or on a vented heater a problem at the special tap that lets cold water mix in.

Dripping from the tap or the heater

On a vented heater, a small drip from the tap spout while the heater is warming up is normal. As water heats it expands, and with nowhere else to go it comes out of the spout. It stops once the water is up to temperature. If it drips all the time, even when the heater is not heating, the washer or cartridge in the tap is letting water past.

On an unvented heater, water should never come out of the tap on its own. Water dripping from the expansion relief valve or its discharge pipe means the heater cannot absorb the expansion as it heats, usually because the small expansion vessel has lost its air charge, the relief valve has a bit of debris or scale on its seat, or the pressure reducing valve has failed and is letting full mains pressure in. It is the same set of faults as on a full size cylinder, and our page on a pressure relief valve leaking explains how each shows up.

Water coming from the body of the heater, from the base or around the casing, is more serious. On a small heater it usually means the tank itself has corroded through, and a leaking tank is replaced rather than repaired.

Noises and smells

A crackle or hiss while the heater is on is usually scale on the element, the same thing that makes a kettle noisy. A loud bang or a knock is more likely to be water hammer in the pipes when the tap is shut quickly. A smell of rotten eggs from the hot water only, not the cold, points to a reaction inside the tank; see our guide if your hot water smells like rotten eggs. A burning or hot plastic smell from the cupboard is different: switch the heater off at the spur and leave it off.

Repair or replace?

Undersink heaters are simple, and a failed element, thermostat, cut out or relief valve can often be replaced. Whether it is worth doing depends on the age of the heater, the state of the tank and whether parts are still made for it. A tank that is leaking, or badly scaled and corroded after years in hard water, is a replacement.

When one does need replacing, it is a good moment to ask whether the same type is still the right answer. A heater that never kept up with a kitchen sink may be better replaced by a larger one, or the sink may be better fed from the main hot water system if it is close enough. Our guide to choosing an electric hot water cylinder covers the options for homes with no gas, and our pricing page explains how we charge before anyone comes out.

Who should work on an unvented undersink heater

An unvented heater is a pressurised vessel with safety valves, just smaller. The Building Regulations rules for unvented hot water, known as G3, apply to installations holding more than 15 litres, which covers some undersink and oversink models. The smaller ones fall outside the notification rules, but the valves on them do exactly the same job, and a heater with a relief valve that has been capped or bypassed is dangerous at any size.

We treat them the same way we treat a full size cylinder. The repair is done under our unvented cylinder repair service, and a G3 qualified plumber checks the relief valve, the pressure reducing valve and the expansion vessel before the job is signed off. A vented heater with a faulty element or thermostat is handled under our immersion heater repair work.

When to call us

Call us on 020 4652 5111 or send an enquiry. Tell us the make and model from the label, whether it has valves on the cold pipe going in, what the fault is and whether anything has tripped. A photo of the label and the pipework under the sink tells us most of what we need. We have 20 engineers across London, and our rate is on the pricing page and agreed before anyone attends.