Heat Pump No Hot Water? What to Check and Who to Call

Check these before you call anyone

Plenty of no hot water problems on heat pump systems turn out to be settings rather than faults. Heat pump controllers are more complicated than a boiler timer, and a power cut, an update or a change to the heating schedule can leave hot water switched off without anyone noticing until the tank runs cold.

• Look at the controller or app. Is hot water (often labelled DHW) switched on, and is the schedule set to heat the tank at the times you expect? Holiday or away mode often turns hot water off completely.
• Check the time and date on the controller. After a power cut some units lose the clock, so the hot water period runs in the middle of the night or not at all.
• Look for an error code or warning light on the controller, and write it down. It is the single most useful thing to have when you ring.
• Check the fuse board. Most heat pumps have their own circuit breaker, and many have an isolator switch outside next to the unit. The immersion usually has a separate switch and breaker too.
• Look at the pressure gauge on the heating circuit, usually near the cylinder or on the indoor unit. Like a boiler, a heat pump sealed system shuts down if the pressure drops too low.
• Go outside. Frost on the fins of a running unit is normal in cold weather and clears itself. A silent, dark unit when it should be running is not.

Heating works but no hot water: the diverter valve

Most heat pump systems have one motorised valve that sends the heated water either to the radiators or to the cylinder coil, not both at once. When the controller asks for hot water, the valve should swing across to the cylinder. If it sticks in the heating position, the heat pump runs and the house gets warm, but the tank never heats. If it sticks halfway, you tend to get lukewarm water and radiators that warm up when only hot water is on. That second pattern is the same fault we describe in our guide to radiators hot when hot water is on, just on a heat pump rather than a boiler.

Valve motors and their microswitches wear out, and the valve body can stick after a quiet summer. On many systems it sits right next to the cylinder and is a straightforward part to test and replace.

The cylinder sensor has moved or failed

A heat pump decides when to heat the tank from a temperature sensor pushed into a pocket in the side of the cylinder. If that sensor has worked loose, been knocked out when someone was in the cupboard, or failed, the controller gets the wrong reading. It may think the tank is already hot and never start a hot water cycle.

The tell tale sign is a controller showing a tank temperature that does not match the tap. If the display says the water is hot and the tap runs cold, the sensor and its wiring are the first things to check.

The immersion has failed or keeps tripping

Heat pump cylinders nearly always have an immersion heater, as a backup and, on many systems, for the regular high temperature cycle that keeps Legionella under control. If it has failed, you may still get warm water from the heat pump alone, but the tank never gets fully hot.

An immersion that trips the electrics every time it switches on usually has a failed element. Our guide on immersion heater keeps tripping explains what that means, and the one on immersion heater not working covers the other faults: a blown thermostat, a tripped high limit or a switch that has burned out. On an unvented heat pump cylinder, changing the element means draining down part of a pressurised vessel, so it is a job for someone qualified to work on unvented systems.

Hot water, but not hot enough, or it runs out

Heat pumps usually store water at a lower temperature than a gas boiler would, because they work far more efficiently that way. Water that feels warm rather than very hot can be normal, especially in the first winter after a boiler was taken out. What is not normal is a tank that never gets up to the target temperature on the controller, or one that suddenly behaves differently from how it did last month.

If the water is lukewarm, check the hot water target temperature on the controller first. Our guide on what temperature should a hot water cylinder be sets out the Legionella side of that setting. If it runs out early in the evening, look at the schedule, the size of the cylinder and whether the heat pump is sharing its output with the heating on a cold day. Our guide to why your hot water runs out quickly covers the cylinder side, and how long does hot water take to heat up explains why a heat pump reheats more slowly than a boiler. A standard boiler cylinder left in place when the heat pump went in will always struggle, and our guide to the heat pump cylinder explains why.

Faults on the unvented side

The cylinder carries the same safety kit as any unvented tank: a pressure reducing valve, an expansion vessel, a temperature and pressure relief valve and a tundish. A dripping tundish, water running from the discharge pipe outside, or hot water pressure that has collapsed are cylinder faults, not heat pump faults.

If you see water in the tundish, read our page on a tundish dripping and never cap or block the discharge pipe. Low pressure at the taps with the heat pump otherwise working is covered in our guide to low hot water pressure, and is very often a blocked strainer in the inlet group.

If the cylinder is making loud banging or rumbling noises, the relief valve is discharging very hot water, or the pipework near the tank is too hot to touch, switch the heat pump and the immersion off at the controller and the fuse board, and call us.

Who fixes what on a heat pump hot water system

This is where people lose time: the installer may not come for days, and a general plumber may not want to touch anything connected to a heat pump. The practical split looks like this.

• The outdoor unit, its refrigerant circuit, fan, compressor, and any error that points to them: the heat pump installer or manufacturer, who will also want to protect the warranty.
• Controller settings and schedules: you can often fix these yourself from the manual or the app, and the installer can confirm the set up.
• The cylinder, its safety valves, expansion vessel, tundish and discharge pipe, the inlet group and strainer: an engineer qualified for unvented cylinder repair.
• The immersion, the diverter valve on the water side, leaks on the cylinder pipework, and the cylinder sensor pocket: our engineers deal with these as routine cylinder work.

Getting hot water back in the meantime

If the cylinder has a working immersion and nothing is leaking or discharging, switching the immersion on will usually heat the tank without the heat pump. It costs more to run, so treat it as a stopgap. If the immersion is on and the tank still does not heat, the immersion is probably part of the problem. Do not keep resetting the high limit thermostat: it trips for a reason, and on an unvented cylinder that reason needs finding.

When to call us

Call 020 4652 5111 or send an enquiry if the tank stays cold with the settings checked, if the immersion keeps tripping, if the tundish is dripping, or if the controller points to the cylinder, the sensor or the diverter valve. A photo of the cylinder data label and of any error on the controller helps us bring the right parts. We have 20 engineers across London used to unvented cylinders, and our rate is on the pricing page, agreed before anyone attends. If the fault turns out to be inside the heat pump itself, we will tell you so rather than charge you to guess.