How Long Does Hot Water Take to Heat Up?

The straight answer, by cylinder size and heat source

Heating water is simple physics and it does not negotiate. Raising one litre of water by one degree takes a fixed amount of energy, so the time is just the energy required divided by the power going in. Heating a full cylinder from cold mains at around 15 degrees up to a storage temperature of 60 degrees means a 45 degree rise, and that gives you the figures below.

These are timings from genuinely cold, which is the worst case. It happens after the system has been off for a day, after a cylinder has been drained and refilled, or after someone has run three baths back to back. Most of the time you are not doing that. You are reheating a tank that is still warm in the upper half, and a partial reheat takes a fraction of the time, often 20 to 30 minutes on an immersion and under 15 minutes on a boiler coil.

The other figure worth knowing is that a cylinder does not need to be fully hot to be useful. Because hot water floats on cold and the layers barely mix, the top third of a cylinder comes up to temperature well before the rest of it. On a 150 litre tank you usually have enough for a shower after around 40 minutes on the immersion, long before the whole thing is up to temperature. An electric thermal store of the Pulsacoil type sits outside this table altogether, because it heats mains water as it passes through a plate exchanger rather than storing the water you wash in, so a Pulsacoil repair in London is normally about flow and the blender rather than about reheat time at all.

• 120 litre cylinder, 3kW immersion, from cold: about 2 to 2 and a half hours
• 150 litre cylinder, 3kW immersion, from cold: about 2 and a half to 3 hours
• 210 litre cylinder, 3kW immersion, from cold: about 3 and a half to 4 hours
• 300 litre cylinder, 3kW immersion, from cold: 5 to 6 hours, which is why big tanks are rarely run on the immersion alone
• 150 litre vented cylinder on a boiler coil: roughly 45 to 90 minutes
• 210 litre unvented cylinder on a high output coil: roughly 25 to 40 minutes
• Partial reheat after a shower, any of the above: a third to a half of the full time

How long does an immersion heater take to heat up

Almost every domestic immersion element in the UK is rated at 3kW. A few are 2.75kW or 3.6kW, and the rating is usually stamped on the head under the plastic cap, but 3kW is the number to assume. That single figure is the reason immersion heating is slow: 3kW is about the same as two kettles, and you are asking it to bring a bathful and a half of water up to temperature.

Work it through for a 150 litre cylinder and you need close to eight kilowatt hours of energy for a 45 degree rise. At 3kW that is a shade over two and a half hours of continuous running, before you allow anything for heat escaping through the insulation while it works. In a warm airing cupboard with a decent factory foam jacket, those losses are small and the real world answer lands near three hours. On an older copper cylinder with a tired lagging jacket, the losses are not small at all and the same tank can take an extra 30 to 45 minutes.

The element itself does not heat up gradually. It reaches full temperature within seconds of switching on, and the pipe leaving the top of the cylinder starts to warm within about 15 to 20 minutes as the first heated water rises. That early warmth is the useful diagnostic. If nothing at the top of the cylinder is even slightly warm after 25 minutes with the immersion switched on, you are not waiting for a slow heater, you are looking at a dead element, a stuck rod thermostat or a tripped thermal cut-out. An immersion heater repair starts from exactly that point, with the element, the rod thermostat and the cut-out each tested in turn on a meter before any of them is condemned, and all three come out of the head without the cylinder being drained.

One detail catches people out on older installations. Many cylinders have two elements, a short one near the top and a long one near the bottom. The top element only heats the water above it, which is perhaps a third of the tank, and it does that in well under an hour. The bottom element heats the lot and takes the full time. If your hot water arrives quickly but runs out after one shower, you may simply be running on the top element while the bottom one sits dead. The complaint that reaches us in that situation is usually no hot water in the shower rather than a slow cylinder, because a third of a tank is exactly the volume a shower gets through before anyone notices.

Never switch an immersion heater on if the cylinder has been drained or is only part filled. The element is designed to be surrounded by water at all times and will destroy itself within seconds if it is run dry, sometimes distorting the boss it screws into as well. If anyone has drained the tank, leave the switch off until it is confirmed full.

How long should an immersion heater be on for

Long enough to satisfy the thermostat, and no longer. That is the honest answer, and it is worth understanding why, because the two habits people settle into are leaving it on permanently and switching it on for a fixed 30 minutes, and both waste money in different directions.

Every immersion has a rod thermostat buried inside the element, adjustable with a small screw behind the cap, normally set to 60 degrees. When the water around it reaches that temperature, the thermostat opens the circuit and the element stops drawing power. It does not keep burning electricity for as long as the switch is on. It cycles on for a few minutes at a time to top the cylinder back up as heat leaks away. So leaving an immersion on all day is not the disaster people imagine, but on a poorly insulated cylinder it does mean paying to reheat water you are not using, several times over.

The useful approach is to match the run time to what you actually need. For one shower, 40 minutes to an hour from cold is normally plenty, because the top of the tank stratifies first. For a full tank of baths, give it the full two to three hours. If you are on Economy 7 or a similar night rate, the sensible pattern is to let the lower element charge the whole cylinder overnight on the cheap rate and use the upper element as a short daytime top up if you run short.

What you should not do is turn the thermostat down below 60 degrees to save money. Storage cylinders are held at 60 specifically because Legionella bacteria cannot survive it, and water sitting for days at 40 to 45 degrees is the one condition that lets them multiply. Turn the storage temperature down and you have traded a small saving for a genuine health risk. If the water at the tap is scaldingly hot, the fix is a thermostatic blending valve on the outlet, not a lower cylinder setting. A thermostat that has failed closed is a different matter again, because it takes the system into hot water cylinder overheating, where the element never switches off at all and the safety chain ends up doing work it was only ever meant to do as a last resort.

• One shower from cold: 40 minutes to an hour on a 3kW immersion
• Full tank for baths: the full two to three hours
• Night rate tariff: let the lower element run the overnight window, top up with the upper element if needed
• Cylinder thermostat: leave it at 60 degrees, fit a blending valve if the tap runs too hot

Why the boiler heats the same cylinder in a fraction of the time

When a boiler heats your cylinder it does not put an element in the water. It pumps hot water from the heating circuit through a coil of pipe sealed inside the tank, and the coil hands its heat across to the stored water through the pipe wall. The pace of that transfer is set by the coil surface area, the flow rate through it and the temperature difference between the two sides.

On an older vented copper cylinder with a modest single coil, the practical transfer rate is somewhere around 5 to 8kW no matter how big the boiler is. That is still double or more what an immersion manages, which is why the same 150 litre tank comes up in 45 to 90 minutes rather than three hours. Fitting a bigger boiler does not speed this up. The coil is the bottleneck, not the heat source.

Modern unvented cylinders are built with much larger finned or twin coils, rated at 20kW and upward, precisely so that a full reheat fits inside the half hour between one person showering and the next. This is the single biggest practical difference people notice when an old vented tank is replaced with a properly sized unvented one, and it matters far more day to day than the pressure improvement everyone talks about.

There is a sequence that has to complete before any of this starts. The programmer has to call for hot water, the cylinder thermostat strapped to the side of the tank has to agree the water is below its setting, the motorised valve has to open fully and close its microswitch, and only then does the boiler fire and the pump circulate. A cylinder that never gets warm at all from the boiler, while the radiators work perfectly, is almost always a break in that chain rather than a slow heat up, and that exact fault, no hot water but the heating works, has its own page working through the valve, the stat and the wiring in the order the signal travels. Where the boiler will not fire for anything at all, heating included, the question stops being about the cylinder and becomes boiler diagnosis instead.

How long a combi boiler takes to give you hot water

A combi stores nothing. It heats water on demand as it flows through a plate heat exchanger, so the correct answer is a few seconds of burner ignition plus however long it takes the already hot water to travel down the pipe to your tap. If the boiler is in the kitchen and the tap is in the kitchen, that is five to ten seconds. If the boiler is in a loft and the tap is in a ground floor cloakroom at the far end of the house, you may be flushing a lot of cold pipework first and waiting 30 seconds or more, which is a pipe run problem and not a boiler problem.

What a combi should never do is take minutes. If you are standing at a shower waiting several minutes for warmth, or the temperature swings hot and cold while you stand there, that points at a fault: a scaled plate heat exchanger, a diverter valve that is not moving fully across to the hot water position, or a flow sensor that is not registering demand reliably. All three are common and all three get worse gradually, so people often do not notice how long the wait has crept out to.

Combi output is also rated in litres per minute at a given temperature rise, and that figure drops in winter. A boiler quoted at 12 litres a minute is quoted on a 35 degree rise, and in January the incoming mains can be close to 5 degrees rather than 15. The same boiler now has to work harder for each litre, so it throttles the flow to hold temperature and the shower feels weaker. That is the appliance protecting the temperature you asked for, not a fault.

Why hot water takes longer to heat up than it used to

If your hot water used to be ready in an hour and now takes three, nothing about the physics has changed. Either less power is going in or more heat is escaping. There is a short list of causes and most of them are visible to an engineer within the first 20 minutes of a visit.

Scale is the first and by far the most common in London. Almost all of the capital sits on hard water, and every litre that passes through leaves a little calcium carbonate behind. On an immersion element the scale builds as a thick crust on the sheath and acts as insulation, so the element runs hotter, transfers less into the water and eventually cooks itself. On a boiler coil the scale forms on the outside of the coil and does exactly the same thing, quietly halving the transfer rate over a few years. A scaled element often announces itself by crackling and popping as pockets of water flash to steam under the crust. A hot water cylinder making noise while it heats is usually that same crust giving itself away, and it is worth acting on at that stage rather than later, because the deposit only ever gets thicker and the element behind it only ever runs hotter.

The second is one dead element out of two. Nothing dramatic happens when the lower element fails. You still get hot water, it just comes from a third of the tank instead of the whole of it, and you get noticeably less of it. Plenty of households run like this for a year or more before anyone puts the two symptoms together.

The third is a system fault on the boiler side: a sludged or partially blocked coil, a pump that has weakened, or a motorised valve that opens only part way so the flow through the coil is reduced. All three leave the cylinder heating, just far more slowly than it should.

The fourth is insulation, and it is the one people forget. A factory foamed cylinder loses very little. An old copper cylinder with a loose or compressed lagging jacket can be shedding a meaningful fraction of the heat you are paying to put in, and in an unheated cupboard on an outside wall it is worse again. Uninsulated pipework leaving the top of the cylinder makes its own contribution, because hot water will happily circulate up the pipe and back by gravity all day, radiating heat into a loft.

• Limescale on the element or the coil, the usual London culprit
• One of two immersion elements failed, usually the lower one
• Sludge in the coil or a weak pump on the boiler side
• A motorised valve opening only part way
• Failed or compressed cylinder insulation and bare pipework at the top of the tank
• A cylinder thermostat set low, or one that has drifted and switches off early

How to time your own cylinder before you call anyone

This is one of the jobs where an hour of your own attention gives an engineer more useful information than any amount of description, and it may well tell you there is nothing wrong at all. It costs nothing and needs no tools beyond a clock and your hand.

Run the hot taps until the water goes cold, so you know you are starting from a genuinely cold tank rather than guessing. Note the time. Switch on one heat source only, either the immersion at its wall switch or the boiler with hot water set to constant, and leave everything else alone. Do not draw any hot water while it runs.

After 20 minutes, put a hand on the pipe leaving the top of the cylinder. It should be clearly warm. If it is not, stop there, because the heat source is not working and waiting longer proves nothing. If it is warm, check again at 45 minutes, then at 90 minutes, then run the hot tap and see what you have got.

Write down what happened, including which heat source you used and whether you have one element or two. Compare it against the timings at the top of this page. A 150 litre tank that reaches full temperature on the immersion in three hours is working exactly as designed and does not need an engineer. The same tank taking five hours, or producing one shower and then running cold, is telling you something specific and has just saved a good part of a diagnostic visit.

If the pipe at the top of the cylinder is completely cold after 25 minutes with the immersion running, switch the immersion off at the wall and leave it off. Repeatedly switching a suspect element back on is how a failing one goes from a simple replacement to a tripping circuit that takes the rest of the house down with it.

Does a slow cylinder mean the tank is near the end of its life

Usually not. Slow heating is a heat input problem, and the heat input parts are all replaceable. Elements, thermostats, pumps and motorised valves are consumables and swapping one does not mean touching the cylinder itself.

Age is still worth knowing, because it changes what is worth spending on. Older vented copper cylinders commonly give well over a decade of service and many go considerably longer, while a good stainless unvented unit that has been serviced properly usually goes longer still. What shortens either of them in London is hard water, which scales the coil and the element continuously, and in an unvented cylinder a failed expansion arrangement that lets the vessel see pressure cycling it was never meant to take.

The signals that point at the cylinder rather than its parts are different from slow heating: rusty or discoloured hot water, weeping from the seam or around the immersion boss, a tundish that drips repeatedly, or repeated element failures over a short period. Slow heating on its own is a reason to test the element and check for scale, not a reason to replace a tank. If a replacement genuinely is the answer, the volume that goes back in deserves a decision of its own rather than a repeat of whatever the original installer happened to fit, because a household that has gained a bathroom or a decent shower since then will run the same tank out faster than it ever ran out before.

When slow hot water is genuinely a fault

Use the normal timings as your yardstick. A wait that matches the figures above is the system working. A wait that is double what it should be, or a heat source that produces nothing at all, is a fault with a short list of causes.

A cylinder that produces nothing at all has stopped being a timing question, because no amount of waiting turns a dead heat source into a warm tank. Call someone when the pipe at the top of the cylinder stays cold with the immersion on, when the breaker or RCD trips the moment you switch the immersion on, when the boiler fires happily for the radiators but never for hot water, when the cylinder crackles or bangs while heating, or when the time to heat has stretched out over a few months without anything else changing. Each of those points at a definite component and each is diagnosed by testing rather than by guesswork.

What an engineer does first is confirm which heat source is meant to be working, then prove whether power or flow is actually reaching it. On the electric side that is a continuity and insulation test on the element and a check of the rod thermostat and thermal cut-out, all of which come out of the head without draining the cylinder. On the boiler side it is the programmer, the cylinder thermostat, the motorised valve and the pump, in that order, because that is the order the signal travels. The rate is fixed before we attend and the total depends on how long the job takes.

• Cylinder pipe cold after 25 minutes with the immersion on
• The immersion trips the breaker or RCD as soon as it is switched on
• Heating works, hot water never does
• Crackling, popping or banging from the cylinder while it heats
• Hot water that lasts one shower and no more
• A heat up time that has roughly doubled over a few months