Hot Water Tank & Storage FAQ

A hot water cylinder is an insulated vessel that stores potable hot water so it is available faster than any heat source could produce it on demand. In an indirect cylinder the heat source — boiler, heat pump or solar collector — circulates through a sealed [coil heat exchanger](knowledge/glossary/coil-heat-exchanger) inside the tank and never mixes with the drinking water; in a direct cylinder an immersion heater heats the stored water itself. Cold water enters at the bottom and hot water leaves from the top, and the temperature layering in between is what makes the volume usable.

Between 150 and 250 litres for most families, following the standard planning figure of 30–50 litres of stored hot water per person. A four-person household with one bathroom typically lands at 180–200 litres; add a second bathroom, a bath or a high-flow shower and 250–300 litres is the safer choice. Heat pump systems sit at the upper end of the range because they reheat more slowly than a boiler.

About five or six in a row. An eight-minute shower at 9 litres a minute uses 72 litres of mixed water at around 40 °C, which draws roughly 43 litres out of a cylinder stored at 60 °C once cold water is blended in — so a 300 litre cylinder, of which perhaps 80% is usable before the outlet temperature falls away, covers five to six showers. A high-flow shower head or a lower storage temperature cuts that number quickly.

Usually yes. Fifty US gallons is about 190 litres, comfortably inside the 30–50 litres-per-person rule for four people. What actually decides it is the first-hour rating on the label — how much hot water the unit delivers in the first hour including reheat — because an electric tank recovers far more slowly than a gas one of the same volume. Two bathrooms in simultaneous morning use argue for the next size up.

That depends on occupants, not bedrooms. Forty US gallons is about 150 litres, which suits two to three people; a three-bedroom house occupied by four or five with one bathroom will run short during the morning peak. Compare the label's first-hour rating against your busiest hour rather than judging by nominal capacity.

Fifteen to twenty-five years for a quality cylinder that is maintained. Enamelled steel tanks only reach the upper end if the sacrificial [magnesium anode](knowledge/glossary/magnesium-anode) is inspected roughly every two years and replaced when about two-thirds consumed; stainless steel achieves comparable life where the water's chloride content suits the alloy. The most common cause of early failure is not age but an exhausted anode that nobody checked.

With anode care, 15–25 years — the same class as stainless steel, despite the "enamel only lasts 5–8 years" claim that circulates online. Enamel is a glass layer fused to the steel at high temperature, and DIN 4753-3 defines it as a system of coating plus cathodic protection, not a coating alone. Enamel without a live anode fails early; enamel with one is a long-life tank, as set out in [enamel vs stainless steel tanks](knowledge/questions/enamel-vs-stainless-steel-tank).

Yes, with one condition: water chemistry. Stainless steel forms a self-repairing passive oxide layer, needs no sacrificial anode and weighs less than an enamelled vessel of the same size. That passive layer is, however, vulnerable to high chloride concentrations, which can cause pitting or stress corrosion cracking at weld seams — so get a water analysis before specifying stainless steel in a coastal or heavily softened supply.

A rotten-egg smell in the hot water, rusty or discoloured water, and, on inspection, a rod whose steel core wire is exposed or whose diameter has dropped from the original 26–33 mm to around 10–15 mm. You cannot judge it from outside without withdrawing the rod or using an anode tester that measures the protective current. Waiting for a leak means the tank is already beyond saving.

Match it to the tank rather than to a generic size: use the manufacturer's part number from the data plate, or failing that the thread — commonly 1 1/4" on European enamelled cylinders — the rod diameter and the installed length. Also measure the headroom above the tank, because a straight rod needs its own length of clearance to withdraw. Where headroom is short, a segmented chain-link anode does the same job in a fraction of the space.

Two numbers add together. Heating the water is fixed physics at 1.16 Wh per litre per degree, so raising 150 litres from 10 °C to 60 °C costs about 8.7 kWh; on top of that, a modern well-insulated 200–300 litre cylinder loses roughly 1–2 kWh a day in [standing loss](knowledge/glossary/standing-loss) whether you use the water or not. Old cylinders with loose jackets can easily lose twice that.

Roughly 20–35% more, not the 56% the volume difference suggests, because standing loss follows surface area and surface area grows more slowly than volume. Insulation thickness and quality matter more than size: the EU energy label reports a tank's standing loss in watts, which is the only figure that lets you compare two products directly. Uninsulated pipe tails, fittings and a bare base often account for more loss than the extra litres.

A modern factory-insulated cylinder loses only about 4–7 °C over a full day — that is what 1.5–2.5 kWh of daily standing loss amounts to in a 300 litre vessel — so water heated in the evening is still comfortably usable the next morning. What normally ends the hot water is drawing it off, not cooling. An older cylinder with a thin or slipped jacket can lose two to three times as much.

Yes if it stands on a cold or uninsulated floor, using a rigid board that can carry the load — a full 300 litre cylinder weighs over 300 kg, so compressible loft-type insulation is unsuitable. Downward loss is smaller than the loss through the sides, but it runs continuously and is cheap to remove. Keep discharge pipework, drain points and safety fittings clear and visible while you do it.

A strap-on cylinder thermostat belongs underneath the insulating jacket, in direct contact with the metal wall — that is exactly where it should sit. What ruins it is losing contact with the wall or resting on top of the foam, because the sensor then reads trapped air and lets the tank overshoot or cut out early. Cylinders with a factory sensor pocket are better still: use the pocket, pushed fully home.

Yes for normal operation. Storing at 60 °C controls [legionella](knowledge/glossary/legionella) fully, while 70 °C accelerates limescale on the coil, increases standing loss and raises the scalding risk with no hygiene gain. Distribute at 50–55 °C through a [thermostatic mixing valve](knowledge/glossary/thermostatic-mixing-valve); in new UK dwellings the water delivered to a fixed bath must not exceed 48 °C.

Off as a standing state, on as a scheduled cycle. Leaving an immersion permanently energised is expensive and pointless; what matters is that the whole stored volume reaches 60 °C often enough, and weekly is the usual recommendation for systems that normally store cooler — heat pump cylinders in particular. The immersion is the standard way to deliver that cycle when the main heat source cannot reach 60 °C efficiently on its own.

A thermal store inverts the usual arrangement: it holds heating water rather than drinking water and produces domestic hot water instantaneously as you draw it through an internal coil or an external plate heat exchanger. That removes stored potable water from the system, which simplifies legionella control, and lets several heat sources charge one vessel. The design point is that the store must be kept hot enough to deliver on demand — natural for solar and solid fuel, and within easy reach of high-temperature heat pumps such as R290 models when the exchanger is generously sized.

Often longer than a potable cylinder — twenty years and more is common — because the stored charge is closed, oxygen-poor water that is never replaced, so corrosion effectively stops once the initial oxygen is consumed. The wear items are the plate heat exchanger or coil, which can scale up in hard water, and the controls. Treating and then leaving the fill water alone is what protects that lifespan.

It is the visible air break between a cylinder's safety valves and the drain, so that any discharge is seen rather than quietly disappearing, and so nothing can siphon back from the drain into the cylinder. Water in the tundish is always a fault signal, never routine. It must be fitted vertically, within about 500–600 mm of the safety device, and remain visible and accessible.

The commonest cause is a waterlogged expansion vessel: once its air charge is lost, water expanding as the cylinder heats has nowhere to go, so the expansion relief valve lifts on every heating cycle even though the static pressure looks normal. Next most common is debris holding a relief valve off its seat, or an inlet pressure-reducing valve that has failed high. A persistently dripping tundish means the safety devices are operating and needs a qualified engineer, not a wait-and-see.

The air pre-charge should match the system's cold inlet pressure — in other words the setting of the pressure-reducing valve, commonly 3–3.5 bar — and it must be measured with the water side fully depressurised, otherwise the gauge simply reads mains pressure. Set too low, the vessel fills with water and stops absorbing expansion; set too high, it never accepts any. In England and Wales this is work for someone holding a current unvented (G3) qualification.

In two parts. The first runs from the safety valve to the tundish and must be short, with the tundish within about 500–600 mm of the valve and visible; the second runs from the tundish with at least 300 mm of straight vertical fall before any bend, in a material rated for boiling water, at least one pipe size larger than the valve outlet, falling continuously to a termination where discharge cannot scald anyone. The permitted length of that second section shrinks with every elbow, so the sizing table in the building regulations has to be followed rather than estimated.

In England and Wales, yes. Any unvented hot water storage vessel over 15 litres is controlled work under Part G3 of the Building Regulations: it must be installed, commissioned and serviced by a competent person holding a current unvented hot water storage qualification, and the work either self-certified through a competent person scheme or notified to building control. Servicing and re-pressurising the expansion vessel fall under the same requirement.

As low in the vessel as the design allows, if you want to heat the whole volume — an element only heats the water above it, so a high element leaves everything beneath it cold. Cylinders built for off-peak tariffs use exactly this arrangement: a low element to charge the whole tank overnight and sometimes a second, higher element for a small daytime boost. Fitting only a high element and expecting a full tank of hot water is the classic disappointment.

No — they sit at different heights precisely because they do different jobs. The lower coil belongs to the solar circuit, which performs best against the coldest water in the tank; the upper coil serves the boiler or heat pump and only needs to guarantee hot water in the top of the vessel. Connecting them in parallel destroys that division of labour and wrecks [stratification](knowledge/glossary/stratification).

Only if it was designed and certified for horizontal mounting. Laying a vertical cylinder on its side collapses the temperature layering it depends on, moves the coil and immersion away from their intended positions and can trap air — the result is more mixing, poorer solar yield and less usable hot water. Purpose-built horizontal vessels do exist, including the horizontal tanks used on [thermosiphon systems](knowledge/glossary/thermosiphon-system); specify one of those rather than improvising.

In the sensor pocket at the level of the bottom of the solar coil, in the lower third of the vessel, so that the controller compares the collector against the coldest water available. A sensor mounted too high reads water the solar circuit has already warmed and switches the pump off far too early, costing yield on exactly the days that matter. Push it fully home in its pocket and use heat-conducting paste where the pocket is oversized.