Hot Water Cylinder Jacket Calculator

Why a £25 jacket beats a £5,000 solar array on ROI

Return on investment has a denominator, and the cylinder jacket's denominator is £25. A bare copper cylinder leaks heat continuously — it is a 120-litre tank of 60°C water sitting in a cool airing cupboard, radiating money all day and night. Fitting an 80mm British Standard jacket cuts that standing loss by roughly three-quarters, saving about £70 a year. The payback is 0.4 years — under five months. No other retrofit on this site, or arguably in the British housing stock, returns its cost faster. The test case (no insulation, £25 jacket) gives £70 a year and a 0.4-year payback; adding £10 of pipe insulation pushes the saving to £85 for £35 total.

The reason this measure is perpetually underrated is that it is invisible and unglamorous. Nobody Instagrams a cylinder jacket. Installers do not upsell it because there is no margin in a £25 DIY product. But the physics is unforgiving: an uninsulated cylinder can lose 2–3 kWh a day as standing loss — 700–1,000 kWh a year, roughly £50–£70 at gas prices and £170–£240 if you heat water electrically. If your cylinder is bare copper, you are paying to heat your airing cupboard.

What to buy and how to fit it

Buy a British Standard 80mm cylinder jacket — the standard is BS 5615, and compliant jackets are sold at every DIY store for £20–£30. Thinner "economy" jackets exist; they are a false economy, saving measurably less for nearly the same effort. Fitting takes 15 minutes: the jacket wraps around the cylinder in sections secured with straps or tape, with cut-outs positioned for the thermostat and pipework. Do not cover the thermostat — it needs to sense the cylinder temperature accurately — and leave the top of the cylinder accessible for the immersion heater if you have one.

Pipe insulation is the companion job: foam sleeves (£1–£2 per metre) slipped over exposed hot water pipes, especially the first metre from the cylinder and any pipework running through cold spaces. Pipes lose heat along their whole length, and uninsulated copper in a cold loft or underfloor void is a radiator you did not ask for. Ten metres of sleeve and half an hour's work adds roughly £15 a year to the saving.

The 60°C rule and legionella

While you are at the cylinder, check the thermostat: it should be set to 60°C. This is the temperature that kills legionella bacteria while limiting scald risk (hot enough at the tap after pipe losses, not dangerously hot). Below 50°C, legionella can multiply in stored water — this is a genuine health consideration, not efficiency folklore. Above 65°C you are wasting energy and raising scald risk for no benefit. If your thermostat is set to 70°C "to get hotter water", turn it down to 60°C today: it is free, safer, and saves money. Combi-boiler households can ignore all of this — with no stored hot water there is no cylinder and no legionella risk — but the pipe-insulation advice still applies to exposed hot runs.

When the answer is a new cylinder, not a jacket

A jacket cannot fix a dying cylinder. If yours is leaking, heavily corroded, or the immersion heater has failed and the unit is decades old, price a replacement: modern cylinders come factory-insulated to high standards (often equivalent to 80mm+ jackets) and a new one costs £400–£800 fitted. The decision rule: jacket the sound cylinder; replace the failing one. And if you are planning a heat pump, flag the cylinder question to your installer — heat pumps want larger cylinders (or thermal stores) with bigger heat-exchanger coils to work at low flow temperatures, so a cylinder replacement may fold into the heat-pump project rather than preceding it.

Renters and the airing cupboard

Like draught-proofing, this is renter-friendly: a jacket is removable, non-damaging, and costs £25. If your rented home has a bare cylinder, fit a jacket (tell the landlord — they will not object to you spending your own money improving their property) and take it with you when you leave, or leave it as goodwill. The payback is months; even a one-year tenancy profits.

The physics of standing loss (and why combis win)

It is worth understanding why the numbers are so lopsided, because it explains a broader retrofit principle. A hot water cylinder loses heat in proportion to its surface area and the temperature difference with its surroundings — a 120-litre cylinder at 60°C in a 15°C airing cupboard has roughly 2 m² of surface radiating across a 45-degree gradient, 24 hours a day. Bare copper has high thermal emissivity; it practically invites heat to leave. An 80mm jacket wraps that surface in low-conductivity insulation, collapsing the loss by around three-quarters. The remaining quarter is mostly pipework and the unavoidable top and bottom — which is why pipe sleeves are the natural second step.

This is also the thermodynamic argument for combi boilers, and it is a strong one: a combi has no stored hot water, so its standing loss is zero by construction. No cylinder, no jacket, no airing cupboard slowly cooling. The trade-off is flow rate — combis heat water as it passes through, so filling a bath is slower and simultaneous showers can overwhelm smaller units — plus the well-known "wait for it to get hot" delay that wastes a litre or two of water down the drain each time. Neither system is universally superior; the point is that if you have stored hot water, insulating the store is non-negotiable, and the jacket is the cheapest way to do most of it.

There is a related efficiency most households miss: hot water timing. Heating a full cylinder twice a day "just in case" when you use one bath is pure waste. If your cylinder is on a timer (system/regular boiler), set it to heat water once, shortly before your main use — the jacket then keeps it hot for the rest of the day at minimal loss. Electrically heated cylinders on Economy 7 should heat overnight on the cheap rate and coast on insulation through the day. A jacket plus sensible timing compounds: less heat made, less heat lost.

Where this sits in the retrofit order

Phase A, first weekend, alongside draught-proofing and LED bulbs: the sub-£150 measures that pay back in under two years and fund — psychologically, if not literally — the bigger phases. There is a case that the cylinder jacket should be the single first recommendation of the entire toolkit: it is the fastest payback, the lowest cost, the easiest DIY, and it works in every home with a hot water cylinder regardless of age, tenure, or wall type. If you have read this far and your cylinder is bare, stop reading and order the jacket.