Is a thermal store better than an unvented cylinder?

Not in general. A thermal store is better where there is no lawful discharge route or where several heat sources feed the same system. An unvented cylinder is usually better for strong mains and heavy simultaneous use, with lower standing heat loss.

Does a thermal store need a G3 qualified engineer?

An open vented thermal store is not an unvented hot water system and does not fall under the unvented cylinder rules in the same way. It still needs a competent engineer, because the store runs hot and the immersion circuits carry heavy loads.

Can I replace my thermal store with an unvented cylinder?

Sometimes. It needs enough incoming mains flow and a lawful route for the safety discharge pipe to a safe point outside. In many flats the discharge route is the obstacle.

Why does a thermal store run so hot?

The store has to heat cold mains water in the few seconds it passes through the heat exchanger. The hotter the store, the more usable hot water it can deliver from a fixed volume.

Which is cheaper to run?

It depends on the heat source and tariff. A thermal store loses more heat standing because it is hotter, but if it is heated on a cheap night rate that loss costs less. Compare on your own tariff and usage.

Do both give mains pressure hot water?

Yes. An unvented cylinder stores mains water under pressure. A thermal store heats mains water as it flows through. Both are limited by the strength of the incoming mains.

Thermal Store vs Unvented Cylinder: Which Is Better?

An unvented cylinder is usually the better choice where the mains supply is strong and there is a lawful route for its safety discharge pipe, because it stores the water you actually use and delivers it at mains pressure with fewer parts in the way. A thermal store is often the better choice in a flat with no gas and no discharge route, or in a house with more than one heat source, because it is open vented, sits outside the unvented cylinder rules, and can take heat from a boiler, immersion elements, solar or a stove at once. The trade offs are real on both sides: a thermal store runs hotter, loses more heat standing and has a pump, flow switch and heat exchanger between the store and your tap; an unvented cylinder needs annual servicing and a qualified installer. Below is how they differ and how to decide for your property.

The difference in one paragraph each

An unvented cylinder is a sealed tank of drinking water fed directly from the cold mains. The water in the tank is the water that comes out of your hot taps. Because it is sealed and under pressure, it carries a set of safety controls: a pressure reducing valve, an expansion vessel or air gap, a temperature and pressure relief valve, and a discharge pipe through a tundish to a safe point outside. Installing one is notifiable building work and must be done by someone qualified for unvented systems.

A thermal store is a tank of water that never comes out of a tap. It is open to the atmosphere through a small cistern or vent and is simply a reservoir of heat. When you open a hot tap, cold mains water passes through a heat exchanger inside or beside the store, picks up heat on the way through, and arrives at the tap hot and at mains pressure. The Gledhill Pulsacoil found in so many London flats is one of these, and our guide on how a Pulsacoil works goes through the parts in detail.

Pressure and flow

Both give you hot water at mains pressure, so on paper they are equal. In practice the limits sit in different places.

An unvented cylinder delivers whatever the mains can supply, reduced to a set working pressure. With a good supply you can run two showers at once. With a weak supply it cannot do better than the mains, and that is the most common reason one disappoints. Our page on low hot water pressure explains how to measure what you have before choosing either.

A thermal store is limited by the mains too, and also by how fast its heat exchanger can transfer heat. As the store cools through the day, the maximum flow at a comfortable temperature drops. A well sized store in a small flat is fine; a small store serving a family bathroom and an ensuite at the same time runs out of heat sooner than a cylinder of similar size runs out of hot water.

Running cost and heat loss

A thermal store has to be held hotter than a cylinder, normally well above the 60 degrees a cylinder is set to, because the store water has to lift cold mains water to tap temperature in a few seconds. Hotter water loses heat faster through the insulation, so a thermal store generally has a higher standing loss than a modern unvented cylinder of similar size.

Where the store is heated on an off peak electricity rate, that loss is paid for at the cheap rate, which narrows the gap. Where it is heated by a gas boiler through the day, it widens it. Neither is automatically cheaper to run; it depends on the heat source, the tariff and how much hot water the household actually uses.

Heat sources: where a thermal store earns its keep

The strongest argument for a thermal store is flexibility. Because the store is open vented and is only holding heat, it can be connected to several heat sources at once: a boiler, immersion elements, a solar thermal panel, a wood burning stove with a back boiler. Some designs also feed the radiators from the store. That is hard or impossible to do safely with a sealed unvented cylinder.

An unvented cylinder normally takes one or two sources through its coils, usually a boiler or heat pump plus an immersion as backup. For most homes that is all they need. If you are only choosing between an electric store and an electric cylinder, the question is closer, and our guide to the electric hot water cylinder covers the all electric options side by side.

Flats, discharge pipes and the rules

This is the point that decides a lot of London jobs before anything else. An unvented cylinder must have a safety discharge pipe running from the relief valves, through a visible tundish, to a safe termination point outside the building. In a house that is usually straightforward. In a flat in the middle of a block, the only route may pass through a neighbour's property or a common area, or there may be no lawful route at all.

A thermal store is open vented, has no pressure relief discharge, and does not fall under the unvented cylinder rules. That is the main reason developers fitted them in purpose built blocks. It is also why swapping a store for an unvented cylinder in a flat is rarely as simple as it sounds.

An unvented cylinder also needs to be notified to building control and serviced each year by someone qualified, which is a small ongoing cost a store does not carry in the same way. Our page on unvented cylinder installation sets out what the work involves.

If someone proposes an unvented cylinder for a flat, ask them to show you where the discharge pipe will run before you agree to anything. If they cannot, the proposal is not finished.

What goes wrong with each

Thermal stores have more between the heat and the tap. A pump, a flow switch, a control board and a heat exchanger all have to work for hot water to arrive. In hard London water the plate heat exchanger scales up and the flow at a good temperature falls off over time. The small cistern on top can stick or overflow. Most of those parts are replaceable, and repairing a sound store is often better value than replacing it; our page on Pulsacoil repair in London covers the common faults.

Unvented cylinders have fewer moving parts but more safety controls, and those need regular attention. Relief valves start to pass, the expansion vessel loses its charge, the pressure reducing valve drifts. The usual symptom is water dripping at the tundish, which is a sign to call someone rather than something to ignore.

So which should you choose?

If you already have one that works, repairing it is usually cheaper and less disruptive than changing the system. The decision matters most when the existing unit has failed badly or you are refitting anyway.

Good mains flow, a clear discharge route, a household using a lot of hot water at once: unvented cylinder
Flat with no gas and no lawful discharge route: thermal store, or a vented cylinder where space allows
More than one heat source, such as solar thermal or a stove: thermal store
Weak mains supply: neither will fix it on its own, measure the supply first
Existing store working but hot water fading in the evening: look at capacity and timing before replacing anything

Get an honest answer for your property

The right answer depends on things only a visit shows: the mains flow, the discharge route, the space, and what is already there. We work on both, so there is no reason for us to push one over the other. If your current store or cylinder has failed, we will tell you whether it is worth repairing first.

Call 020 4652 5111 to book. Our rates are on the pricing page and are agreed before we attend.

Questions we get asked

Is a thermal store better than an unvented cylinder?: Not in general. A thermal store is better where there is no lawful discharge route or where several heat sources feed the same system. An unvented cylinder is usually better for strong mains and heavy simultaneous use, with lower standing heat loss.
Does a thermal store need a G3 qualified engineer?: An open vented thermal store is not an unvented hot water system and does not fall under the unvented cylinder rules in the same way. It still needs a competent engineer, because the store runs hot and the immersion circuits carry heavy loads.
Can I replace my thermal store with an unvented cylinder?: Sometimes. It needs enough incoming mains flow and a lawful route for the safety discharge pipe to a safe point outside. In many flats the discharge route is the obstacle.
Why does a thermal store run so hot?: The store has to heat cold mains water in the few seconds it passes through the heat exchanger. The hotter the store, the more usable hot water it can deliver from a fixed volume.
Which is cheaper to run?: It depends on the heat source and tariff. A thermal store loses more heat standing because it is hotter, but if it is heated on a cheap night rate that loss costs less. Compare on your own tariff and usage.
Do both give mains pressure hot water?: Yes. An unvented cylinder stores mains water under pressure. A thermal store heats mains water as it flows through. Both are limited by the strength of the incoming mains.