Horizontal and Slimline Hot Water Cylinders: Which Fits?

Standard, slimline or horizontal: the short answer

Measure the space first, then pick the shape in this order. A standard upright cylinder if it fits, because it gives the most usable hot water for the money and the widest choice of sizes. A slimline cylinder if the width is the problem. A horizontal cylinder only if the height is the problem. If neither fits, the answer is a different position or a different system, not a cylinder squeezed in where it cannot be serviced.

• Fits upright at standard diameter: standard cylinder
• Narrow cupboard with good ceiling height: slimline
• Low space such as a loft, eaves or ceiling void: horizontal
• Neither fits with room to service: move the cylinder or rethink the system

What a slimline cylinder is

A slimline cylinder is an ordinary upright cylinder with a smaller diameter. Standard cylinders are commonly around 545 millimetres across, and slimline versions around 450. The slim one gets its litres back by being taller.

Because it still stands upright, it behaves like a standard cylinder. Hot water collects at the top, cold enters at the bottom, and you can draw most of the tank at full temperature before it goes cool. Recovery depends on the coil, as with any cylinder.

The catches are height and cost. A slim cylinder of a useful size is tall, so check the ceiling height and allow room above it to withdraw the immersion heater. Many London conversion cupboards have the height but not the width, which is exactly where slimline earns its place. It also costs more than a standard cylinder of the same capacity, and the range of sizes is smaller.

What a horizontal cylinder is

A horizontal cylinder is built to lie on its side, with the connections, coil and immersion arranged for that position. It is designed for spaces with plenty of length and little height: a loft, the eaves behind a loft conversion knee wall, a void above a bathroom ceiling, or a low cupboard.

You cannot lay a standard cylinder on its side and call it horizontal. The internal layout, the position of the cold inlet and hot outlet, and the way the coil heats the water are all designed for one orientation. A horizontal cylinder cannot be stood up either. The choice is made at purchase.

Several of the manufacturers we work on make horizontal versions, including Telford, Gledhill, Joule and Heatrae Sadia. Availability of sizes varies by maker and changes over time, which matters later if a like for like replacement is ever needed.

Why a horizontal cylinder gives less hot water

This is the trade off most people are not told. In an upright cylinder, hot water floats on top of cold in distinct layers. As you draw hot water from the top, cold water enters at the bottom and the layer between them stays thin, so you can use most of the tank before the water at the tap cools.

Lie the same volume on its side and the column is short and wide. The hot and cold mix more, the warm band between them is a larger share of the tank, and the amount you can draw at full temperature is a smaller share of the nominal capacity. Manufacturers publish a usable or delivered volume for their horizontal models, and that is the figure to compare, not the size on the label.

In practice that means a horizontal cylinder is usually specified a size larger than the upright cylinder it replaces, and reheat speed matters more. If the household already runs out of hot water, our guide on hot water runs out quickly explains how recovery and volume interact.

If you are replacing a vertical cylinder with a horizontal one of the same nominal size, expect less hot water, not the same. Ask for the delivered volume in writing.

What to measure before you choose

Most shape decisions are made by a tape measure, and the mistakes come from measuring only the cylinder and not the space around it.

For a slimline, measure the cupboard width and depth, the height to the ceiling, and the clearance needed above the cylinder to pull out the immersion heater. For a horizontal, measure the length and height of the space, the clearance at the end where the connections and immersion sit, and the route in, since a long cylinder has to turn corners on stairs and pass through a hatch.

On an unvented cylinder, allow room for the kit around the vessel: the inlet control group on the cold feed, an external expansion vessel on many designs, and the tundish, which has to be visible. Our guide on how to choose a hot water cylinder lists the full set of measurements.

• Width, depth and height of the space
• Clearance to withdraw the immersion heater, at the top for upright or at the end for horizontal
• Room for the inlet controls, expansion vessel and a visible tundish
• The narrowest point on the route in: door, stair turn or loft hatch
• Whether the floor or platform can carry the full weight

Mistakes we see with awkward cylinders

Choosing on litres alone. A horizontal or slimline cylinder of the same nominal size as the old one does not always give the same hot water, and a cylinder that fits but runs cold on the second shower has not solved anything. Size it properly first; our guide to what size hot water cylinder do I need works it out by bathrooms and habits.

Leaving no room to service it. An unvented cylinder needs an annual service, and a cylinder boxed in so tightly that the immersion cannot be withdrawn or the valves cannot be reached costs more every time anyone works on it.

Forgetting the discharge. An unvented cylinder in a loft or ceiling void still needs a discharge pipe that falls all the way to a safe point outside. That route often decides the position more than the shape does. Our article on putting a hot water cylinder in the loft covers it in detail.

Is it worth the extra cost?

If the alternative is losing a bedroom cupboard, a slimline is usually worth it, because it works like a standard cylinder in a smaller footprint. A horizontal is worth it when it unlocks a space that could not otherwise take a cylinder at all, and you accept that it needs to be larger to deliver the same hot water.

It is not worth it when a standard cylinder would fit with a little joinery, or when a better position exists elsewhere. Moving the cylinder to a utility room or garage, or converting to a combi boiler in a small flat, are sometimes the cleaner answers. What a cylinder replacement actually costs goes through the drivers that make one route dearer than another.

Getting the right shape fitted

If your airing cupboard is too small for the cylinder you need, or you want a cylinder in a loft or eaves space, call 020 4652 5111. We will measure the space and the route in, check the incoming main and the discharge route, and tell you which shape will actually work before anything is ordered. Our hot water cylinder replacement and unvented cylinder installation pages explain how we fit them, and our rates are on the pricing page and agreed before we attend.