A Homeowner's Guide to Indirect Hot Water Cylinders

So you've decided to replace your old indirect hot water cylinder but you're not sure what you need? Maybe you've just moved into a new home, maybe your old cylinder isn't providing the heat it used to, or maybe you've got a new arrival on the way?

Maybe you've seen that Megaflow cylinders advert on TV and it's got you thinking?

Whatever your needs, TradePlumbing is here to help, with this simple to understand, illustrated guide to indirect hot water cylinders.?

Indirect Hot Water Cylinder

An indirect hot water cylinder works in a very different way to a traditional direct cylinder. An indirect hot water cylinder is fed from the cold water mains but the heat is generated by a coil of conductive pipe which passes through the cylinder and back out again.

This coil's main purpose is to carry hot water to radiators on the central heating system, but as hot water travels through this coil, the hot pipe conducts heat, "indirectly" heating the water inside indirect hot water cylinders, hence the name.

This type of cylinder comes in two different systems; vented and unvented.

Vented Hot Water Cylinder

Traditional vented indirect hot water cylinders rely on a Feed and Expansion (F & E) tank positioned above the highest outlet (tap or radiator), and use gravity to generate sufficient water pressure to push water around the home.

The strength of pressure depends entirely on the height difference between the tank and your outlets (such as taps and radiators); the higher the F & E tank is above your outlets, the greater your water pressure will be.

As heating causes the water within the hot water cylinder to expand, a vent pipe allows a safe route for excess pressure, air bubbles and steam should the system overheat.

This vent pipe runs from the top of the cylinder back up to the cold water storage header tank, where the open vent is located just above the water level.

Unvented Hot Water Cylinder

There are no cold water storage tanks, vent pipes or expansion tanks with unvented indirect hot water cylinders, instead cold water is supplied at mains pressure directly into the base of the cylinder, allowing the supply of both hot and cold water at mains pressure to the entire property.

A pressure reducing valve is often used to limit the pressure of the incoming mains water to a safe level should the unvented indirect hot water cylinder only operate under a certain pressure.

Obviously this type of hot water cylinder needs to be much more resilient than a traditional vented indirect hot water cylinder does, and is made with either extra-thick copper or stainless steel to allow it to withstand mains pressure.

An unvented indirect hot water cylinder is completely sealed from the atmosphere and has no vent pipe. Instead a pressure vessel inside the cylinder, in the form of a rubber diaphragm, allows for expansion as the water is heated.

We Recommend

TradePlumbing recommends the Megaflow Cylinder range from Heatrae Sadia for all your Indirect Hot Water Cylinder needs. Their Megaflow cylinders are the brand leader in unvented indirect domestic water heating.