Central Heating Systems Explained

Central Heating Systems Explained

A simple guide to household heating systems

When you consider household heating systems you should differentiate between your household heating requirements and your domestic hot water (DHW) requirement. Whilst heating the property is often left to the trusty old radiator (or more recently to Under Floor Heating) there are different ways to provide domestic hot water.

It is possible to mix and match as it were the different systems to some degree. For example, you could have a gravity hot water system which is open vented coupled with a sealed radiator system, or a fully open vented system where hot water and radiator circuit are both vented (albeit separately).

This guide is intended to take a little of the mystery out of heating systems and help you to understand your options.

There main types of household heating and water systems include:

Fully vented / Gravity Fed System

In a fully vented system (with gravity cold water feed to hot water cylinder and radiator circuit venting into a small cistern — often sited in the loft) water is typically heated by a boiler then circulated via (zone valves) two 2 port valve or a single 3 port valve (depending on whether you have a ‘Y’ or ‘S’ plan configuration) to either:

  1. The hot water cylinder’s boiler coil (heat exchanger) to generate hot water, and/or
  2. To the radiators for heating.

Control of what your boiler heats is by virtue of a programmer and the zone valves.

The term ‘fully vented’ comes from the fact that no part of this type of system is pressurised. There are usually two water tanks situated in the loft (one smaller than the other). The larger cold water cistern (or tank) supplies water to the hot water cylinder and does so purely by the force of gravity. The domestic hot water system (DHW) and cylinder vent back into this cistern. The smaller ‘feed and expansion’ tank (F&E) that vents the radiator circuit (and also keeps it topped up) also operates courtesy of gravity. Both cisterns will usually be fitted with a float valve so that a level may be maintained within the cistern and fed from a mains supply.

A gravity system is inherently safe. Because it should be impossible to have a dangerous pressure build-up in any part of the system there is no risk of something bursting under pressure — or at worst. an explosion! That’s the good news. There are however draw backs (trade-offs) as there are with each and any system.

The main draw back with a fully vented gravity system is the pressure you can expect from your domestic hot water. As mentioned, the cold water feed to your hot water cylinder is under the force of gravity. It is therefore going to be relatively low pressure.

To explain this in a more tangible way you should consider the following:

As you elevate a vessel of water it gains potential energy. Keeping the outlet point lets say at ground level and raising the vessel of water, you increase the pressure at the outlet the higher you raise the vessel. For the purpose of this example when you raise the water vessel by approx 33 feet (10 metres) you create 1 bar of pressure out the outlet.

Central Heating Systems Explained

The pressure throughout a gravity system will differ between outlets as the height differential between the cistern and the outlet changes. For example, the height difference between a cistern in a loft and an outlet on the ground floor of a two a storey house will be greater than the height difference of an outlet on the first floor of the same house when fed from the same cistern. The pressure at the outlet will therefore be greater on the ground floor than on the first floor by virtue of the height differential.

This is why the performance of a gravity fed shower on the first floor of a house is generally pretty poor — especially if compared to a shower on the ground floor when fed on the same system. The height differential between the loft cistern and the first floor outlet (in this case the shower handset) can be extremely small — especially if you raise the handset as high as it will go. The nearer you go to the ceiling with it the nearer you get to the under side of the cold water cistern.

It is important to realise that it is the height differential between the cistern and the outlet that makes the difference — not the size of the cistern or the amount of water in it. The position of the hot water cylinder will for intents and purposes not affect the pressure at the outlet by virtue of its relative position in the system.

Providing the cold water cistern is higher than the outlet point you have what is known as a ‘positive head’. If you attempt to create an outlet higher than the cistern you will have a ‘negative head’ scenario. In this situation you will not get water to flow from the outlet under normal gravity conditions — for the simple reason that water will not voluntarily travel up hill.

Gravity systems (open vented systems) as mentioned are simple and safe. They may be low pressure, but they do have the advantage of being to ONLY type of system to which you may add a booster pump. You can even get ‘negative head pumps’ to cope with the problems of outlets above cistern level.

By comparison, Water Bye Laws prohibit the use of pumps on mains pressure systems to increase flow or pressure. Combi systems are ains fed and will already be working flat out so attempting to increase flow here is folly. So, if you have a low pressure gravity system and are fed up with the poor performance of your shower you can add a booster pump — that’s the good news.

To summarise:

Positives: Inherently safe. Suitable for addition of booster and power shower pump to provide exceptional flow rates.

Negatives: Require loft tanks and a hot water cylinder, so provision must be made for the space these will require. An unpumped system will offer low pressure at taps and outlets.

Economy: Water must be heated and stored whether or not it’s used. Important to make sure your system and storage is insulated well to minimise standing heat losses. No less economical to produce hot water than any other system.

Typical System Requirements: Boiler, Hot Water Cylinder, Loft Tank/s, Rads.

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