Overview

[webmaster]

Modern Combi boilers come in two main flavours: Direct and Indirect. Some Combi’s also work with water storage systems, of which more later.

The Direct Combi has one heat exchanger with two channels, and can heat both the central heating and domestic hot water, but only one at a time. In principle they are simpler than Indirect Combi’s, but not necessarily less complicated.

An Indirect Combi has two heat exchangers. One heats the circulating water and the other utilises this heated water to heat the domestic hot water (similar to a traditional tanked system).

A fundamental flaw with combi’s is that they can only effectively supply one tap at a time, because in order to heat the water adequately the boiler has to slow the flow to an acceptable level. Your requirements may be such that this is not a draw back - for example, a property with one bathroom and kitchen will not be too demanding. Two bathrooms or a separate shower-room may need further consideration. A simple rule is that water is lazy and will take the shortest and easiest route to the drain.

A key advantage of Combi’s is that there is no need for motorised valves, storage tanks, external pumps and wiring centres. Also (although still under debate) the absence of a store of hot water to maintain should reduce bills.

The appliance should last twelve years or so. If you are planning extra baths or showers, then consider the other options.

Although technology is improving continuously, there are some rules of thumb for choosing a Combi. Hot water output directly correlates to the amount of gas consumed. A Combi is a powerful appliance and will heat most domestic properties. If it is not ‘man enough for the job’ then the chances are that a domestic hot water supply will also be inadequate. Consider the following outputs of hot water:

· 24kW = 9 litres per minute
· 28kW = 11 litres per minute
· 33+kW = 14 litres per minute.
· Storage Combi’s (mentioned later) = 18 litres per minute.

These appliances need a lot of gas, so do not be surprised if the gas pipe in your property needs increasing.

For brand new systems Combi’s are much simpler and quicker to install. A conventional tanked installation will need some conversion work, removing the storage tanks and converting the heating system to a sealed type. This will increase the number of pipes going to the boiler.

A Combi needs six pipes (seven if it is condensing):

· Mains cold water in,
· Domestic hot water out,
· Central heating flow,
· Central heating return,
· Gas,
· Safety discharge pipe (SDP),
· Condensate run-off (Condensing boilers only)

The SDP requires a nearby outside wall, which should not be too much of a problem due to the flue requirements.

When choosing a boiler look at the features that come with it. These may include frost thermostats, scale reducers, bypasses, filling loops, timers etc. They will reduce installation time but not always the overall price or quality. The key issue is often the manner in which the flue can be run.

Storage Combi’s are different, and are generally one of the following:

· Thermal storage
· Domestic hot water storage

The latter stores hot water in the conventional manner, but thermal storage Combi’s use a store of hot water to augment the burners in order to increase their production of domestic hot water. When the heat store runs out the boiler will revert to a lower flow rate. The initial performance is better than that of a non-thermal storage Combi, and subsequent heating should be adequate for most needs. Recharging the store takes some time, but not too much. Thermal storage boilers are often similar in size to an average conventional boiler, but Vokera and Worcester Bosch (to name but two) make larger floor standing boilers – similar in size to a washing machine – with slightly higher performance. If a supply of 12 litres per minute is all that is needed then such a boiler is probably unnecessary. For that sort of consumption, look at a 28 or 33kW conventional Combi boiler.

There are many manufacturers, each with models of differing outputs and appearance, and their own particular problems that change from year to year. It is worth talking to a range of professionals to gauge their collective opinion, as the latest is not always the best and design problems may creep in or become evident in service - this is not always the fault of the installer. At the end of the day you have to pick one that suits your needs with regard to output, aesthetics, cost and features.

A final note - Combi’s are great for flats and small houses with few domestic hot water outlets or irregular hot water demands. They may not be a good idea if you have multiple outlets, young children or a poor or unreliable mains water supply – ask your installer.


Regards, Webmaster
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Regards, Webmaster