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19 July 2010 - Combination Boiler Installation

Combination boilers or “Combis” are a particularly attractive form of heating because they dispense with the need for the hot water cylinder in the airing cupboard, and also the large cold water cistern (“tank”) plus small header cistern in the loft which are otherwise needed in conventional vented heating systems. All the major components are housed in one unit which makes them very convenient to the installer. They are especially suitable for smaller households with undemanding lifestyles. They are less suited to larger households although the very latest models seem more capable than their recent predecessors.

The diagram below shows a simplified typical combi installation. Unlike more conventional boilers, the casing normally houses the pump and water/heating timer. Because it is a sealed system, Central Heating (CH) water expansion/contraction has to be accommodated in a pressure vessel (not shown) which is also within the casing. Instead of an open safety-vent pipe looping over the expansion cistern in the loft there will be a pressure-relief valve. There must also be an overheat cut-out within the casing to prevent boiling should the boiler’s internal thermostat fail. A gauge is often provided to indicate that the CH pipework is under pressure and not admitting air. It will need occasional re-pressurising from the mains water supply so a facility for a filling loop (flexible braided pipe) is provided. To comply with the water regulations the loop must be physically removed upon completion. However, some recent models have the CH circuit filling components internal to the boiler and the regulations are observed by removing a large (hand sized!) plastic key.

When Hot Water (HW) is demanded, a HW-priority diverter valve is used to direct the output from the boiler away from the CH circuit into the small secondary heat exchanger where it heats the Domestic Hot Water (DHW). The valve is generally not motorized and a variety of mechanisms are used to operate it, e.g. wax pellet expansion, water flow venturi etc. Often the temperature of the primary heat exchanger is also increased when DHW is demanded.

The secondary DHW exchanger is less likely to form lime scale hot spots because it is heated by hot water rather than very hot gases, and is much smaller than the primary because it is a liquid-liquid heat transfer rather than gas-liquid.

It must be emphasised that not all combis operate identically. For example there are some which have two independent gas-fired heat exchangers rather than one primary and one secondary and this type dispenses with the Diverter valve.

Water to the hot taps/shower comes out at mains pressure although it has to pass through the boiler slowly for it to be heated sufficiently. The flow rate is therefore limited for a given temperature rise. The high pressure / low flow is perfect for showers. It dispenses with the need for power shower pumps which might be necessary in a conventional system with a low pressure head (vertical distance between shower head and outlet of the cistern).

It is generally the power needed to heat the tap/shower water which determines the specification of a combi boiler, not the power needed for the CH.

Some points to think about:

  • Combis are easy to install because they eliminate the need for both an expansion tank in the loft and a hot water cylinder in an airing cupboard. The pump and maybe the timer are also placed within the boiler casing which makes a combi easier to install but more difficult to maintain.
  • If the heat dissipated by a HW cylinder in a conventional system cannot be used usefully then a combi system, with little or negligible stored hot water, may be cheaper to run given identical heat-exchanger efficiencies.
  • Combis generates hot water as and when it is needed, avoiding running out of it. The hot water is at mains pressure, which is an advantage for showers.
  • Standard combis take 40 seconds to heat water and this is a considerable wastage of water. However, slightly more sophisticated combis have a device which restricts the flow of water until it is hot enough to use. Others (storage combis) keep a small reservoir of water permanently hot.
  • Standard combis provide maximum pressure through only one tap at a time. If you have two taps running the flow rate diminishes. This can be partly overcome with a high power or a storage combi.
  • If gas-powered, very large combis or multiple combis impose a huge demand on the gas supply pipework within the house. If two combis are running simultaneously the installation has to be good enough that a gas hob etc does not go out and then fill the room with gas when the boilers shut off. It generally means separate gas feeds taken from as near as possible to the gas meter. Seek professional advice for pipe sizing.
  • The flow rate for *all* the appliances in the house is limited by the capacity of the rising main. Flushing a toilet may thus lead to the hot water temperature fluctuating, or even going cold. Do not install a combination boiler unless your rising main is sufficiently good – flow rate may not be adequate for washing machines/dishwashers that require a hot feed.
  • An airing cupboard is a godsend to some people for the final drying/storage of clothes. A combi dispenses with the cylinder and hence the space needed for it. However, if an airing cupboard is available, it may be possible to keep it warm by installing the boiler within it. The usual trick of putting a small radiator on the boiler’s bypass circuit is not really adequate because, unlike other types of boiler, heat will only be available when the central heating system is needed, but not when only hot water is demanded.
  • An immersion heater cannot be used as an emergency backup in case of the boiler failing as there is no cylinder for it to go in. An instantaneous electric shower could be used although it would need its own water supply as it cannot be put in series with the combi’s output. It certainly won’t be as cheap as adding an immersion element to a normal cylinder and will need a beefier electricity supply.
  • If the system is getting a bit sludgy due to poor installation, poor maintenance or abuse, then some diverter valves on some models can show problems with lack of CH or DHW.
  • When hot water is being used no heating is available (although this would only be significant if the radiators are not already hot when hot water is required
  • Pressure variations within a sealed system are quite high, which can encourage leaks from radiator valves, etc.

When comparing flow rates of different combis make sure that they are specified for identical temperature rises. They are commonly specified at either 35°C or 30°C rises, the lower the rise the greater the flow. However, the temperature rise required in practice is often a good deal higher than either of these figures. If the mains water temperature is 10°C (which it was at 21:00, on 20th November, 2000 in Reading) then with the boiler’s output set to 57°C the required thermal rise across the boiler is 47°C. It would therefore be more realistic but less persuasive if boilers were specified at higher temperature rises.

Be aware that a boiler with a flow rate of 14 litres a minute for a 30°C temperature rise may have half that for a rise of 55°C (required in winter when mains temperatures are not far from zero).

A combi’s output temperature is set by a temperature control on the boiler and, once set, is nominally constant (although early combis fluctuate badly). The setting should a few degrees higher than the maximum tap temperature to allow for thermal losses from pipework. At outlets requiring lower water temperatures, cold water needs to be mixed in.

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