Note: Some useful information on the different types of Central Heating system can be found on this commercial website:



By Dave Williams, a.k.a. "Trollard", 21/5/96 edited by Matthew Marks

This comprises a boiler and a hot water storage cylinder having a capacity of typically 25-40 gallons. The water in the cylinder is heated by an internal coiled tube fed from the boiler. Circulation between them is either gravity (relying on convection to produce circulation) or pumped. The cylinder will be fed from a cold water storage/expansion cistern with a capacity equal to or greater than it. This is supplemented by a separate cistern, typically 10 gallons, which acts as a feed and expansion vessel for the boiler circuit and space heating system. The water in the boiler and radiators is thus kept separate from the domestic hot water, reducing problems of scale build-up or corrosion from dissolved air or impurities, and allowing corrosion inhibitor to be added.

Gravity systems usually have no control specific to the hot water, and use the pump as control for the heating system. This means that the cylinder is heated whenever the boiler is on, leading to very hot water when heating is required, and it is also possible for some radiators to heat up when the pump is off, due to convection. Control can be provided by mechanical means (valve on return to boiler, sensing return water temperature) or electrical (cylinder thermostat operating motorised valve). In any case, the boiler always (except in very special cases such as Agas) has its own thermostat to limit the temperature of the water passing through it.

Pumped systems usually have a three-port motorised valve (q.v.), which can supply for the cylinder, for heating or for both. The valve, pump and boiler are controlled by the timer and room and cylinder stats. More complicated arrangements can include extra two-port valves for different heating zones.

There are variations on this theme. "Un-vented" systems (as opposed to "open" systems) exist that do away with the need for a cold water storage cistern. These are fed direct from the mains water supply and are supplied as a packaged unit comprising all necessary controls and safety devices. It is important to note that this type of system comes under the jurisdiction of Building Regulations and appropriate approval should be sought. The system can only be installed by a competent person i.e. the holder of a Registered Operative Identity Card.

Another type of hot water storage cylinder is known as a Primatic. This differs from a conventional cylinder in that the heating water from the boiler is not contained within a coil (indirect system) but is instead contained within a dome which separates the two water systems by an air bubble. The advantage of this system is that the need for two separate tanks is done away with and a single tank provides cold water storage only, with the air bubble providing expansion capacity for the heating system, and allowing it to be topped up from the hot water supply. Additives cannot be placed in a heating system with this type of cylinder. Its use is not advised, and often prohibited, by most boiler manufacturers.


Combination Boilers

Click here for the FAQ on Combination Boilers

Recommendations as to choice of system

This will vary depending on individual circumstances, but generally the following will act as a guideline:

Combi boilers are best suited to flats, small properties and/or families, low/infrequent hot water consumption.

Storage hot water systems are best suited to large properties and/or families, high/frequent hot water consumption

Can I attach a power (pumped) shower to my combi boiler?

By Ian Smith

No. A power shower needs to draw its supply, both hot and cold, from tanks so that it is not subject to problems with mains pressure variations. Since a combi effectively gives you mains hot water it is incompatible with a power shower.


By Dave Williams, a.k.a. "Trollard"

Review of Conventional Boilers

Before describing a condensing boiler it is important to understand how a "conventional" boiler works.

A conventional boiler comprises a burner and heat exchanger to transfer the heat to the water. Typically the heat exchanger is cast iron or tinned copper (the latter particularly popular on wall mounted boilers). These 2 simple components are supplemented by equally simple controls, a boiler thermostat and gas valve. The thermostat controls the temperature of hot water generated (typically 82C) by switching the gas valve.

There are variations on this theme e.g. conventional flue/balanced flue/fanned flue, on/off and modulating control but all operate with the same basic principles.

Of the heat put into the boiler 70-80% is passed to the water (including some heat loss to the boiler's surroundings) and the remaining 20-30% goes out of the flue. (The efficiency of the boiler is dependent on the age of its design.) On this type of boiler the flue temperature is typically 220C. An important criterion on these boilers is that the return water temperature should not fall below 55-60C (on a normal system the return would be 71C). If the water falls below that temperature then condensation will occur in the flue. This is bad news because the condensation will contain weak acids that will lead to premature failure of the heat exchanger. In fact, it can halve the boiler's life.

Additions in a Condensing Boiler

A condensing boiler attempts to recover the 20-30% heat loss through the flue by deliberately inducing condensation. This is why condensing boilers operate most efficiently with return water temperatures below 55-60C. This is achieved by adding a second heat exchanger on the flue and because of the acids (typical pH3.6 - similar to tomato juice) the material used tends to be stainless steel or aluminium. This second heat exchanger is the main reason why condensing boilers tend to be bigger and certainly more expensive (typically double the cost) than a conventional boiler. The introduction of the second heat exchanger means that the higher resistance for flue gases often necessitates the use of a fan. This does have the added advantage that siting of the boiler is more flexible as longer flues are allowed. In all other respects a condensing boiler works on the same principle as a "conventional" boiler.

The efficiency of a condensing boiler is from 86% upwards (some industrial boilers achieve 99%) although better than 90% can only be achieved by reducing the return water temperature below 60C. The heat emission from the boiler casing is negligible on condensing boilers due to the addition of insulation. Typically a condensing boiler will take 3-7 years to recover the additional capital outlay.

Considerations when Installing a Condensing Boiler

With all the extra heat being squeezed out of the flue gases the temperature is far lower, typically 50-60C, and results in considerable pluming (steam) from the flue. Condensing boilers are best suited to gas as oil and solid fuel fumes have higher acidity levels and lower moisture content. A condensing boiler requires a drain to be installed on the flue to remove the condensation, but because of the low temperatures this can be run in plastic. It should not be routed externally due to the risk of freezing.

Condensing boilers do introduce some special design considerations. Thermostatic radiator valves will reduce boiler efficiency as by shutting down they raise the return water temperature. Outside weather compensation control systems are recommended although impractical for most domestic installations as a separate pumped circuit is then required for domestic hot water. If lower water temperatures are used then a greater surface area of radiators will be required. Condensing boilers can only be used with fully pumped systems.



By Rick Hughes 25/9/1997

These are open vented hot water cylinders, not unlike a domestic hot water cylinder. The big difference is the internal heat transfer tubing, and the way the hot water take off occurs.

In a conventional DHW tank, the transfer coil is connected to the boiler, and the boiler is also connected to the central heating pipe feed and return. The hot water is run off from the DHW cylinder, being replaced by the cold water cistern, and this is heated by the transfer coil. The coil has to transfer all the boiler heat and this is the weak point, resulting in long warm up times and long recovery periods.

The Thermal Store differs in that the boiler is connected directly to the cylinder, circulating the whole cylinder contents. This direct transfer means faster response (mass of hot water = thermal store).

The transfer coils are typically much larger in surface area than in typical DHW tanks and are 10's of metres long, coiled up and usually finned. There are two of these transfer coils. The one at the lower part of the tank feeds the heating system and is typically set to be around 60 degrees C. The upper transfer loop has one end connected to the incoming mains water supply, and its other end is the domestic hot water feed.

The upper part of the tank is set to run at around 75 degrees, the large store of hot water in the tank allowing you to run the cold water through the coil at 5 or more gallons per minute. Note, however, that this will only be the case if your rising main can supply it! (See section 5.2). Some even include a return loop and water pump to give instantaneous hot water at mains pressure, as in hotels. So you get mains pressure hot water, which is one big benefit of this system.

The other big benefit is that there is a large thermal mass of hot water in the cylinder. This means that the boiler can run for a long efficient burn, rather than the repeated small cycles that are inherent with a traditional coil connection.

Thermal stores can be used with any wet heating system, and are almost mandatory in Under-floor heating systems: the store provides the thermal mass at a LOW temperature that makes it ideal for use with such a system. Otherwise, the small volume of water in the UFH pipes would cause repeated cycling of the boiler. The temperature differential in the taller cylinder with a lower and upper stat also means they can be used effectively with condensing boilers, which otherwise present a problem for under floor heating.

For further information, get a specification sheet off one of the Thermal Store manufacturers, such as Nu-Heat (01395 578482).

I'll attempt ASCII art but not my strongest subject :-

  Thermal Store
        ------    blender valve              Hot water take offs
       |      |________X_____________|________|_______|______
-------|      |        |                                     |
       |      |        |                                     |return
       |      |________|_____>O<_____________________________|loop
Boiler |      |    |        pump
Loop   |      |    |
       |      |    |-----  mains  water feed
       |      |              
       |      |----------------  
       |      |            Under-floor heating loop
 ->O<--|      |-------->O<-----
  pump |      |        pump


By Matthew Marks 7/11/1996

Corrosion in central heating systems is usually caused by air getting in. This may happen directly, or it may be via oxygenated water.

Direct entry of air

Air can get in directly if part of the system goes below atmospheric pressure. This will happen if the following combination of circumstances occurs:

  • The speed of the pump is sufficient (more speed gives more head)
  • The flow resistance round the circuit is great enough (more resistance gives more head)
  • The head provided by the feed/expansion cistern is insufficient (vented system - as used with most conventional boiler installations) or the pressure vessel pressure is insufficient (sealed system - as used in most combi boiler installations [q.v.])
  • The pressure reference point (the feed/expansion cistern or pressure vessel connection) is in the wrong place (i.e. near the outlet of the pump, so that the pump subtracts from rather than adds to the pressure).

Air can enter via compression fittings, valve stems or the vent pipe, and it is often very hard to find where this is occurring, because when the pump is switched off, water will not necessarily leak from the entry points. It is therefore a good idea for none of the system to be below atmospheric pressure, and this can be achieved by making sure that not all of the above conditions are satisfied. Moving a feed/expansion connection or a vent arbitrarily is not a good idea, however, because it may pose a safety risk if the boiler overheats, and may lead to pumping over (described later). One advantage of a sealed system is that air entry via the vent pipe and pumping over cannot occur. The pre-charge pressure (normally 1 atmosphere: equivalent to 10m) is also usually greater than the feed/expansion head, although this will slightly increase the risk of leaks.

Air dissolved in water

Fresh water from the mains contains a lot of air, which is why it is not fed through your radiators or conventional boiler, and is one reason why the hot water cylinder has a vent - the air is expelled when the water is heated. (Another advantage of having an isolated body of water in the central heating system is that there will not be much scaling in hard water areas.) After filling a drained system, it can take weeks for all the air to be expelled, because if it is not caught by vents when the water is hot, it may re-dissolve, and most vents don't function effectively with the pump operating, as bubbles are swept past them in the water flow. (Centrifugal and other types of vent which actually have the flow passing through them rather than being teed off, ought to be better in this respect.) Although intuitively it would appear that a radiator would make an ideal air trap, this doesn't seem to happen either!

So apart from not draining the system every other week, leaks should be avoided, as these cause more dissolved air to be introduced as the system is topped up. Leaks are much easier to detect than air entry, but can often go unnoticed if they are in obscure places or are slight. Another advantage of a sealed system is that leaks are not topped up (or not topped up indefinitely), so that not only is the damage caused by a catastrophic leak minimised, but minor leaks will make themselves known by the reading on the pressure gauge dropping. Of course, if regular checks are not made on the system pressure, the leaks will not be noticed until the system is in danger of running dry, which is why sealed system boilers need overheat thermostats.

A small amount of dissolved air will be introduced to a conventional system via the open feed/expansion cistern, if there is enough thermal contraction to draw water all the way from the cistern to the primary circuit when the system cools. Far more air will be dissolved if pumping over occurs - the pump forces water through the vent pipe and into the expansion cistern, becoming thoroughly oxygenated, circulating the already-oxygenated water in the cistern around the system, losing heat and introducing fresh water as clouds of steam are emitted, and for good measure causing mildew or dry rot in the roof timbers as the steam condenses on them. As you can see, pumping over is not A Good Thing (TM). It is caused by a similar set of circumstances to parts of the system being below atmospheric pressure, but laying out the feed and vent positioned to avoid one problem makes it more susceptible to the other.

Have you got corrosion?

If corrosion has progressed to the point where radiators are starting to pinhole, it's pretty bad.