By CliveE, with guidance from Ed Sirret
This FAQ was put first together in November 2000 following much debate in the uk.d-i-y newsgroup on the topic of Combination boilers, their strengths and weaknesses, how to chose them, and the latest trends in design. This introduction is written by a non-expert in an attempt to cut the hype and to distil the discussions into just the pertinent points.
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:
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.
Washing up and shaving require the hottest water temperature of 55°C.
A bath typically contains 120 litres of water at 42°C.
An average shower uses 35 litres of water in 5 minutes with a temperature of 42°C at the showerhead.
A modern 100,000 BTU/h (29.0kW) combi can typically provide 14 litres/min at 30°C, 12.5 litres/min at 35°C, and 7 litres/min at 55°C rise.
Calculating for a bath:
I calculate that if the cold tap is 10°C and the hot tap is 55°C, then you'll need 35 litres of cold and 85 litres of hot for 42°C bath water. The 100kBTU combi above is specified at 9.5 litres a minute for a 47°C rise (allowing 2°C loss in pipework, 55+2-10) and this will take 9 minutes to fill. If the bath is iron then you'll need less cold, more hot and more time. If the combi is older and less capable then times can double or even treble.
Calculating for a shower:
I calculate that from a cold feed at 10°C you'll need 2 litres/min and from a hot feed at 55°C you'll need 5 litres/minute. A combi specified at 9.5 litres a minute for a 47°C rise is therefore easily capable of this throughput.
As a comparison to all this, I can run a good bath in one and a quarter minutes from my unvented Megaflo cylinder, heated with a Potterton Envoy condensing boiler. Showers are also at mains pressure. Of course, this comes at a price and with its own merits and demerits.
By Ed Sirret 14/11/2000
If you shell out enough for the better models (not the stuff you invariably see installed) there are essentially only a few drawbacks:
1) Scaling of the DHW heat exchanger in hard water areas (i.e. the bulk of our water), the replacement part is not a negotiable price - either you buy it at the price requested by the manufacturer or you replace the boiler. However, I hear that the latest models are designed to reduce scaling. Also you could run softened water through them but you'd probably invalidate the warranty. Although a HW cylinder also scales up this is less of a problem in practice.
2) There is no possibility of an electric immersion heater as a backup should the boiler fail. However, an electric shower and kettle will do for a while. A back-up electric instantaneous shower does not necessitate a header tank and benefits from a reasonable supply pressure (not usually available from a tank).
3) Although installation is relatively simple you might find that you need to upgrade the gas supply pipe diameter, at least to comply with regulations if not in practice.
In practice, in terms of what actually is installed in most places I actually see, the cheaper models dominate. For cheaper models without stored hot water then you might also add the following drawbacks:
4) Modest flow rate of hot water good for a shower but possibly tedious for a bath.
5) Increased delay before hot water comes through - irritating for small quantities e.g.. hand basin.
A number of benefits claimed for combis are actually benefits that all modern boilers have - sealed pressurized CH circuit, all major components integrated into system box (is this an advantage?).
The real benefits are:
1) No tanks and cylinders - but no warmth in airing cupboard
2) Mains pressure hot and cold at all outlets.
For instant hot water in a large house a combi could be put to good use by either supplying HW to a nearby kitchen tap and also supplying HW and CH with a conventional boiler, avoiding the delay of HW on a long pipe run to the kitchen. Or perhaps in another arrangement a combi might provide the HW for a shower only (giving all the benefits of a good shower but without a noisy and expensive pump).
David Shepherd 15-11-2000
Water pressure and flow rate which, though interdependent, are not the same. It's a common misunderstanding, and something I initially struggled to get my head round when I did my fluid mechanics many moons ago.
The flow rate is proportional to the resistance to flow and to the pressure. This means that:
(a) for a given pressure, the flow rate is dependent on the resistance to flow.
(b) for a given resistance to flow, the flow rate is dependent on the pressure.
The explanation detailed below is based upon simple combi boilers. Modern combis of modest power output and many older combis would fall into this category. As detailed elsewhere in this FAQ, higher powered combis and more sophisticated combis are now available which have performance characteristics much closer to those of a conventional boiler, and thus the differences outlined below will be far less noticeable. I have used simple combi system for the purposes of this explanation since it is with these systems that the differences are most exaggerated.
To compare hot and cold water for the same system, we need to refer to point (a).
In a combi system, both the hot and cold water will be fed from the mains and will be at mains pressure. However the resistance to flow in the hot and cold supplies is different. The resistance due to the pipework will be more or less the same for both. But the boiler itself restricts the flow rate of hot water so that the water remains in the boiler long enough to be heated to the correct temperature. Therefore, although the pressure at the hot and cold taps is the same, the flow rate from the hot tap will be less than from the cold tap. That is the difference you notice.
In a conventional system with a hot water cylinder fed from the cold water tank in the attic, the pressure will again be the same at the hot and cold taps. The resistance to flow will be similar in both the hot and cold system and thus the flow rates will be more or less the same. The pressure of the water in this case will be due to the difference in height between the cold water tank and the tap you are using. This pressure will be considerably less than mains pressure, an important point when it comes to showers and baths (see below).
You will get a better shower with a combi than from a conventional system.
To compare a shower fed by a combi boiler with one fed by a hot water cylinder, we need to consider point (b) above.
The shower head will provide a large resistance to flow. With a combi boiler the hot water is at mains pressure so the flow rate to the shower will be good (but will never be higher than the boiler itself will allow). With a hot water cylinder, the pressure is significantly lower than mains pressure so the flow rate will be comparatively low
To compare a bath fed by a combi boiler with one fed by a hot water cylinder we need to refer to both (a) and (b).
Bath taps do not give a huge restriction to flow. Therefore from a hot water cylinder we get a reasonable flow rate despite the low pressure. However, with a combi boiler, although the pressure is higher, the flow resistance (due to the boiler) is also much higher, and this more than offsets the higher pressure. The flow rate is therefore much lower.
To sum up:
Note that I have assumed a simple basic combi boiler; a simple hot water tank in an upstairs airing cupboard with cold water tank in the attic; decent mains water pressure; houses that are not 10 stories high!!! All those parameters can vary (e.g. you have a fairly high powered combi) and as such the differences I've outlined will be more marked in some situations than others.
To come back (finally!!) to the original topic....
As far as the choice between boiler types goes, it's a question of personal priorities. Someone (like myself) who takes mostly showers and only uses the bath for a long leisurely soak now and then, might consider the high pressure shower a considerable benefit and the slow filling bath no more than a minor irritation at most, and therefore be swayed towards a combi system. On the other hand someone who has few showers (or maybe has a high powered electric shower already) and for whom a slow filling bath is a major irritation, might be swayed towards a conventional system.
There are, however, several other factors to consider, and again, much of it boils down to personal preference.
I don't think I would go for a cheap one. I fitted a 95,000BTU Ariston combi very recently, and am very happy with it. Comes with built in by-pass, filling loop, frost protection, fully modulating, clock (although Mickey mouse), pump, etc - and one of the smallest dimensions you will find.
The hot water comes through quickly with the Ariston set on comfort mode - which keeps the water in the heat exchanger warm. Many combis have this feature now. I don't use it in this mode personally, as once water has been drawn the first time in the morning, the water is generally still warm in the pipes / heat exchanger. I don't see the point in the boiler cycling 24 hours a day just to keep the heat exchanger hot.
Filling a bath takes 10-15 minutes with the Ariston. Not a problem in our household, showers (which are excellent) being used most often.
In addition to Ed's comments (above) I would add:
Drawback: No stored cold water for mains failure (although this could be catered for)
Advantage: unlimited supply of hot water
I fitted the combi in the same airing cupboard that I removed the hot water tank from (on the back wall) - cupboard is still as warm, with more space than before.
One thing I would add is if you are going for a combi powered shower, get a thermostatic mixer. They aren't cheap (c. 150-200+ quid) - but will prevent the fluctuations when a toilet is flushed / hot is drawn elsewhere. 
A few more advantages / disadvantages -
 (CliveE) This feature is not a consequence of having a combi boiler but rather one of the consequences of mains-fed WC cisterns in general. It's just the fact that fresh water entering the cistern is colder if the water is coming directly from the mains, causing water vapour in the air to condense on it. If it comes from the cistern (tank) in the loft it should be a bit warmer because the thermal insulation beneath the cistern should have been removed to keep the water from freezing in winter.
 (CliveE) Someone's advantage of having no cold water cistern in the loft is another's disadvantage of having no water when the water supply goes off. This "disadvantage" has to be balanced against the usually small risk of the water mains failing.
 (Adam) Fluctuations can be greatly minimized: 1) Split the cold water supply to
cold taps from the cold water combi supply. As the mains enter the house go straight to the combi
inlet. As it enters the house, tee off a feed to all the cold taps. You could put a gate valve where
the cold line tees off using this to restrict the whole cold supply line. 2) Put in-line restrictors
at cold taps (about £3 each). Basin taps and toilets don't need high flow rates so throttle them
back. 3) Throttle back hot taps too that don't require great flow, if the combi is operating on a
water flow detector as the Ariston and Ferroli do. Few hot taps require great flow. The shower and
bath tap are about the only ones that do. People tend to over turn the taps. Just restrict it.
We have a 3 bed semi (2 adults and a baby on the way, with a 100,000 BTU combi - the minimum size I reckon for HW supply (I can't remember the listed flow rates off hand).
The only downside for me really is that if you run some hot water, then stop, then run a bit more, you get a slug of cold in the middle. Yes baths do fill a bit slower, but it's not that slow, and we are shower people here mostly, baths being the occasional good long soak so it's not an issue.
As for speed of HW arriving, I don't think it takes a great deal longer than it took for the HW to get from the cylinder to the kitchen sink.
I don't have any problem with the dish/clothes washers - a lot of the time they use the cold fill anyway. I keep meaning to just use a cold fill anyway, by the time the combi has heated some water they have stopped filling.
The shower is of course great (thermostatic), and even if the toilet is flushed, or the dishwasher kicks in, the flow rate drops a bit but it's still ok. The HW never runs out, and we don't have the waste of space of an airing cupboard.
If a high pressure shower is required then this can be achieved with :
A well designed shower with short, direct runs of 22mm pipes off a conventional vented system can also give extremely good results. It will tend to be high flow/lower pressure, but a good pressure can be simultaneously achieved if the outlet of the cistern (tank in the loft) is at least 1.5 metres above the shower head. Remember that high flow showers cost more to run due to the extra volume of water heated (and consumed if on a water meter).
High pressure, low flow showers produce jets which are like needles on the skin and can be uncomfortable. Ideally shower heads should be matched to the likely pressure and flow for best results.
I am having problems with my original low pressure mixer now it is on the combi - so don't assume you can fit a combi without replacing / uprating existing low pressure mixer showers...