"Christian McArdle" wrote in message
. net...

A combi is a heating and hot water system all in one box with no
external Tanks or cylinders. Hence the name "combi" from
combination.

Yes, but the term does now seem to mean an
instantaneously heated system,
rather than a storage based one.

To clarify for you:

Firstly, a combi is a "combination" of the heating and water system in one
case, eliminating external tanks and cylinders, and generally supply hot
water at high main pressure. To confuse a little, some can run at very low
pressures and even off tanks. Generally most are fed from the mains. It is
generally a matter of mounting the boiler and connect up the pipes. The
expert designers have done the hard work for you and put all in one case.

There are three types of combi:

1) The Infinitely Continuous Combi -

Heats cold mains water instantly as it runs through the combi. It never runs
out of hot water. This is the most common type of combi, generally having
lower flowrates than Nos 2 & 3 below. The largest flow rate instant combi is
a two bathroom model, 22 litres/min ECO-Hometec. Being a condenser it is
very economical too. http://www.eco-hometec.co.uk

2) Unvented Cylinder Combi -

An unvented cylinder is a similar to a conventional cylinder but run off the
high-pressure cold mains. A combi with an integral unvented cylinder has
approx 60 litre cylinder heated to approx 80C, with a quick recovery coil
that takes all the boilers output. A fast acting cylinder thermostat ensures
the boiler pumps heat into the cylinder ASAP with a recovery rate from cold
around 5-8 mins (Ariston claim 8 mins). The 80C water is blended down to
about 45-50C. e.g's, Ariston Genus 27 Plus, Glow Worm, Powermax.

3) Heat Bank Combi -

Incoming water is instantly heated running through a plate heat exchanger
(as is most instantaneous combi's) that takes its heat from a "domestic hot
water only" store of water at approx 80C (instantaneous combi's take the
heat from a heat-exchanger heater via the burner). A fast acting thermostat
ensures the boiler pumps all of its heat into the store ASAP with a recovery
rate about 5-8 mins from cold. The 80C water is blended down to about
45-50C. They are generally two stage, in that when the thermal store is
exhausted it reverts to what the bunrer can produce, which isapprox 11-12
litre/minute. e.g. Vokera & Worcester floor standing models (standard
washing machine sizes).

N.B. The heat bank is a variation of a thermal store, but is "not" a thermal
store in the conventional sense in that a coil carrying cold mains water
runs though a store of hot water kept at about 80C. Heat-banks are far more
efficient and give higher flowrates than conventional coiled thermal stores.
The stainless steel plate heat-exchangers do not scale up so easily.

4) Combined Primary Storage Unit
(Not classed as a combi, but a derivative of a combi, but still a one box
solution, so still in the same family)

These are a combination of a large thermal store and boiler in one casing.
The units are large (larger than standard washing machine size) and floor
mounted. The heating is taken off the thermal store, with in many cases the
DHW taken off the store using a plate heat-exchanger (heat-bank). Unlike
the Heat-bank in 3) above the thermal store supplies heating and DHW. They
are available from 1 to 2.5 bathroom models. Gledhill do an excellent
condensing version, the Gulfsream 2000. http://www.gledhill.net

Nos. 2) 3) & 4) have high flowrates. No. 1 "generally" has low flowrates
but there are always exceptions and some can be high - e.g. ECO-Hometec
infinitely continuous combi; actually has a very high flowrate.

Nos 2), 3) & 4) use stored water, but in different ways. Unlike No. 1 they
will eventually run cold, but that takes quite a time, hence they are
referred to as "two bathroom" models, having the ability to fill two baths
with very fast recovery rates. As hot water is being drawn off the high
rating burner is also reheating. Very rare do these combi's run out of hot
water in average use. When taking one shower the burner may be re-heating
faster than what can be drawn-off.

There are combi's that give hot water and heating simultaneously - Combined
Primary Storage Units do. Most don't as they are hot water priority.