2. Introduction
One of the elements in both Natural Gas
and Propane is hydrogen. A gas burning
appliance mixes the gas with air and during
the combustion process hydrogen and
oxygen combine together and produce heat
(143,100kJ/kg) and water vapour (H
2
O).
For every kg of hydrogen burned 9kg of
water vapour is produced. The
temperature in the heat exchanger of a
boiler can reach 1400
0
C. The water vapour
produced is so hot it turns into superheated
steam.
2.1
The Condensing Process
This superheated steam contains both
sensible (available heat) and latent heat
(heat locked up in the flue gases). A
conventional boiler will recover some of the
sensible heat by passing these hot gases
over a heat exchanger.
The heat exchanger in non-condensing
boilers is generally made of cast iron and
cools the gases to between 250
0
C and
350
0
C. A conventional boiler does not
recover any of the latent heat and this
energy is simply lost to the atmosphere
through a metal flue. These flue gases are
extremely hot and the lost energy wasted
can equate to up to half of the annual
running costs.
A simple condensing boiler will however
extract more of the sensible heat and some
of the latent heat by cooling the flue gases
down to below 100
0
C.
When a condensing boiler is operating in its
most efficient manner flue gas temperatures
of around 50
0
C will be achieved and the
boiler will now start to condense the flue
gases. The superheated steam is cooled to
its dew point, typically around 55
0
C the flue
gases give up their latent heat to the boiler
and condense out.
The critical factor that ensures maximum
efficiency from a condensing boiler is the
water return temperature. The water return
temperature determines whether the boiler
operates in condensing mode, which in turn
controls the boilers efficiency.
To sum up, with water return temperature of
55
0
C or less, the latent heat is condensed
out of the flue gases.
A typical, non-condensing, central heating
system is designed with a water flow
temperature of 80
0
C and a return water
differential of 10
0
C. This design differential
is critical.
System designs had to incorporate high
return temperatures (typically 70
0
C) to stop
any unwanted condensing of the flue gases.
The flue gases leaving a conventional boiler
have to be discharged very hot for the
following reasons.
To propel the flue gases up a chimney or
through a flue they have to be discharged
hot to give them buoyancy and enough
thermal lift to overcome the flues natural
resistance.
If the flue gases are not hot enough the
effectiveness of the flue system is reduced
and harmful by products of combustion
could enter the building via the appliance or
its flue.
A conventional boiler has to discharge the
flue gases hot to prevent any unwanted
condensing. If the flue gases are not kept
hot enough they will condense allowing
water to run back down the flue and into the
boiler.
Clearly this has to be avoided. Heat
exchangers made of cast iron, or boiler
designs not equipped to discharge this
water would suffer imminent failure.
The flue gas discharge from conventional
boilers has to be maintained at high
temperatures.
Questions you might be asking yourself now
might include:
Question?
Why were such inefficient
appliances designed?
Answer:
Fossil fuels were
cheap and the environmental
consequences of burning and wasting so
much fuel were not fully appreciated.
EC 25 Compact Technical Manual page
5
