©
EUROHEAT
DISTRIBUTORS (H.B.S) LTD. March 2006
Technical Guide IN1087 Edition C
5
No flue operates in isolation, but is a
component part of a complete system,
with the stove and house affecting its
performance. Whilst we normally think of
flues only when the stove is operating, the
same rules of heat induced flow in the flue
apply when the stove is not lit. If the house
is hotter than the outside air, the flue will
perform as if the stove was lit, with the
warm air from the house travelling up the
flue. However, on those odd days when it
suddenly becomes warm and the outside
temperature is higher than the temperature
in the house, the flue will operate in reverse
with the possibility of sooty smells being introduced into the house, and will make it very difficult to light
the stove. This reversed air flow may provide sufficient air for the stove to ignite and burn poorly, but
the products of combustion will spill into the house. Because none of the heat from the fire is passing
into the flue it will not warm to correct the air flow until the heat from the fire has warmed the stove
to allow conduction of this heat to the flue. If reverse flow is noticed before lighting the stove a fan
heater should be directed into the stove through the open door for several minutes before attempting
to light the stove. (see page 28, Spring and Autumn syndrome)
X
A more complex example of reversed flow in flues is caused by multiple flues and differences in
flue heights and again the effects will become evident even before any fire is lit. All the flues will
tend to aspirate as in the previous example, but with increasing numbers of flues the throughput of
air increases resulting in considerable loss of heat from the house. Reducing these heat losses, by
improving the sealing of doors and windows has led to the loss of numerous small ventilation areas
and unless provision has been made for specific ventilation to each flue the air flow in each flue will
depend upon the balance of temperature and height of them and will inevitably result in the coldest
or shortest flue acting as the incoming air supply. This will not only cause smells and difficulty in
lighting the reverse flowing flue, this imbalance has the potential of becoming dangerous. A typical
example would be where an open fire is in use and a stove is installed with a shorter flue. Because
an open fire allows so much of its heat to escape up the flue the flue operates at a very much higher
temperature than that of a stove’s flue resulting in an imbalance of draft motivation. If this imbalance
is increased by a difference in height it is quite possible for the shorter flue to stall or even reverse
flow when operating, allowing the oxygen depleted air from the stove to spill into the house.
Adequate permanent ventilation must always be provided for each fire in the house, whether open
fires or stoves.
Ventilation must allow for any air extraction systems such as kitchen or shower room fans.
The heights of all flue terminals should be equal.
