GB2310715A - Fire box with convection duct and high-level hood - Google Patents

Abstract

Combustion products collected from a combustion chamber 2 and mixed with secondary air B pass to a flue through a hood 5 via upper and lower outlets 12 and 14. The upper outlet 12 provides an upward flow path to the flue when the fire is lit, but when the fire heats up the additional suction through the flue draws hot air and gases through the lower outlet 14. This produces more efficient space heating by increasing heat transfer to air which is drawn through a convection duct 6, 7. The area of the outlets 12 and 14 can be changed to compensate for different flue characteristics. In addition, a baffle plate 20 located within the hood 5 further increases efficiency by maximising flow through the hood.

Description

FIRE BOXES TECHNICAL FIELD OF THE INVENTION This invention relates to fire boxes, and more particularly, those which comprise a combustion chamber provided with a flue outlet, and a convection duct at least partially surrounding the combustion chamber.

BACKGROUND In such fire boxes, gaseous combustion products are generally collected by a hood mounted at the top of the combustion chamber. Along with secondary air drawn in from a living space, the waste gases collected by the hood pass to the flue through the rear wall of the file box.

Aesthetic considerations sometimes dictate that the hood should be relatively small and mounted high in the combustion chamber. There is therefore a large gap between the hood and the fire, which has two consequences.

Firstly, large quantities of cold air are drawn in from the living space, which cools the convention duct and thus reduces the heating efficiency of the fire.

Secondly, the drawing ability of the fire is low, particularly when it has just been lit.

The present invention seeks to provide a fire box which is patentably different from known fire boxes.

SUMMARY OF THE INVENTION The present invention proposes a fire box for heating a living space comprising a forwardly open combustion chamber having a hood at its top coupled to a flue outlet, and a convection duct in heat exchange relationship with the combustion chamber to draw air from and return heated air to the living space, in which the said flue outlet comprises an upper flue outlet and a further flue outlet, lower than the upper outlet, arranged to cause a proportion of the hot combustion products from the combustion chamber to travel in heat exchange relationship with the convention duct before passing through said further flue outlet.

In general, the upper flue opening ensures an initial flow of combustion products to the flue when the fire has just been lit and the fire box and flue are relatively cool. Once the fire box and flue become hot, the suction on the lower opening increases causing a flow of combustion gases through the lower opening transferring heat to the air flowing in the convention duct. As a result, the overall efficiency of the fire is high even if the hood is relatively high up in the combustion chamber.

BRIEF DESCRIPTION OF THE DRAWINGS The following description and the accompanying drawings referred to therein are included by way of non-limiting example in order to illustrate how the invention may be put into practice. In the drawings: Figure 1 is a vertical section through a fire box in accordance with the invention, and Figure 2 is a general rear view of the upper part of the fire box.

DETAILED DESCRIPTION OF THE DRAWINGS The fire box 1 is intended to be installed in a wall opening provided with a rear flue. The fire box comprises a combustion chamber 2 for containing a solid fuel fire 3 or an open solid-fuel-effect gas fire for example. The chamber 2 is formed by a box 4 having rear, side and bottom walls, leaving the front of the chamber open to a living space. The top of the combustion chamber is defined by an elongate, downwardly-open hood 5 arranged to collect gaseous combustion products A from the chamber 2 together with secondary air B drawn into the combustion chamber from the living space.

The combustion box 4 is partially surrounded by a convection duct 6 which has a bottom opening 7 positioned below the combustion box 4 for drawing air C in from the living space to be heated by the fire. The duct 6 passes up the rear of the combustion chamber where the air is heated by direct heat exchange from the combustion chamber 2. After passing along a forwardly and upwardly inclined section 7 the convection duct again travels upwards and then passes forwardly over the hood 5. The heated air D returns to the living space via an outlet 8 located above the hood.

The rear of the hood 5 opens via a duct 10 which passes through the convection duct 6 to carry waste gases into a box 11 forming a rear chamber immediately behind the convection duct. The rear wall of the box 11 has upper and lower flue outlets 12 and 14 which conduct the gases from the chamber 1 1 to the flue (not shown). The upper outlet 12 may be formed by a single opening or by a number of separate openings (e.g. the three openings 1 2a, 1 2b and 1 2c - Fig. 2) disposed in a horizontal row, whereas the lower outlet will generally be formed by a single horizontal slot. The total area of the outlet 12 will normally be less than that of the outlet 14. The upper flue outlet is located at the top of the box 11, slightly higher than the duct 10.

The gases thus have a substantially continuous upward path from the combustion chamber 2 into the flue via the outlet 1 2 as indicated at E, which ensures that there is an initial flow of combustion products to the flue when the fire has just been lit and the fire box and flue are relatively cool.

However, once the flue and fire box become hot the suction from the flue increases so that there is an increased flow to the flue. As a consequence of this increased flow, the combustion gases additionally flow through the lower opening 14 as indicated at F, after passing downwardly over the inclined portion 7 of the convention duct. As a result, the waste gases heat the air in the convection duct, subst.lntially increasing that overall sp.4ce-heatinK efficiency of the fire.

Performance can be further improved by incorporating a baffle plate 20 within the hood 5. The baffle comprises a substantially vertical portion 21 disposed within the entrance to the hood with its lower edge 22 projecting below the bottom of the hood. The upper portion 23 of the baffle plate is angled rearwardly to direct the flow of gases towards the rear duct 10. The baffle reduces turbulence within the hood, and in the general area of the hood at the top of the combustion chamber 2, thus ensuring maximum flow into the hood 5 and reducing the risk of spillage of combustion products into the room.

In Fig. 2 it will be noted that a restrictor plate 15 is mounted on the rear of the chamber 11 to completely cover the middle opening 12b and partially cover the lower parts of the other two top openings 1 2a and 1 2c. The plate also covers the top part of the lower opening 14. This plate is removed for flues having a low pull, i.e. precast flues, to increase the area of the openings, but is left in place with flues having a relatively high pull. The increased surface area provided by the plate 15 also effectively increases the length of the duct 6 over which the combustion gases flow to the lower opening 14.

This provides a larger area to be heated by the waste gases, thereby further increasing heat recovery in the convected air stream flowing through duct 6.

It will be appreciated that the features disclosed herein may be present in any feasible combination. Whilst the above description lays emphasis on those areas which, in combination, are believed to be new, protection is claimed for any inventive combination of the features disclosed herein.

Claims (8)

1. A fire box for heating a living space comprising a forwardly open combustion chamber having a hood at its top coupled to a flue outlet, and a convection duct in heat exchange relationship with the combustion chamber to draw air from and return heated air to the living space, in which the said flue outlet comprises an upper flue outlet and a further flue outlet, lower than the upper outlet, arranged to cause a proportion of the hot combustion products from the combustion chamber to travel in heat exchange relationship with the convention duct before passing through said further flue outlet.

2. A fire box according to Claim 1, in which the total area of the upper flue outlet is less than that of the lower flue outlet.

3. A fire box according to Claim 1 or 2, provided with means for altering the areas of the upper and lower flue outlets.

4. A fire box according to Claim 3, in which said means for altering the areas of the flue outlets comprises a removable restrictor plate.

5. A fire box according to any preceding claim, in which the hood contains a baffle plate.

6. A fire box according to Claim 5, in which the baffle plate projects into the combustion chamber below the entrance to the hood.

7. A fire box according to Claim 5 or 6, in which the baffle plate comprises an upwardly extending portion and a rearwardly extending portion.

8. A fire box substantially as described with reference to the drawings.