Heating system and boiler therefor

Abstract

A method of heating a building in which a boiler provides hot water for circulating through radiators in the building and a condensing heat exchanger recovers heat from the boiler flue gases. Air entering the building is heated by an additional heat exchanger through which water flows which has been heated by the condensing heat exchanger. Cold air supplied to the combustion chamber is also employed to cool the flue gases. The condensing heat exchanger comprises a first exchanger in which the hot flue gases are cooled to a temperature above the dew point and a second in which the previously cooled flue gases are cooled to below the dew point. The second exchanger includes a primary water circuit which cools water in a secondary circuit, in which water is in contact with the flue gases and the secondary circuit water passes through a heat exchanger in an air inlet to the building. Some of the water returning from the radiators is cooled before supply to the primary circuit. A building and heating system comprises a boiler which provides a source of hot water, a pump for circulating the hot water through radiators in the building, a condensing heat exchanger which recovers heat from the boiler flue gases, and in which water which has been heated in the condensing heat exchanger is employed to heat air entering the building.

1 . A method of recovering latent heat from water vapour in the flue gases from a hydrocarbon fuel-burning boiler adapted to heat water to be circulated through radiators to heat a building, in which hot flue gases from the boiler are cooled first by a heat exchanger through which water returning from the radiators is caused to flow and then by a second heat exchanger through which water at a temperature below the dew point of the water vapour in the flue gases is caused to flow in direct contact with the flue gases, so that not only is heat extracted from the latter but soluble products of combustion (such as SO 2 ) are dissolved in the water and are thereby separated from the flue gases before they exit to atmosphere, and wherein before the water returning from the radiators is supplied to the first heat exchanger a small proportion of the returning water is diverted through an additional heat exchange device located in a relatively cool region in the building being heated, and thereafter the diverted water is further cooled as it passes through an air to water heat exchanger through which air which is to support combustion in the burner of the boiler passes en route to the combustion chamber, such that the exit water temperature therefrom will be in the range of 20°-40° C., so as to be below the said dew point and the exiting water is supplied to cool the water in the second heat exchanger.  
   
   
       2 . A method as claimed in  claim 1  wherein the second heat exchanger includes a subsidiary water to water heat exchanger through which the said exiting water flows to cool water which flows around a closed circuit defined by the second heat exchanger, and after passing through the subsidiary heat exchanger the water is returned to the boiler.  
   
   
       3 . A boiler for supplying hot water to radiators in a building having first and second heat exchange means for cooling the exhaust gases through the first of which water returning from the radiators is caused to flow before returning to the boiler, which includes a water path by which a fraction of the water returning from the radiators to the first heat exchanger is first caused to flow direct to an additional heat exchange device instead of to the first heat exchanger, after passing therethrough to pass through an air to water heat exchanger in an air inlet to the boiler supplying air to support combustion therein, thereafter to pass through a water to water heat exchanger adapted to cool water circulating in the said second heat exchanger, and lastly to return the water to the water inlet to the boiler, where it is mixed with water passing thereto from the first heat exchanger.  
   
   
       4 . A method of heating a building in which a water heating boiler which in use burns a hydrocarbon fuel in a combustion chamber from which hot products of combustion leave to vent to atmosphere via a flue, and which provides a source of hot water for circulating through radiators in the building, and a secondary condensing heat exchange means operates to lower the temperature of the products of combustion below the dew point of water vapour therein thereby to gain the latent heat of evaporation of the water content thereof to the flue, and in which there is an air inlet to the building, comprising the step of warming air entering the building by a heat exchanger in the air inlet through which water is caused to flow which in use is heated in the secondary heat exchange means using heat from the hot products of combustion, thereby to transfer heat therefrom to air entering the building.  
   
   
       5 . A method as claimed in  claim 4  wherein the air inlet to the building also comprises an air intake for supplying air to the combustion chamber of the boiler, so that after passing through or around the heat exchanger in the inlet some of the warmed air goes to the combustion chamber in the boiler to support combustion and the remainder of the warmed air is diverted into the building, to directly warm the latter.  
   
   
       6 . A method as claimed in  claim 4  wherein the secondary condensing heat exchange means comprises two separate heat exchangers, a first in which the hot flue gases are cooled by water flowing therethrough to a temperature which is above the dew point temperature and a second through which the previously cooled flue gases pass and in which they are cooled to below the dew point temperature before escaping to atmosphere, and in which the second heat exchanger includes primary and secondary water circuits, water flowing around the secondary circuit being cooled by water flowing through the primary circuit, the secondary circuit water being in contact with the flue gases to cool them below the dew point, and water from the secondary circuit is caused to pass through the heat exchanger in the air inlet, to cool the secondary circuit water and warm the incoming air, before being returned to rejoin the water flowing around the secondary circuit, and water which is to flow through the primary circuit is obtained by diverting some of the water returning from the radiators in the building to the boiler, and this diverted water is cooled by flowing through at least one further heat exchange means before being supplied to an inlet to the primary circuit.  
   
   
       7 . A method as claimed in  claim 4  wherein the secondary condensing heat exchange means comprises two separate heat exchangers, a first in which the hot flue gases are cooled by water flowing therethrough to a temperature which is above the dew point temperature and a second through which the previously cooled flue gases pass and in which they are cooled to below the dew point temperature before escaping to atmosphere, and in which the second heat exchanger includes primary and secondary water circuits, water flowing around the secondary circuit being cooled by water flowing through the primary circuit, the secondary circuit water being in contact with the flue gases to cool them below the dew point, and water from the secondary circuit is caused to pass through the heat exchanger in the air inlet, to cool the secondary circuit water and warm the incoming air, before being returned to rejoin the water flowing around the secondary circuit, and water which is to flow through the primary circuit is obtained by diverting some of the water returning from the radiators in the building to the boiler, and this diverted water is cooled by flowing through at least one further heat exchange means before being supplied to an inlet to the primary circuit, wherein the said at least one further heat exchange means comprises a radiator located within a cool region of the building.  
   
   
       8 . A method as claimed in  claim 4  wherein the secondary condensing heat exchange means comprises two separate heat exchangers, a first in which the hot flue gases are cooled by water flowing therethrough to a temperature which is above the dew point temperature and a second through which the previously cooled flue gases pass and in which they are cooled to below the dew point temperature before escaping to atmosphere, and in which the second heat exchanger includes primary and secondary water circuits, water flowing around the secondary circuit being cooled by water flowing through the primary circuit, the secondary circuit water being in contact with the flue gases to cool them below the dew point, and water from the secondary circuit is caused to pass through the heat exchanger in the air inlet, to cool the secondary circuit water and warm the incoming air, before being returned to rejoin the water flowing around the secondary circuit, and water which is to flow through the primary circuit is obtained by diverting some of the water returning from the radiators in the building to the boiler, and this diverted water is cooled by flowing through at least one further heat exchange means before being supplied to an inlet to the primary circuit, wherein the building includes a reservoir of cold water, a heated water cylinder and taps to which water is supplied therefrom, and the further heat exchange means comprises a further heat exchanger associated with the cold water reservoir, through which the diverted water is caused to flow before it is supplied to the inlet to the said primary circuit.  
   
   
       9 . A method as claimed in  claim 4  wherein the secondary condensing heat exchange means comprises two separate heat exchangers, a first in which the hot flue gases are cooled by water flowing therethrough to a temperature which is above the dew point temperature and a second through which the previously cooled flue gases pass and in which they are cooled to below the dew point temperature before escaping to atmosphere, and in which the second heat exchanger includes primary and secondary water circuits, water flowing around the secondary circuit being cooled by water flowing through the primary circuit, the secondary circuit water being in contact with the flue gases to cool them below the dew point, and water from the secondary circuit is caused to pass through the heat exchanger in the air inlet, to cool the secondary circuit water and warm the incoming air, before being returned to rejoin the water flowing around the secondary circuit, and water which is to flow through the primary circuit is obtained by diverting some of the water returning from the radiators in the building to the boiler, and this diverted water is cooled by flowing through at least one further heat exchange means before being supplied to an inlet to the primary circuit, and wherein the diverted water returning from the radiators is forced to pass through a further heat exchanger in an air inlet to the boiler combustion chamber.  
   
   
       10 . A method as claimed in  claim 4  wherein the secondary condensing heat exchange means comprises two separate heat exchangers, a first in which the hot flue gases are cooled by water flowing therethrough to a temperature which is above the dew point temperature and a second through which the previously cooled flue gases pass and in which they are cooled to below the dew point temperature before escaping to atmosphere, and in which the second heat exchanger includes primary and secondary water circuits, water flowing around the secondary circuit being cooled by water flowing through the primary circuit, the secondary circuit water being in contact with the flue gases to cool them below the dew point, and water from the secondary circuit is caused to pass through the heat exchanger in the air inlet, to cool the secondary circuit water and warm the incoming air, before being returned to rejoin the water flowing around the secondary circuit, and water which is to flow through the primary circuit is obtained by diverting some of the water returning from the radiators in the building to the boiler, and this diverted water is cooled by flowing through at least one further heat exchange means before being supplied to an inlet to the primary circuit, and wherein a pump operates to pump secondary circuit water through the second heat exchanger and through the building air inlet heat exchanger and wherein only some of the secondary circuit water is circulated through the air inlet heat exchanger before it returns to the secondary circuit, and the remainder of the secondary circuit water is simply circulated around the secondary circuit by the pump.  
   
   
       11 . A method as claimed in  claim 4  wherein the secondary condensing heat exchange means comprises two separate heat exchangers, a first in which the hot flue gases are cooled by water flowing therethrough to a temperature which is above the dew point temperature and a second through which the previously cooled flue gases pass and in which they are cooled to below the dew point temperature before escaping to atmosphere, and in which the second heat exchanger includes primary and secondary water circuits, water flowing around the secondary circuit being cooled by water flowing through the primary circuit, the secondary circuit water being in contact with the flue gases to cool them below the dew point, and water from the secondary circuit is caused to pass through the heat exchanger in the air inlet, to cool the secondary circuit water and warm the incoming air, before being returned to rejoin the water flowing around the secondary circuit, and water which is to flow through the primary circuit is obtained by diverting some of the water returning from the radiators in the building to the boiler, and this diverted water is cooled by flowing through at least one further heat exchange means before being supplied to an inlet to the primary circuit, and wherein a further pump is provided which in use pumps the diverted water through the further heat exchange means and through the primary circuit, to allow the flow rate of the diverted returning water to be controlled.  
   
   
       12 . A secondary heat exchange means for recovering latent heat of evaporation from hot products of combustion from a water boiler which in use burns a hydrocarbon fuel in a combustion chamber to provide a source of hot water for circulating through radiators in the building comprising a closed housing divided internally into two compartments, an upper compartment containing a first heat exchanger, a flue gas inlet at its upper end, and a flue gas outlet at its lower end, and a lower compartment having upper and lower regions, a second heat exchanger forming the said primary circuit and located in the lower region thereof with water inlet and outlet connections thereto, a plurality of baffles defining a tortuous path between the top and bottom of the upper region of the lower compartment, duct means communicating between the flue gas outlet of the upper compartment and a flue gas inlet to the lower compartment for conveying flue gases between the two compartments, a flue gas outlet conveying flue gases from the lower compartment to atmosphere, pipe means for conveying water from a water outlet at the bottom of the lower region of the lower compartment to an inlet to a water spray means at or near the top of the upper region of the lower compartment, so that in use, water cascades downwardly over and around the baffles and is in contact with flue gases following the tortuous path around the baffles, to comprise said secondary circuit, and further pipe means for conveying water from an outlet of the second heat exchanger to an inlet of the first heat exchanger, so that in use the water flowing through the latter is warmed before it leaves the secondary heat exchange means to return to the boiler.  
   
   
       13 . A central heating boiler in combination with secondary condensing heat exchange means as claimed in  claim 12  when forming part of a heating system for a building.  
   
   
       14 . A building when heated by a heating system operating as claimed in  claim 4  to which heat is supplied from the combustion process by means of at least one of a plurality of different heat exchange processes, wherein air entering the building is heated, water which is supplied to radiator heat exchangers in the building is heated, cold water stored in the building is heated, and heat is gained from hot gaseous products of combustion by cooling them to a temperature below the dew point of water vapour therein before being exhausted to atmosphere, the heat thereby given up by the hot products of combustion and the latent heat of vaporisation of their water content also being employed to heat water and/or air employed in heating the building.  
   
   
       15 . A heating system for a building in which a hydrocarbon fuel is burnt in a combustion chamber in a boiler to heat water for circulating through radiators in the building, which produces hot gaseous products of combustion, and in which heat is exchanged between those products of combustion and secondary water which is cooled below the dew point temperature of water vapour in the products of combustion by primary water diverted from water returning from the radiators to the boiler, to achieve condensation of the hot exhaust gas water vapour content, wherein the diverted primary water is itself cooled to below the said dew point temperature and in use serves to cool the secondary water, and before the diverted primary water returns to the boiler, heat is exchanged between it and the hot gaseous products of combustion as a first step in the cooling of the latter and to raise the temperature of the primary water before it is remixed with the water returning to the boiler from the radiators.  
   
   
       16 . A building and heating system therefor which comprises a water heating boiler which in use burns a hydrocarbon fuel in a combustion chamber which produces hot products of combustion which vent to atmosphere via a flue and which provides a source of hot water, pump means for circulating the hot water through radiators in the building, a secondary condensing heat exchange means which in use operates to lower the temperature of the flue gases below the dew point of water vapour therein thereby to gain a latent heat of evaporation therefrom before they pass to a flue, and in which the building includes an inlet through which air enters the building in which a heat exchanger is located through which in use water which has been heated in the secondary heat exchange means is caused to flow, thereby to transfer heat from the latter to air entering the building, to warm the latter.  
   
   
       17 . A building and heating system as claimed in  claim 16  wherein the air inlet to the building also comprises an air intake for supplying air to the combustion chamber of the boiler so that after passing through or around the heat exchanger in the air inlet some of the warmed air goes to the combustion chamber in the boiler to support combustion and the remainder of the warmed air is diverted into the building, to directly warm the latter.  
   
   
       18 . A building and heating system as claimed in  claim 16  wherein an air inlet to the boiler combustion chamber is separate from the air inlet to the building and a heat exchanger is associated with each of the air inlets, and in use heated water is caused to flow through both heat exchangers.  
   
   
       19 . A building and heating system as claimed in  claim 16  wherein the secondary condensing heat exchange means comprises two separate heat exchangers, a first in which the hot flue gases are cooled by water flowing therethrough to a temperature which is above the dew point temperature and a second through which the previously cooled flue gases pass and in which they are cooled to below the dew point temperature before escaping to atmosphere, and in which the second heat exchanger includes primary and secondary water circuits, water flowing around the secondary circuit in use being cooled by water flowing through the primary circuit, wherein the secondary circuit water is in contact with the flue gases to cool them below the dew point, and wherein water from the secondary circuit in use is caused to pass through the heat exchanger in the air inlet, to warm the incoming air and cool the secondary circuit water, before it returns to the secondary circuit.  
   
   
       20 . A building and heating system as claimed in  claim 16  wherein the secondary condensing heat exchange means comprises two separate heat exchangers, a first in which the hot flue gases are cooled by water flowing therethrough to a temperature which is above the dew point temperature and a second through which the previously cooled flue gases pass and in which they are cooled to below the dew point temperature before escaping to atmosphere, and in which the second heat exchanger includes primary and secondary water circuits, water flowing around the secondary circuit in use being cooled by water flowing through the primary circuit, wherein the secondary circuit water is in contact with the flue gases to cool them below the dew point, and wherein water from the secondary circuit in use is caused to pass through the heat exchanger in the air inlet, to warm the incoming air and cool the secondary circuit water, before it returns to the secondary circuit and further comprising diverting means whereby in use some of the water returning from the radiators in the building to the boiler, is diverted through at least one further heat exchange means to be cooled therein before being supplied to an inlet to the primary circuit.  
   
   
       21 . A building and heating system as claimed in  claim 16  wherein the secondary condensing heat exchange means comprises two separate heat exchangers, a first in which the hot flue gases are cooled by water flowing therethrough to a temperature which is above the dew point temperature and a second through which the previously cooled flue gases pass and in which they are cooled to below the dew point temperature before escaping to atmosphere, and in which the second heat exchanger includes primary and secondary water circuits, water flowing around the secondary circuit in use being cooled by water flowing through the primary circuit, wherein the secondary circuit water is in contact with the flue gases to cool them below the dew point, and wherein water from the secondary circuit in use is caused to pass through the heat exchanger in the air inlet, to warm the incoming air and cool the secondary circuit water, before it returns to the secondary circuit and further comprising diverting means whereby in use some of the water returning from the radiators in the building to the boiler, is diverted through at least one further heat exchange means to be cooled therein before being supplied to an inlet to the primary circuit and wherein the said at least one further heat exchange means comprises a radiator located within a cool region of the building.  
   
   
       22 . A building and heating system as claimed in  claim 16  wherein the secondary condensing heat exchange means comprises two separate heat exchangers, a first in which the hot flue gases are cooled by water flowing therethrough to a temperature which is above the dew point temperature and a second through which the previously cooled flue gases pass and in which they are cooled to below the dew point temperature before escaping to atmosphere, and in which the second heat exchanger includes primary and secondary water circuits, water flowing around the secondary circuit in use being cooled by water flowing through the primary circuit, wherein the secondary circuit water is in contact with the flue gases to cool them below the dew point, and wherein water from the secondary circuit in use is caused to pass through the heat exchanger in the air inlet, to warm the incoming air and cool the secondary circuit water, before it returns to the secondary circuit and further comprising diverting means whereby in use some of the water returning from the radiators in the building to the boiler, is diverted through at least one further heat exchange means to be cooled therein before being supplied to an inlet to the primary circuit and wherein the building includes a reservoir of cold water, a heated water cylinder to which water from the cold water reservoir is supplied, and taps to which water is supplied from the cylinder, and the said further heat exchange means is a further heat exchanger associated with the cold water reservoir, through which the diverted water is caused to flow before it is supplied to the inlet to the said primary circuit.  
   
   
       23 . A building and heating system as claimed in  claim 16  wherein the secondary condensing heat exchange means comprises two separate heat exchangers, a first in which the hot flue gases are cooled by water flowing therethrough to a temperature which is above the dew point temperature and a second through which the previously cooled flue gases pass and in which they are cooled to below the dew point temperature before escaping to atmosphere, and in which the second heat exchanger includes primary and secondary water circuits, water flowing around the secondary circuit in use being cooled by water flowing through the primary circuit, wherein the secondary circuit water is in contact with the flue gases to cool them below the dew point, and wherein water from the secondary circuit in use is caused to pass through the heat exchanger in the air inlet, to warm the incoming air and cool the secondary circuit water, before it returns to the secondary circuit and further comprising diverting means whereby in use some of the water returning from the radiators in the building to the boiler, is diverted through at least one further heat exchange means to be cooled therein before being supplied to an inlet to the primary circuit and wherein the building includes a reservoir of cold water, a heated water cylinder to which water from the cold water reservoir is supplied, and taps to which water is supplied from the cylinder, and the said further heat exchange means is a further heat exchanger associated with the cold water reservoir, through which the diverted water is caused to flow before it is supplied to the inlet to the said primary circuit and wherein the said further heat exchanger is a contra flow heat exchanger in which there are two water paths which are essentially parallel and in use the flow of water to be warmed along one path is opposite to the direction of the flow of water which is to do the warming and which flows along the other path.  
   
   
       24 . A building and heating system as claimed in  claim 16  wherein the secondary condensing heat exchange means comprises two separate heat exchangers, a first in which the hot flue gases are cooled by water flowing therethrough to a temperature which is above the dew point temperature and a second through which the previously cooled flue gases pass and in which they are cooled to below the dew point temperature before escaping to atmosphere, and in which the second heat exchanger includes primary and secondary water circuits, water flowing around the secondary circuit in use being cooled by water flowing through the primary circuit, wherein the secondary circuit water is in contact with the flue gases to cool them below the dew point, and wherein water from the secondary circuit in use is caused to pass through the heat exchanger in the air inlet, to warm the incoming air and cool the secondary circuit water, before it returns to the secondary circuit and further comprising diverting means whereby in use some of the water returning from the radiators in the building to the boiler, is diverted through at least one further heat exchange means to be cooled therein before being supplied to an inlet to the primary circuit and wherein the building includes a reservoir of cold water, a heated water cylinder to which water from the cold water reservoir is supplied and taps to which water is supplied from the cylinder, and the said further heat exchange means is a further heat exchanger associated with the cold water reservoir, through which the diverted water is caused to flow before it is supplied to the inlet to the said primary circuit and wherein the diverted water is first caused to flow through a radiator located in a cool region of the building and thereafter through the heat exchanger associated with the cold water reservoir.  
   
   
       25 . A building and heating system as claimed in  claim 16  wherein the secondary condensing heat exchange means comprises two separate heat exchangers, a first in which the hot flue gases are cooled by water flowing therethrough to a temperature which is above the dew point temperature and a second through which the previously cooled flue gases pass and in which they are cooled to below the dew point temperature before escaping to atmosphere, and in which the second heat exchanger includes primary and secondary water circuits, water flowing around the secondary circuit in use being cooled by water flowing through the primary circuit, wherein the secondary circuit water is in contact with the flue gases to cool them below the dew point, and wherein water from the secondary circuit in use is caused to pass through the heat exchanger in the air inlet, to warm the incoming air and cool the secondary circuit water, before it returns to the secondary circuit and further comprising diverting means whereby in use some of the water returning from the radiators in the building to the boiler, is diverted through at least one further heat exchange means to be cooled therein before being supplied to an inlet to the primary circuit and wherein the building includes a reservoir of cold water, a heated water cylinder to which water from the cold water reservoir is supplied, and taps to which water is supplied from the cylinder, and the said further heat exchange means is a further heat exchanger associated with the cold water reservoir, through which the diverted water is caused to flow before it is supplied to the inlet to the said primary circuit and wherein the further heat exchanger comprises two lengths of copper tube one inside the other and defining an annular water path between the two tubes which is closed at each end and includes a water inlet to the inner tube at one end and a water outlet therefrom at the other end, and a water inlet is provided to the annular water path near the outlet from the inner tube and an outlet from the annular path is provided near the inlet to the inner tube, with pipe connections for supplying the diverted water to the inlet and retrieving diverted water from the outlet of the annular path, and a pump is provided for circulating water from the reservoir through the inner pipe to re-enter the reservoir as it exits from the inner tube outlet, thereby to create a contra flow heat exchanger in which the two water paths are essentially parallel, so that in use the flow of water to be warmed along one path is opposite to the direction of the flow of water which is to do the warming and which flows along the other path.  
   
   
       26 . A building and heating system as claimed in  claim 16  wherein the secondary condensing heat exchange means comprises two separate heat exchangers, a first in which the hot flue gases are cooled by water flowing therethrough to a temperature which is above the dew point temperature and a second through which the previously cooled flue gases pass and in which they are cooled to below the dew point temperature before escaping to atmosphere, and in which the second heat exchanger includes primary and secondary water circuits, water flowing around the secondary circuit in use being cooled by water flowing through the primary circuit, wherein the secondary circuit water is in contact with the flue gases to cool them below the dew point, and wherein water from the secondary circuit in use is caused to pass through the heat exchanger in the air inlet, to warm the incoming air and cool the secondary circuit water, before it returns to the secondary circuit and further comprising diverting means whereby in use some of the water returning from the radiators in the building to the boiler, is diverted through at least one further heat exchange means to be cooled therein before being supplied to an inlet to the primary circuit and further comprising a further heat exchanger in an air inlet to the boiler combustion chamber through which in use, the diverted water returning from the radiators is forced to pass to be cooled thereby.  
   
   
       27 . A building and heating system as claimed in  claim 16  wherein the secondary condensing heat exchange means comprises two separate heat exchangers, a first in which the hot flue gases are cooled by water flowing therethrough to a temperature which is above the dew point temperature and a second through which the previously cooled flue gases pass and in which they are cooled to below the dew point temperature before escaping to atmosphere, and in which the second heat exchanger includes primary and secondary water circuits, water flowing around the secondary circuit in use being cooled by water flowing through the primary circuit, wherein the secondary circuit water is in contact with the flue gases to cool them below the dew point, and wherein water from the secondary circuit in use is caused to pass through the heat exchanger in the air inlet, to warm the incoming air and cool the secondary circuit water, before it returns to the secondary circuit and further comprising a secondary circuit pump which operates to pump secondary circuit water through the second heat exchanger and through the heat exchanger in the air inlet to the building and further comprising diverting means by which in use some of the secondary circuit water is caused to circulate through the heat exchanger in the air inlet to the building before it returns to the secondary circuit, and the remainder of the water is simply circulated around the secondary circuit by the secondary circuit pump.  
   
   
       28 . A building and heating system as claimed in  claim 16  wherein the secondary condensing heat exchange means comprises two separate heat exchangers, a first in which the hot flue gases are cooled by water flowing therethrough to a temperature which is above the dew point temperature and a second through which the previously cooled flue gases pass and in which they are cooled to below the dew point temperature before escaping to atmosphere, and in which the second heat exchanger includes primary and secondary water circuits, water flowing around the secondary circuit in use being cooled by water flowing through the primary circuit, wherein the secondary circuit water is in contact with the flue gases to cool them below the dew point, and wherein water from the secondary circuit in use is caused to pass through the heat exchanger in the air inlet, to warm the incoming air and cool the secondary circuit water, before it returns to the secondary circuit and further comprising diverting means whereby in use some of the water returning from the radiators in the building to the boiler, is diverted through at least one further heat exchange means to be cooled therein before being supplied to an inlet to the primary circuit and further comprising an overall control system which includes temperature sensors and/or flow sensors and electrically controlled valves, whereby in use the volume and/or rate of flow of any diverted water associated with the secondary or the primary circuits is control.  
   
   
       29 . A building and heating system as claimed in  claim 16  wherein the secondary condensing heat exchange means comprises two separate heat exchangers, a first in which the hot flue gases are cooled by water flowing therethrough to a temperature which is above the dew point temperature and a second through which the previously cooled flue gases pass and in which they are cooled to below the dew point temperature before escaping to atmosphere, and in which the second heat exchanger includes primary and secondary water circuits, water flowing around the secondary circuit in use being cooled by water flowing through the primary circuit, wherein the secondary circuit water is in contact with the flue gases to cool them below the dew point, and wherein water from the secondary circuit in use is caused to pass through the heat exchanger in the air inlet, to warm the incoming air and cool the secondary circuit water, before it returns to the secondary circuit and further comprising diverting means whereby in use some of the water returning from the radiators in the building to the boiler, is diverted through at least one further heat exchange means to be cooled therein before being supplied to an inlet to the primary circuit and wherein a first pump circulates heated water from the boiler through the radiators and back to the boiler and a second pump is provided which in use pumps the diverted water through the or each further heat exchanger and through the primary circuit, to allow greater control over the flow rate of the diverted returning water.