CA1120465A

CA1120465A - Method for the recovery of heat from the flue gas of a furnace

Info

Publication number: CA1120465A
Application number: CA000336971A
Authority: CA
Inventors: Hartmut Behrens
Original assignee: PEPER KLAUS
Current assignee: PEPER KLAUS
Priority date: 1978-12-22
Filing date: 1979-10-04
Publication date: 1982-03-23
Also published as: NO152268C; NO152268B; SE438547B; SE7909528L; FI793650A7; EP0013018B1; DK547979A; EP0013018A1; NO793745L

Abstract

ABSTRACT

A method for the recovery of heat from the flue gas of a furnace, especially of an oil or gas furnace, and for the purification of these gases, with a heating circuit connected with the furnace. Flue gases of the furnace are lead to the outside of the evaporator of a heat pump circulating a refrigerant, cooled to below the condensation point of all noxious substances contained in the flue gas and then given off into the atmosphere.
The heat absorbed by the evaporator is given off directly into the heating circuit via the heat pump condenser which is in a heat exchange relationship with the heat-ing circuit.

Description

6~

e s c r i p -t i o n =--===================

The invention concerns the design and t:he operation of hea-t-ing installations, particularly hea-ting ins-tallations for residences, and per-tains especially to the recovery of the hea-t contained in -the flue gases of the heating furnace of such heating installa-tions.
In -the conventional heating ins-tallations for dwellings which are fired with oil or gas, during the normal heating period up to 20% of the energy used escapes into the atmos-phere with the flue gases. During the transitional periods lo in the spring and fall, when such heating is not fully car-ried out, because the heat requirement in the building is lower, the efficiency is even fur-ther diminished.
It is a known prac-tice -to recover -to a greater or lesser degree the heat contained in the flue gases. In addition to the regenera-tive heat recovery which is used especially in indus-trial furnaces, so-called recuperators are also known in which the flue heat heats the air of combus-tion of the furnace by means of heat exchange. Since the flue gas contains harmful subs-tances in considerable amounts, es-pecially sulfur compounds, the cooling of the flue gas is problematic, because it can lead to the precipitation of the harmful substances in the chimney and hereby to the destruction of the chimney. The situation resulting from the above-mentioned causes is especially unfavorable when ~5 only relatively small amounts of hea-t are required for the heating of the building.
The main problem, then, of the invention is to better uti-lize the fuel energy used, by means of the recovery of the heat from the furnace flue gas, without endangering the 30 existence of the heating equipment and/or giving off noxious ~, 4~5 substances to a critical extent to the atmosphere.
According to one aspect of the inven~ion there is provided metho~ for the recovery of heat from tile flue gas of a furnace, especially of an oil or gas ~urnace, as well as Eor tt~e puri~ication of these gases, with a heating circuit connected with ~he ~urnace, charac~erized by the fact that the Elue gases of the furnace are led to the outside of the evaporator of a heat pump circulating a refrigerant, cooled down in this to below the condensation point of all noxious substances contained in the 1ue gas and then given off into the atmosphere; and that the heat absorbed by the eva~orator is given off directly into the heating circuit via the heat pump condenser which is in a heat exchange relationship with the heating circuit.
According to another aspect of the invention there is provided heating installation with a heating circuit an~
a furnace located in it, which has an exhaust gas flue, characterized by the fact that in the exhaust gas flue the evaporator of a refrigerant-circulating heat pump is installed for heat exchange, whose condenser is in a heat exchange relationship with the heating circuit.
Thus the invention, at least in its preferred forms, solves the problem referred to above in that the furnace flue gases act upon the evaporator of a rerigerant-circulating heat pump, located in the exhaust flue, so that the flue gas heat is absorbed by the refrigerant through heat exchange, with the flue gases being cooled off essentially below the condensation point of all noxious substances contained therein and therefore able to be given off directly into the atmosphere. The heat absorbed in the evaporator from the refrigerant is brought - - 2 ~

~:L2~46S

in the usual manner in heat pumps with the help of a com-pessor to a higher temperature level and fed by renewed heat exchange clirectly ,into the heating circuit of the heating installation.
Throu~h this new application of a heat pump several advantages are ohtained: The Elue gas is cooled oEf at the heat pump evaporator to the point--as is obvious--where the noxious substances carried along in the flue gas are precipitated in a controlled manner and can be led off. The remaining cooled gas is purified to the extent that it no longer requires a c'nimney in the real sense of the word. The heat energy obtained from the flue gas can--as a result of the temperature increase through the heat pump principle--be introduced directly into the heating circuit.
Furthermore, the heat pump used Eor the flue gas heat recovery can be used during the time when the heating circuit has only a relatively low heating demand (transi-tion period, cool days, etc.) as the sole heat source by introducing the ambient outside air directly into the flue and past the evaporator. In this operational state of the heating installation based on the invention, the heat pump works in the fashion of an ordinary heat pump.
A combination of these two operational states of the heat pump makes it possible, according to a further development - 2a --3~ 4~5 of -t}-le inven-tion, -to rnix air into -the flue gas from the furnace before the lat-ter is conduc-ted to the evapora-tor.
In this way, on the one hand, the high through~pu-t -that a heat pump working in even pure air operation requires is 5 obtained, and on -the other, overhea-ting of the qvaporator is avoided.
This me-thod of opera-tion can be developed further by re--introducing, instead of fresh ou-tside air, -the cooled ex-haust air which collects behind -the blower which forces the necessary flue gas/air current -through -the evapora-tor.
This has the additional advan-tage tha-t -the condensation poin-t drop tha-t occurs with fresh air admixing does no-t take place and only a tempera-ture drop occurs.
Finally, the use of a heat pump as prescribed by the inven-5 tion is especially advantageous because it makes possible an especially simple basic control of the heating instal-lation. Because the pressure ~and in like manner the tem-perature) is a reliable indicator of -the s-ta-te of the re-frigerant in -the heat pump and thus also of -the -temperature ~O in the evapor~-tor on the one hand, and of -the heat con-veyance effected by the heat pump, on the o-ther, a simple control device is sufficient, which, in dependence on the pressllre or -the tempera-ture of -the refrigeran-t in the eva-poratorregulates the opera-tion of -the furnace and the 25 blower in such a way that with continuous blowing and heat exchange be-tween outside air and refrigerant the furnace is turned on only when a predetermined lower limit value of the pressure or of the temperature is exceeded, but that the furnace is turned off when a predeterrnined upper limit 30 value of the pressure or of the temperature is exceeded.
On -the basis of the invention, this arrangement can be de-veloped into a system with polyvalen-t heat utiliza-tion, firs-t of all because additional heat energy sources can be in-tegrated directly or indirectly in-to the heating circuit 35 in such a way tha-t without any greater control measures a 4~5 _L~_ "logical regulation" resu:L-ts from any given temperature drop an~ new primary energy (wi-th the help of the furnace burner) is used only when o-ther supplementary energies are no-t sufficiently available.
S In par-ticular, a solar hea-t device is provided, which (to the exten-t -that solar heat energy collects) warms up the return of -the heating circui-t before it en-ters -the con-denser and there absorbes -the heat of the refrigeran-t.
Only -then does the re-turn reach -the boiler, which, however, .
goes into opera-tion only when -the return temperature is be-low the required forward flow -temperature. An advan-tageous secondary effect of this arrangement lies in the fact -tha-t the boiler is kept warm and no corrosion damage occurs as a result of non-operation.
But more important than -this utilization of energy supplied by a solar-heat device is the direct hea-ting of normal sup-ply water ~ho-t wa-ter). If -the solar heat vehicle has an adequate temperature, -then firs-t of all -the normal wa-ter supply is heated and only if either enough solar heat is available or the waterls temperature level is too low is -the above-described heating of the hea-ting circuit return undertaken.
If the solar heat is at a still lower temperature level, so that it cannot be made available for either the normal 2S wa-ter supply heating or the return pre-heating, then its u-tilization is provided through pre-heating of the air which is fed either as fresh air or as returned cooled flue gas/air mixture to the heat pump evaporator. Instead of fresh air (or return air), the heated exhaust air of 3~ an air conditioner can also be used, which receives cold fresh air through heat exchange wi-th at least a part of the cold exhaust air leaving the heat pump evaporator (or of the escaping cold flue gas/air mixture).
In a similar manner, the hea-t energy contained in the ex-3S haust air from warm or heated areas, such as, e.g., swim-ming pools or the like, can be made useful by mixing this , --5---clir directly wi-th -the air elowing -through -Lhe hea-t pump evaporator (OI' with -the ~lue gas/air mix-l,ure).
On t:he o-ther hand, -the utilization of the hea-t energy obtained can be :improved by having the peak hea-ting o~
5 -the normal wa-ter supply -take place in a heat exchanger which on -the primary side has -the highly-heated re~rige-rant of -the heat pump circui-t coming :~'rom the heat pu~p compressor flowing -through it. Since here the re~rigeran-t is cooled down from7 e.g., ~5C -to 80C and the required peak heat -transferred to the normal water supply~ there results at -the same tIme an improvement in the econom~ o~
the hea-t pump which is all the greater 7 -the lower the re~
frigerant condensa-tion -tempera-ture.
In order to take into consideration the di~erent condl~
t~ tions under summer and winter operation within the frame-work of this arrangement, a cross oonnection can be pro~
vided between the normal water supply return and the heat~
ing circuit return, as well as a cross connec-tion in the forward flow in such a way tha-t in -the summer OperatIon

2~ wi-th the heat shut o~ the hea-t exchangers serylng o-ther~
wise for the prehea-ting of the heating circuit return are available to the normal water supply cirCuit~ Here all that is needed, in addition to a thermostatically controlled valve in the forward ~low cross connectl`on is two check ~5 valves, namely in -the return cross connection and between this and the heating circult circulating pump, because the la-tter is considerably stronger than the no~mal water su~-ply circulatlon pumpO lf the heating ci~rcui-t pump turns on, it opens with its pressure the check ~alve loca-ted be~ -30 tween it and the cross C~nnectiQn and close$ the check valye located in -the cross connec-tion, even when the ~
weaker--normal water supply pump opposes it, ~s SOQn~ how-ever, as the heating circuit circula-tion pump shuts o~f, the normal water supply pump can open the check yalye in -the cross connection~ whereupon it simultaneously closes 346~

the other check valve and prevents -the incorpora-tion of the rest of the heating circui-t into -the normal water supply circuit.
A Eurther source of help for heat energy is the waste water. According to a further developmen-t of the inven--tion, a branch is provided in -the heat pump circuit which leads Wit}l i-ts own expansion valve to an evaporator which is ins-talled in a was-te water reservoir and is in a heat exchange rela-tionship with the waste wa-ter. Instead of waste water, any other water (or any o-ther available fluid) of appropria-te tempera-ture can be used.
A further u-ti]ization possibility of recovered heat energy can be found in -the fact -that in the re-turn flow of the refrigerant to the expansion valve and evaporator there is 15 located a heat exchanger by means of which the heat con-tained in the still warm refrigerant condensate can be re-leased to -the ambient air or---using a blower--for heating the air. Such a utilization of the residual heat in the refrigerant is more advantageous than a countercurrent heat 2~ exchange with the cold refrigerant flowing from -the eva-porator to -the compressor~ because this leads to an in-crease in -the compressor -tempera-ture.
This withdrawal of heat from -the heat pump circuit can also be used in the summer, when the heat pump runs only for the 25 operation of an air conditioner (because normal water has already been sufficien-tly heated), to carry off the heat of condensation.
In order to prevent any heat being carried out of the building via the heating circuit, it is advantageous -to

3~ provide a heat pump control in the heating circui-t in such a way that the pump is shut off when the pressure or the temperature in the condenser of the heat pump circuit goes below a cer-tain value. If pressure or temperat~re again go above this value, -the heat pump is again switched on.
3~ The drawings illustrate the invention on -the basis of the embodiments.

4~;
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Fig. 1 shows a flue gas heat recovery ins-tallation of a very simple -type in schematic represen-ta-tion.
Fig. 2 shows likewise a schematic represen-tation of the structure oE a compact apparatus with es-sentially -the same characteris-tics, but with an addi-tional control device.
~ig. 3 shows a representa-tion co-rresponding -to Fig. 2 of a an appara-tus equipped with further devices ~ for polyvalent hea-t utilization.
Fig. ~ shows a sh~meatic circuit diagram of the installation shown in Fig. 3 for a better clarification of the mode of operation.
Fig. 1 shows a boiler 1 with its furnace, which is not re-t5 presented in detail. The heat-carrying vehicle heated in -the boiler 1, usually water~ flows through the heating cir-cuit 2, in which several radiators 3 are ineorporated;in this case only 1 radiator is represented. A circulation pump 4 provides for -the forced circulation of the heat ve-2~ hicle.
The flue gases from the furnace escape into -the open air vi,a the flue 6. Of the building in which the furnace is installed, only the roof 7 is indicated.
An adjustable damper 8 in the exhaust flue 6 makes it pos-25 sible to léad the flue gas into a branch 9 through the eva~porator 12 of a hea-t pump 13 described below, as well as a further branch 10 back into the exhaust flue 6. Through the connection 19, in which there is an adjusting damper 21, ambient air can be led in-to the branch 9 and mixed with 3~ the flue gas flowing,to the evaporator 12. A blower 22 is in the branch 10 to provide the necessary flow of the'flue gas or flue gas/air mixture, It is advantageous if the speed of -the blower is adjustable so that when air only is being blown it provides, e.g,~ 10 3~ times more -through-put than when flue gas/air is being blown; further, a return line 29 branching off in back of the blower 22 can be proYided in the connection 19 for cold air ~or cold air/flue gas mix-ture); the adjustable damper 21 -:
' ' ' ' . .

l~LZ~16~

is so designed and arrallged -that i-t opens either -the fresh air or the cold air supply.
In addition to the evapora-tor 12, the heat purnp 13 also has a compressor lL~, a condenser 15 and an expansion valve 16.

5 With their piping~ these componen-ts cons-ti-tute a secondary circuit which is filled with a refrigerant that is circula-ted by the compressor.
The refrigerant hea-ted in the evapora-tor 12 flows through the piping 17 into the compressor 14. With the compression of the refrigerant the tempera-ture rises sharply. In the con-denser 15, which is in a heat exchange relationship with the heat in circuit 2, the heat of the refrigerant is extensively given off to the heat carrying medium being circulated in the heating circuit 2. Upon the expansion of the refrigerant 1~ in the expansion valve 16, it cools off greatly so that the refrigerant is ready -to absorb heat again. Generally the temperatures in the evaporator are from o to +5C, in the condenser, from 80 -to 90~.
The numeral 25 indicates a valve arrangement by means of ~o which a short circuit connection 26 of forward flow and re-turn flow of the heating circuit 2 can be made when the heat-ing of -the hea-ting circuit 2 is done only by such heat as the heat pump 13 extracts ~rom the ambient air drawn in through the connection 19 by the blower 22.
a 5 Fig. 2 shows--though only schematically--the design of a hea-ting installa-tion with the features seen in Fig. 1. Like par-ts bear like reference numerals and require no re-expla-nation here. Deviations and additions are described below.
In the heating circuit 2 the forward flow is indicated by 3~ 5' and -the return flow by 5 " ; the dashed line 30 indicates what belongs to a heating installation of simple design based on the invention and can be set up as a compact unit in the place of utilization.
At -the boiler 1 is shown the burner 38, which is under the 35 control of the control device 37; by way of an ins-trument lead 36 the pressure in hea-t pump 13 evaporator~ is measured, ~ ~9 ~ 5 and wherl a predeterln;rled Ininimum pressure is passed the control device 37 turns on the burner 38 via the indicator lead 36'. Instead of -the pressure of the refrigerant in the evaporator, i-ts -tempera-ture can also be used for the S control of -the burner 38.
A base irame 33 res-ting on the floor 3L~ carries the boiler 1 as well as---par-tially by way of uprigh-ts 32 for an upper frame 31--all other components of the heating ins-talla-tion.
The evapora-tor 12 of the heat pump 13 is suppor-ted on the IV top frame 31. Underneath i-t is a drip pan 23 tha-t catches the water of condensation and condensed noxious substances.
The exhaus-t fl~e is reduced -to an exhaus-t stack 6'. But the exhaust flue con-tinues on in a connecting pipe 11 which surrounds the exhaust flue stack 6' with an enlarged cross sec-tion 11' in such a way that ambient air can enter the connecting pipe as indicated by the arrows 18 and there-~if the burner 38 is in operation--mix with the flue gas.
The air (or -the flue gas/air mixture) propelled by the blower 22 leaves the building through the exhaus-t s-tack 2 in -the building wall 25, There is no need for a chimney 7 since -the escaping gas has been cooled off and cleaned up.
The hea-ting installation shown in Fig. 3 is, compared -to that of Fig. 2, enlarged by numerous supplementary devices for the purpose of polyvalent heat u-tilization. Again, ~5 like parts are designated with the same reference numerals as in the preceding figures. This applies also to ~ig. 4, which clarifies in terms of the circuitdiagram the design of the heating installa-tion represen-ted in Fig, 3, The blower 22 installed in the exhaust stack 2~ draws 3~ here, too 7 either air as per the arrows 18 or air and also flue gas as per the arrows 18~ in the connecting flue ll and through the evaporator 12 of the heat pump. In addition, an inle-t 56 is provided, -through which ~the warm outside air (e.g., ~rom a swimming pool) can be introduced. At the in-3~ le-t for the ambient air a heat exchanger is provided which is acted upon on the primary side via a llne 59 by a heat carrying medium heated by solar energy (not shown in further -10- ~ 0~5 de~ail here~. The air entering as indicated by -the ar-rows 18 can -thus be prehea-ted. Further, a connecting pipe (not shown here~ can lead from the exhaust stack 24 to the inlet connection with the hea-t exchanger 48, so that the cooled exhaust air is led back in-to the connecting pipe 11. The exhaust air escaping from the exhaust stack 24 as per the arrows 24' can be further used for the air condi-tioning (cooling) of rooms if necessary, in hea-t exchange with fresh air. The air path represen-ted on -the left in Fig. 4 illustrates this, with the heat exchanger 67 and the air path 68 of an air conditioner.
By means of a swi-tching device 60 the heat exchanger 48 is activated only when a solar heat of low temperature is available. In general, the solar heat is led direct:Ly to the normal water supply (hot wa-ter~; this is not shown in Fig. 3, but is indicated in Fig. 4. If there is sufficient solar heat, or if this is not suitable for normal water supply heating~ then the heat carrying medium in question is conducted via -the line 41 to a hea-t exchanger 40 which is acted on secondarily by the re-turn flow 5~ of the heat-ing circui-t 2. The latter -then enters the heat pump conden-ser 15 and leaves it at its connection with a hea-t exchanger 42 that is likewise located in the hea-t pump circuit and which conducts normal supply water via the line 43 on the secondary side.
The refrigerant circulated by the compressor 14 in the heat pump circuit flows, after leaving the compressor 15, into a collector 28, and from this into a heat exchanger 44, which by means of a blower 46 heats the ambient air or--3~ by means of a conduit that is not shown--the air in another space. Before reaching the expansion valve 16 a branching 51 is provided which leads via an expansion valve 16' to an eva~orator 12' shown only in Fig, 4 t The return circuit bears -the reference numeral 52.

-- 1 1 r ~ 4~;S

The CirCUlatiOIl pUlllp 55 in -the hea-ting circuit 2 is con--trolled by a pressure sw:i-tch 57 which measures the pres-sure (or the tempera-ture) in the condenser 15 and turns off -the circulation pump 55 when a minimum pressure in 5 the condenser lS is exceeded.
Fig. 4 supplemen-ts -the representation in Fig. 3 somewhat further. Thus a line 41~ is provided which ex-tends -the line 41 from the solar device 61 to a heat exchanger 62 9 which serves for the basic hea-ting of the normal wa-ter supply. Furthermore, the circula-tion pump 63 for the circulation of -the hea-t carrying medium -that heats the normal water supply is marked. On the pressure side the circulation pumps 63 and 65 are connected by a line 64.
In the line 64 there is a check valve 65, and an additional check valYe 66 is located between circulation pump SS (in the heating circuit 2) and the point where the line 64 branches off. The function of -the cross connection wi-th the line 64 and the check valves 65, 66 has already been explained above. The cross connection of the forward ~O flows consists of a line 69 with a valve 70, which is opened by a thermoplastically controlled priority swi-tch when the normal wa-ter supply requires heat.
In the case of-failure of the burner 38, the whole instal-lation, above all the heat pump, is shut off by means of an electrical in-terlocking switch.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Method for the recovery of heat from the flue gas of a furnace, especially of an oil or gas furnace, as well as for the purification of these gases, with a heating circuit connected with the furnace, characterized by the fact that the flue gases of the furnace are led to the outside of the evaporator of a heat pump circulating a refrigerant, cooled down in this to below the condensation point of all noxious substances contained in the flue gas and then given off into the atmosphere; and that the heat absorbed by the evaporator is given off directly into the heating circuit via the heat pump condenser which is in a heat exchange relationship with the heating circuit.

2. Method as defined in claim 1, characterized by the fact that outside air is mixed with the flue gas before it acts on the heat pump evaporator.

3. Method as defined in claim 1, characterized by the fact that the returned exhaust air is added to the flue gas before it acts on the heat pump evaporator.

4. Method as defined in any of claims 1 to 3, char-acterized by the fact that the flue gas or the flue gas/air mixture is mechanically accelerated.

5. Heating installation with a heating circuit and a furnace located in it, which has an exhaust gas flue, characterized by the fact that in the exhaust gas flue the evaporator of a refrigerant-circulating heat pump is installed for heat exchange, whose condenser is in a heat exchange relationship with the heating circuit.

6. Heating installation as defined in claim 5, char-acterized by the fact that the evaporator is located in a by-pass of the exhaust gas flue and is provided with a first adjustable damper at the branching point.

7. Heating installation as defined in claim 6,character-ized by the fact that in the exhaust gas flue or its by-pass, a connection as well as a blower are located which accelerate the exhaust of the flue gases and/or air.

8. Heating installation as defined in claim 7, character-ized by the fact that the opening width of the connection is adjustable by means of a second adjustable damper.

9. Heating installation as defined in claim 8, character-ized by the fact that the first and second adjustable dampers have a common operating element.

10. Heating installation as defined in claims 5 and 8, characterized by the fact that a diversion damper in one position closes off the exhaust gas flue to the furnace and opens the connection fully, and in its other position opens the exhaust gas flue and throttles the connection.

11. Heating installation as defined in claim 7, character-ized by the fact that a control device in dependence on the pressure or the temperature of the refrigerant in the evaporator controls the operation of the furnace and of the blower in such a way that with blower running and heat exchange taking place between outside air and refrigerant the furnace is switched on when a predetermined lower limit value of the pressure or of the temperature is exceeded, and that the furnace is switched off when an upper limit value of the pressure or temperature is exceeded.

12. Heating installation as defined in claim 11, character-ized by the fact that the through-put volume of the blower is adjustable and is decreased when the furnace is turned on, so that a smaller amount of outside air is drawn in and mixed with the flue gas.

13. Heating installation as defined in claim 7 or one of the following claims, characterized by the fact that a return line beginning behind the blower in the exhaust gas flue or its by-pass opens into the connection.

14. Heating installation as defined in claim 1, claim 2 or claim 3, characterized by the fact that means are located on the evaporator for carrying off the condensate.

15. Heating installation as defined in claim 1, character-ized by the fact that in the heating circuit, in the flow direction before the condenser of the heat pump, there is incorporated a heat exchanger which is acted on on the primary side by a solar heat device.

16. Heating installation as defined in claim 15, character-ized by the fact that a heat exchanger is provided for normal water supply heating which is incorporated into the heating circuit with priority over the heat exchanger.

17. Heating installation as defined in claim 15, char-acterized by the fact that a heat exchanger is provided which receives on the secondary side air intended to be introduced into the evaporator of the heat pump.

18. Heating installation as defined in claim 1, claim 2 or claim 3, characterized by the fact that a connection is provided for the introduction of warm exhaust air into the evaporator of the heat pump.

19. Heating installation as defined in claim 1, char-acterized by the fact that between the compressor and the condenser of the heat pump there is incorporated a heat exchanger which is acted on on the secondary side by normal supply water or heat carrying medium used for \
its heating.

20. Heating installation as defined in claim 19, char-acterized by a pressure-side cross connection of the normal water supply return flow and the heating circuit return flow, and by check valves in the cross connection, as well as between the latter and the circulation pump of the heating circuit return flow.

21. Heating installation as defined by claim 1, claim 2 or claim 3, characterized by a branching of the heat pump circuit, in which its own expansion valve and an evapor-ator located in a waste water container are provided.

22. Heating installation as defined in claim 1, claim 2 or claim 3, characterized by control device dependant on the cooling agent pressure in the condensor for the shutting off of the circulation pump of the heating cir-cuit when a predetermined minimum pressure is exceeded.

23. Heating installation as defined in claim 1, claim 2 or claim 3, characterized that in case of failure of the burner as a result of electrical breakdown the entire installation shuts off.

24.A method for the recovery of heat from the flue gases of a furnace and the removal of undesired gases from said flue gases, the furnace including a heating circuit and an exhaust gas flue, said method comprising mixing outside air with the flue gases, leading the flue gases past an evaporator of a refrigerant-circulating heat pump assembly, using said evaporator to remove heat from and cool the flue gases to a temperature below the condensa-tion point of the undesired gases, pumping said removed heat to a condenser in a heat exchanging relationship with said heating circuit, and controlling the operation of said furnace in response to a given condition of a refrigerant in said refrigerant-circulating heat pump.

25. A heating system for the recovery of heat from the flue gases of a furnace and the purification of said gases, the furnace including a heating circuit and an exhaust gas flue, said heating system comprising a refrigerant-circulating heat pump having an evaporator in a heat exchanging relationship with the exhaust gas flue and a condenser in a heat exchanging relationship with the heating circuit and means controlling the op-eration of said furnace in response to a given condition of a refrigerant in said refrigerant-circulating heat pump.