with continuing attention to the drawings wherein applied reference numerals indicate parts similarly hereinafter identified , the applied reference numeral 1 indicates generally a structure for exposure to a heat source such as solar radiation at e . details of a housing and supporting means for structure 1 are dispensed with as same are not believed pertinent to an understanding of the present invention which is applicable to a wide range of heating systems . when structure 1 is adapted for the absorption of radiant energy , the structure comprises a series of conduit means or riser tubes 2 orientated for optimum absorption . a fluid supply manifold or header 3 is in communication with the lower ends of the several riser tubes 2 . the uppermost end segments at 2a of the inclined conduits terminate in communication with the interior at 4a of a collection header 4 . the heated fluid received in header 4 , as later explained , is directed via tubing at 5 toward the structure to be heated such , as for example , a fluid - to - fluid heat exchanger at 6 and thence redirected via tubing at 7 toward vapor condenser means at 8 . condenser means 8 functions to condense vapors discharged from the riser tubes 2 with the vapor condenser arrangement shown having one or more passageways 9 communicating header 4 with the condenser means it is understood that while the foregoing heating system corresponds generally to that disclosed in u . s . pat . no . 4 , 478 , 211 , various other heating systems may advantageously utilize the present invention which is particularly suited to providing fluid circulation through a system wherein multiple fluid heads exist resulting in a fluid imbalanced condition . first conduit means or riser tubes 2 each have a portion which defines conjointly with second conduit means at 11 , a vapor formation chamber 10 . the second conduit means 11 is shown as being interiorly disposed within and of lesser section than conduit means 2 with chamber 10 defined by the closely spaced , opposed surfaces of conduit means 2 and 11 . the conduit spacing maybe such as to promote fluid charging of the chamber by capillary action . at the upper or discharge end of conduit means 2 , a closure at 12 blocks the upper end of vapor formation chamber 10 . when the inclined conduits 2 and 11 are circular in section closure 12 will be of annular shape . to enhance heat conduction to the conduit means 2 each may be provided with an energy collector plate 13 affixed as by brazing at 14 . in one suitable embodiment of the invention the tubes 2 have a three - eights inch outside diameter . tube wall thickness is such as to be conducive to heat transfer and of requisite strength . the upper end segment of riser tube 2 , partially defining chamber 10 , may be two feet or so in length when in a solar panel sized for use in a home . tube surface spacing maybe such to promote chamber filling by capillary action such as for example one sixteenth or one sixty fourth of an inch . the system is preferably evacuated to cause fluid or the fluid and its vapor to occupy chamber 10 . the application of heat , such as from solar radiation , causes fluid in chamber 10 to vaporize as steam which is discharged from the lower end of the chamber to provide steam bubbles for passage into the lower end of second conduit means 11 . the fluid trapped between successive bubbles in conduit means 11 is lifted past a fluid level at l1 and discharged in collection header 4 whereat it gravitates toward a fluid level at l2 to provide an imbalanced fluid system wherein gravity causes circulation of the heating system fluid . the condenser means 8 , as earlier noted , may be otherwise embodied , as for example , in a relatively cool fluid flow routed lengthwise along the collection header 4 and serving to condense steam discharged into the collection header . accordingly , vapor pressure within collection header 4 is at all times , during system operation , at a lower value than the pressure exerted on a fluid slug by a steam bubble moving in conduit means 11 . the steam vapor discharged downwardly from chamber 10 is believed to be discharged in small increments each of which serves as a nucleation source for a vapor bubble . the bubble vapor is ultimately condensed in condenser means 8 for return to lower header 3 while assuring a pressure in header 4 conducive to bubble formation . retention of conduit means 11 against axial displacement in conduit means 2 may be by random points of contact between the conduit means . the conduit means may be of horizontal , vertical or inclined disposition for optimum heating by a heat source . suitable fluids for use in the foregoing system may be a mixture of water and methanol or water and glycol . while we have shown but one embodiment of the invention , it will be apparent to those skilled in the art that the invention may be embodied still otherwise without departing from the spirit and scope of the invention .