Let’s look at a system design that will not work: Imagine the four dots on dice: the flow attaches to either of the top two pipes whilst the return attaches to the opposite corner on the bottom of the stove (in this case the other two are for the radiator circuit or can be capped). If a boiler stove has four pipes then the primary circuit will attach to two of the four, corner to corner. If a boiler stove has two pipes then the primary circuit attaches to those two pipes (flow to higher pipe). Of course the cylinder water will gradually increase in temperature and will, at some stage, not cool the coil – then then gravity circuit will slow and will eventually stop (leading eventually to everything boiling). This means that the red pipe (FLOW) is hotter than the orange pipe (RETURN) and as long as this continues the circuit will flow. The cylinder is acting as a HEAT SINK (a term we will come across again), in other words it is stealing heat from the primary circuit. Note that the flow pipe is always connected to the top of the cylinder coil (from the top of the stove). Hence the red pipe (we call this the FLOW) is hotter than the orange pipe (we call this the RETURN). The stove does not boil because the hot water rises up the red pipe and is cooled whilst it travels DOWN through the coil (because the water in the cylinder itself is cold and therefore cools the coil). Note that the cylinder in the above picture can be replaced by a radiator should domestic hot water not be required (this detailed further down). For stoves under 35kw 28mm diameter is recommended. Hence the primary circuit pipework will increase in diameter as the stove power increases. If the water rising up the red pipe cannot get away from the stove fast enough then it will stay in the stove longer and might boil (not good). It should now be obvious that the diameter of the pipe for the gravity circuit is important. Note that the water in our primary circuit never mixes with the main mass of water in the cylinder (primary circuit water has a chemical known as “inhibitor” added to it to prevent rust).
Note that no pump is required! The benefit of course is that the water in the cylinder itself is gradually heated by the hot coil (the cylinder is known as an “indirect cylinder”).
This is what is often called “gravity circulation”. As things heat up so the speed of this process increases and we end up with a circuit of fast flowing water: up the red copper pipe, through the copper coil and down the orange copper pipe. Because warm water is leaving the stove, cooler water from the orange pipe flows into the stove to replace what has left. The heated water rises to the top of the stove and starts to rise up the pipe that is connected to the top of the stove (the red pipe in our scribble). In the scribble above the stove has been lit and the water within the stove (in a metal box within the stove or in the jacket of the stove) heats up.
Wet gravity circuit full#
It is important to realise that all of the pipes are full of cold water almost to the top of the vent pipe (circuits fill from loft header tanks and will circuits will fill to same level as header tank).Īll solid fuel stove circuits centre around one basic principle: hot water rises through cold water (hence why when you swim in the Med the water gets colder if you dive down a few metres).