Central heating Queries. Open vented conventional S-Plan system.

[Logic]

Alright.

I'm an apprentice currently studying the central heating unit of my course. I have a couple of queries regarding conventional S-Plan open vented systems. (Even more specifically close coupled systems).

Why is it requisite that the feed and vent pipes running to a typical F&E cistern or tank should not be more than 150 mm apart?

Also, why is it so that there should always be a cold water feed path to the boiler's return connection when all automatic valves (if fitted) are closed? By automatic valves I intend your common two port motorised valves (S-Plan as opposed to Y with the treble port).

Advice/Commentary much appreciated.

Thanks in advance.

Paul.

[grahamep]

We only did this to pass the course. You don't really expect us to remember it do you?

Seriously though, 1) to prevent pump over (water up the vent and back down the feed introducing nice fresh corroding oxygen) and 2) the boiler is the heat source. If the stat goes awol and it overheats it will lose its pressure/steam/whatever through the vent. Nice cooler water needs to get back to the boiler to save its life. And possibly yours if you are standing next to it.

[Logic]

Cheers Graham.

Although I don't understand in full your first point. If pump over occurs you say water in the heating circuit could or would be unnecessarily pushed up the vent pipe even to the point where it could find itself back in the F&E tank or cistern? As a result of that the heating circuit would require replenishment to the approx. volume of water lost. This new water from the F&E tank/cistern is oxygenated and as such will cause undesirable corrosion within the system. Yes? The only part I'm missing then is what the above has to do with the 150mm max. distancing? What is the significance?

[kes36]

The pump in an open vented system must be located as to give the heating or/and primeries a positive pressure giving the vent on the flow and the feed on the return a negative pressure to stop over pumping, i can only think that the 150mm is a minimum distance from the tee feeding the vent to the pump.

[Dal5band]

I think this is related to pressure along the pipe - There must be different pressures along the pipe for water to flow. If the two points (fe and exp) were further apart than 150mm, the difference is pressure may create enough of a difference in level such that the expansion pipe will start overflowing into the expansion tank causing circulation through the expansion tank etc.
Best cross check this theory as I am not an expert - just a DIY