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[cab]

Hello-

Perhaps a stupid question, but could someone explain how an open system works? I have googled and seen plenty of systems with an open tank positioned above the boiler but these are all thermosyphon systems (gravity feed). It seems to me that open system boilers located outside must have a way to pump to the house??? How does one run a pump to circulate the hot water out of the boiler if the system is open?

[RSI]

Open system just means that it is not pressurized. They just have an overflow pipe instead.

[cab]

Then how do you circulate the water? Only by gravity?

[tulenutn2o]

Water circulation by pumps.

[cab]

if there are pumps, there is pressure, so how can it be open?

[beeman]

the pump only push or pull the water, the water tank on boiler is open so no presser  try this take one gallon jug and a big pot put  the jug on a chair  and the pot on the floor . take a small pump put into pot on the floor hose from pump to the top of jug on chair fill jug with water poke hole in side of jug close to the bottom so water comming out falls out into pot on floor turn the pump on water will move , no presser gary

[RSI]

What does pump and pressure have to do with each other?
There are 2 fittings on the boiler tank. The loop to the house and back connect between these and the pump circulates through the loop. The pump may make a small amount of pressure and if the house is lower than the boiler then there will be some pressure due to gravity but that is not what the pressure vs open is talking about.

A pressurized system has an expansion tank to allow for temperature changes and a pressure relief to allow for overheating. The open system just isn't completely full so there is room for expansion and if overfilled or overheated it just goes out the overflow tube.
Other than this they both operate exactly the same.