Zoned Systems

Wilson

I have been reading posts at this site for months marveling especially at the pictures of the elaborate zoned boiler systems. But after studying these photos I have a question: what prevents water from circulating in all the zones when one zone calls for heat? Doesn't the circulator at the boiler (or even the water pressure from the supply line) have sufficient pressure to push water through all of the lines coming out of the manifold even if the pump for an individual zone is not on? And if the circulator itself stops the flow of water when idle, then why do you need one-way valves to prevent backflow?

Bob Bona_4

dumb

and lazy, that's what water is. It will go the path of least resistance (circ). The circ doesn't block the flow of water when idle, the hot bouyant water wants to rise-thus flochecks.

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Dan Peel

Hydronic Voo Doo

Once you've read most of Dan's books (Books and more on the left) you'll have the answers to these and a plethora of other mysterys of why heating fluids flow as they do. The short answer is that flow is governed by differentials in pressure and temperature. By piping to achieve a balance - using check valves, zone valves, thermal drops, closely spaced tees, etc. - and a bunch of other design strategies, you can take control over flow. Physics always wins........ Dan

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Frank_3

Trick question: If you turn off all the faucets in your house, how much water does the street supply move through the pipes? None. There's no place for the water to go, so all you have is pressure, not flow.

The same thing holds in a heating system -- it's a closed system so the supply can't create *flow*. All it can do is create pressure, but it's not even doing that. Typically, once the system has been filled the supply valve is shut off and pressure is maintained by the expansion tank.

When the primary circulator is running it's manipulating the pressures in the primary loop only, so water only moves through those pipes and doesn't go to the zones. Only when a zone circulator is turned on does the zone loop see a pressure change which makes the water move around.

Lastly, the one-way check valves are there to prevent gravity circulation of hot water *into* the zone when the circulator is off. They're not there to prevent backflow of water *out* of the zone -- the water in the zone loops stays where it is until the zone circulator kicks in again.

Ok, there is contraction and possibly expansion of the water in the zone loop when the zone circulator is idle and when that happens water will naturally flow in or out of the zone because the pressure is changing. However, that will happen regardless of whether the primary circulator is running at the time and the flow rate is negligible.

Did that help any?

Frank_3

Trick question: If you turn off all the faucets in your house, how much water does the street supply move through the pipes? None. There's no place for the water to go, so all you have is pressure, not flow.

The same thing holds in a heating system -- it's a closed system so the supply can't create *flow*. All it can do is create pressure, but it's not even doing that. Typically, once the system has been filled the supply valve is shut off and pressure is maintained by the expansion tank.

When the primary circulator is running it's manipulating the pressures in the primary loop only, so water only moves through those pipes (and the boiler) and doesn't go to the zones. Only when a zone circulator is turned on does the zone loop see a pressure change which makes the water move around.

Lastly, the one-way check valves are there to prevent gravity circulation of hot water *into* the zone when the circulator is off. They're not there to prevent backflow of water *out* of the zone -- the water in the zone loops stays where it is until the zone circulator kicks in again.

Ok, there is contraction and possibly expansion of the water in the zone loop when the zone circulator is idle and when that happens water will naturally flow in or out of the zone because the pressure is changing. However, that will happen regardless of whether the primary circulator is running at the time and the flow rate is negligible.

Did that help any?

Boilerpro

One way check valves do prevent backflow.

When a pump on the supply to one zone kicks on it creates a low pressure point at the inlet to the pump and a higher pressure point at the ouletof the pump. The water will try to find the easiest path between the two.....either through the zone piping and then through the boiler or backflowing through aother zone and usually both, if not piped properly.

Boilerpro