Pump pressure and static height in central heating

[TSN79]

I was told recently that in a central heating system, the pressure on the suction side of the circulation pump should be at least equal to the static height of the system. I've never heard this stated before, and I don't intuitively get why this should be the case. If there is actually something to it, maybe someone here can explain it to me?

 

[JRMichler]

All centrifugal pumps have a minimum NPSH (net positive suction head). If the absolute pressure at the suction is too low, it cavitates. Cavitation is bad, it destroys the impeller.

What the OP was told is an oversimplified rule of thumb that avoids the need to find / calculate the pressure at the pump suction, the (worst case) vapor pressure at the pump suction, and the minimum NPSH for that pump.

 

[CWatters]

Doesnt this mean that in a vented system the pump must be at the lowest point? In my vented heating system I have circulating pumps on both floors. The pressure at the input to the top floor pump is therefore about half the max static height.

 

[JRMichler]

With positive static pressure at the pump suction (suction is below the vent), water temperature below boiling, reasonably low line losses between the pump suction and the vent, and a typical hydronic circulating pump, you should be good.

Most hydronic systems do not have NPSH problems. It's the high performance industrial applications that need to be checked, especially systems with high line losses in the pump suction line.

 

[russ_watters]

I was told recently that in a central heating system, the pressure on the suction side of the circulation pump should be at least equal to the static height of the system.

That doesn't make any sense. The only way it can even be true (for a basement-located system...) is when the pump is off; the pressure on both sides is equal to the stack height. When you turn the pump on, the pressure on the suction side goes down!

I think you must have heard wrong.

 

[TSN79]

Thanks for the feedback guys. In addition I have a related question regarding the pump pressure. As mentioned, when the pump starts, the pressure on the suction side decreases, and increases on the pressure side. I can calculate the loss through the entire loop, but is there a way to predict exactly how much it will decrease and increase on the suction and pressure side?

 

[JRMichler]

The exact calculation is system dependent, but in general the procedure is:
1) Find the pressure difference across the pump from the pump curve and the calculated flow rate.
2) Calculate the line loss between a point of known pressure and the nearest side (suction or discharge) of the pump. Typical points of known pressure are an atmospheric vent or an expansion tank.
3) From (2), find the pressure on one side of the pump.
4) From (3) and (1), find the pressure on the other side of the pump.
5) Compare to the static pressure at the pump (pressure with pump off).

 

[TSN79]

I suddenly became unsure of a question I was previously sure about :P

Say a closed system has a height of 10 meters. When the pump is off one can observe a pressure difference of 1 bar between the top and bottom of the system. Will this difference stay constant also when the pump is started? I'm quite sure it does not and that I only need to calculate the pipe's head loss as if it was a horizontal piee of pipe, but for some reason I sudden became unsure of this...

 

[JRMichler]

Three steps:
1) Calculate head loss while ignoring gravity.
2) Calculate static head at zero flow.
3) Add the two together.

Most hydronic heating systems operate at 10 to 20 PSI, most potable water systems are at 40 to 50 PSI. This makes for challenges in taller buildings.