# Pump pressure and static height in central heating

• TSN79
In summary, the conversation discussed the importance of maintaining a minimum NPSH (net positive suction head) in a central heating system to avoid cavitation and damage to the pump. The rule of thumb is to have the pressure on the suction side of the pump at least equal to the static height of the system. However, this rule may not apply to all systems and it is best to calculate the pressure difference across the pump and the line loss to ensure proper functioning. The conversation also touched on the challenges of maintaining sufficient pressure in taller buildings for hydronic heating systems.
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?

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.

Asymptotic
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.

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.

TSN79 said:
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.

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?

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).

Asymptotic and russ_watters
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...

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.

## 1. What is pump pressure in central heating and why is it important?

Pump pressure refers to the force that is exerted by the pump in a central heating system to distribute hot water throughout the system. It is important because it ensures that the hot water reaches all the radiators and other components of the system to provide efficient heating.

## 2. How is pump pressure measured in a central heating system?

Pump pressure is typically measured in bar units using a pressure gauge attached to the boiler or pump. This gauge displays the pressure of the water as it leaves the pump and enters the system.

## 3. What is static height in central heating and how does it affect pump pressure?

Static height refers to the vertical distance between the pump and the highest point in the central heating system, such as the highest radiator. It affects pump pressure because the higher the static height, the more pressure is needed to push the water up to the highest point.

## 4. How can I adjust the pump pressure in my central heating system?

The pump pressure can be adjusted by using the pump speed or flow rate settings on the pump itself. Increasing the speed or flow rate will increase the pump pressure, while decreasing it will lower the pressure. It is important to consult a professional or refer to the manufacturer's manual before making any adjustments.

## 5. What are the consequences of having incorrect pump pressure and static height in central heating?

If the pump pressure is too low, the hot water may not reach all the radiators and the heating system may not work efficiently. On the other hand, if the pump pressure is too high, it can put unnecessary strain on the system and may cause leaks or other issues. Incorrect static height can also lead to uneven heating and poor performance of the central heating system.

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