Understanding pressure and flow

May 12, 2020




This article is supplementary to the Ultimate balancing article, check it out!

rules of the pump!

This states the relationship between flow, pressure and power consumption.

If you double the flow rate but keep the pressure loss the same (by increasing pipework size for example), you will double pump power consumption.

If you double the pressure loss (aka differential pressure or DP) but keep the flow rate the same (by closing a valve for example), you will again double the power consumption.

If you double the flow and double the pressure you increase power consumption by 4x.

However, the real exponential increase in power consumption comes from the square rule:

The 'square rule' states that the system resistance is proportional to the flow rate squared. It’s a quadratic relationship.

In other words, if you double your flow rate through fixed size pipework, you double your velocity (speed the water moves through the pipe). This doubling of speed exponentially increases the frictional resistance against the flow by 4x! Which requires an exponential increase in pump power to push against. Surprising how quickly it adds up right?

So the one to really remember...

If you double the flow rate, you quadruple the resistance across the system, and octuple (x8) your power consumption! 😳

This was the reason ERP pumps were put in.

Also remember, this is also relevant for fans and compressors and further elaborates the requirement to modulate our systems.

This process actually brings heating systems to somewhat self-balance within reason, otherwise all the water flow would move around the first circuit and no movement of water would reach any of the other emitters/radiators.

Pressure and flow when balancing

Understanding the link between pressure and flow is not necessary for balancing. However having a grasp of what is happening in the system when you balance will help with later on this article and for future posts. For any fixed system, flow rate and pressure (or pressure difference) are joined at the hip.

In other words, the higher the pressure differential created either side of the pump, the higher the flow rate you will have around the system. Conversely, the higher the flow rate you have the higher the pressure loss (or pressure differential) across the system. Pressure loss and pressure differential are almost one and the same.

When we say pressure differential we are referring to the difference in pressure between any two points in the system. This could be the pressure difference either side of the pump which would be an increase in pressure as electrical energy is added, or the pressure difference across any part of the rest of the system which will always be a drop in pressure.

Pressure loss, only refers to the drop in pressure. This is caused as the flow of water moves through the resistance of the fittings and pipe, however this can cause some confusion as although there is a pressure loss, without this differential of pressure either side of the fitting or pipe, you won't get any flow of water! This can be a hard one to get your head around, you can't have one without the other (unless you've mastered perpetual motion).

If you reduce the resistance across a system or circuit by increasing the pipework size for example you will be able to have the same flow rate, for a lower Differential pressure, or dP.

By adding restrictions (closing valves or making pipework smaller or longer) you will need to increase the dP across the circuit to obtain the same flow rate.

When you balance a heating system you are effectively making the resistance across each radiator circuit the same as the radiator circuit the furthest away and/or with the most resistance when it's receiving it's full required flow rate.

Below we have a much exaggerated example of the pressure differentials in an unbalanced larger system, say a 5/6 bed house.

You can see that to get the correct amount of flow through the boiler (DT20) and system we require a differential pressure of 4 meters head.

All the lockshields on this system are currently wide open. There are 2 interesting differentials to look at here, the dP across the circuit, and the dP across the radiator. The radiators nearer the boiler have a much higher pressure differential either side than the ones further away, and so these have a much higher flow rate than required. Because the higher the pressure differential, the higher the flow rate.

Fig 3

In this exagerated situation A and B have a pressure differential of 2.7 and 1.8 Meters head, this gives them a very high flow rate.

C has 1 meter head pressure difference across it and has the correct amount of flow

D+E would have too little flow and a very narrow pressure differential of 0.5 and 0.2.

From the pump, around any circuits individually however will always have the same total pressure loss, 4 m in this instance at their current flow rates. The system always finds balance.

As the old adage goes, 'water will always take the path of least resistance'. Because there is less resistance to flow through Radiator A, more water passes through that circuit until the resistance (or pressure difference) builds to match 4m head. Radiator B has slightly further away so has more fittings and pipe for the water to go through. This receives slightly less flow in order to balance at a DP of 4m, and so the flow is shared amongst the radiators in varying amounts that give a 4m pressure loss, or pressure differential, around each circuit.

Assuming for now that the radiators are all the same size, what we actually do when balancing, is make the resistance or DP across all the radiators the same as what the index radiator requires when it's operating at full required flow rate.

We do this by closing down valves and adding resistance to replicate the extra pipework and fittings (resistance) in the index radiator. For this reason, the index radiator valves should always be left fully open as to make sure we are not adding unnecessary resistance to the pump.

In our example above (fig 3), radiator C has the correct flow rate and has 1m differential across the radiator. Therefore if we can adjust the first lockshield untill this differential is reached and move along the system we should reach balance without touching the last radiator and leaving E's lockshield fully open.

When we start to close these radiator valves on radiators with too much flow, more water is having to go further round the system. This means more resistance to the flow, and so to achieve the same flow rate for the boiler, the pump will have to put in more energy.

You can see in Fig 4 below the radiators have been balanced at the lockshield and the new DP across the system is 6.1 metres head. However, the resistance across each radiator is now exactly the same (1m) and so flow is evenly distributed.

Fig 4

Notice that Radiator C's lockshield has had to close enough to give 2m resistance across it which is how much the pump head is increased by.

Larger radiators will need slightly wider dp to get more flow, and smaller, lower dp. No matter what size your radiator though, the total DP across each circuit and back to the pump is always the same regardless (6.1m here). The system always finds balance.

Smaller systems will not get this much of a swing in dP after balancing, but larger systems may need the pump adjusted a second time after balancing.

The main two reasons engineers will struggle with balancing is when they are working on large systems and when systems have smaller bore pipework.

When one valve is balanced down it shifts the dynamics of the system and can throw other radiators or emitters out. Another one of the main reasons for this, apart from system layout and design is valve authority.

Heat Geek is the one stop to find out everything from how to bleed a radiator to selecting the right boiler, we don’t have any bias and value the facts above everything else.
May 12, 2020
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