Pump Circulation, Pressurisation and the Pressure Gradient Line

1. What happens in a closed circuit

In building services we are mostly dealing with closed pump circuits such as the ones for heating and cooling.

In a closed, full circuit, the water in the flow riser and the water in the return riser have the same height. Their static heads balance. So the pump doesn’t have to lift water and it only has to provide the energy lost to friction as water travels round the network.

There are two consequences for this:

• The pump creates a pressure difference between its suction and its discharge.
• Building height does not appear in the pump head calculation for a closed circuit. Pump head will be the circuit friction loss.

When we say the pump creates a pressure difference, it doesn’t mean the pump sets the pressure level in the loop. For example, let’s say we have a network that is 25m high and the pump is located at the bottom of the network. When water is filled in the network, the bottom portion will have this 25m static head. If the friction loss in the network is 10m, then the pump will create a pressure difference of 10m across its suction and discharge. But it will have no control over the pressure level of the network.

2. What sets the pressure level and why is it needed

The pressure level is set by the pressurisation unit or in smaller network an expansion vessel.

Two things make this necessary.

First is the air. The pressurisation unit keeps the pressure level above the atmospheric pressure and prevents air from entering the system.

The second is expansion. Water expands by about 3.6 percent between 10 °C and 90 °C. On a 4000 L system the expansion would be around 143L. Water is almost incompressible and hence a very small increase in volume creates a very large increase in pressure. The expansion vessel helps in this situation by accommodating the extra volume of water and keeps the pressure steady.

3. The neutral point

3.1 Definition

The neutral point, or point of no pressure change, is the point where the expansion vessel connects to the system. The pressure at this point remains the same at all times (Doesn’t matter if the pump is on or off)

3.2 Why the pressure there does not change

The gas pressure in the vessel is fixed by the gas volume (Boyle’s law, $pV = \text{constant}$ at constant temperature). Compressing the gas requires water to enter the vessel. Water can only enter the vessel if the same volume leaves the pipework. The pipes are rigid, the water is nearly incompressible, and the total water content does not change when the pump starts. So water doesn’t enter the vessel and gas pressure stays the same. (This is only the case when temperature does not change. more discussion in later sections)

The pump cannot pressurise the whole loop. The pump only redistributes pressure. It will be higher on the one side of the neutral point and lower on the other. This can be understood by a pressure gradient line or also called Hydraulic gradient line which can be found below. (For more go to the tools sections to test it out) Whatever the differential pressure that pump creates will be consumed by friction and finally end up as heat.

3.3 Caveat: a little water does move

At start-up a small transient flow does enter or leave the vessel, because water and steel have finite elasticity. The quantity is negligible. Skip the below example if not interested.

Worked estimate: 1000 L of water in the part of the circuit where the pressure changes. The effective bulk modulus of water in steel pipe ≈ 1.5 GPa, pressure change = 10 m (98 kPa):

$$\Delta V = \frac{V \times \Delta p}{K} = \frac{1.0\ \text{m}^3 \times 98\,000}{1.5 \times 10^{9}} \approx 65\ \text{mL}$$

A 50 L gas volume at 3.5 bar absolute gaining or losing 65 mL changes by:

$$\frac{3.5 \times 0.065}{50} \approx 0.005\ \text{bar} \approx 0.05\ \text{m}$$

The other side of the circuit changes by 10 m. The vessel changes by just 0.05 m. So we can consider the connection neutral for all practical purposes.

3.4 The consequence

If we walk in the flow direction from the neutral point, we can see that pressure falls with friction and rises across each pump.

• Between the pump discharge and the neutral point: above static.
• Between the neutral point and the pump suction: below static.

So the location of the neutral point is extremely important which is discussed in the coming sections.

The model below shows this on the circuit from section 5. Move the vessel round the loop and start and stop the pump. The vessel reading does not change when the pump starts or stops.

1Interactive model

Where the pump raises and lowers the pressure

The section 5 circuit: 20 m high, filled to 25 m at plant level. Move the vessel connection and switch the pump on and off.

Pump running

Vessel (neutral point)

25.0 m

does not move with the pump

Highest point

10.0 m

gauge, 5.0 m when static

Pump suction

25.0 m

gauge

Vessel connectionpump suctionPump head10 m

Every point sits at or above its static value. The top is at 10.0 m instead of 5.0 m.

4. Drawing the pressure gradient line

Two forms of the same line.

Form	Definition	Use it for
Piezometric head	pressure + elevation above datum	Checking the logic. Elevation cancels, so the line falls steadily with friction and jumps up only at pumps.
Gauge pressure	what a gauge reads at that point	Checking sub-atmospheric high points, NPSH (Net Positive Suction Head), and component ratings.

Procedure

1. Find the neutral point. Fix its pressure at the fill value.
2. Walk round the circuit in the flow direction. Subtract friction losses and add pump heads as you cross them.
3. Convert to gauge pressure by subtracting each point’s height above datum.
4. Check the lowest gauge value against the minimum required pressure, including NPSH at every pump suction.
5. Check the highest gauge value against component pressure ratings and the safety valve setting.

5. Worked example: single circuit

Consider a Plant at level 0 and having the highest point 20 m above. Cold static requirement will be: 5 m at the top (We are keeping at least 5 m at the top for a safe margin above atmospheric pressure), so fill pressure at plant level = 25 m. Pump head is 10 m to overcome friction losses. Friction losses are: flow riser 3 m, across the top 2 m, return riser 3 m, plant and boiler 2 m. Total 10 m.

Case A: neutral point at the pump suction

Point	Gauge head (m)
Pump suction (neutral)	25
Pump discharge	35
Top of flow riser	35 − 20 − 3 = 12
Top of return riser	10
Bottom of return riser	10 + 20 − 3 = 27
After boiler → suction	25

Piezometric: 25 → 35 → 32 → 30 → 27 → 25. Constant fall and a jump at the pump.

Case B: neutral point at the pump discharge

Point	Gauge head (m)
Pump discharge (neutral)	25
Pump suction	15
Top of flow riser	2
Top of return riser	0

Every point other than the neutral point falls by the pump head, including the suction side and the top of the system.

Test the above using the Pressure Gradient Tool in the Tools Section

Result

Case A keeps every point at or above its static value. The high point rises from 5 m to 10 m. Case B takes the high point to 0 m gauge. The problem here is that any further friction, or any reduction in the fill margin, makes it sub-atmospheric and this will cause air drawn in at the automatic air vents and dissolved gas comes out of solution. The pump suction also loses the full pump head, which reduces its margin against cavitation.

Rule: connect the pressurisation unit at the main pump suction.

Here are both cases drawn as the two lines from section 4. The marker walks round the circuit in the flow direction. Change the pump head or the fill pressure and see how close case B gets to atmospheric pressure at the top.

2Interactive model

Which connection keeps the top of the system positive?

The worked example walked round in the flow direction, as piezometric head and as gauge pressure.

Case A: at the pump suctionCase B: at the pump discharge

Highest point

10.0 m

gauge, pumps running

Pump suction

25.0 m

gauge

Pump discharge

35.0 m

gauge, the highest in the circuit

Pump head (= circuit friction)10 mFill pressure at plant level25 m

The line is anchored at the pump suction, so the pump head is added on top of the fill. The top of the system sits at 10.0 m.

6. The expansion vessel

6.1 Construction

It is a sealed steel shell divided by a butyl diaphragm or a replaceable bag and has Nitrogen on one side and system water on the other.

• Diaphragm type: It has a fixed membrane and the water touches the shell
• Bag type: It has water fully enclosed in a replaceable bag, so the water does not touch the shell. It is used on potable systems and also where replacement is required.

6.2 Diaphragm position

The diaphragm has no fixed position. It sits wherever the two pressures balance, and its position indicates how much water the vessel holds. It is not “halfway” by design.

At pre-charge, since the vessel is empty of water, the gas fills the shell and the diaphragm is pressed flat against the water-side wall. In that position the vessel is effectively disconnected: the water side can be at any pressure from zero up to the precharge and nothing moves, because the gas cannot expand further.

6.3 Precharge, cold fill and the water cushion

• Precharge = static height from the vessel to the highest point, plus 0.3 to 0.5 bar so the top stays positive.
• Cold fill = precharge + 0.2 to 0.3 bar. This pushes a small quantity of water into the vessel and lifts the diaphragm off the wall. That quantity can act as a water cushion.

The cushion does two jobs: it keeps the vessel live and responsive, and it provides a reserve against small losses.

Why it matters: numerical comparison. 100 L vessel, precharge 2.5 bar gauge (3.5 bar absolute):

Fill pressure	Water in vessel	Lose 2 L → new pressure
2.5 bar (= precharge)	0 L	≈ 0 bar. The pressure collapses because there is no reserve.
2.7 bar	5.4 L	2.62 bar. A 0.08 bar reduction which is visible on the gauge.

The cushion for the 500 L vessel in the sizing example is checked in section 11.5.

The same comparison with both vessels side by side. Play the slow leak, or drag the loss yourself.

3Interactive model

What the water cushion buys

Two 100 L vessels, both precharged to 2.5 bar. Let some water leak out of the system and watch each gauge.

Play a slow leak

Water lost

0.0 L

from the system

No cushion

2.50 bar

filled to 2.50 bar

With cushion

2.70 bar

filled to 2.70 bar

Water lost from the system0 LRight vessel cold fill2.70 bar

Nothing lost yet. The left-hand vessel holds no water at all, so it has nothing to give when the system loses some.

6.4 Operation with temperature

As the water heats up and expands (see section 2), the extra water enters the vessel and it will compress the gas. When cooling, the gas pushes the water back. The vessel needs to be sized such that the diaphragm never reaches either wall, and so that the final pressure stays below the safety valve setting by the required margin.

6.5 Site adjustment

Both sides of the vessel are adjustable:

• Gas side: There will be a Schrader valve. The vessel will be Factory precharged typically to 1.5 bar. It can be set correctly on site as required.
• Water side: It is set during the fill using a filling loop and gauge, or a set point in the pressurisation unit controller.

6.6 Some points to note

• Try to keep the vessel on the return, where the water is coolest. Butyl is generally limited to about 70 °C. Fit an uninsulated cooling leg if the return is still hot.
• Take the connection from a low-velocity point (header or buffer vessel, 0.1 to 0.3 m/s) so the reading is stable.
• Do not connect at the top of a buffer vessel because it might be an air pocket.

7. The pressurisation unit

7.1 Components

• Break tank fed from the mains through a Type AB air gap
• One or two make-up pumps (duty/standby on critical plant)
• Pressure sensor and controller
• Discharge or spill solenoid valve
• Make-up counter or level sensing for leak detection
• Optional vacuum degasser

7.2 Break tank functions

1. Backflow protection. The air gap physically separates treated system water from the mains. EN 1717 and the UK water regulations require this level of protection for any closed system containing inhibitor or glycol.
2. Suction reservoir for the make-up pump, at atmospheric pressure and independent of mains pressure variation.
3. Dosing point for inhibitor and glycol.
4. Measurement point for leak detection.

7.3 Control sequence

1. Pressure below the lower set point → make-up pump runs until the set point is restored.
2. Pressure falls further → low-pressure alarm will be triggered and heat source interlocked off.
3. Pressure above the upper set point → discharge valve dumps a small quantity.
4. Make-up volume or pump starts logged for leak detection.

7.4 Why the unit must sit at the neutral point

The vessel connection is the neutral point, so the unit’s sensor must sit at the same point. There the sensor sees no pressure change when pumps start or stop. If it is connected elsewhere, it sees a step change at every pump start, then tops up on the drop and dumps on the rise.

8. Buffer vessel and low loss header

A normal scenario in modern heating systems is the presence of a buffer vessel and a low loss header. They do the same hydraulic job. The header gives separation with almost no stored water. The buffer vessel adds thermal capacity, which a heat pump needs for defrost and for minimum run time.

8.1 What separation is

A short pipe of very low resistance between the two circuits. Its loss is near zero, so the pressure difference between the flow and return connections is near zero. So neither of the pumps can influence the flow in the other.

8.2 Where the pressurisation connects

At the header or buffer vessel, on the return side, close to the suction of the largest pump.

Example: heat pump plant, primary pump 4 m, secondary pump 12 m, header loss 0.2 m, fill 25 m at plant level, high point 20 m above.

	Connection at header return	Connection on secondary flow, after the pump
Header	25 m	13 m
Both pump suctions	≈ 24.9 m	≈ 12.9 m
Heat pump operating range	25 to 29 m (2.5 to 2.8 bar)	13 to 17 m (1.3 to 1.7 bar)
Building high point	≈ 10 m gauge	≈ −2 m gauge (below atmospheric)

The heat pump is the critical point usually missed. Manufacturers set a minimum operating pressure, often 1 bar or more, for the plate heat exchanger and the internal pump. A wrongly placed connection can put the unit below its minimum point.

8.3 One neutral point per hydraulically connected system

A header or buffer vessel connects the circuits hydraulically. They share water, so they are one system. So they require only one pressurisation unit and expansion vessel. Two units on the same water content will fight so be careful with this.

Exception: the plate heat exchanger. There is a separation here and each circuit requires its own unit.

9. Multiple secondary circuits

If the system has multiple secondary circuits, for One neutral point there would be more suctions to check.

Arrangement. Header, primary circuit, three secondaries, each with its own pump: low zone 8 m; high zone 22 m with a riser to 40 m; AHU coil circuit 12 m. Fill 45 m at plant level, so the high zone top is at 5 m gauge cold and static.

Point	Case A: neutral at header	Case B: neutral at high zone pump discharge
Header	45	23
LZ suction	45	23
LZ discharge	53	31
HZ suction	45	23
HZ discharge	67	45
HZ top of riser (piezo)	57	35
HZ top of riser (gauge)	+17	−5
AHU suction	45	23
AHU discharge	57	35

In Case B the whole plant drops by 22 m, the head of the pump the connection sits behind. The header falls to 2.25 bar, so the heat source approaches its minimum operating pressure. The fault is not confined to the circuit that was connected wrong. Every secondary suction falls with the header. The high zone riser top goes sub-atmospheric.

Try each connection point. The drop always equals the head of the pump the unit sits behind, and the unit’s own gauge never changes.

4Interactive model

What one wrong connection does to the whole plant

A low loss header with the primary on one side and three secondary circuits on the other, filled to 45 m. Choose where the pressurisation unit connects.

HeaderAfter LZ pumpAfter HZ pumpAfter AHU pump

Unit's own gauge

45 m

always, so no alarm

Header and every suction

45 m

4.41 bar

High zone top

+17 m

gauge, pumps running

Every secondary suction is reached from the header with almost no friction and no pump in between, so they all sit at the neutral pressure.

9.1 Rule for locating the worst point

Walk the line from the neutral point as described in section 4. The lowest value reached is the critical suction.

With the neutral point at the header, every secondary suction is reached with almost no friction and no intervening pump, so all suctions sit at the neutral pressure. That is the best possible outcome, and it is why the header is the correct connection point.

10. Limits on the pump discharge pressure

The pump has no upper stop. Give it 25 m at the suction and it delivers 35 m. Give it 80 m and it delivers 90 m. Everything else sets the limit.

10.1 The limits, roughly in the order they bite

1. Safety valve setting. Everything must stay below it at all times, including at full thermal expansion with pumps running.
2. Component pressure ratings. Boilers, heat pumps, plate exchangers, cylinders, valves and flexible connections, typically 3, 6 or 10 bar. Boilers are usually the vulnerable point, often rated at only 3 or 4 bar. Also check the items near to the pump discharge at the lowest level.
3. Pump casing rating and maximum inlet pressure. Inline bodies are often 10 or 16 bar, with a separate stated maximum suction pressure (a seal limit).
4. Mechanical seal, whose life shortens under high pressure and high temperature.
5. Expansion vessel rating, if the vessel is anywhere near the discharge side.

10.2 The usable pressure band

• Minimum: enough positive pressure at the highest point and at every pump suction with the pumps running. Typically 5 m gauge at the highest point, and NPSHr plus margin at each suction.
• Maximum: the safety valve setting less 10 % or 0.5 bar (whichever is greater), at the worst point in the system, hot, with pumps running.

Usually in tall buildings if the pressure requirements exceed the safety limit, heat exchangers are used to hydraulically separate the circuits.

10.3 Where the maximum pressure occurs

Three terms add at the same place:

• Elevation: every metre below the neutral point adds a metre of head
• Pump head: the discharge side sits above the neutral point
• Thermal expansion: the whole line lifts when hot. The rise depends on the vessel size and the set points. It can be a few tenths of a bar, or well over 1 bar (see section 11.5).

So the worst case is: hot, pumps running, lowest elevation, immediately after a pump.

10.4 Plant at the top vs plant in the basement

There might be situations where it would be best to keep the plants on the roof rather than in the basement purely from a pressure safety perspective. Moving the plant does not reduce the peak system pressure. It moves the weak component out of the high-pressure region.

60 m building, 10 m pump, 5 m required at the top, 0.3 bar rise when hot:

	Plant in basement	Plant on roof
Fill pressure at plant	65 m (6.4 bar)	5 m (0.5 bar)
Pump discharge	75 m (7.4 bar)	15 m (1.5 bar)
Boiler sees (hot)	≈ 7.7 bar	≈ 1.8 bar
Pressure at base of risers	≈ 6.7 to 7.7 bar	≈ 6.7 to 7.7 bar
Verdict	4 or 6 bar plant unusable	4 bar boiler comfortable; 7.7 bar appears only in 10 bar pipework

Two consequences:

• The safety valve protects at its own elevation. A rooftop valve set to 2.5 bar means the base of the system sees about 8.4 bar under relief conditions. Rate the low-level pipework for that.
• The problems associated with rooftop option is that it costs structural load, access, flue routing, noise and vibration to the top floor, and freezing risk. If the plant must stay in the basement, heat exchanger options can be pursued.

10.5 Two cases that catch people out

• Basement plant in a tall building. Static head alone might be 60 m before anything is added.
• A large secondary pump added to an existing system. The original safety valve and fill pressure suited the old pump. The new pump adds head to the discharge side and the relief valve triggers when hot. The site diagnosis is usually “faulty safety valve”. But the root cause is that nobody re-checked the pressure band :)

11. Sizing the expansion vessel

Calculated using Boyle’s law.

11.1 System water volume

System water volume shall include water present in the pipes, equipment, buffer vessels etc and it is good to add a 10% safety.

Early-stage rules of thumb only:

System type	L/kW
Radiator LTHW	10 to 12
Fan coil / AHU dominated	5 to 7
Underfloor heating	15 to 20
Heat pump	Calculated pipe volume + buffer vessel (usually dominant)

A 1000 L buffer on a 100 kW heat pump can double the system volume and double the expansion vessel.

11.2 Expansion volume

$V_e = V \times e$, where $e$ is the expansion coefficient from fill temperature to maximum temperature. From 10 °C:

Max temperature	$e$
55 °C	0.0142
70 °C	0.0224
80 °C	0.0287
90 °C	0.0357

Use the maximum temperature the system can reach. For a boiler, the high limit thermostat setting (typically 90 °C) and not the design flow temperature.

Multiply by about 1.1 for 25 % glycol and 1.2 for 40 % glycol, and check diaphragm compatibility.

For chilled water the design case is usually the system standing idle in summer: take the expansion from the lowest operating or fill temperature up to the maximum standstill temperature, often 30 to 40 °C in a plant room.

11.3 The pressures (all absolute for the calculation: gauge + 1 bar)

Symbol	Meaning	Value
$P_i$	Precharge	Static height vessel → highest point, + 0.3 to 0.5 bar
$P_c$	Cold fill	$P_i$ + 0.2 to 0.3 bar (creates the water cushion)
$P_f$	Final / maximum vessel pressure	Spill setting, or safety valve setting less margin
	Safety valve setting	$P_f$ + margin

On the margin, references differ. CIBSE and common UK practice use the greater of 10 % of the setting or 0.5 bar. Some manufacturers work to 0.35 bar. Use 10 % or 0.5 bar.

11.4 The formula

$$V_v \geq \frac{V_e \times P_c \times P_f}{P_i \times (P_f - P_c)}$$

Setting $P_c = P_i$ reduces this to the familiar $V_e \times P_f / (P_f - P_i)$. That simpler form assumes the vessel starts empty, so it omits the water cushion and typically undersizes by 15 to 25 %, more when the pressure band is narrow. Use the full form.

11.5 Worked example

LTHW, 4000 L, fill at 10 °C, boiler high limit 90 °C, vessel at plant level, highest point 20 m above.

$$\begin{aligned} V_e &= 4000 \times 0.0357 = 143\ \text{L} \\ \text{Static at vessel} &= 1.96\ \text{bar};\ + 0.4\ \text{bar margin} \\ P_i &= 2.4\ \text{bar gauge} = 3.4\ \text{bar absolute} \\ P_c &= 2.7\ \text{bar gauge} = 3.7\ \text{bar absolute} \end{aligned}$$

Attempt 1: 4 bar safety valve, margin 0.5 bar, $P_f$ = 3.5 bar gauge = 4.5 bar abs

$$V_v = \frac{143 \times 3.7 \times 4.5}{3.4 \times (4.5 - 3.7)} = 875\ \text{L}$$

Absurd for a 4000 L system. The working band between 2.7 and 3.5 bar is only 0.8 bar, so the gas barely compresses. The conclusion is that the 4 bar valve is wrong, not that 875 L is needed.

Attempt 2: 6 bar safety valve, unit spilling at 5.0 bar gauge, $P_f$ = 6.0 bar abs

$$V_v = \frac{143 \times 3.7 \times 6.0}{3.4 \times (6.0 - 3.7)} = 406\ \text{L}$$

Select 500 L, or two 300 L vessels. Two vessels allow one to be serviced while the other keeps the system pressurised. One 300 L vessel alone is below the 406 L needed, so do this with the system cold.

Verification of the 500 L selection

$$\begin{aligned} \text{Cushion at cold fill} &= 500 - \frac{500 \times 3.4}{3.7} = 41\ \text{L} \\ \text{Water at full expansion} &= 41 + 143 = 184\ \text{L} \\ \text{Gas at full expansion} &= 500 - 184 = 316\ \text{L} \\ \text{Pressure at full expansion} &= \frac{3.4 \times 500}{316} = 5.4\ \text{bar abs} = 4.4\ \text{bar gauge} \end{aligned}$$

4.4 bar gauge is below the 5.0 bar spill setting. Comfortable.

The lesson: vessel size is dominated by the pressure band, not by the water volume. Doubling the band roughly halves the vessel. Active pressurisation units can use much smaller vessels for another reason: their spill valve sends the expansion water back to the break tank.

Heat the example up below. Try the 4 bar valve with the 500 L vessel, and see how big the vessel has to be before the valve stops lifting.

5Interactive model

Will the vessel hold the expansion?

The worked example heating from 10 °C to 90 °C. Precharge 2.4 bar, cold fill 2.7 bar. Change the vessel, the system volume and the safety valve.

6 bar valve, spill at 5.04 bar valve, final 3.5

Expansion volume

143 L

4000 L × 0.0357

Vessel needed

405 L

band 2.7 to 5.0 bar

Pressure at 90 °C

4.36 bar

with 500 L

Vessel volume500 LSystem water volume4000 L

500 L holds the 143 L of expansion on top of its 41 L cushion and reaches 4.36 bar at 90 °C, 0.64 bar under the 5.0 bar limit.

11.6 Checking the rest of the system against $P_f$

Walk the gradient line at the worst condition: hot, pumps running, lowest plant item.

For the example, with the boiler at plant level on a decoupled primary and a 3 m shunt pump: header at up to 5.0 bar gauge (the spill setting), boiler at about 5.3 bar. A 6 bar boiler is acceptable with a 6 bar safety valve. A 4 bar boiler is not, and no vessel sizing can fix that.

11.7 Pressurisation unit set points (for the worked example, bar gauge)

Function	Setting
Vessel precharge	2.4
Low pressure alarm / plant interlock	2.5
Make-up pump start	2.6
Make-up pump stop	2.8
Spill valve	5.0
High pressure alarm	5.2
Safety valve	6.0

Keep at least 0.2 bar between the pump stop and any nearby high alarm, or the unit trips unwantedly. Keep the low alarm above the precharge, so it operates while the vessel still holds water and before the diaphragm reaches the wall.

12. The safety valve

12.1 Setting and capacity are different properties

• Setting: the pressure at which it opens.
• Capacity: how much it can pass once open.

The valve exists for one failure mode: the heat source at full output with no heat being taken away (control failure, seized contactor, all secondary pumps stopped while the boiler fires). The water heats, expands, and if it reaches saturation it flashes to steam which is about 1600 times the volume of the water that made it (at atmospheric pressure).

So the valve must pass enough mass flow to remove the full heat input of the plant as steam, with no more than 10 % overpressure above the setting.

12.2 Sizing procedure

1. Total the rated input of every heat source on that side of the system. Use the boiler gross input, not the output. Add immersion heaters and any heat pump sharing the same water.
2. Take the required capacity in kW from the valve manufacturer’s data at your setting. A typical DN20 heating safety valve is rated in the order of 100 kW; DN25 passes more. Use the catalogue instead of a rule of thumb.
3. Confirm certification to EN ISO 4126 with a stated capacity. A generic valve with no capacity data is not a safety valve.

13. Design checklist

1. Establish system water volume and expansion volume.
2. Locate the neutral point: header or buffer vessel, return side, at the suction of the largest pump. One per hydraulically connected system.
3. Set the precharge from the static height above the vessel, plus 0.3 to 0.5 bar.
4. Set cold fill at precharge plus 0.2 to 0.3 bar.
5. Choose the safety valve setting so the pressure band gives a practical vessel volume, and so every component rating is respected. Iterate: the first attempt often fails.
6. Size the vessel with the full formula, including the cushion. Verify by Boyle’s law.
7. Draw the piezometric line from the neutral point, adding every pump and subtracting every friction loss, including small shunt pumps.
8. Convert to gauge pressure and check:
   • Highest point, pumps running and pumps off
   • Every pump suction against NPSHr plus margin, at design temperature
   • Lowest component against its rating, hot, pumps running, at full expansion
9. Size the safety valve capacity against total gross heat input and check the discharge route.
10. Set the unit set points with adequate gaps, low alarm above the precharge.
11. Confirm commissioning sequence: drain vessel, set precharge, fill, vent, top up.