what pressure should be after installing a new filter
Why is pressure after installing a new filter a critical parameter
Heating is a system that operates on the principle of balance. If this balance is disrupted, the consequences are often not immediately visible, but fatal in the long-term perspective. One of the most common myths when installing filtration into heating systems is the assumption that a new filter should have zero or negligible resistance. The reality, however, is that every new filter, regardless of whether it is a simple mesh or an advanced magnetic separator, represents a certain hydraulic limitation. The correct pressure after installing a new filter is not just a number on a pressure gauge, but a key indicator of the system's proper operation and the lifespan of your pump.
In practice, I have seen dozens of cases where the installation of a new filter caused an "unexpected" boiler shutdown due to a lack of heat or circulation. The cause was not a fault of the device itself, but an incorrect choice of installation location or ignoring the resistance of the new component. When you install Filter 5" - 1"F; without insert, you must know that this component immediately changes the hydraulic characteristics of your circuit. The difference between what you expected and reality will be revealed precisely in the pressure difference before and after the filter.
This article does not focus on marketing promises about energy savings, but on technical reality. We will clarify what pressure should be in the system immediately after startup, how to calculate the acceptable pressure drop (difference), and what it means when pressure starts to change during operation. We will also look at the differences between various types of filters commonly used in households and industry.
Difference between absolute pressure and pressure drop
The first step to understanding the problem is clearly distinguishing two terms that are often confused in everyday language, but have completely different meanings in hydraulics. Absolute pressure is the value you measure on a pressure gauge at any point in the pipe. For example, if your system has a static pressure of 1.8 bar, it means that water is pressing against the pipe walls with a force of 1.8 bar. This pressure depends on the height of the water column and the filling of the expansion tank.
Pressure drop (or differential pressure), often denoted as ΔP (Delta P), is the difference between the pressure before and after the filter. This number is critical for assessing the condition of the filter. If you have 2.0 bar before the filter and 1.95 bar after the filter, the pressure drop is 0.05 bar. If the drop is 0.3 bar, the filter is already blocking the flow. When installing a new filter, we usually aim to achieve the minimum possible drop to avoid overloading the pump's impeller.
In the figure above, you see a schematic of a simple water flow through a filter. The red arrow visualizes the pressure loss that occurs when overcoming the resistance of the mesh. This loss is necessary because water must flow through the mesh. If the filter were completely free of resistance, water would bypass it and filtration would not occur. The goal is for this resistance to be as small as possible, but sufficient to capture impurities.
What pressure should be after installing a new filter? Real values
We will directly answer the question that probably concerns you. After installing a new filter, with clean inserts and a properly dimensioned system, the pressure drop (the difference in pressure before and after the filter) should be in the range of 0.02 to 0.05 bar. This corresponds to approximately 2 to 5 centimeters of water column.
These values are valid for standard polypropylene filters with a 20-micron insert, which are installed in typical family homes. If you measure a difference higher than 0.1 bar (10 cm of water column) on a completely new and clean filter, there is a problem. It may be due to:
- Incorrect cross-section dimension: If you have a DN25 (1") pipe in the system, but installed a filter with DN15 (1/2") connection or a tight mesh, the pressure drop will be high.
- Installation error: The filter may be installed in a way that restricts the cross-section (e.g., improperly tightened fittings or the use of a reducer that narrows the diameter).
- Defective insert: Sometimes, a new insert has a manufacturing defect and is too dense.
It is important to emphasize that the pressure in the system (absolute pressure) can vary between 1.5 and 3.0 bar depending on the type of boiler and the height of the building. What interests us with the filter is only the difference. Because if you have 3 bar in the entire system and 2.95 bar after the filter, the system is working perfectly. If you have 1.5 bar and 1.45 bar after the filter, it is also fine. The key is the relative value.
Practical example from the field: A customer with low pressure
A few months ago, I dealt with a case in a family home where the owner installed Filter 5" - 3/4"F; without insert with the aim of protecting a gas condensing boiler. The system was older, with copper and aluminum radiators. After installation, the boiler reported a "lack of flow".
The owner claimed that the pressure in the system was normal (2.0 bar). During the inspection, we found that the pressure before the filter was 2.0 bar, but immediately after the filter it dropped to 1.4 bar. The pressure drop was therefore 0.6 bar! This is catastrophic for modern gas boilers. The internal three-way valve in the boiler closed because it sensed too much pressure loss and the boiler disconnected.
The cause was that the customer used a 5-micron insert (too fine) and, in addition, the system already had settled dirt, which was released and immediately clogged this fine mesh. The solution was to replace the insert with a standard 20-micron one and to flush the system. The pressure drop decreased to 0.04 bar and the boiler ran smoothly. This example shows that a "new filter" does not necessarily mean a "functioning filter", if its capacity is not chosen correctly.
Factors affecting pressure drop in a new filter
Not every filter is the same. Even if both are new, they can have radically different effects on the system's hydraulics. Here we must analyze three main factors that determine what pressure will be after installation.
1. Cross-sectional size (filter diameter)
This is the most common mistake in design. If you have a DN32 (1 1/4 inch) pipe in the main supply to the boiler, you have no right to install a filter with a DN15 (1/2 inch) connection. Every reduction creates turbulence and increases resistance. For comparison, a 1/2" diameter filter has approximately four times the resistance of a 3/4" diameter filter at the same flow rate.
Therefore, we always choose a filter according to the pipe diameter, or one size larger if possible. You will find a wide range of options in our online shop, for example Filter 5" - 3/4"F; stainless steel insert - special offer, which offers not only the correct cross-section, but also resistance to mechanical damage. If you have DN25 pipe in your system, choose a 3/4" filter. If you have DN32, consider a 1" filter or at least a high-quality 3/4" filter with low resistance.
2. Type of filter insert (mesh vs. PP fiber)
This is where the biggest difference in pressure drop occurs. There are two main types of inserts used in these filters:
- PP (polypropylene) inserts: These are cylindrical inserts made of wound fibers. They work on the principle of deep filtration. They capture particles inside the insert. They have a higher initial resistance, but better capture fine dirt and sand. Resistance increases quickly with saturation.
- Stainless steel meshes: These are metal meshes with precise holes (e.g. 100 microns, 200 microns). Filtration is surface-based – impurities remain on the surface. Their initial resistance is lower and changes less over time, unless they are completely clogged.
For standard heating, where the goal is to protect pumps and boilers from coarse dirt, a stainless steel mesh is often recommended, as it has a lower pressure drop. If you need protection against very fine dirt that could clog valves, you will use a PP insert, but you must expect a higher initial pressure drop.
3. Temperature and viscosity of water
Theoretically, we should be talking about cold water, but in heating we work with hot water (60 - 80 °C). Water at higher temperatures has lower viscosity (it is "thinner"), which means it flows through the filter more easily. This is good news for hydraulics. However, if the filter is installed in cold water (e.g. in the supply of drinking water or in a system that is not yet heated), the resistance will be slightly higher. When installing a new filter, you should measure pressure at operating temperature, not in cold water, to get an accurate picture of the conditions in which the filter will work.
The graph illustrates how pressure drop changes over time. The PP insert (blue line) starts with higher resistance and increases quickly, as impurities settle deep in the fibers. The stainless steel mesh (red line) has stable resistance, which only increases dramatically when the mesh is completely clogged and stops letting water through. For long-term pressure stability in the system, a stainless steel mesh is often more advantageous, especially if you cannot replace the inserts regularly.
Pressure check procedure after installation: Step by step
Since the installation of a new filter is the final phase of assembly, it is important to follow a procedure to avoid errors. The following procedure has been verified on hundreds of installations and works flawlessly.
Step 1: Check the flow direction
Before you turn on the pump, check the flow direction. Most filters have an arrow on the body indicating the direction of water flow. If you install the filter backwards, it may cause deformation of the insert or the water may not pass through the filter, but bypass it (if the construction is such). This is a basic, yet often forgotten step. If you have Filter 5" - 3/4"F; PP insert - special offer, check that the arrow on the cover is pointing towards the boiler (if the filter is before the boiler) or away from the boiler (if it is a return filter).
Step 2: Filling the system and bleeding air
The new filter must be full of water. If there is air inside, it will cause excessive pressure and bubbling. Before turning on the pump, open the bleed valve on the filter cover (if available) and let the water flow out until it runs without air. Then close the valve. If you do not have a bleed valve, you must fill the filter carefully to avoid emptying the entire system.
Step 3: Measuring pressure before and after the filter
Turn on the pump and let the system run for 15 minutes to stabilize the flow. Then measure the pressure on the manometer before the filter (P1) and on the manometer after the filter (P2). If you do not have manometers on both sides, you can use a universal pressure gauge with a hose and connect it to both sides in turn (provided the system has accessible measurement points).
Calculate the difference: ΔP = P1 - P2.
- If ΔP < 0.05 bar: Everything is fine, the filter is clean and properly sized.
- If ΔP is between 0.05 and 0.15 bar: The filter is fine, but may be slightly dirty or this is a natural state for the given insert type. Monitor it.
- If ΔP > 0.15 bar: The filter is either clogged or improperly sized. You must open it and check the condition of the insert.
Step 4: Checking tightness and vibrations
When installing a new filter, especially if it is a threaded connection, it is important to check that the filter is not vibrating. If the filter vibrates, it means there is air in the system or the flow is unstable. Vibrations can cause connections to loosen and lead to leaks. Make sure all connections are tight, but not over-tightened, to avoid damaging plastic parts.
What to do if pressure drops after installation?
There is a situation that can be confusing. Some customers complain that after installing a new filter, the pressure in the entire system drops. It is important to distinguish whether it is a drop in the overall system pressure (absolute pressure), or a drop in pressure after the filter (differential pressure).
Scenario A: Drop in overall system pressure. If you have installed a filter and the pressure in the entire system has dropped by 0.2 bar, you have probably lost water during installation. A new filter has a volume that needs to be filled, and if you opened the system during installation, water escaped. The solution is to add water to the desired pressure (usually 1.5 - 2.0 bar). This is not a problem with the filter, but an error in filling the system.
Scenario B: Drop in pressure after the filter (high ΔP). If the pressure before the filter is stable (e.g. 2.0 bar), but the pressure after the filter is low (e.g. 1.4 bar), it means the filter is blocking the flow. As we have already mentioned, it may be a too fine insert or an incorrect diameter. In this case, it is necessary to open the filter and check the condition of the insert. If the insert is clean and the pressure drop is still high, the filter is likely undersized for your system.
In practice, I have seen a case where the installation of Filter 5" - 1/2"F; without insert into a system with a 1" diameter caused the pressure after the filter to drop so low that the pump "choked". The solution was to replace it with a filter with a 1" or 3/4" diameter and a larger cross-section.
Comparative table of resistance of different filters
To get a better idea of what pressure drop you can expect with different types of filters, I present the following table based on experimental measurements at a flow rate of 10 liters per minute (which is a typical flow rate in smaller households).
| Type of filter / Insert | Connection diameter | Initial pressure drop (ΔP) | Resistance at saturation (max) |
|---|---|---|---|
| PP insert 20 µm | 1/2" | 0.08 - 0.12 bar | > 0.5 bar (very high) |
| PP insert 20 µm | 3/4" | 0.04 - 0.06 bar | 0.3 - 0.4 bar |
| PP insert 20 µm | 1" | 0.02 - 0.03 bar | 0.15 - 0.2 bar |
| Stainless steel mesh 100 µm | 3/4" | 0.01 - 0.02 bar | 0.1 - 0.15 bar |
| Stainless steel mesh 200 µm | 3/4" | 0.005 - 0.01 bar | 0.05 - 0.1 bar |
From the table it is clear that the diameter of the filter has a decisive influence on the pressure drop. Increasing the diameter from 1/2" to 3/4" reduces the resistance by almost half. Therefore, it is always advisable to choose the largest possible diameter that fits into the installation space.
Hydraulic characteristic and operating point
The installation of a filter shifts the overall hydraulic characteristic of the system, which means that the pump has to operate at a lower flow rate. The diagram illustrates how the operating point (the intersection of the pump curve and the system resistance) changes after adding a new component.
In the graph you can see that the addition of a filter (red line) increases the overall resistance of the system. As a result, the operating point shifts from point A to point B – the pressure at the pump outlet slightly increases, but the flow decreases. Therefore, it is critical that this flow drop is negligible (usually less than 5 %), otherwise the circulation system will be underdimensioned.
Influence of pipe length before and after the filter
Hydraulic resistance is not determined only by the filter itself, but also by the way it is connected to the pipe. An incorrect distance from bends or valves can create turbulence, which increases the pressure drop above the theoretical value of the filter. For smooth flow, there should be a sufficient length of straight pipe before the filter and the filter should be placed so that the water does not strike the internal parts at a sharp angle.
The left part of the diagram shows the ideal condition, where the filter is placed in a sufficiently long straight section of pipe, ensuring laminar flow. The right part demonstrates an error when the filter is too close to a bend; this causes swirling water movement (turbulence), which dramatically increases the local pressure drop and may cause noise or material erosion.
Frequently Asked Questions (FAQ)
Can a filter cause a pressure drop in the entire system?
The filter itself does not cause a drop in the overall system pressure if it is properly filled with water. If the pressure drops after installation, it means that water was lost during installation (leak or air venting). It is sufficient to refill the system to the desired pressure. However, the filter does cause a local pressure drop (differential pressure) behind it, which is normal.
How often should I check the pressure after installing a new filter?
After installation, it is advisable to check the pressure immediately after startup, then after 24 hours (to allow the system to stabilize), and subsequently once a month during the heating season. If you notice that the pressure drop (the difference in pressure) starts to increase, it means the filter is clogging and it is time to replace the insert.
Which is better in terms of pressure: a stainless steel mesh filter or a PP insert?
To maintain stable pressure and minimal resistance, a stainless steel mesh is more suitable. It has a lower initial pressure drop and its resistance changes less dramatically over time. PP inserts have higher resistance but filter fine sediment more effectively. The choice depends on the water quality in the system.
What if the pressure after the filter drops to zero?
This is a critical situation. It means the filter is completely clogged or there is an air lock in the system. Immediately turn off the pump to avoid damaging the motor. Check whether the filter is filled with water and whether the insert is clogged. If the insert is clean, check whether the filter is installed incorrectly or whether a valve before the filter is closed.
Can the pressure drop change with a change in water temperature?
Yes, as the water temperature increases, its viscosity decreases, which means the pressure drop will slightly decrease. At 80 °C, the pressure drop will be lower than at 20 °C, but the difference is usually negligible (about 10-15%). The main factor remains the condition of the insert and the pipe diameter.
How do I know if the pressure drop is too high for my pump?
Most modern circulation pumps can overcome a pressure drop of up to 0.5 to 0.6 bar without problems. If your pressure drop is higher than 0.2 bar on a clean filter, it is a sign that the filter is either undersized or incorrectly selected. In such a case, the pump will be operating at the edge of its capabilities, which shortens its lifespan and reduces the efficiency of heating.
Conclusion: Pressure is the key to efficient filtration
Installing a new filter is a simple task, but its success depends on attention to detail, especially regarding the system hydraulics. The correct pressure after installing a new filter should be in the range of 0.02 to 0.05 bar differential pressure. If these values differ, it is necessary to find the cause—whether it is incorrect sizing, insert type, or an installation error.
Remember that a filter is not just a passive element that "catches something." It is an active component of the hydraulic circuit that affects the performance of the entire system. Choosing the right filter, such as Filter 5" - 3/4"F; stainless steel insert - special offer, together with careful pressure monitoring, ensures that your heating system will operate smoothly, efficiently, and without unnecessary faults. An investment in quality filtration pays off in the form of a longer boiler lifespan and lower pump repair costs. Do not neglect pressure, as it tells you more about the health of your system than any other check.
Do you have a question about this topic?
Not sure or dealing with a specific situation in your home? Write to us—we are happy to help.