What diameter of a cast iron filter do I need for my water supply
What diameter of a cast iron filter do I need for my water supply system?
This question is among the most common ones we address with customers even before placing an order. At first glance, the answer seems simple – after all, it's enough to measure the pipe, right? In practice, however, it can be more complicated. The diameter of the cast iron filter directly determines whether your water supply will function correctly, what water pressure and flow you will achieve in your household, and ultimately whether the entire installation will be reliable and last for many years. In this article, we will look at the topic of the diameter of the cast iron filter in great detail – from basic terminology through selecting the correct size to typical mistakes people make in practice.
Why the diameter of the filter matters more than many people think
A cast iron filter is not just a passive obstacle in your water supply. It is an active element that affects the overall hydraulic resistance of the system. If you choose a diameter that is too small, the filter becomes a so-called throttling point – pressure loss increases, water flow decreases, and the pump in your home water supply will work under greater load, which shortens its lifespan. On the other hand, if you choose a diameter that is too large, the filter will not fit hydraulically well in the system, the threads may not connect properly, and the entire installation will be aesthetically and functionally unsatisfactory.
From experience, I know that a large portion of problems with home water supplies – pressure drop, pump noise, low flow at multiple points simultaneously – stem precisely from an incorrectly chosen diameter of the filter element. That is why we are dedicating so much attention to this topic, and also in the next article How to choose a cast iron filter for a home water supply.
Basic terminology: DN, inches, external and internal thread diameter
Before we proceed to selecting a specific size, we need to clarify the terminology. In the field of water installation and filtration, we encounter several systems for marking the diameter, which can be a source of confusion even for experienced installers.
DN – nominal diameter
DN (Diameter Nominal) is a European standard designation that expresses the nominal internal diameter of the pipe in millimeters. It is a rounded value, so DN 25 does not mean that the internal diameter of the pipe is exactly 25 mm – it could be, for example, 26.4 mm or 24.3 mm depending on the material and standard. For domestic filtration, the following values are most important:
- DN 15 – corresponds to a 1/2" thread, typical for apartment water supplies and smaller households
- DN 20 – corresponds to a 3/4" thread, most common in family homes
- DN 25 – corresponds to a 1" thread, suitable for larger family homes, garden water supplies, and home water supplies with higher performance
- DN 32 – corresponds to a 1 1/4" thread, used in larger installations
- DN 40 – corresponds to a 1 1/2" thread, commercial and semi-industrial applications
- DN 50 – corresponds to a 2" thread, industrial applications, larger facilities
Inch system (BSP / NPT threads)
In practice, the inch system BSP (British Standard Pipe) is commonly used in the Slovak and Czech environments. Here is a trap to be aware of: the inch in the thread designation does not refer to the external diameter of the thread, but to a historically established value of the internal diameter of the pipe around which the thread was developed. Thus, a 1" thread (G 1) has an external thread diameter of 33.249 mm, not 25.4 mm (which is the actual one inch). This mistake is also made by experienced tradesmen, who measure the thread with a caliper and the result does not match the filter designation.
What to measure: external thread diameter
To correctly determine the filter diameter, measure the external diameter of the thread on your existing pipe or valve. The following table will help you convert the measured value to the correct designation:
| External thread diameter (mm) | Thread designation (BSP/G) | DN (nominal diameter) | Typical use |
|---|---|---|---|
| ~9.7 mm | 1/4" | DN 8 | appliances, pressure switches |
| ~13.2 mm | 3/8" | DN 10 | refrigerators with ice makers, condensers |
| ~20.9 mm | 1/2" | DN 15 | apartments, small households, one branch of the system |
| ~26.4 mm | 3/4" | DN 20 | family homes, garden water supplies |
| ~33.2 mm | 1" | DN 25 | larger family homes, home water supplies |
| ~41.9 mm | 1 1/4" | DN 32 | large family homes, small facilities |
| ~47.8 mm | 1 1/2" | DN 40 | commercial facilities, hotel equipment |
| ~59.6 mm | 2" | DN 50 | industry, large facilities |
How to determine the correct filter diameter for your specific system
The selection of the diameter of a cast iron filter depends on several basic parameters. Let's go through them step by step, as I do for every customer.
Step 1: Determine the diameter of the existing pipe or fitting
The most important step – and the most common mistake at the same time. People often measure the outer diameter of the pipe (not the thread!), compare it with a table, and get the wrong result. That's why I emphasize: always measure the external diameter of the threaded part, not the pipe body itself.
If the pipe is threadless and you are doing a renovation, measure the outer diameter of the pipe and check the relevant standard for the material (steel, copper, PE-X, PPR) to find out what thread is typically cut on it. For example, the outer diameter of a 22 mm copper pipe corresponds to a 3/4" thread. On a steel pipe DN 25, a 1" thread is cut.
Step 2: Determine the capacity of your home water supply (Q max)
Every pump or home water supply has a maximum flow rate Q in m³/h or l/min stated in the technical documentation. This data is key. A simple rule of thumb from practice:
- Pumps up to 1.5 m³/h (25 l/min) – a 3/4" or 1" filter is usually sufficient
- Pumps 1.5 – 3 m³/h (25 – 50 l/min) – we recommend a 1" filter
- Pumps 3 – 5 m³/h (50 – 83 l/min) – a 1 1/4" or 1 1/2" filter
- Pumps over 5 m³/h – a 1 1/2" or 2" filter, or even a pair of filters
The reason is simple: every filter has a pressure loss (ΔP) at a certain flow rate. Manufacturers state this in technical documentation for various flow rates. The larger the filter diameter, the lower the pressure loss at the same flow rate. For a home water supply, an acceptable pressure loss on the filter is typically up to 0.3 – 0.5 bar at the maximum flow rate of the pump.
Step 3: Take into account the number of draw-off points and simultaneous draw-off
This is a parameter that people often underestimate. If you have a family house with two bathrooms, a kitchen, a garden, and a washing machine, the actual simultaneous draw-off during a morning peak (when people are showering, cooking, and the washing machine is running) can easily be 1.5 – 2.5 m³/h. Based on this, I recommend:
- 1 – 2 people, apartment or cottage: 1/2" or 3/4" filter
- 3 – 4 people, family house, basic equipment: 3/4" or 1" filter
- 4 – 6 people, larger family house, garden, washing machine, dishwasher: 1" filter
- More draw-off points, garden irrigation system, small business: 1 1/4" filter
- Guest house, small production, intensive irrigation: 1 1/2" or 2" filter
Filter diameter versus pipe diameter – a crucial mistake in practice
Many customers believe that the filter diameter must be the same as the diameter of their main distribution pipe. This is only partially true. The actual rule is: the filter diameter should not be smaller than the pump discharge nozzle diameter, ideally it should be the same or one size larger.
Why? Because the filter is a hydraulic obstacle. The larger its flow cross-section (and thus the larger the diameter), the lower the pressure loss. In practice: if you have a pump with a 1" discharge and install a 3/4" filter after it, you will immediately lose an additional 0.3 – 0.8 bar of pressure compared to a 1" filter. You will notice this at the end of the distribution system as weaker water flow in the shower or slower filling of a garden tank.
On the other hand, if you have a pump with a 3/4" discharge and install a 1" filter, you won’t cause any damage – the pressure loss will be lower, although you will naturally have to use reducers, which is a small extra cost. In this case, however, it is a reasonable investment for the future, for example, if you plan to replace the pump in the near future with a more powerful one.
Concrete examples from practice
Example 1: Older family house, well, single-pump domestic water supply
The customer had an old domestic water supply system with a 3/4" discharge nozzle and steel pipes. He installed a cast iron filter of 1/2", because he thought the pipes in the house were thinner. Result: after filtration, the pressure dropped from 3.8 bar to 2.9 bar, showers were weak, and the pump was switching on every 30 seconds (short cycling). After replacing the filter with the correct size of 3/4", the problem immediately disappeared. The mistake was inaccurate measurement – steel pipes DN 20 (3/4") have an outer diameter of ~26.9 mm, but with insulation around it, the customer mistakenly measured a different diameter.
Example 2: New construction, submersible pump, PE-X distribution
The customer had new PE-X distribution pipes with a diameter of 25 mm (corresponding to a 3/4" thread for PE-X in a push-fit system). The pump had a 1" discharge nozzle. The correct choice was a 1" filter with reducers to 3/4" on both sides – this preserved the full flow through the filter and the reduction was only for connecting to the existing distribution system. The customer originally wanted the opposite (a 3/4" filter), which would have resulted in unnecessary pressure loss.
Example 3: Garden distribution, irrigation system, larger garden
A guesthouse with a large garden (approx. 2000 m²) needed a garden distribution system with filtration for the irrigation system. The pump had a Q max = 4.5 m³/h. We used a cast iron filter of 1 1/4" – a smaller filter would unnecessarily restrict the system and in summer peak during intensive irrigation, the pressure would have been insufficient. It was also important that the cast iron filter in this size can handle a higher operating pressure (typically 10 – 16 bar) compared to plastic filters of the same diameter.
Example 4: Apartment building, inlet filtration into the building
During the renovation of an apartment building (8 apartments, approx. 20 people), there was a requirement to install a central cast iron filter after the main shut-off valve. The main supply was DN 40 (1 1/2"). A filter of the same size was the correct choice – it ensured filtration for the entire building without restricting the flow during simultaneous consumption from multiple apartments.
Pressure loss on the filter: a technical view
For technically knowledgeable customers – and for those who want to really understand what they are buying – here are approximate values of pressure losses on cast iron filters depending on diameter and flow. These values are typical for filter inserts with a mesh of 100 – 150 microns (standard filtration of coarse impurities).
| Filter diameter | Flow 0.5 m³/h | Flow 1 m³/h | Flow 2 m³/h | Flow 4 m³/h |
|---|---|---|---|---|
| 1/2" (DN 15) | ~0.15 bar | ~0.55 bar | — | — |
| 3/4" (DN 20) | ~0.06 bar | ~0.20 bar | ~0.70 bar | — |
| 1" (DN 25) | ~0.02 bar | ~0.07 bar | ~0.25 bar | ~0.90 bar |
| 1 1/4" (DN 32) | <0.01 bar | ~0.03 bar | ~0.10 bar | ~0.35 bar |
| 1 1/2" (DN 40) | <0.01 bar | ~0.02 bar | ~0.05 bar | ~0.15 bar |
Note: The values given are approximate for a clean filter insert. With a clogged insert (after a few weeks of operation), pressure losses can be 2 – 5 times higher. This is another reason to choose a filter one size larger than minimally necessary.
Reductions: when the filter diameter does not match the piping
Very common situations in practice: the customer has a 3/4" piping system, but wants to install a 1" filter (for lower pressure loss and future reserves). Or the opposite – existing pipe flanges are 1", but the filter in this size is out of stock and the customer is considering a smaller size. Both situations can be solved with reduction couplings and reducing nickel-plated or brass fittings with threads.
The following rules apply when using reductions:
- Reduction from smaller to larger (before the filter): hydraulically advantageous, the cross-sectional area increases – minimal pressure loss
- Reduction from larger to smaller (after the filter): less hydraulically advantageous, but negligible for short pipe runs (pipe length up to 20 cm)
- Never use a reduction before the filter from a larger to a smaller diameter unless absolutely necessary – it unnecessarily increases the resistance of the system
- Always seal reductions with a sealing tape (flax/hemp rope) with sealing compound or PTFE tape – cast iron filters have BSP (parallel) threads, not NPT (tapered) threads
The topic of installation and sealing details is discussed in more detail in the article Installation of a cast iron filter on a home water supply step by step, where you will also find the procedure for sealing threads on cast iron.
Special situations when choosing the diameter
Well with submersible pump
Submersible pumps are usually connected to a discharge hose or PEX pipe, which has a different dimension system. The pump flange itself is usually 1" (thread), but the discharge pipe can be PEX 25 mm or 32 mm. In this case, it is first necessary to determine what thread is on the pump discharge flange (usually 1" external thread), and choose the filter accordingly – usually 1" internal thread (G 1" F).
Water supply connection from the public network
When connecting to the public water supply, the filter diameter is determined by the diameter of the water supply connection, which was installed by the water utility company. Standard household connections are usually DN 25 (1") or DN 32 (1 1/4"). The filter diameter should be identical to the connection diameter – reducing the diameter of the filtration is undesirable and in some cases may even be in conflict with the requirements of the water utility company.
Irrigation systems
For garden irrigation, it is important to consider not only the pump flow rate, but also the pressure range in which the irrigation system should operate (typically 2.5 – 4.5 bar). Cast iron filters are advantageous for garden applications mainly because they can withstand UV radiation without degradation (unlike plastic ones) and have a long service life in outdoor installations. More about material comparisons can be found in the article Cast iron filters vs. plastic water filters – what is better.
Hard water and limescale deposition
If you have hard water in your area (hardness above 2.0 mmol/l), keep in mind that limescale will also deposit inside the filter and on the filter insert. This means that the actual free passage of the filter will gradually decrease and the pressure loss will increase faster than it would with clean water. In such cases, I recommend choosing a filter one size larger than the minimum required. This issue is discussed in a separate article Cast iron filters for hard water – protection against limescale.
Internal vs. external filter thread – how it all fits together
Cast iron filters are produced in two basic variants of threaded ends:
- Internal thread (G – Female): pipe or fittings with external threads are screwed onto both ends. This is the standard and most common version for single-family homes and home water supplies.
- External thread (G – Male): less common, used in specific configurations or when connecting to manifolds.
When I refer to a 1" filter diameter, I usually mean a filter with internal G 1" thread on both the inlet and outlet. Pipe, valves or reductions with external G 1" thread are screwed onto this filter. The direction of flow must be observed – the correct direction of water flow (from inlet to outlet) is usually indicated by an arrow on the filter. If the filter is connected in the reverse direction, it will not function properly and damage to the filter insert is possible.
Most frequently asked questions (FAQ)
Can I use a filter with a smaller diameter than my piping, if I use reductions?
Technically yes, but it is not hydraulically suitable. Reducing the filter diameter compared to the piping unnecessarily increases the pressure loss, limits the maximum flow and loads the pump. In case of emergency (temporary solution, spare part) it is acceptable, but I do not recommend it as a permanent solution. You will save a few euros on the filter, but you will pay for it with higher pump energy consumption and reduced pump lifespan.
What filter diameter do I need if my pump has a 3/4" discharge, but I want to have a reserve for the future?
A very reasonable question. In this case, I recommend installing a 1" filter right away – with reductions G 1" → G 3/4" on both ends. The additional cost for the reductions is minimal (a few euros), but if in a few years you replace the pump with a more powerful one with a 1" flange, you will not need to change the filter – just unscrew the reductions.
I measured the external thread diameter of my valve and got 33.5 mm. What filter should I buy?
An external thread diameter of 33.2 – 33.5 mm corresponds to a G 1" thread (BSP). Your cast iron filter should have an internal G 1" thread (also marked as G 1" F or G 1" IG). This is one of the most commonly sold sizes for home water supplies and larger single-family homes.
Are cast iron filters also available with NPT thread (American standard)?
On the European market, filters with BSP (G) thread are dominant. NPT thread is tapered and mainly used in the American environment. Although you can find BSP/NPT adapters, I do not recommend them for pressurized water installations – the risk of leakage is higher due to the different thread geometry. For the Slovak market, always choose a filter with BSP (G) thread.
I have an apartment in an apartment building with central filtration. Is one filter enough or should there be one for each apartment?
For an apartment building, it is standard to have one central filter at the inlet after the main shut-off valve – with a diameter corresponding to the riser or horizontal distribution pipe (typically DN 32 – DN 50). Individual filters in the apartments are then an optional addition for fine filtration (e.g., before the kitchen faucet). In this case, the central cast iron filter performs coarse filtration and protects the entire network, while individual filters address fine filtration for specific users.
How can I find out whether my existing cast iron filter has the correct diameter and is not causing pressure problems?
The simplest test: install pressure gauges before and after the filter (or use one portable pressure gauge with a ball valve for a T-piece). Measure the pressure before and after the filter at maximum water consumption (all outlets open). The difference (pressure loss) should not exceed 0.3 bar with a clean insert. If it is higher, the filter is either clogged (solution: clean the insert – procedure in the article Cleaning and maintenance of a cast iron water filter) or the filter diameter is too small for your piping system.
Conclusion: main rules for choosing the diameter of a cast iron filter
Choosing the correct diameter of a cast iron filter is not complicated if you proceed systematically. Let's summarize the key points that are the result of experience from dozens of similar installations:
- The filter diameter should be the same as the pump discharge flange diameter, ideally one size larger
- Never install a filter smaller than the pump flange – pressure loss increases quadratically with flow
- When in doubt between two sizes, always choose the larger one – pressure loss will be lower and the filter will stay clean longer
- Always measure the external thread diameter, not the pipe body or insulation
- For a standard single-family home with a home water supply, a 1" filter (DN 25) is the most common and suitable choice
- Reductions are acceptable, but never as a permanent diameter reduction – only for connecting to a different sized piping system
- Consider water hardness and contamination – for hard or heavily contaminated water, choose a larger diameter and plan for more frequent maintenance
- For commercial and semi-industrial applications, consult a specialist – a poorly designed filtration system can significantly increase operating costs