For a refillable Siliphos housing that opens downward — the familiar bowl-and-cartridge design — the space below the housing must be at least the full vertical drop of the removable bowl, plus working room for hands, tools, and a container to catch water and spent media. A practical planning rule is bowl length + 100–150 mm. That puts a typical 10-inch housing at roughly 350–400 mm (14–16 in) of clear space beneath it, and a typical 20-inch housing at roughly 550–650 mm (22–26 in).

Those figures are planning estimates based on common bowl dimensions and normal service practice — they are not manufacturer specifications. The binding number is always the dimension drawing for the exact housing model you own or intend to buy, combined with the way your piping is arranged. This article explains how to work that number out properly, what else the clearance has to accommodate, and what to do when the space is already too tight.

What the clearance under the housing actually has to cover

Installers often think of “clearance” as a single measurement. In practice it is the sum of three separate requirements, and only the first one is truly non-negotiable.

1. Drop height — the hard limit

Measure from the sealing face of the housing head (where the bowl threads or the O-ring seat begins) to the lowest point of the bowl, including any drain plug or sump cap. When you unscrew the bowl, that entire length has to travel straight down before it clears the head. If the bowl hits a floor, a shelf, a pump, or a pipe run before that point, you cannot open the housing at all — regardless of how much room your hands have.

If the housing uses a refillable cartridge that slides out of a fixed bowl, the drop requirement is the same, because you still need to remove the bowl to reach the cartridge. Only housings designed for top access avoid this.

2. Working height — for hands and tools

Once the bowl is free, you still need to grip it, tip it, and — in most designs — hold it steady while you unscrew or pry out the refillable cartridge. Add 100–150 mm for this in normal installations. Add more if the housing sits in a plant room where the operator will be wearing gloves, or if the bowl is heavy when full of wet media.

3. Catchment height — for water and spent media

Opening a housing always releases some water, and refilling usually spills a little crystal. A shallow tray or bucket needs 80–150 mm of vertical space. You can reduce this requirement by fitting a drain valve and hose, or by placing the housing over a floor drain or a bunded tray, but you cannot reduce it to zero in a finished space.

How to calculate the figure for your installation

1. Find the drawing. Get the dimensioned drawing or the physical bowl length for the exact housing model. Do not scale from a photo or assume that a “10-inch” housing measures 10 inches externally — nominal sizes refer to the cartridge, not the bowl.
2. Measure the drop. Identify the lowest point of the bowl when installed, including drain fittings, and measure vertically to the finished floor or to the nearest obstruction.
3. Add working and catchment allowance. Add 100–150 mm for hands and tools, and a further 80–150 mm if you will catch water in a tray rather than drain it away.
4. Check the diagonal. If the bowl must be tilted during removal, or if the housing is mounted on a swing bracket, measure the arc it sweeps, not just the straight-down distance.
5. Verify horizontal access too. You need room to rotate the bowl and to reach the inlet and outlet unions. A housing with generous vertical clearance but piping pressed against its side is still awkward to service.

Worked planning examples

The bowl-drop column reflects commonly seen dimensions for each housing class. Confirm the real value on the drawing before you commit to a mounting height, because bowl depths vary between manufacturers and between pressure ratings.

When you do not need clearance below the housing

Not every refillable housing is serviced from underneath. Clearance below is largely irrelevant in these cases:

• Top-fill vessels. The head is opened from above and media is poured in. Here you need headroom above the vessel, and only enough room underneath for the drain valve and for lifting the vessel off its bracket if it ever has to be replaced.
• Refillable cartridges with a quick-release head. Some designs let you release the whole bowl assembly sideways once the inlet and outlet are isolated. Check whether the manufacturer permits this; many do not, because of the risk of stressing the connections.
• Housings mounted over an open drain or trench. The physical drop is still required, but the catchment allowance disappears.
• Duplex or parallel installations. If one housing can be isolated and the other left in service, the servicing space can be shared between them — provided the layout still lets you remove one bowl without disturbing the other.

If the clearance is already too tight

This is common in retrofit work, where a housing was fitted into an existing plant room. The options, roughly in order of cost:

• Re-site the housing. Move it higher on the wall or to a different section of pipe. Usually the cleanest fix if pipework allows.
• Rebuild the connection with a flexible section. A short flexible connector or a union with a spool piece lets the bowl be removed once the spool is taken out. Confirm that the flexible section is rated for the system pressure and temperature, and that it does not create a low point where debris collects.
• Switch to a top-access design. If the space below cannot be changed but the space above can, a top-fill refillable vessel may solve the problem.
• Use a different media format. Where a refillable housing genuinely cannot be accommodated, a pre-filled cartridge or a different treatment approach may be more practical. This changes the maintenance routine and the cost profile, so treat it as a system decision rather than a like-for-like swap.

Do not simply mount the housing so low that the bowl can only be partially unscrewed. Forcing a bowl off at an angle distorts the threads and the O-ring seat, and a housing that leaks at the seal is a far bigger problem than a cramped plant room.

Other installation factors that affect mounting height

• Flow direction and bypass. A bypass valve set adds pipework and handles around the housing. Plan the clearance around the completed assembly, not just the housing body.
• Isolation valves. You need to be able to close the inlet and outlet before opening the bowl. If the valves are behind the housing, clearance alone will not make the job practical.
• Freeze protection. Housings in unheated spaces need insulation and, in some cases, drainage during cold periods. Insulation increases the effective bowl diameter.
• Support. A large bowl full of wet media is heavy. The bracket or mounting point, not the pipework, should carry the weight.
• Water hammer. Downstream of a pump or a quick-closing valve, surge can stress the housing head. Follow the housing manufacturer’s installation guidance on this point.

Refilling the housing: what the clearance is really for

Understanding the refill routine makes the clearance requirement easier to judge. In a typical refillable housing:

1. Isolate the housing and, if a drain is fitted, release pressure and drain the bowl.
2. Unscrew the bowl and lower it clear of the head.
3. Remove the refillable cartridge or basket and empty the spent media into a container.
4. Rinse the cartridge and refill it with fresh phosphate crystal, following the media supplier’s fill guidance. Overfilling can reduce the effective surface area exposed to flow; underfilling can cause channelling.
5. Check the O-ring, refit the cartridge, and screw the bowl back on hand-tight as instructed.
6. Open the inlet slowly, check for leaks, then restore full flow and confirm that downstream flow and pressure are normal.

Steps 2 and 3 are the ones that set the minimum clearance. Everything else can be done with less room.

Choosing the media that goes in the housing

Housing clearance and media selection are separate decisions, but they are often made together. It helps to be precise about what each material does, because the terms are frequently mixed up.

Note also that “Siliphos” is a trade name. Phosphate crystal products sold under different names can differ in composition, dissolution rate, and recommended applications, so performance data from one product cannot be transferred to another. Ask for the technical data sheet for the specific grade you intend to use.

If the treated water is intended for drinking or food preparation, food-grade status alone is not sufficient evidence of suitability. Potable-water approval depends on the exact product, the dosage, the contact time, and the regulations of the destination market, so this must be verified for each case rather than assumed.

Frequently asked questions

Is there a standard clearance figure for refillable housings?

No universal figure exists, because bowl lengths differ between models and manufacturers. The reliable approach is to take the bowl drop from the housing’s dimension drawing and add 100–150 mm of working room plus space for a catchment container.

Can I open the bowl if it only clears the floor by a few millimetres?

Technically the bowl may drop free, but you will have no room to grip it, tip it, or catch the water. In practice, a few millimetres of clearance means the housing cannot be serviced properly, and the seal is likely to be damaged during repeated awkward refills.

Do top-fill phosphate vessels need clearance below?

Not for refilling. You need headroom above the vessel instead. Below the vessel, allow enough space to reach the drain valve and to lift the vessel off its mounting if it needs replacement.

Does adding a drain valve reduce the clearance I need?

It reduces the amount of water you have to catch, so you can remove the catchment allowance. It does not reduce the drop height or the working room, and the drain fitting itself may extend the lowest point of the assembly.

What information should I have ready before ordering media?

The housing model and bowl size, the source-water analysis (hardness, pH, and any relevant ions), the target flow rate, the treatment objective, and the intended end use of the water. These determine whether a phosphate crystal is appropriate at all, and which grade fits the application.

Getting the media specification right

Clearance is a mechanical question with a mechanical answer: measure the bowl, add working room, and check the drawing. The media side needs the same discipline. ENVO CHEMICAL supplies phosphate crystal and related water-treatment materials, and can discuss which grade suits a given housing and water condition once the application details are clear. If you are specifying a refill, send us the housing type and bowl dimensions, your source-water analysis, the target flow rate, your treatment objective, the required quantity, and the destination market — with those details we can respond on the specific material rather than on general assumptions.