In short: if water keeps running into the toilet from your Grohe concealed cistern, the cause is almost always the seal on the flush valve or the fill valve (the float valve). You can usually replace either one yourself through the access opening behind the flush plate, with no building work. First, though, you need to work out which cistern family you have, because Grohe spare parts are not interchangeable between models.
In this guide:
- How a Grohe concealed cistern is built
- How to identify your cistern model
- Water keeps running into the toilet: causes and fixes
- The cistern won't flush or the button does nothing
- How to replace the flush valve
- How to replace the fill valve
- Other useful spare parts
- When to call a plumber
- FAQ
How a Grohe concealed cistern is built
A concealed cistern is a tank hidden inside the wall (or a stud or frame wall) and connected to the toilet. Everything you need for maintenance is reached through the access opening behind the plate with the flush buttons. Here is what you will find:
- Flush plate: the buttons you press to flush. It covers the access opening.
- Flush valve: the central mechanism that lifts when you press the button and releases water into the bowl. At the bottom it has a seal that keeps the cistern closed.
- Fill valve (float valve): refills the cistern after flushing and shuts off when the water reaches the right level.
- Isolation valve: shuts off the water supply to the cistern. Close it before any work.
- Lever or pneumatic tube: carries the push of the button to the flush valve. Older cisterns use a mechanical lever; newer ones often use a thin air tube.
- Flush pipe connector: the pipe that connects the cistern to the toilet.
- Flow restrictor: a small accessory, not always fitted, that reduces the force of the flush.
How to identify your Grohe cistern model
This is the most important step: if you buy the wrong part, it simply won't fit. Over the years Grohe has made very different cisterns, which fall into three families.
The cistern number
Start by looking for the cistern number (for example 37051, 37594 or 38863). It may be on a label, a sticker or in the installation paperwork. Where exactly depends on the model, so remove the plate and take a careful look inside the access opening too. If there is no number, or it is no longer readable, a few visual clues will tell you the family.
Old single-flush cistern
This is the Grohe/DAL 37051 series cistern, 6 litres, fitted in many bathrooms in past decades. It usually has a single button, and inside the opening you will see a mechanical lever that lifts a "plunger" valve: an upright tube with a rubber seal at the bottom. Typical flush plates for this family come from the Classic and Colani ranges. If you are searching for an "old Grohe flush valve", in most cases you need a part for the 37051.
Dual-flush cistern with a large plate
This is the family of the 37594 cistern: it has two buttons (full and reduced flush) and a large plate of about 330 × 190 mm. The buttons often operate the valve through two tubes. A typical plate for this cistern is the Grohe Surf G 37859SH0 flush plate (if it is out of stock, ask us about availability).
GD2, Rapid SL and Uniset cisterns
These are the more recent cisterns, such as the GD2 38863000 cistern, often fitted inside a frame (Rapid SL) or a module built into the wall (Uniset). They have two buttons and the AV1 valve, operated by a single pneumatic tube or mechanically. The flush plates can also be small.
| If you see… | You probably have… |
|---|---|
| One button, a mechanical lever and a plunger valve with a rubber seal | The old single-flush cistern (37051 series) |
| Two buttons, a large plate of about 330 × 190 mm, often operated by two tubes | The dual-flush cistern with a large plate (37594 series) |
| Two buttons, an AV1 valve, a single pneumatic tube or mechanical operation | A GD2, Rapid SL or Uniset cistern |
If you can't identify it
Take photos of the inside of the opening (valve, lever or tubes, fill valve) and of the flush plate, then compare them with the spare-part pages or with Grohe's official spare-parts finder. Still unsure? Send the photos to our customer service team before you order.
Water keeps running into the toilet: causes and fixes
A constant trickle into the bowl is the most common fault, and it is almost always fixed by replacing a single part. Start from the symptom:
| Symptom | Likely cause | What to replace |
|---|---|---|
| A thin stream of water runs into the bowl all the time | Flush valve seal worn, deformed or scaled up | The seal or the whole flush valve |
| The cistern keeps refilling, hisses, or water runs down the overflow | Fill valve not shutting off (dirty or damaged diaphragm, float set wrongly) | The fill valve |
| Water escapes between the toilet and the wall when you flush | Flush pipe connector or its seal damaged or badly fitted | The flush pipe connector or its seal |
A quick way to find the leak: close the isolation valve and watch. If the level in the cistern drops while water keeps running into the bowl, the flush valve is the problem. If the running stops straight away, the fill valve is more likely not shutting off properly.
Limescale is often the real culprit, because it hardens the seals. Don't pour acid into the cistern: it damages seals and plastic parts. To understand when acid makes sense and when it doesn't, read our guide to muriatic acid in the toilet and the alternatives.
If the leak is between the toilet and the wall, the part you need is the Grohe flush pipe connector for concealed cisterns. Before ordering, check that it fits both your cistern and your toilet.
The cistern won't flush or the button does nothing
If you press the button and nothing happens, or the flush only half works, the problem is almost always in the link between the plate and the valve.
Old single-flush cistern: the actuation lever
In the 37051 series the button pushes a plastic lever that lifts the valve. Over the years the lever cracks or comes unhooked, and the button does nothing. Remove the plate and check: if the lever is broken, the part you need is the Grohe 43896000 flush actuation lever (if it is out of stock, ask us about availability).
Cisterns with an AV1 valve: tube and valve
On pneumatically operated GD2, Rapid SL and Uniset cisterns, the button pushes air down the tube, which lifts the valve. Check that the tube is firmly connected at both the plate and the valve, and that it isn't kinked or split. If the tube is fine but the valve sticks or flushes weakly, limescale may be blocking it: clean or replace it as described below.
The flush is too weak or too strong
On cisterns with an AV1 valve the flush volume can be adjusted on the valve itself; check the Grohe instructions for your model before changing the setting. If the flush is so strong that water splashes out of the bowl, see the tip on the flow restrictor below.
How to replace the flush valve, step by step
The basic procedure is similar for every family. You will usually need a screwdriver, a cloth or sponge and, if there is limescale, a soft brush.
- Close the isolation valve inside the cistern opening.
- Empty the cistern by pressing the button, and soak up the remaining water with a sponge.
- Remove the flush plate and, if there is one, the mounting frame, following the instructions for your model.
- Unhook the lever or tube from the valve.
- Take out the flush valve: it usually releases from its seat with a twist or by unclipping, then lifts out through the opening.
- Clean the seat at the bottom of the cistern, removing limescale and dirt without scratching it.
- Fit the new valve and lock it into the seat.
- Reconnect the lever or tube and refit the plate.
- Turn the water back on, let the cistern fill and flush several times, checking that nothing runs into the bowl afterwards.
Old single-flush cistern
For the 37051 cistern the part you need is the Grohe 42961000 flush valve, complete with overflow pipe. If the valve itself is fine and only the rubber seal at the bottom leaks, the seal is also available on its own (number 43917). Watch out: cisterns from the 1980s use a different valve (number 43992), so check your cistern number before ordering.
Dual-flush cistern with a large plate
The 37594 cistern uses its own valve (number 42253). The valve from the old single-flush cistern does not fit, so don't mix them up.
GD2, Rapid SL and Uniset cisterns
Here the part you need is the Grohe AV1 42320000 flush valve. It comes complete with a pipe extension and has an adjustable flush volume (Grohe states 6–9 l), but it does not include the valve seat. For GD2 cisterns built before 2008, Grohe offers a retro-fit set that includes the seat (number 38736000): check which of the two your model needs.
How to replace the fill valve (float valve)
It is worth replacing the fill valve when the cistern hisses for a long time, fills very slowly or never stops filling. Sometimes descaling the diaphragm is enough, but if the valve is old, replacing it is the safer option.
- Close the isolation valve and empty the cistern.
- Remove the plate and disconnect the supply hose from the fill valve.
- Release the old valve from its holder and lift it out through the opening.
- Fit the new valve and reconnect the supply. Check that the connection on the isolation valve matches the new fill valve.
- Turn the water on and check that the cistern stops at the right level. If needed, adjust the float following the part's instructions.
The same rule applies as for flush valves: check your cistern number before you buy. The Grohe 43537000 fill valve (if it is out of stock, ask us about availability) is designed for 6-litre concealed cisterns, such as the Uniset 37588 and 37582 and the 37050 and 37049 cisterns. It is not suitable for the old 37051 cistern, which uses a different fill valve (number 37092).
Other useful spare parts: flow restrictor, connector, flush plate
Flow restrictor. A practical tip: if your toilet splashes every time you flush – a rimless pan, for example – the Grohe 42300000 flow restrictor reduces the flow from the AV1 valve. It fits into the valve seat, with no need to change the cistern.
Flush pipe connector. If the leak is at the joint between cistern and toilet, go back to the section on running water and leaks, where you will find the part and what to check before ordering.
Flush plate. If the plate is damaged or the buttons stick, choose a plate that matches your cistern family. For GD2, Rapid SL and Uniset cisterns with the pneumatic AV1 valve there are the Skate Cosmopolitan plates in white or chrome. They are not suitable for the other two families. You will find all our plates in the toilet flush plates collection.
When to call a plumber
Most faults can be fixed through the plate opening, but in some cases it is better to get help:
- You see damp patches or stains on the wall around the cistern: the leak may be inside the wall, not in the bowl.
- The cistern is cracked or leaks from its internal connections.
- The isolation valve is seized and you can't shut off the water.
- You can't identify the model and the parts you find don't match what you see.
If the cistern is very old and parts are no longer available, consider a new one, such as the Grohe concealed cistern for 80 mm walls, or choose from our other toilet cisterns. In that case it is best to use a professional installer. If you are changing the toilet as well, our step-by-step toilet installation guide will help you avoid the most common mistakes.
FAQ
How do I tell which model my Grohe concealed cistern is?
Look for the cistern number (for example 37051, 37594 or 38863) and check the mechanism. Grohe cisterns fall into three families: the old single-flush cistern, with a mechanical lever and plunger valve; the dual-flush cistern with a large plate; and the GD2, Rapid SL and Uniset cisterns with the AV1 valve. The spare parts differ from one family to another.
Why does my Grohe concealed cistern keep running into the toilet?
Usually the seal at the base of the flush valve is worn or scaled up, so water gets through even when the cistern is closed. On old single-flush cisterns the seal is also sold on its own. If instead the cistern keeps refilling or hissing, the fill valve is the problem. Both are replaced through the opening behind the flush plate.
Can I replace a Grohe flush valve myself?
Yes, if you are reasonably handy. Close the isolation valve, empty the cistern, remove the plate, lift out the old valve and fit a new one that matches your cistern family. Then reconnect the lever or tube. No building work is needed: everything is done through the access opening.
What is the difference between the old Grohe cistern valve and the AV1?
The valve in the old Grohe/DAL single-flush cistern is a plunger valve with an overflow pipe. The AV1 is the dual-flush valve used in GD2, Rapid SL and Uniset cisterns, operated pneumatically or mechanically, with an adjustable flush volume. They are not interchangeable: check your cistern number before ordering.
What does the Grohe flow restrictor do?
It reduces the flow of water coming out of the AV1 valve. It is useful, for example, with rimless toilets, which splash out of the bowl when the flush is too strong. It fits on the valve, with no need to change the cistern.
My Grohe cistern won't flush when I press the button: what should I check?
On old single-flush cisterns, check the actuation lever between the plate and the valve, which can break. On cisterns with an AV1 valve, check that the pneumatic tube is connected and undamaged. If the lever and tube are fine, the valve may be blocked by limescale and needs cleaning or replacing.
To sum up: identify your cistern family, find the part from the symptom and replace it through the plate opening. You will find all our Grohe spare parts for concealed cisterns in the installation accessories collection.
