Keston Boiler Problems – What They Mean, And What To Do Next

Welcome to our 5-minute guide to Keston boiler problems.

If you’ve got a problem with your boiler, it should lock out and display a fault code. So below, we’ve covered the most common problems we’ve come across with Keston boilers, the error codes that are displayed, and what you can do next.

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Top Keston Boiler Problems

#1 – Low Water Pressure

The most common Keston boiler fault, is low water pressure. The amount of water in your boiler and the pressure are directly connected. So, if you add water, you’ll also increase the pressure. You’ll be able to see this on the gauge that’s located on the front of your boiler. 

The error codes for the low water pressure fault vary depending on the Keston model you have. For instance, it would be F1 on a S30 system boiler, and E44 on a C40 combi or C55 combi.

Boilers work best at 1.3 bar of pressure. If your boiler is currently well below 1 bar, there’s a good chance that low water pressure is the problem.

You need to determine if your boiler is losing pressure, or simply HAS low pressure. If it’s losing pressure constantly, there’s a leak in the heating system.

You’ll want to have a heating engineer find this leak before topping up the pressure. If you don’t, water could be leaking onto floorboards, plaster, and even boiler components.

The Fix

Assuming you have either, found and fixed the leak, or you simply need to top up your boiler’s pressure, you’ll need to locate the external filling loop.

This a small braided hose that’s located below your boiler. There’s a valve on the filling loop that will allow water to enter your heating system. As you open it, you’ll see the pressure rise. Aim for 1.3 bar, but up to 1.5 bar is fine.

If you accidentally overfill your boiler, you can drain water (and pressure) from the system via the drain cock. If you can’t locate the drain cock, you can release pressure (and water) via the radiators using a drain key.

Remember to protect carpets and walls and have a container ready to catch the water that escapes.

#2 – Water Pressure Too High

If your pressure gauge is above 1.5 bar, or even worse, in the red area of the gauge, your boiler pressure is too high. Excess boiler pressure is unnecessary, especially as it causes excess strain on parts such as the heating pump’s seals and soldered copper joints.

On a Keston boiler you’ll expect to see the E40 fault code for combis such as the C36.

The first thing is the determine whether your boiler’s pressure is always high, or it’s rising during boiler operation. If it’s always too high, you need to reduce it.

If your boiler rises by 0.2-0.3 bar as it heats up, that’s normal. As parts heat up, they expand and that increases boiler pressure. It should drop as the boiler cools down.

A rise of 0.5-1 bar might indicate that the auto air vent or pressure release valve have failed. These are designed to release excess pressure. So, if they fail, that pressure is going to be sealed in the system.

The Fix

For boilers that rise dramatically in pressure during operation, you’ll need to get a Gas Safe engineer to inspect the air vent and PRV.

However, if your boiler doesn’t rise in pressure during operation significantly, you’ll need to remove pressure (and water) from the system. It’s simply been overfilled.

Pressure can be released via the heating drain cock. If you can’t find the drain cock in your property, the easy solution is to drain water via the radiators.

Each radiator has a bleed valve. By using a bleed key, you’ll be able to release water and reduce the pressure. Aim to get the pressure to 1.3 bar when cold. If you drain too much water from the system, you can simply top up the water using the external filling loop on your boiler.

#3 – Boiler Won’t Fire Up

If the problem with your Keston boiler relates to it not firing up, it’s likely there’s a fault with your fan. You’ll see a F3 error code on a S30 system boiler, and E05 fault on a C36 combi.

The fan on your boiler creates a small draught. This allows dangerous gases to be expelled via the flue. Your boiler won’t ignite unless the printed circuit board recognises the fan running.

Essentially, if a faulty fan is the problem, the boiler won’t sound like it’s even trying to ignite. But the problem is, the fan can also be the fault if it’s running too slow.

This won’t create a big enough draft to expel these flue gases. The air pressure switch (which senses this draught) notices that the boiler is unsafe, and it locks out the boiler.

The Fix

Unfortunately, this isn’t a case of a simple DIY fix. You shouldn’t be working on your boiler unless you are registered with Gas Safe.

So, you’ll need to call a Gas Safe registered engineer. They’ll be able to diagnose the problem with the fan. If the fan needs replacing, this could cost £250 and upwards (including parts and labour).

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#4 – Boiler Works Intermittently, Or Doesn’t Respond At All

If you’ve noticed your boiler working intermittently, the problem here is likely to be the control panel and circuit board (PCB). The PCB makes sure that all electronic boiler parts are working in symphony. So, when this part breaks, a boiler won’t work the way it should.

You might have noticed your boiler:

  • Starting up or turning off when it shouldn’t
  • Taps and radiators constantly going hot/cold
  • The display panel LEDs flickering on and off

Anything that seems unusual, is likely to be connected to an electronic fault, and one that’s linked to the PCB. In these cases, you’ll see an error code of E09, E10, E11 and even E12 on C40 combis and C55 combis.

The Fix

A boiler engineer will need to test the PCB to check that this is the problem. They’ll use a multi-meter, which can determine whether the PCB is getting power.

Besides checking connections and wires to/from the PCB, there’s not much else that can be done, other than a replacement. And, a replacement could cost around £500.

Before having a replacement PCB fitted on your Keston boiler, make sure there are no other faults.

Most damaged PCBs are linked to boiler leaks. It could be that the main fault with your Keston boiler is a leak, and the broken PCB is a by-product. You’ll want to get this fixed before spending big money on a PCB.

It’s also worth assessing the overall condition of your boiler. There’s little point investing £500+ on a boiler that’s out of warranty and continuing to deteriorate. Instead, we’d suggest getting a fixed price online from Heatable. That’s going to help you determine what the most cost effective route is going to be long-term.

#5 – Boiler Won’t Ignite

Another frequent problem with Keston boilers is an ignition failure. For the C36 it will show a E01 fault code, whereas the C40 combi and C55 combi will show the E02 fault code.

Ignition failures are not particularly specific. They could be related to:

  • Fan faults
  • Blocked air flow (flue)
  • Blocked condensate pipe (likely in freezing temperatures)
  • Moisture on the ignition lead and probe
  • Gas supply issue

The Fix

The most likely problem is a gas supply issue. In winter months, regulators on gas meters can freeze. When they do, that can restrict the gas flow to the boiler.

However, when it’s not cold, the gas valve is the likely culprit. These valves are prone to sticking as they get old and worn out. If they stick closed, they won’t be giving the boiler’s burner the amount of gas that it really needs. The PCB recognises this issue and locks out, displaying an error code.

You’ll need to get a gas engineer to free the valve and check its overall condition. If the valve can’t be freed and is in poor condition, a replacement can be fitted. This will cost roughly £250.

#6 – No Hot Water / No Heating

If you have no hot water, no central heating, or radiators are heating up slightly even when the heating is off, it’s likely the diverter valve is the cause.

The diverter valve helps a combi boiler switch between hot water and heating. If it’s sticking, as you call for hot water, it will be letting by some of this heated water into radiators, and vice versa.

The Fix

Like a gas valve fault, the diverter valve can sometimes be freed.

However, it’s much more likely that a replacement is going to be needed. Typical prices for a diverter valve replacement on Keston boilers is around £350, although the price will fluctuate depending on the model of your boiler.

As we mentioned with the PCB, with these sort of repair costs for your Keston boiler, it might be worth considering a replacement. You can get a price on-screen from Heatable in less than two minutes, using their handy clickable form.

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What’s Next?

Thanks for reading our 5-minute guide to Keston boiler problems. If you’re looking for further information on boiler faults, check out our boiler fault finding guide here.

Got a boiler problem that’s not listed above, or a question?

Leave a comment below and we’ll respond as soon as we can.

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  1. Hi, how I do release an air lock in the pump in the Keston boiler?
    The temperature fluctuates, where I get warm water (not hot water) and the radiators don’t get hot.


    1. It’s quite easy. Switch it on and unscrew the big screw in the middle. Air will come out and then you screw it back in. That’s it, air is out of your system.

  2. Sorry, this may be a stupid question but my Keaton boiler was recently serviced and it sounds like it is “on” all the time, even when the flame is not on. Is that right? Should it be making a sound all day?

  3. Hello Alex,
    My boiler, not working for a long time, started humming again a few days ago and it displayed all sorts of temperatures, then displayed the code E01 and has now settled for the letters LU, any idea what LU means? I know what E01 means (I called an emergency gas service shortly after hearing the boiler and the man who came switched off the gas from the mains as he could not turn my Keston C36 boiler off, hence the E01 fault code appearing). Many thanks in advance for your help.

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