Helpful Tips


Boiler breakdown info from Mulgas

High boiler system pressure

Low system pressure boiler breakdown

Frozen condense boiler breakdown from Mulgas

Unvented hot water tank discharging


Power flushing

A power flush machine is connected to the central heating system which generates very high flow rates. Powerful chemicals can also be used to aid the process. This is the industry’s preferred method for removing sludge and debris from systems. These machines have a reverse flow option meaning you can circulate the water backwards and forwards. The idea is you flush each radiator one at a time until the system water runs clear. Obviously the larger and dirtier the system the more time this process will take. Typical problems a power flush would resolve is cold spots on radiators or radiators not heating completely.


 Be careful though because power flushing does get over diagnosed in my opinion. For example poor circulation can be caused by a blocked or restricted section of pipe work. A power flush is very unlikely to unblock this pipe. The location of the blockage would need to be found and cleared 1st and then maybe a power flush could be discussed to avoid a repeat blockage.


 Clear system water is the most important factor for a trouble free future of your new boiler.

Open flued boilers

These boilers are dying out now but there are still some out there. This type of boiler takes the oxygen the gas requires for complete combustion from the room it’s installed into. As a consequence the boiler needs to be open to the room and the room needs an adequate supply of oxygen from outside. This all works very well but from an efficiency and safety point of view isn’t great. Ventilation to outside undoes all the good insulation of a house such as double glazing and wall insulation. Added to this if the combustion of the gas or the flue operation became faulty for any reason it could become extremely dangerous with the potential for carbon monoxide entering the room. Carbon monoxide alarms are readily available and cheap so this helps but these appliances are also very inefficient and old, our advise is to replace this appliance as soon as possible. We’d recommend you get these appliances service annually to avoid a dangerous situation.

Fan flued appliances

This is how the modern boiler operates although the earliest fan flued boilers were available probably as long ago as 30/40 years ago. Meaning just because the flue is a fan flue doesn’t mean the boiler isn’t inefficient and requires replacing. The appliance is room sealed again and the flue still has 2 separate tubes for the combustion gasses and oxygen. Because the flue is fan assisted this means the flue tubes can be much smaller and the flue can travel longer distances (not all models). However some of the older fan flue boilers can pose a danger because as the appliance seals become older combustion gasses can be forced out through the leaking seals. We’d recommend you get these appliances service annually to avoid a dangerous situation.

System boiler

These boilers are heat only boilers that are prepared for pressurised systems. Pressurised systems need extra controls and these controls (expansion vessel, filling loop, pressure gauge and pressure relief valve) or already fitted in the boiler. The boiler also has a pump fitted internally meaning if your chosen system is a sealed type the system boiler makes for easier and neater installation.

Open vented heating systems

This is the more typical/conventional system in this country and this type of system has a header tank, typically in the loft. The tank normally has a ball valve and will automatically fill the system. If you bleed the radiators the header tank should fill the system until the correct level of water is reached. These systems have lower pressures than sealed/pressurised systems which can be an advantage on old systems. They are open to air though and as water absorbs air the chances of air in the system is higher and this can result in increased corrosion and therefore more sludge.

My heating system keeps loosing pressure

You would only be aware of this problem if your system was a pressurised type. If the system pressure gauge is continuously showing a decrease in pressure requiring you to re pressurise the system frequently, this indicates you have a leak. The leak could be anywhere on the system but obviously you normally see some visible sign of this. If not the favourite place for invisible pressure loss is the pressure relief valve which discharges to outside. This needs attention from an engineer because it’s no good for the long term of the system if you are continually refilling with fresh water.

System re pressurising

One of our most common breakdowns is low system pressure. This only applies to sealed pressurised central heating systems. This is the type of system which doesn’t have a little header tank in the loft. The other way to identify which system you have is to check if you have a red expansion vessel some where on the system (like a little cylinder) or a pressure gauge. If the system is pressurised the system pressure needs to be kept at 1 bar. If the system pressure falls below this the system can perform poorly or even breakdown. To re pressurise the system you need to find the filling loop and open both taps until the black needle is at 1 bar (see also our video help guides).

Boiler error codes

The modern boilers all have their own on board diagnostics. They are continually monitoring all internal components and the boilers operation. If it detects a fault or gets strange readings from a component the boiler will display a code. The boiler may continue to operate correctly but the code will be alerting you to a potential fault. Alternatively the boiler could stop working and display a fault relevant to the particular problem. If this is the case the boiler will need resetting to cancel this fault code before the boiler can continue to operate. However if the fault still exists the boiler will likely return to the same fault code after reset. In this case an engineer would be required. 

Frozen condense

During the very cold periods it’s possible the condense outlet for the boiler can become blocked by ice. This would in fact cause the boiler to breakdown eventually. If you experience a boiler fault and suspect this to be the cause it’s quite simple to resolve. The condense outlet is the white plastic pipe under the boiler. Follow this pipe if you can to it’s termination point. This won’t always be possible depending on it’s route. If you can follow the pipe to outside and the end of the pipe appears to have ice forming check this for a blockage. If the pipe is indeed blocked with ice you will need to thaw this with hot/warm water. The boiler may then need resetting if a fault code is present but the boiler should then operate. See also our video for further assistance on this.

What is an accumulator

These are used in addition to unvented hot water tanks if the flow rate on the water main isn’t satisfactory. Their purpose is to smooth out the dip in flow rate when 2 or more taps are used consecutively. They don’t however increase the pressure or flow rate, although accumulators with booster pumps can be purchased if this is required. This would be an expensive option however and both an accumulator and unvented tank will take up a large footprint of your property. An alternative would be to install a new water main from outside but this is also costly and disruptive.

Water flow rate/pressure

These both have an effect on each other but are also separate. The pressure is the force and the flow rate the quantity. The larger the diameter of pipe the more quantity of water can pass, therefore the better the flow rate and visa versa. Of course if you improve if your pressure your flow rate will also improve and again visa versa. Most older properties have a water main of 15 mm diameter and no matter how good your pressure is the flow rate is limited to this size. Normally the pressure is also average meaning you’ll probably have a  flow rate of 12/15 litres/minute. This is because the properties were originally designed for the water main to supply cold water to the kitchen cold tap and the cold water storage tank ball valve only, great pressure & flow rate therefore wouldn’t be necessary. This is why you need to explore your options carefully when either considering updating to a combination boiler or unvented hot water installation.


Scale isn’t something your central heating system or boiler should be suffering from, even in hard water areas (including combination boilers). This is because boilers and systems use the same heated water over and over again. There is only so much scale in water and once the scale has deposited it won’t again. The only time scale can become a problem is if the system has fresh water introduced frequently, therefore bringing fresh scale with it. Only systems with a water leak would continuously have fresh water filling the system though. Normally a leak would be noticeable and repaired quite quickly therefore not causing a vast amount of fresh water to be introduced to the system. Open vented systems (described above) do have the ability to refill themselves though and if a leak is not detected (under floor) the situation could arise that water is changing frequently refilling the system. This is unusual and if this was happening the ball valve on your header tank would be continuously filling, this shouldn’t be happening even a little. If you suspect you have a scale problem check your header tank and if this is even dripping continuously you need to call an engineer. If your system was pressurised (explained above) you’d be continuously refilling the system which should alert you to this problem, an engineer would be required if this was the case. Even combination boiler use the same heated water over again. The only part of a combination boiler that has fresh heated water passing through it continuously is the plate heat exchanger. In the past these heat exchangers have suffered with scale problems but their stainless steel plate design means this no longer is a problem. A scale reducer can be fitted to the incoming water main though in hard water areas and some boiler manufacturers do recommend this.  

Not to current standards

This is something that isn’t 100 % correct according the manufacturers instructions, building regs or Gas Safe but isn’t serious enough for action to be insisted upon other than reporting it to the responsible person. This type of situation would crop up on boiler services or gas safety inspections normally. It’s usually because regulations are continuously updated, and old installation may well have complied with the regulations at the time of install but no longer does. For example the boiler flue terminal maybe to close to something but isn’t causing an issue.

Immediately Dangerous

This is a situation that not only requires reporting but also action. It’s the type of issue that will have consequences on the occupiers safety. An example of this would be a gas leak. If a gas leak is detected this would require repairing. No other action is acceptable other than capping the appliance/pipe work causing the leak. Again you’d be required to leave the appropriate labels/notices with the responsible person explaining the issue and describing the corrective action.

Boiler servicing

It’s important you have your boiler serviced annually not only because the boiler manufacturers recommend this but for safety and efficiency reasons also. Faults can develop slowly with your system or boiler and doesn’t always result in a complete and sudden breakdown. A service would usually identify this issue before it becomes a large costly one. A lot of the modern boilers also have long manufacturer guarantees these days. For example our preferred boiler is Worcester Bosch and they have 10 year parts and labour guarantee’s. All the manufacturers do however insist the boiler is serviced once a year otherwise this invalidates their guarantee.

Room sealed or balanced flue appliances

This type of appliance takes the air required for combustion from its flue. This means the flue is different from the open flue appliances. The combustion gasses along with the oxygen requirement both pass through the flue. This means the flue has 2 tubes, the combustion gasses and oxygen intake. These tubes have to be completely sealed from one another and the appliance has to be sealed from the room. This is an advantage because this is safer. The earliest types of room sealed appliance are still very old and these are no longer available as boilers. This means they are also very inefficient and also require upgrading when possible.We’d recommend you get these appliances service annually to avoid a dangerous situation.

Heat only/conventional boiler

These boilers do what they say on the tin, they produce heat only. The controls to circulate and control this heat are all external to the boiler. These controls would normally be a pump, motorised valves, programmer and thermostats. They also require a hot water cylinder to store the hot water.

Combination boiler

These are boilers which require no storage water tanks because they heat mains water for your hot taps. The boiler detects the flow of water at your hot tap and immediately fires heating the water almost immediately. This is convenient for space because only the radiators are external to the boiler. This is neat, tidy and saves on installation cost when installing a new system. The general rule with a combination boiler however is not to install into a property with more than 1 bathroom. This is because the average flow rate on the water main is about 12 litre/minute. When you run 2 taps consecutively this flow rate halves at each tap meaning quite poor flow rate. It therefore maybe viewed a more conventional tank fed system could be the better option for larger properties where the chances of multiple taps running are higher.


A general misconception of combination boilers is if you increase the KW output you will also increase the flow rate but this is incorrect. The combination boiler can’t increase the flow rate of your water main. If you want to increase the pressure/flow rate on your water main the only way is either replacing the mains pipe from outside with a larger pipe or investing in a booster pump/accumulator type of set up. Both these options are quite costly and disruptive. It is however worth investing in a higher output combination boiler if your property does already have high flow rates (20 litres/minute or higher). This is because the water is heated instantly, so the higher the flow rate (water passes through the boiler quicker) the less heat will be picked up unless the KW output of the boiler is also increased. However if you only have average flow rate it’s pointless increasing the output of the boiler. This will just cost your more money on installation and these extra KW will not be used.

Pressurised/sealed heating systems

This type of system does not have a header tank, typically found in the loft. The radiator circuit and boiler are filled with water from a filling loop. This is a small section of flexible tube which connects the heating circuit to the water main. The heating circuit is filled when both taps of the filling loop are turned on. Fill and vent the radiators until 1 bar pressure is reached. This system is under more pressure than an open vented vented which could be an disadvantage if the system is old. However these systems have less air ingress which usually means less corrosion therefore less sludge. These systems also need extra controls to maintain the pressure such as expansion vessel, filling loop, pressure gauge and pressure relief valve. These controls need to fitted externally to the boiler unless using a system boiler.

My heating system pressure keeps rising

This would mean your pressure gauge is gaining pressure. An increase in pressure is normal with an increase in temperature however the pressure shouldn’t get any where near the red areas of your pressure gauge (3 bar). If this is happening this indicates either your filling loop is letting bye or your expansion vessel is low on air charge, either way you will require an engineer visit to avoid further damage.

Upgrade your boiler

The new generation of condensing boiler has seen a sudden increase in efficiencies. The fact they are called condensing boilers is directly linked to their increased efficiency. The flue gases which exit the boiler to outside (therefore lost energy) have water vapour. On the older boilers they remained a vapour because their temperature was so high. The new condensing boiler however reuses virtually all the combustion heat meaning these vapours are cooled sufficiently enough to return the vapours to liquid. This fluid is collected inside the boiler and is then dumped to a drain. Most boiler manufacturers claim their boilers are somewhere between 90 to 95% efficient. Some older boiler efficiencies could be as low as 60% meaning a very nice slice off your annual gas bill to be saved if you upgrade your boiler.

Low system pressure

This is the cause of most of a breakdown call outs. If the system is of the pressurised type the boiler will have a pressure measuring/flow rate devise. The purpose of this is to prevent the boiler firing if the pressure is lower than the required amount. This means when the pressure inevitably reduces (virtually all systems have small leaks) to a point lower than the boilers minimum the boiler will not operate. The cure is normally quite simple, you just need to re pressurise the system ( see system re pressurising) and reset the boiler if your boiler has a reset.

What is an unvented hot water tank

This is a hot water cylinder which is fed with mains water only. This type of system therefore wouldn’t have the traditional cold water storage tank in the loft. This obviously saves space and offers the customer flexibility to extend into the loft for example. All water is therefore mains water meaning this is clean fresh drinkable water which hasn’t been sat in an open tank in a dirty loft. 


In my opinion there’s is a misconception with these tanks. They are associated with high flow rates and pressure’s at your taps, in fact the most famous of these tanks is called a Megaflo. However these tanks don’t increase your pressure or flow rate, they just deliver whatever pressure and flow rate you have on your incoming water main.


If you have very good mains pressure and flow rate (1.5 bar and 20 litre/min or above) in my opinion these tanks deliver a very good performance and are well worth the money. They negate the requirement of expensive noisy pumps to raise pressure for power showers or quickly flowing baths.They are very well insulated and the heat recovery times are impressive making them also efficient. 


On the flip side if you don’t have great pressure/flow rate it’s questionable the outlay is worth it. This is because with average to poor flow rates and pressure (1 bar and 15 litre/min or below) your taps will not have great force, added to this if 2 taps are run consecutively the flow and pressure will drop at both noticeably. For example if the shower was in use and another tap was also used the shower would noticeably drop off in pressure and flow. They are quite expensive and their installation isn’t as simple as replacing your existing cylinder. The existing gravity tanks have to be capped and removed, they need additional safety controls, discharge pipe work to outside, and the installer also has to have additional qualifications to install. Also to be considered is old gravity fed plumbing and fittings suddenly subjected to mains water pressure can potentially become faulty or leak.

My unvented tank keeps discharging water

This indicates the internal or external expansion vessel is either low or lost it’s air charge. This air pressure would need to be replaced and we would recommend you call an engineer to service your tank.


Sludge is a result of corrosion which takes place in all central heating systems. Corrosion can be limited by the use of chemicals and correct installation of the boiler and system. Normally if the installation is correct and chemicals are used the corrosion is minimal and doesn’t cause a problem. If the system suffers from increased levels of corrosion (normally caused by elevated levels of oxygen in the water) the sludge content will increase. Sludge is mainly very small pieces of iron oxide and if this is floating around your system passing through your boiler, pump, motorised valves, radiator valves, pipes and radiators it is only likely to cause trouble. Typical problems are noisy boilers, poor circulation, frequent pump replacements, valves letting bye or leaking components. If this is describing your system it’s probably time to consider a power flush. However you also need an engineer to diagnose why your system had elevated levels of corrosion in the first place otherwise you’ll end up back in the same place before to long. 

Earth bonding

This is an electrical regulation which says all metallic pipe work in the property should be earthed. This is in case a damaged electric cable happens to touch a pipe and passes electric current. If this were to happen and the pipe was touched you would receive a electric shock. If the pipe is earthed even if current is present you won’t receive a electric shock because electricity will always travel to the path of least resistance. We mainly come across this when carrying out gas safety inspections. The meter is supposed to be earth bonded and the earth bonding should be attached within 60 mm of the meter or before the 1st branch. However as usual with our industry it’s a little confusing because it’s only classed as not to current standards if the bonding isn’t correct which is report able only and no corrective action is necessary.

At risk

This is a situation the not only requires reporting but also requires an action. It’s the type of issue that could have a consequence on safety but isn’t felt to be affecting the safety of the occupier at this present time. For example if the boiler is open flued and the ventilation into the room isn’t adequate the boiler may still operate safely on the day tested. However with a known lack of adequate ventilation it’s possible at some point during it’s operation on another day the situation may change and the flue or combustion operation could be affected adversely. The correct action for this situation is to turn the appliance off and issue warning labels/notices to the responsible person explaining the issue and describing the corrective action.