Central Heating FAQ

Here you will see FAQ’s covering Central Heating, and Boilers. This may also be cross referred to some of the other plumbing sections, such as pumps and radiators, and some other “HOW TO” hints.

Please read the DISCLAIMER HERE before continuing. WARNING: you are strongly advised not to attempt any job or perform a test or check on any boiler that needs the case removed. This is because re-fitting some cases can be tricky for the untrained, and unless you know how to check case-seals for soundness and know how to check for Carbon Monoxide (CO) leaks with a KANE 250 or similar CO meter, it is best left to the CORGI professional. CO poisoning results in around 50 deaths per year in the UK alone. For good advice on GAS SAFETY please see http://www.gas-guide.org.uk/emergencies.html

For GAS EMERGENCIES (leaks, or smell of gas): National Grid Gas Emergencies – 0800 111 999 (This is a 24 hour emergency line)

Your attention is also drawn to the Gas Safety (Installation and Use) Regulations 1998 at: http://www.opsi.gov.uk/si/si1998/19982451.htm and http://www.opsi.gov.uk/si/si1998/98245102.htm © Crown Copyright 1998

Q: How do I calculate the radiator sizes I need for my rooms?

A: I have found a very useful link with this information in here at INSPIRED HEATING : http://www.inspiredheating.co.uk/radiators.htm

Q: I have been told that the case gasket has failed, and a spare cannot be obtained, and it is leaking fumes and carbon monoxide. Is this common?

A: I have to say, unlikely, unless it has been deliberately damaged. The case gasket material, in common with many items requiring a high temperature gasket, uses a special silicone rubber to meet the requirements. Even then, there is a standard sized gasket material available from trade outlets, and some plumbers merchants might stock it. See http://www.abtap.com and check the catalogue. You will this item : “10 Meter Roll 15mm x 3mm Adhesive Backed Rubber for combustion chambers etc “,fairly well down the list. I would get a second opinion, especially if they put in a smoke pellets with the case-work loose, that always looks dramatic (lots of smoke coming out of the boiler), when actually those are for testing the flue airflow. The CORGI PDF on this subject (case seals), mentions only TAPERS/MATCHES to check for flicker, due to air blowing past the case seal when the fan is running. Other methods, like smoke pellets are still being validated. Also in this PDF file, there is a BIG list of boiler models where parts relevant to case seals ARE STILL AVAILABLE. The proper test for Carbon Monoxide (CO) leakage is to use calibrated professional instruments, such as the Dwyer 450, or Kane 250, with probes to test around the case, and flue. Hopefully it was not condemned to get you to sign up for a new boiler. If so, contact the CORGI Registration site, and ask for advice. The link to the case testing information is HERE. Here is the link to the main CORGI site: http://www.corgi-gas-safety.com Also see this useful link about getting an Inspector to check any work done by CORGI installers (free I believe):


Q: How do I change the central heating pump?

A: See the PLUMBING FAQ section on changing the pump HERE

Q: How do I drain down the central heating system?

A: See the PLUMBING FAQ section on draining the system HERE

Q: The water and radiators are only getting luke warm, but the boiler is cycling on and off, what can be the cause of this?

A: This is most likely the Flue Fan. Sometimes these can work for a short while, then the thermal cut-out in the fan motor operates prematurely, causing it to stop. Sometimes the bearings become partially siezed up, causing insufficient air flow. The air-vane sensor switch (or APS) in the duct will detect this and the electronic safety circuits will just close the gas-valve for a time. However,There are other more technical reasons for a boiler short cycling like this, outside the scope of this FAQ. The fan-motor IS a part that you can replace yourself, but it will cost around £100, and since it is such a major safety item, it is best left to professional Corgi engineers. Ensure that you replace the sealing gasket that should come with the new motor (it sits between the main motor body, and the duct case). If yours is covered on insurance, then obviously do not touch it, just call out your registered Corgi specialist. Use THIS LINK to find yours. NOTE Many fan issisted flue boilers operate at positive pressure, and therefore it is imperative that the boiler casing is replaced properly, and has a sound gasket, that does NOT leak CO and other combustion products into the room. Unless you have good experience with gas appliances, and know how to check for CO leaks, consider this a NON DIY job. Just because it may be legal, does NOT mean it is safe. Also consider that if you cause a major problem, your household insurance might not cover it, nor any liability for problems caused to other parties.

Q: The pilot light in my boiler has a very small flame and tends to go out frequently (or it will not stay lit after releasing the button) , what can cause this?

A: This can be caused by gradual deposit build-up in the pilot light gas tube, or the heat sensing thermocouple that detects its lit condition, has deteriorated. Try first adjusting the pilot light flame regulator screw a small amount, to try to get the flame to the correct height as per your manual. Usually 15 to 20mm is enough. If it will not adjust to the correct height, then you will need to replace the pilot light thermocouple. This will certainly be the case if you find that holding the interlock button in allows you to light the pilot light, but it goes out after releasing it. Try the SCREWFIX 16098 (Super Universal Thermocouple, around £4). Ensure that the correct coupling nut is used (the kit comes with about 6 different types), and that the sensing tip is located properly in the pilot flame area. NOTE: See HERE for warnings on case removal and leaks.

Q: The boiler tends to kettle a bit, and makes popping noises. What can cause this, and how do I fix it?

A: With regards to kettling, banging or popping noises in the central heating boiler, this is usually caused by either lime scale and corrosion debris becoming baked onto the inside surfaces of the heat exchanger, leading to localised boiling (due to hot-spots).

This is a common problem with low-water content wall-mounted boilers over about 10 years old. Provided that the heat exchanger is not too scaled up, or damaged, then by following the procedure below, may restore it back to normal, for a few years more.

A good method to clean the boiler and system to prevent a reoccurrence of the noise problem is to use Fernox Heavy duty Restorer and Fernox DS-40 boiler descaler as outlined by Fernox, so avoiding boiler replacement and re-plumbing. Note that FERNOX DS-3 SHOULD NOT BE USED, THAT IS FOR WATER HEATERS. ONLY DS-40 OR SIMILAR PRODUCTS SPECIFICALLY FORMULATED FOR CENTRAL HEATING SYSTEMS SHOULD BE USED. See the FERNOX site at THIS LINK, and note that PLUMBWORLD and other good plumbers merchants stock these products

  1. Drain and flush the entire heating system with plain water until the water comes out clear and not frothy (that is, let it continually fill from the header tank, while you drain the water off from the lowest drain point off one of the radiators)
  2. after leaving the system about half empty (so you ensure that the Fernox Restorer gets into the system and is circulating, and you can add at least 2 litres), circulate a 2% solution (2 Litres per ten radiators) of Fernox Heavy Duty Restorer in the whole central heating system, then allow to fill up again from the header tank.
  3. Either let this circulate continuously for 48 hours or intermittently under normal load conditions for a week (i.e. 7 days on a 6 hour heating cycle).
  4. Drain and flush the whole system until the water is clear and to ensure de-flocculated sludge and cleaner are removed from the system (At least three complete changes of water are likely to be required. If there is some pipe-work t below the level of the drain-off points, operate the pump for 10minutes between each flush. This will ensure that any sludge and sediment remaining in the low points is diluted further)
  5. To clean any baked on debris that remains in the boiler after cleaning, also do a separate boiler descale by isolating flow and return pipes to the heating circuit, or by closing the lock shield and wheel head valves to all radiators except one. For systems with a 3-port motorised changeover valve, this is easier to do by just turning down the ROOM THERMOSTAT, and turning UP the hot-water cylinder thermostat, to about 70 degrees.
  6. Use a 2% solution of Fernox DS-40 (for typical domestic system, a restricted circuit on 2% DS-40, will require only half a 2Kg container). You can introduce this as a dissolved solution, by draining down to about the level with the bottom of the pump, then pouring it into the header tank. Allow the header tank to fill just enough with fresh water to fill the pipework that leads down to the pump, but ensure that the feed pipe hole seen from the inside of the header tank is immersed in water, so that the pump does not run dry.
  7. Allow this to circulate for 24 hours at 70 degrees C.
  8. Drain the system while it is still hot and flush with plain water about 3 times. You may need to circulate with the pump on (but the boiler thermostat to the “off” or “minimum” position) for about 30 minutes between flushes.
  9. When the third flush is empty (stop the header tank from filling up), turn off the drain off point, allow the header tank to start filling, and add the correct amount of a good quality inhibitor to the system (such as Fernox MB/1).
  10. Bleed all radiators, then put the boiler thermostat to its normal position, all other controls to normal position, and balance all radiators if needed (you may have to bleed the radiators a few more times over a few days)

Q: I have seen a device advertised that will trap the iron particles in a central heating system. What is it?

A: This is the Magnaclean®. It is fairly new, and costs about £89. It looks like a very useful device, and not only traps and holds the iron particles, to reduce sludge build up in your radiators, but also allows easy introduction of FERNOX concentrated cleansers and inhibitors (using the new cartridge and adapter system), by using a one-way valve. See this, and a multi-media presentation at the PLUMBWORLD SITE at this link: http://www.plumbworld.co.uk/1374-0000

Q: I have bled some radiators on my COMBI system, and now the boiler will not fire up.

A: This is a common problem, and it is due to the pressure drop in the radiator section. Some boilers wil not fire up if the water pressure drops to below a certain level (usually 0.8 to 0.9 BAR). If you can locate your COMBI FILLING LOOP (usually near the boiler) then it is a simple matter of slowly opening a couple of valves, and noting the pressue on the pressure gauge. Use the correct pressure as stated in your user manual. It is usually between 1.25 and 1.5 BAR. DO NOT INCREASE THE PRESSURE ABOVE THE LIMIT FOR YOUR BOILER, OTHERWISE THE OVERPRESSURE VALVE WILL ACTIVATE, DUMPING WATER TO THE OUTSIDE. SOMETIMES THESE WILL NOT FULLY CLOSE OFF ONCE OPENED, SO YOU MAY STILL LOOSE PRESSURE. IF THIS HAPPENS, AND THE CORRECT PRESSURE CANNOT BE MAINTAINED, REPLACE THE OPV. Note that as in the picture, some gauges show the upper limit with a red line. Once the correct pressure has been reached, CLOSE OFF BOTH VALVES (IF THERE ARE TWO). In theory the filling loop should be disconnected after use, but many people leave them connected up. If there is a non-return valve, there should not be a problem doing this, and it can prevent drips that might occur and ruin clothes in the airing-cupboard.

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