Sort-of off topic.... why doesn't my central heating work?

[haxby]

Well it might be an electronics problem.... so what the hey, I'll post it here!

In Melbourne it has been very cold lately, and of course my gas central heating has had to break down on me. Figures!

The gas is lit by a silicon carbide ignitor, also called a hot surface ignitor. It warms up and glows red hot for a few seconds before the gas turns on.

These ignitors have to be replaced every few years, and the one in the system was well past its due date, so I thought it would be an easy fix.....
Not so. I had a look to see what was happening, and the old ignitor was glowing just fine, but it wasn't lighting the gas!!

So I thought that it would be worth replacing it anyway, as sometimes they get a white coating on them which decreases their ability to light the gas.

Well the new one came, and it's doing the same thing. The ignitor is glowing like a star but it is not igniting the gas. The system knows the gas hasnt been lit, so it turns the gas off.



I wonder if there are any people on the forum that have experience with this stuff?

I have filmed a youtube video of the heater trying to turn on. The first part shows the heater attempting (and failing) to turn on with the hot surface ignitor. The second part of the video shows that if I light a match at the right time, the gas turns on no problem.

[Fraser]

I must first state that I am not a heating engineer so read my comments in that light. I do understand vehicle based fuel ignition principles though  

In order to ignite flammable gas there has to be the correct gas/air mixture to permit ignition combined with a heat source of high enough temperature to initiate combustion. Much like a car engine.

In your case you have the gas present and a match ignites it so there should be a roughly correct gas/air ratio present. That leaves your ignitor as chief suspect for the fault. It is very difficult to judge temperature using the human eye so bear this in mind. I would suggest that your ignitor is not running hot enough to cause the required ignition event.

Just a thought..... Once lit with a match, check that the gas flame is the expected blue colour and not bright orange (I am working on my experience of UK gas boilers here). A yellow flame would indicate incorrect gas/air mixture and could be caused by jet problems, the gas regulator or a blocked flu vent.

The ignitor unit must apply a high current to the ignition device so you have the potential for corrosion problems or a failed ignitor driver that is producing an insufficient current and therefore insufficient heat in the ignitor for gas ignition. It would be worth while checking all connections in the ignitor circuit for corrosion (causing series resistance) and if you can find out what current these ignitors require you should be able to carefully measure it with an appropriate meter set to it's current range.

This should lead you to the source of the problem but as I have no idea how the ignitor is driven I can offer no thoughts on the potential failures in such a circuit.


As a final comment.... I am not a gas appliance installer so take my comments as amateur and not professional. Where gas is involved, it is a very good idea to engage the services of a registered gas engineer as gas explosions can be very nasty ! I would never suggest that anyone messes with the gas regulator or gas pipework as that is an area that can cause a world of hurt if something goes wrong. It is also illegal to do so in many countries as you risk your neigbours lives as well as your own.

[haxby]

I guess the ignitor is the chief suspect. The only other thing it can be is the air/gas mixture as you say.

The ignitor is not in the flames, its off to one side, I thought this was strange, but this position has worked for over 20 years. I was tempted to relocate it just to experiment but that really wouldn't prove where the fault was.

The last 20% of the video shows the flames while the system is running. The flames do look a bit yellow. Well mostly blue but a good amount of yellow.... So it could be the air mixture too.

[Fraser]

The ignitor is designed to ignite the pocket of gas that forms just above and around the jets and should not be in direct line with the gas jets as that would decrease it's operational life.

To my unqualified eye, the main jet flame appears to have too much yellow in it indicating an unstable air/gas mixture but that could be caused by the fact that you have the boiler covers removed.

Air/Gas mixture is another area where a heating engineer is recommended as they can use a mixture or flu gas meter to see what is going on. You can do some basic checks though. It would be worth checking that your jets are not sooted up and that any air extraction in the exhaust is working correctly (extraction fan etc.). Check for any blockages in the exhaust as well. I have seen an exhaust liner collapse due to corrosion and partially block the outflow of gasses from the combustion chamber.

Good luck.

[PetrosA]

I don't think it's an ignitor problem. I would try removing and cleaning the jet tubes (they should lift out) as there's probably debris accumulated in them impeding the gas flow. Other than that it could be a pressure problem in the gas line. The flames do seem a little yellow which would support either theory.

[Zero999]

All I can say is be very careful. Working on a gas powered device is much more dangerous than a purely electrical one. Electricity can shock and kill or even start a fire but a gas explosion can literally blow your house to pieces or an incorrect gas/air mixture and poor ventilation can release deadly carbon-monoxide poisoning you and your family. Check the laws in your jurisdiction. Here in the UK it's illegal to carry out work on a gas appliance unless you've got certain qualifications and are registered with the appropriate body. I'm pretty sure that even in countries which lack such regulation you're still liable for damage done by work you do and if it goes wrong you're probably not be covered under your house insurance.

If it's legal and you're confident in what you're doing then by all means continue. If you have any doubts, get a professional in who's been recommended by a friend and can prove they're suitably qualified. It may cost more to get the work done professionally but at least it's not your head on the block if it goes wrong.

[Cj1corbystarlet]

Your gas burners could blocked and need cleaning,your gas pressure regulator may have wandered and need adjustment, also put the cover back on the appliance before you light it (This will disturb the gas/air mixture).

Since your unit is using HSI ign, you should have a Optical eye or similar device which looks at the HSI glowbar and click over when it's hot enough and open the gas valves, if this is the case it sounds like it needs replacement.

When the HSI glow bar is glowing bright give the unit a good Bang and the gas should come on, once the gas is on then it will switch over and monitor the flame through the flame sensing rod on the left hand side with a millivolt return to a saftety board.

Ps, Get a licensed gas fitter to look at it !

regards

Ben
"Licensed Gas Fitter"

[Fraser]

As Ben has stated, it appears the best way forward is to engage the services of a professional.

If your boiler is 20 years old and has not had a recent service, give it a present in the form of a damned good professional service to thank it for all it's hard work over the years  You will likely thank yourself in the long run as at 20 years of age things can start to wear/burn out. Even the jets begin to corrode and the flame profile can change.

[haxby]

Just thought I'd update the situation, as so many times, people find the solution but don't write back to the forum that helped them....

The igniter is a 110 V unit and it draws 3A while on. As this is an Australian heater, the control circuitry pulses the 110V hot surface igniter with 240V (50% duty cycle). So another theory I had was that the control circuitry was pulsing incorrectly. This could be valid as if it used a simple resistor capacitor transistor circuit, it could be possible that the capacitors have lost their values over time, changing the time constant, and therefore not heating up the igniter enough. So to rule out that part of it, I connected the igniter to a 110V transformer. The heater still didn't fire up.

So I called a central heating tradie, and he discovered that the heat exchanger inside the system had rusted through. This altered the flow rate of air through the combustion chamber, which altered the air/gas mixture enough to stop it from igniting, so the solution has been to change the whole system.

Thanks to all that replied! Shame it wasn't an electronics problem!

[Zero999]

Thanks for coming back, glad you got it sorted.