Furance problem

Hi to you all
I had a problem with a gas heater and hope to find here someone with expiriance in the field that can give me a tip.

The problem is this:
Gas ducted heater......besic one with no draft inducer etc'
Consist of stand alone pilot,gas valve,simple thermostat(on-off,temp set).
The problem is:
the pilot is lit and when you call for heat the burner lit--no problem.
After a while I shut it and wait a few minutes and call for heat again------sometimes its re lit ok but some times there is a big WOOOF and a big flame an thats it.....probably the big WOOF is shuting the pilot and the unit stops working.
I must say again: old fusion heater, no pc boards just a basic heater.
I must say that I have checked the press' and its ok
Also the fan is working ok and continue to work for a while after temp is met.
I am trying to think about faults but allways come to:"if there was a problem then it will do it in the first time and not in the second or third"
I will appriciate any comment on that.....maybe someone came across problem like this:confused:
Thank you all

What you really need is a combustion engineer to look at it for you.

It sounds like problem either with the burner pressure or the jets.

But these type of appliances are dangerous and it's no way I can diagnose it thousand of miles away.

Could be a flueing problem, as The Viking suggests, get in a local expert.

It will be cheaper than an exploded heater or a hospital visit.

Thanks guys

Your problem is delayed ignition, this can be caused by a mutitude of things. the most commom in your case is since it fires the first time but delays after that is the carry over tracks that connect the burners to each other and to the pilot are worn and are not properly carrying the gas to the pilot to ignite. Since it ignites the first time but not after that they are expanding and thus becoming open to much and not properly carrying the gas to the pilot for ignition and it builds up in the burner compartment till it reaches the pilot and ignites. The bes cure for this problem is to replace the burner rods, at the least they should be cleaned and adjusted and realigned for proper carry over to each other and to the pilot. Also verify the manifold gas pressure, Maximum 3.5" natural, 10" Lpg then measure your temp rise and adjust your pressure down if your tem rise is to high or your blower down if it's to low.
Hope this helps...

**************Experience: Something aquired after you need it....************

Thank mate
Sounds right......I checked the pressure and its ok
Any way this machin is to old to start that kind of operation......I belive they will buy a new one
Thanks any way
Cheers

hi Did you check the pressure wile burnenr is on
usually the big wooof sound is a sign of unsuficient air-gas mixture
Best if don by experts for gas burners

Some other considerations:
If the pilot lights but the flame is a weak yellow flame, it will not get hot enough to heat the thermocouple to its set point allowing the gas valve to open.
A natural gas flame should be a bright blue with the tip of the flame having just a tinge of yellow. A propane flame should have a bluish green flame with a tinge of yellow at the tip. The flame should be strong enough to hit the thermocouple tip about 1/2 inch from the tip end. If the flame is weak or shaky looking, check to see that a breeze or draft is not blowing on it.

Adjusting the Flame
There is usually a small screw on the pilot valve body that will adjust the flame. You may have to refer to the manufacturer's instructions to find the screw. Turn the screw as needed to adjust the flame throw.

Yellow Flame
A yellow flame is caused by lack of air and incomplete combustion. It can be caused by a dirty pilot tube tip.

Split Flame
This is caused by dirt in the pilot tube. Take a needle or small nail and gently clean the tube.

Flickering or Wavering Flame
A flame that flickers is usually caused by a draft.

Thankes for your answers.
I did check the press' and clean the pilot but still the same.
I belive that the problem is what mr Mcguiver descibe.
Any way this machine saw better days and probably will be replaced.
Thanks for all your answers
Cheers

Theres a cluey fridgey in australia that posts in the www.whirlpool.net.au forums, in the gadgets section, you might have to register to see that section. When you register and see the gadget lounge, scroll to the bottom and do a search for "air conditioning" or "gas heating"
He's been doing refrigeration for a long time, plus he uses gas heating (as it's so cheap here).
He posts under the name "youbloodlybeauty" :D
See if you can get his attention there, or look him up as I think he's from melbourne. Send him a PM.
Being in perth where the temps never go low enough to bother with gas or electrical heating (reverse cycle works fine), I've never seriously looked into gas heaters, or oil, or other furnaces as I can install a reverse cycle a/c pretty cheaply.

UPDATE:
First of all thanks for all the reply.....you are all great.
I am doing a 2 day course to get my gas ticket(dissconnect&reconnect)
I asked the teacer about this problem and he said simple as that:your problem is dirt.
Clean all the assy' and it will be ok.
What do you think?
I am talking about a very old brivis wombat 92