Zanussi gas oven slow to preheat

[Marko the idiot]

Hello,

Having moved house recently, I am now the proud owner of a Zanussi ZCG664GWC gas oven. Product number 94890505800

After a few cooking disasters involving cold food and hungry children, I tested the oven using an oven thermometer.

At first, I thought it was a thermostat issue as the oven didn't seem to ever get to the correct temperature but having left the oven to preheat for 45 minutes, it showed that it will reach the heady heights of 240C when asked for gas mark 7, at which point, the flame reduces. This suggests to me that the thermostat is functioning correctly.

I do feel that 45 minutes is a very long time to wait for an oven to preheat. Another factor is, once the door is opened to put food in, the temperature drops to 180C and it can take another 30 minutes to get back up to 200C.

I thought it seemed to fit that the igniter was failing but having checked the parts diagram on Zanussi's website, it doesn't seem to have an igniter, it has a thermocouple controlling the gas valve. Is it possible that this is failing and allowing a reduced amount of gas to pass the valve to create a big enough flame, or is a thermocouple a strictly on or off device? Is it possible to test a thermocouple?

The oven lights immediately but I'm unsure whether it is stuck on a low flame as I don't know what size the flame should be!

Does anyone have any ideas on what I can check before I call the repair man?

 

[Mark_FixCookers]

Thermocouple
This is a safety device its job is to allow the gas to flow to the burner while it thinks the burner is lit or to shut off the gas when the burner flame goes out, it does this by having part of the thermocouple sat in the flame and being kept hot by the flame. This either works or it doesn't when you release the control knonb after holding it in for a few seconds after it has lit and has no bearing on the amount of gas that passes to the burner so you can rule that out.

Thermostat
It's job is to control the temperature in the oven. It does this by reducing the flame height, which should be a full flame when you first light the oven burner as the temperature gets closer to whatever the thermostat is set at by reducing a valve inside the thermostat reducing the gas flow to the burner. It would appear from your description that the Temperature regulation side of the thermostat is working correctly however this does not rule out your thermostat as the cause of your low flame height but it is unlikely to be the cause.

Blockage
Your issue is most likely a partial blockage somewhere between and including the thermostat and the gas oven burner.
The most likely place this blockage occurs is the oven injector
Second most likely is the oven burner
Most unlikely but cannot rule it out is the oven thermostat

Low Working Pressure
It is also possible that you have what is called "Low working gas pressure" at the appliance although you would most likely have noticed this as you put more and more hob burners on, you would see the hob flame lower.
You didnt mention this so I just added it as the alternative cause.

If I were to come out and look at it I wouldn't prep the call by ordering a thermostat I would suspect it to be a blocked injector which is easily fixed with a pin.
So I would strip out the burner to get access to the injector, take it off hold it up against the sunlight so i could see through it, wiggle a pin through it then look through it again to see if the hole was any bigger.
I would then clean the oven burner out reassemble and test.
If that did not improve it I would then suspect the thermostat.
You need a gas safe engineer to attend, one that knows about cookers. This is not something you should touch yourself

 

[Marko the idiot]

Mark, wow, what a brilliantly informative answer! Thank you! The hob burners seem to be working OK, although the smallest one won't light at all which I don't think is related. I don't think we have low working pressure as the working hobs all maintain a strong flame whether one is on or 3.
I have no intention of taking anything apart as gas is not something I want to play with.
It's a shame you're not local to us, we're in Wiltshire or you would have been getting a call. I will use your answer to talk through the problem with a local gas oven repair man, hopefully he'll turn up with the same knowledge and diagnostic nous that you clearly have. Thanks again, I'll post once I have any update.

 

[Mark_FixCookers]

Your welcome
Hob ignition issue. If there is no spark at the electrode tip Spark plug) then you have an ignition issue typically a trapped ignition lead so it's arcing to earth where it is trapped or been rubbing against a sharp edge. You should be able to light the burner with a match.
Should be fixable by the engineer
If there is a spark there and you also cannot light it with a match then you most likely have a completely blocked injector.
If this is the case take of the burner look at the small brass nipple in the middle, this is the injector. Wiggle a pin in it this should clear the blockage
Nothing bigger than a pin or fine needle and don't force it through as with very small diameter injectors such as the small burners or the inner wok burner injectors you can make the hole bigger than it is supposed to


Good luck