OT Gas Oven Query

[Phil Allison]

** Hi all,

anyone here familiar with modern, domestic gas ovens ?

Seems many of them have gone high tech and use AC powered igniters and gas
valves.

The igniters in question run from 125 VAC ( via an auto-tranny ) and draw
about 3 amps when hot - while the *really odd* thing is they have a
negative tempco, starting off at about 1.5 amps and rising to 3 amps after
about 60 seconds.

Google tells me the technology is called " flat surface igniter " and the
conductor used is Silicon Carbide, a semiconductor - hence the negative
tempco. SiC igniters are all the rage for gas ovens, driers and furnaces in
the USA - explaining the 125VAC rating.

Thing is, they run continuously and sit right in the gas flame which
naturally causes the SiC to deteriorate and rise in resistance, lose power
and then run at a lower surface temp creating a hazard in that the gas may
not ignite.

To solve this, makers have linked the current draw of the igniter to the gas
valve operating current making the combination fail safe but rather critical
on the igniter being in good shape.

Replacement ignites are about $100 each and real pain to fit.

You can guess how I came to know this ...

Any similar experiences ?



..... Phil

 

[Phil Allison]

** Pic of typical SiC gas igniter.

http://midwestapplianceparts.com/images/5303937186.jpg




..... Phil

 

[I'm Fvckin' Frank]

On Mon, 11 Jul 2011 18:04:34 +1000, "Phil Allison" <phil_a@tpg.com.au>
wrote:

Damn. I was hoping you were looking for suicide advice.

 

[Mr Mac]

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:97vp4pF9vpU1@mid.individual.net...

** Hi all,

anyone here familiar with modern, domestic gas ovens ?

Seems many of them have gone high tech and use AC powered igniters and gas
valves.

The igniters in question run from 125 VAC ( via an auto-tranny ) and draw
about 3 amps when hot - while the *really odd* thing is they have a
negative tempco, starting off at about 1.5 amps and rising to 3 amps after
about 60 seconds.

Google tells me the technology is called " flat surface igniter " and the
conductor used is Silicon Carbide, a semiconductor - hence the negative
tempco. SiC igniters are all the rage for gas ovens, driers and furnaces
in the USA - explaining the 125VAC rating.

Thing is, they run continuously and sit right in the gas flame which
naturally causes the SiC to deteriorate and rise in resistance, lose power
and then run at a lower surface temp creating a hazard in that the gas may
not ignite.

To solve this, makers have linked the current draw of the igniter to the
gas valve operating current making the combination fail safe but rather
critical on the igniter being in good shape.

Replacement ignites are about $100 each and real pain to fit.

You can guess how I came to know this ...

Any similar experiences ?



.... Phil

I posted a similar-ish question on Dave's EEV blog along with a youtube
video.

http://www.eevblog.com/forum/index.php?topic=3496.msg49656#msg49656

My 25 year old central heating system had a hot surface silicon carbide
ignitor (HSI). It looked different though, it was tubular shaped and it was
not in contact with the flames once lit.

There was a separate wire that was used to sense if the flame was lit.

There was also a circuit to sense the current going through the HSI. If the
current was not large enough, it would not attempt to light.

I went through about 5 HSI's during the central heating units 25 year life.
So not too bad I suppose. The unit died when the combustion chamber rusted
out, which affected the air/gas mixture apparently, so it wouldn't light.

Unlike your oven, it was very easy to change over the HSI. I purchased two
HSI's from the states (ebay) at around $50 each inc shipping. Suggest you
order a couple and keep at least one spare.

By the way, my HSI was also rated at 110V but it didn't use an
autotransformer. It had a simple circuit that pulsed the full 240V at about
2Hz. The lights would flicker in the house just before the heat would come
through

The thermal mass of the HSI was enough for it to glow quite uniformly
despite the very low frequency switching.

Our system was either a Northern Air or Southern Air.,, I forget. The
installer of the new system said that he had seen a model like ours that
blew up after failing to ignite. He noted that the exhaust flu on this model
was very small for his liking.... goes to show despite all the safety
features, they still go BOOM every once in a while.

 

[Phil Allison]

"Mr Mac"

I posted a similar-ish question on Dave's EEV blog along with a youtube
video.

http://www.eevblog.com/forum/index.php?topic=3496.msg49656#msg49656

My 25 year old central heating system had a hot surface silicon carbide
ignitor (HSI).

** My error for calling it a "flat surface igniter".

It looked different though, it was tubular shaped and it was not in
contact with the flames once lit.

** Much better idea.

By the way, my HSI was also rated at 110V but it didn't use an
autotransformer. It had a simple circuit that pulsed the full 240V at
about 2Hz. The lights would flicker in the house just before the heat
would come through

** That requires the duty cycle to be 25% .

When ON the power is 4 times rated, so spending 75% of the time off does the
trick.

The thermal mass of the HSI was enough for it to glow quite uniformly
despite the very low frequency switching.

** Was this done with an oscillating relay ??

Our system was either a Northern Air or Southern Air.,, I forget. The
installer of the new system said that he had seen a model like ours that
blew up after failing to ignite. He noted that the exhaust flu on this
model was very small for his liking.... goes to show despite all the
safety features, they still go BOOM every once in a while.

** How very comforting to hear ...........

The "old fashioned" idea of having a continuous pilot flame seems to have
gotten the flick from the safety experts ( funny how my instantaneous, gas
water heater still has one) to be replaced by a variety of dodgy and
unreliable alternative systems that can still result in an explosion.

Anecdote:

Some years back, a meter reader for AGL knocked on my door cos he could not
find the gas meters for the block.

Being the nice guy I am, I personally escorted him to the hidden room under
the stair well that contained all the meters and opened the door for him.

The room was very dark and had no light.

So the meter reader pulled a butane cigarette lighter from his pocket and
used it to read the meter's dials.



..... Phil

 

[Metro]

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:984o4uF2a0U1@mid.individual.net...

8<----------------------------------------------------------

Anecdote:

Some years back, a meter reader for AGL knocked on my door cos he could not
find the gas meters for the block.

Being the nice guy I am, I personally escorted him to the hidden room under
the stair well that contained all the meters and opened the door for him.

The room was very dark and had no light.

So the meter reader pulled a butane cigarette lighter from his pocket and
used it to read the meter's dials.
___________________________________________________________________________

Well, it was a gas lighter......

Metro.....

 

[Mr Mac]

** That requires the duty cycle to be 25% .

When ON the power is 4 times rated, so spending 75% of the time off does
the trick.


The thermal mass of the HSI was enough for it to glow quite uniformly
despite the very low frequency switching.

** Was this done with an oscillating relay ??

No, I think it was a triac or something. Definitely not a relay.

Our system was either a Northern Air or Southern Air.,, I forget. The
installer of the new system said that he had seen a model like ours that
blew up after failing to ignite. He noted that the exhaust flu on this
model was very small for his liking.... goes to show despite all the
safety features, they still go BOOM every once in a while.

** How very comforting to hear ...........

The "old fashioned" idea of having a continuous pilot flame seems to have
gotten the flick from the safety experts ( funny how my instantaneous, gas
water heater still has one) to be replaced by a variety of dodgy and
unreliable alternative systems that can still result in an explosion.

A friend's new-ish gas oven uses a spark ignition, which is OK but it
regulates the temperature by completely turning off and turning on again
many times during the cooking time. Every once in a while, the spark fails
to ignite the gas and the oven just sits there and doesn't attempt to
re-ignite. The result is that you come back after an hour to check on your
food, and find that it's still completely raw!!!!!!!

You can imagine the fury of a hungry engineer when his meat is denied to be
delivered in a timely manner.

I have no idea why the electronics don't just keep the flame going and set
it to low. Maybe some bullshit efficiency star rating benefit.

 

[Phil Allison]

"Mr Mac"

** That requires the duty cycle to be 25% .

When ON the power is 4 times rated, so spending 75% of the time off does
the trick.


The thermal mass of the HSI was enough for it to glow quite uniformly
despite the very low frequency switching.

** Was this done with an oscillating relay ??

No, I think it was a triac or something. Definitely not a relay.

** Triacs * are * cheaper........

A friend's new-ish gas oven uses a spark ignition, which is OK but it
regulates the temperature by completely turning off and turning on again
many times during the cooking time. Every once in a while, the spark fails
to ignite the gas and the oven just sits there and doesn't attempt to
re-ignite. The result is that you come back after an hour to check on your
food, and find that it's still completely raw!!!!!!!

You can imagine the fury of a hungry engineer when his meat is denied to
be delivered in a timely manner.

** Hell hath no Fury like an hungry Engineer let down by ELECTRONICS !!

I have no idea why the electronics don't just keep the flame going and set
it to low. Maybe some bullshit efficiency star rating benefit.

** CO2 pollution.

Destroying the planet - you know .........




..... Phil