Blodgett convection oven heats up to temperature and maintains it for about two hours and then it shuts off

[ectofix]

Don’t mean to bust you in the chops, but sounds like you’re just throwing parts at it instead of troubleshooting it.

 

The first thing I would’ve done is to have access to electrical test points behind the control panel and have the kick panel off to monitor the flames.  I’d do a visual inspection of all the wire connections.  I’d pay particular attention to the burner’s ground connection.  It’s too easy to overlook that and if it’s corroded or deteriorated, then that deters proper flame current in the flame proof circuit.  Furthermore, I’d inspect the burners, their orifices and the primary air inlets for any buildup that would need cleaned out.

 

If all looks good to that point, I’d then operate it until the problem occurs.  I’d check gas pressures and inspect for proper flame quality.  There’s two circuits at play in there.  The circuit that calls for heat and then the circuit IT energizes that calls for flame.  After hopefully catching it drop out prematurely and LEAVING it that way (without resetting it with the door), I’d then test the circuit with my voltmeter to see why it stopped heating.

 

Your problem seems to hint at an ignition lockout, since you’re able to reset that by opening/closing the door.  The presence of a good pilot is what proves the flame.  If its a weak flame, then maybe crud within the pilot burner that’s affecting it.   A blast of nitrogen usually clears that.  Poor supply pressure (like a bad line regulator) can also hinder the pilot flame when the main burners being supplied gas starves the pilot.

 

If its a temp control circuit issue (the circuit that calls for heat), then about the only thing you HAVEN’T replaced is the blower motor.  It has a centrifugal switch in it that must close for the heat circuit to energize the ignition module.   If, for some reason IT loses contact while the motor’s still running, you can test for that.  I’ve never seen it happen, but since you already replaced just about everything else, I thought I’d throw that in there.

 

There’s other various components you MIGHT have to look at too, depending on how old your oven is.

 

So IF you do what I said by laying the thing open to have test points and at the ready with your meter when your problem arises, you can probe through the circuit and narrow it down to the exact point of failure.

[olivero]

Well, if its anything like my Vulcan ones then they don’t prove a flame before giving it all shes got.

 

It does sound like a module lockout like Ectofix said, I would also check the Mili amps coming back from the monitoring electrode of flame rod or whatever you call it, your meter should have that function and make sure its in the right range, normally its 1.5 and above is acceptable. Could be its loosing MA in the 2 hours and then drops out, don;t know if it auotomatically reignites at that point but thats another thing to check. Just dont measure the mili amps from the igniter wire.

[fixbear]

Isn’t the door switch also involved with ignition proofing? Whatever it is it has to be from heat saturation. That can be anywhere not in direct contact.

[coby1]

I worked on testing the Blodgett oven out this morning by testing the microamps to the flame sensor, checked the manifold gas pressure and it was 3.5″,restripped the ground wires and crimped new terminal ends on, checked that I was getting proper voltage, 120V, checked the back of the fan motor for blockage and cleaned the dust out of it, cooling fan was clogged with grease and dust, checked the transformer secondary amp draw 0.62amps, fan motor amps on high 3.70 amps, capacitor mfd’s was 7.8 mfd, rated for 10 mfd. The thought than occurred to me with this weak run capacitor on fan motor, is the motor somehow the culprit? Changed out the run capacitor with a new 10/440v.

I have continued to watch the oven cycle on and off at 375°f when the oven reaches temp the red light goes off and then when the temperature starts to drop the oven ignites , the red light comes back on until it reaches temp and goes off again.

So far, so good.

[ectofix]

The door switch energizes the motor with 120v.  Once motor is at speed, its centrifugal switch closes to passed 24v to the ignition module.

[ectofix]

Sounds like you meticulated (is that a word?) about as well as I would’ve.  Hopefully you got it.

[ectofix]

Yes, the Vulcan ovens are direct spark ignition, so no pilot.  Just the main burner.  The Vulcans also use a Fenwal module.  The two oven variations that I’m familiar with are their VG Series and SG Series:

The VG has just one spark electrode. The SG (their modern-day Snorkel ovens) has two spark electrodes, so it uses a two-channel module.

The minimum flame current for both of those Fenwal modules is 1.0μA.

BOTH modules do 3 trials for 7 seconds – with 15 seconds between each.

 

The Blodgett DFG100 & 200 ovens are intermittent pilot ignition, so there IS a pilot which impinges upon the flame sensor rod.  The module must prove pilot before energizing the main valve.  Blodgett’s Fenwal module requires a minimum of 0.3μA.  That is readable with most multimeters that are designed to read microamps.

That Fenwal module is 1 trial for 50 seconds.  If that initial trial fails, the module locks out.  However, if flame signal is lost during MAIN valve operation, the module will do a RECYCLE (another trial for ignition).  That module will do 5 RECYCLES and then locks out.

 

The old Johnson Controls module that Blodgett once used were different.  ITS flame current was such a low value that a transducer was required to read flame current.  I don’t know what other operational parameters those modules had, since they’re pretty much defunct parts now…so I don’t care.

I have one of those transducers, but never used it.  Heck, I could tell by the extended duration of ignition spark that the module wasn’t sensing very well.  I always kept a spare flame rod in stock on my truck.

[olivero]

Well, I guess it could be the case. Your oven techincally would not be heating too much without the convection fan as it moves the heat around and would then just get really hot in the center part of the oven but would take a bit to get to the thermostat in the top part of the oven so maybe that was it. Never would have thought of that myself though, that’s why I  come here and offer my help, sometimes you learn something new

 

Oh, and what was the MA on the flame rod?

[coby1]

The milliamps where 1.7 micro-amps DC on the flame rod.

[coby1]

Apparently, The Blodgett oven will not run if the fan motor does not turn, due to the centrifugal switch in the motor. If the motor was shutting down due to a weak run capacitor, this would cause the oven to shut down until it was restarted again by cycling power to the unit. It has a two speed motor and the oven must run with either low or high speed of the blower motor running at all times.