Heater Problem - running out of options


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Old 05-25-12, 05:26 AM
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Heater Problem - running out of options

I'm running out of ideas to fix a heater problem I have

Problem is that the system doesnít light the main furnace anymore. To summarise I have diagnosed:

The unit sits idle and fan comes on ~ 90 seconds after Heat On. [good]
It takes some time to turn off once Heat is switched to off. [good]
Rollout cable is isolated [good]
Overheat is closed circuit [good]

However Iím not getting 24Volts DC from the light blue or Orange wire or when I directly measure the control board pins. Iím using the ground connection for this.

Thinking this was a controller issue I organised a replacement (the previous was 12months old) and the same issue is present.

I can hear the switching when Heat On is selected and the test LED illuminates and main fan operates so I cant see it being a supply issue.

Could this be related to the thermostat controller as the issue strangely coincides when my wife changed the batteries for the controller. I have taken that apart to clean the contacts.

Can this be short circuited on the controller to test cabling/thermostat controller?

I guess I could get a 24Volt supply to apply to the valves to help diagnose, but probably overkill.

Any thoughts.. lucky wife is down the coast this weekend its getting too -5 Celsius below at night at the moment.

Any thoughts would be appreciated


 
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Old 05-25-12, 09:28 AM
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Well, of course you are not getting 24 volts DC because furnace control voltages are AC. As for the rest of it, start by posting the manufacturer and model number of your furnace.
 
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Old 05-25-12, 04:45 PM
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Well I'm in Australia and its a Brivis Wombat 92, 15 yeas old.

Btw what is the best way to measure the 24 V AC. Obviously its the blue/orange wire but not sure where I can access netural can I use ground.
 

Last edited by zzkazu; 05-25-12 at 06:56 PM.
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Old 05-25-12, 06:56 PM
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I'm not there to read the schematic and the abbreviated schematic in the picture you posted has a bright light washout. If you could post a better picture of the schematic, full schematic if possible, I might be able to help.

Furnaces are fairly simple things once you break them down into component blocks. What makes them seem formidable is the myriad of safety devices.
 
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Old 05-25-12, 07:43 PM
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Thanks Furd for your response I have since worked out and measured 24VAC is being provided to the Orange and one of the blacks. So I can conclude the redundant and economy valve solenoid are getting power.

So I need to work out if I can replace the solenoid or maybe the whole gas control unit.
 
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Old 05-25-12, 08:02 PM
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It depends on the particular gas valve but often even when replacement is possible the availability of the replacement parts (and their costs) is so limited as to making replacement of the complete assembly the more economical choice.

Myself, if I determined that the fault was in the gas valve solenoid I would replace the entire assembly rather than fiddle with the parts.
 
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Old 05-27-12, 07:16 PM
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Ok replacing the VR800A 1160 with a Honeywell VR8301A, I have the parts at this stage no one is really interested in installing this. Does anyone know it that are form/function equivalent in size etc.. ie unbolt and insert.
 
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Old 05-28-12, 09:25 AM
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Don't change the gas valve yourself.

It's fine to deal with electrical problems if you know what you're doing but anything gas or refrigerant related should always be done by a tech.
 
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Old 05-28-12, 03:13 PM
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Yes I was thinking the same..
 
 

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