Majestic fireplace main burner not coming on

[Azo]

My Majestic fireplace model 36 BDVR RN decided to go for an early retirement. More exactly the main burner does not light. I read some treads related to this type of fireplace and fault but I cannot come to a conclusion.
The pilot is working fine and the voltage looks good for switch in off position only.
I have another identical fireplace in the house that is working fine and I compared the readings.
Switch off:
TPTH (top contact) to TH (bottom contact) = 630 mV for both
TPTH to TP (mid contact) = 630 mV for both
TP to TH = 0 mV for both.
Switch on:
TPTH to TH = 0 mV for both (normal since the switch is on)
TPTH to TP = 280 mV for working one and 0 mV for faulty one.
TP to TH = 250 mV for working one and 0 mV for faulty one.
It looks that the main valve is working. When I turn off the pilot and the thermopile starts cooling down and the voltage drops, at about 300 mV I can hear the characteristic sound of the solenoid.
What went wrong?

[HayZee518]

furnace

change the thermocouple.

[Azo]

The thermocouple is keeping alive the pilot and the pilot is working fine. Is the thermocouple related in any way to the main burner circuit?

[HayZee518]

thermocouple

the thermocouple is the main safety device for the whole burner. when the flame goes out, the thermocouple closes the pilot valve and in turn closes the main valve and energizes the lockout circuit [relay]'if you say the thermocouple is working fine, then the relay it feeds is shot.
your furnace may have a millivolt generator which operates the relay and main valve.
if you can find the leads that come from the millivolt generator, temporarily connect up a small mercury cell to the terminals. this will simulate the millivolt generator and the main valve should open.

[Mabscotthandyman]

Oxygen problem?

It could be a oxygen depletion problem is the flue clean and clear? Every thing is electronic these days possibly a circut board. Is it a standing pilot or electronic light pilot some have a electronic igniter that lights a pilot that lights the burner there is a flame sensor that detects flame to open the valve for the burner if the flame sensor is bad the unit will not light.

Your unit is vented correct. There are some that are vent free that have an oxygen depletion sensor that will prevent the unit from working if it is bad.

Dan

[Azo]

Thank you for advice. TP is not capable to supply the required current to open the solenoid.
I used a variable power supply set on 600 mV. Main burner lights and the voltage readings when switch is ON position is: TPTH to TP = 280 mV and TP to TH = 250 mV. The power supply showed me that required current consumed by the solenoid is 130 mA. The thermopile is not completely dead since if I disconnect the power supply the main burner still lights (not enough power to move it, but enough to hold it).
I cannot find in Canada a parts supplier. It may be a law that only qualified/licensed people can service these units. Can you recommend me a good parts supply store? I saw TP sold between $40 (Amazon.com) and $101(Vermont). I also want to add a fan kit since without a fan the fireplace is just for decor. The other fireplace has a fan and this saved me. Over the weekend it was a power outage in Ontario, over 250,000 houses were left in dark due to freezing rain. I used a small inverter to power the fan and around the fireplace I've got around 75F. Good enough to avoid going to a hotel. Vermont sells the fan for $380...this is quite a rip off.
Thank you one more time.

[Mabscotthandyman]

It wont let me post urls till I have made more post but the two sites are ibuyfireplaceparts
And fireplacepartsworld

You will need to add the www and the dot com

Hope this will help you
Dan

[HayZee518]

thermocouple, thermopile concepts

both of these devices generate a voltage. How they are applied to the external circuit is different.

Gas appliance safety

Many gas-fed heating appliances such as ovens and water heaters make use of a pilot flame to ignite the main gas burner when required. If the pilot flame goes out, unburned gas may be released, which is an explosion risk and a health hazard. To prevent this, some appliances use a thermocouple in a fail-safe circuit to sense when the pilot light is burning. The tip of the thermocouple is placed in the pilot flame, generating a voltage which operates the supply valve which feeds gas to the pilot. So long as the pilot flame remains lit, the thermocouple remains hot, and the pilot gas valve is held open. If the pilot light goes out, the thermocouple temperature falls, causing the voltage across the thermocouple to drop and the valve to close.

Some combined main burner and pilot gas valves (mainly by Honeywell) reduce the power demand to within the range of a single universal thermocouple heated by a pilot (25 mV open circuit falling by half with the coil connected to a 10–12 mV, 0.2–0.25 A source, typically) by sizing the coil to be able to hold the valve open against a light spring, but only after the initial turning-on force is provided by the user pressing and holding a knob to compress the spring during lighting of the pilot. These systems are identifiable by the 'press and hold for x minutes' in the pilot lighting instructions. (The holding current requirement of such a valve is much less than a bigger solenoid designed for pulling the valve in from a closed position would require.) Special test sets are made to confirm the valve let-go and holding currents, because an ordinary milliammeter cannot be used as it introduces more resistance than the gas valve coil. Apart from testing the open circuit voltage of the thermocouple, and the near short-circuit DC continuity through the thermocouple gas valve coil, the easiest non-specialist test is substitution of a known good gas valve.

Some systems, known as millivolt control systems, extend the thermocouple concept to both open and close the main gas valve as well. Not only does the voltage created by the pilot thermocouple activate the pilot gas valve, it is also routed through a thermostat to power the main gas valve as well. Here, a larger voltage is needed than in a pilot flame safety system described above, and a thermopile is used rather than a single thermocouple. Such a system requires no external source of electricity for its operation and thus can operate during a power failure, provided that all the other related system components allow for this. This excludes common forced air furnaces because external electrical power is required to operate the blower motor, but this feature is especially useful for un-powered convection heaters. A similar gas shut-off safety mechanism using a thermocouple is sometimes employed to ensure that the main burner ignites within a certain time period, shutting off the main burner gas supply valve should that not happen.

Out of concern about energy wasted by the standing pilot flame, designers of many newer appliances have switched to an electronically controlled pilot-less ignition, also called intermittent ignition. With no standing pilot flame, there is no risk of gas buildup should the flame go out, so these appliances do not need thermocouple-based pilot safety switches. As these designs lose the benefit of operation without a continuous source of electricity, standing pilots are still used in some appliances. The exception is later model instantaneous (aka "tankless") water heaters that use the flow of water to generate the current required to ignite the gas burner; these designs also use a thermocouple as a safety cut-off device in the event the gas fails to ignite, or if the flame is extinguished.

[Azo]

Thank you for the general overview of the two systems: thermocouple and thermopile.
My fireplace contains both. In this case I believe the only source of electrical power is the thermopile. When heated by the pilot it will generate 600-650 mV and probably at least 200 mA to open the main burner gas valve (the power supply I used to diagnose the thermopile showed 130mA). In these systems the thermocouple should not generate any power. I believe the thermocouple is just a thermistor that vary its resistance with temperature. It is using the power generated by the thermopile, and it will hold open the pilot gas valve. The current required for the pilot gas valve should be very small compared to the main burner gas valve. I disconnected the thermocouple and its tip is isolated from the main brass pipe, it looks like a thermistor for me.
Please note that I used the word "I believe". Before I tried to repair my fireplace I had no idea how is working. I was amazed that the fireplace lights despite the power outage (common sense: anything with a switch need power...wrong).
These are my conclusions after reading many related threads and the answers I received.
Please let me know if my suppositions are correct. I cannot find an electrical diagram showing the connections for gas valve, main valve, thermopile and thermocouple.

All the best,

[HayZee518]

parts

that site fireplacepartsworld is a good site
just type in your nomenclature and it will find the valve you need.
further down the page it has the thermocouple too.