Heatnglow TownsendII-LP

Some Heat & Glo products are designed for non-vented (vent-free) applications. In those models, the flame input is typically limited — often under 30,000 BTU/hr — because vent-free appliances must meet strict combustion and safety standards.

What model number is your gas log?

If you have a vent-free model, it can generally be installed in a vented fireplace behind glass doors, but it is specifically engineered to operate without a chimney draft. In order to be approved as a vent-free gas appliance, it must include a safety device that shuts down the burner if oxygen levels in the room drop below a safe threshold.

That safety device is called an Oxygen Depletion Sensor (ODS).

The ODS system typically consists of a precisely engineered pilot burner and a carefully calibrated orifice. It works in combination with a thermocouple (or thermopile). As the oxygen level in the room decreases, the pilot flame characteristics change — it becomes smaller and lifts slightly off the burner. When the flame can no longer properly heat the thermocouple, the millivolt signal drops, the gas valve closes, and the appliance shuts down.

It does not wait until all breathable oxygen is consumed. The shutdown occurs well before dangerous oxygen deprivation levels are reached. It is a preventive safety measure.

Given the location of your gas log inside the fireplace — particularly if glass doors are closed or airflow is restricted — is it possible that the oxygen level around the burner is dropping during a 30-minute burn? If so, the ODS may simply be doing exactly what it was designed to do.

It appears the Townsend II has a Thermocouple and a Thermopile.

Just to be clear, so when it acts up

" it will run for 10-30 mins and then the gas valve will shut down completely "

The pilot flame goes out ?

https://manuals.fire-parts.com/aws-assets/heat-n-glo-townsend-ii-townsend_ii-pdf.pdf?inline=true

Can you watch the pilot flame as the milliVolts drop ?

Since you replaced the pilot assembly I wonder if they sent you a pilot assembly that has the Oxygen Depletion Sensors (ODS) built in. And that is not correct for your unit.

Apparently Oxygen Depletion Sensors (ODS) effectively moves the flame away from the thermocouple to cool the thermocouple enough so the gas valve shuts off the pilot flame.

I wonder if this all started with a bad thermocouple then other issues were introduced into the situation by changing other parts causing similar but different problems.

With a good pilot flame sufficiently and consistently heating the thermocouple the milliVolts should not drop.

Well if the flame seems consistent and is sufficiently engulfing the end 1/2 inch of the thermocouple until the instant 'it just goes out', I try a different thermocouple, probably the least expensive thing commonly available. No kinks in the thermocouple cable.

Image from an old boiler manual.

I can think of a bunch of independent tests I would do in your situation. However I have many Electrical / Electronic toys to play with.

One thing I think you could do is remove the thermocouple voltage test adapter from the gas valve and monitor the thermocouple voltage while holding the gas valve's pilot push to light control and monitor the thermocouple's voltage with the pilot lit. This effectively removes the load of the gas valve's pilot solenoid. See what the voltage does.

You could also do an independent test with a propane torch heating the thermocouple. You have to be careful to not let any raw propane gas into the living space. Either light the pilot also or shut the gas off the the heater.

With the multiple thermocouples you tried were some from different manufactures ?

The reason ODS keeps coming up it's that what you're describing is the expected behavior of an ODS pilot during an oxygen deficiency event. Don't be offended, but you're sure you've got the correct pilot assembly, not an upgraded assembly with ODS?

The next things I'd check are pilot gas pressure, to see if it's dropping due to perhaps an obstruction in the pilot tube or orifice, and power delivered to the valve (voltage and current together) up to pilot failure. Maybe cold & hot resistance (before pilot ignition & immediately after pilot failure) of the gas valve itself, even though it sounds like you already changed it. Two identical failures is very unlikely—but not completely impossible, and would explain the issue.

That is a common misconception among non-professionals. The fact that an appliance is installed in a vented fireplace does not necessarily mean it does not use an ODS (Oxygen Depletion Sensor) pilot assembly.

While a vented appliance cannot be safely operated in a non-vented configuration, a vent-free (non-vented) appliance may be installed in a vented fireplace, provided it is listed and installed according to the manufacturer’s instructions.

The part number you referenced is, in fact, an ODS pilot thermocouple assembly. There is a distinct design difference between a standard pilot assembly and an ODS pilot. The ODS pilot is specifically engineered with a calibrated air intake and orifice to respond to changes in ambient oxygen levels.

The difference is noticeable: the thermocouple on the left is smooth and allows for adjusting how far the tip can be pushed up into the pilot flame. The one on the right has a precise dimension on the sensing tip (Red Arrow) and two machined ridges (Blue Arrows) to ensure that thermocouple insertion can NOT be adjusted.

Based on the model number you provided, your appliance is equipped with an ODS system. Given the symptoms you are describing, it is very likely that the unit is shutting down due to oxygen depletion or restricted pilot combustion air.

One common cause of this condition is lint or dust accumulation in the pilot air intake opening. Even minor debris can alter the pilot flame characteristics enough to reduce thermocouple output and cause nuisance shutdowns.

I recall resolving a similar issue for a customer by carefully clearing lint from the pilot’s combustion air opening. by blowing the lint away with a plastic straw. In that particular case, the issue recurred approximately every two years due to environmental dust buildup. The customer called it the McDonalds straw procedure

You might ask: "Then why did appliance operate properly for 30 years and has only recently begun exhibiting this problem?" That suggests an environmental change in the room. Possible contributing factors could include:

• New carpeting or flooring.
• A new pet that sheds.
• Changes in cleaning methods (for example, no longer using a central vacuum system)
• Increased airborne dust or lint in the space.
• New HVAC system with an air filter that is not as good as the previous system.

Something in the environment is likely contributing to debris accumulation at the pilot air intake. Identifying that change may help prevent recurrence, or maybe you just need to clean the intake more often, as a result of a change that you just can't pinpoint.

Hope this helps

Mr. Ed