OT? Furnace Thermocouple Life?

Check the pilot flame it should be a blue flame. If it is yellow or partly yellow you will have to take the pilot assembly apart and clean it. Fine dust and lint will block the air to the pilot. This is more prevalent on boilers and counterflow furnaces because the burners are closer to the floor. Furnaces don"t end up in the cleanest rooms in the house.

I have been in the HVAC industry nearly 40 years and haven"t seen a furnace blow up do to the pilot going out and the burner staying on. I am not saying it couldn"t happen but I haven"t seen it. I have seen gas valves stick open usally do to pipe joint compond in the valve. Gas appliances are probably safer than the tractors we play with.

For the sake of this discussion, lets assume we have a single stage gas valve and need to install a thermocouple.

1) Please list the install procedures for the thermocouple?

2) Please list ALL tests that the thermocouple must pass after it has been installed and give a brief over view of that test?

3) Please give a brief explanation of how this gas valve orperates?

I will give you my follow up post after you answered these basic questions!

Retired member of ASHRE (American Society of Heating And Refrigeration Engineers)

T_Bone

Sounds like to me who ever did the conversion from natural gas to LP gas didn't set the correct pilot flame impringment onto the thermocouple.

This does two things:

1)it severely shortens the life of the thermocouple

2) When the pilot flame goes out, the main gas valve is held open to long thus dumping a extra raw gas into the burner area and defeats the saftey factor of using a thermocouple.

As you can see reason #2 is of a huge concern as it could/will cause a explosion.

This problem is way beyond the scope of this forum and you need to call a service tech. I suggest calling the gas supplier as they will test the thermocouple for proper orperation since it was clearly not done during the conversion setup.

If you listen to the responeses below, you will get someone killed!

Please get this taken care of as soon as possible as this isn't a small problem that can wait!

T_Bone

I was with you up 'til your #2. I'm afraid I don't understand why a misadjusted pilot would cause a hazard. Could you elaborate?

There's many different types of gas valves, varible volume, constant volume, multi-stage and single stage and just as many control designs. These type of vlaves can be found on residentual heating units and probably a few more as the burner units get more efficient.

For this example I'll use a simple single stage gas valve with thermocouple.

A typical thermocouple generates 350mv when heated to design tempature. This voltage then is appiled to a coil on the main gas valve holding the valve open so a volume of gas can flow to the burners.

If the thermocouple has too much heat beyond it's design limit, then it will not cool down fast enough to quit generating electricity thus holding open the main gas valve too long dumping excessive raw gas into the combustion chamber.

For the sake of this discussion, lets say the mfg designed the combuston chamber for when the thermocouple at correct design tempature would allow .125cf of gas into the combustion chamber upon pilot flame failure.

Now lets heat the thermocouple to twice the mfg design flame impringement length. In the real world it will take approx 3times as long for the thermocouple to cool down and shut off the main gas valve and now .5cf of raw gas is now dumped into the combustion chamber upom pilot flame failure or 4 times the mfg limit for the combustion chamber design.

There are tests that have to be completed on a typical thermocouple system to insure the thermocouple is installed correct. To many variables in both parts and controls to cover here and is why there's books written on the subject that can't be covered on the internet.

4 times the gas volume beyond mfg design is alot of BOOM!

T_Bone

Hang tight as your question will be answered shortly.

T_Bone