Hot water heater, LPG . Replaced the thermocouple twice, cleaned all orifices and even replaced gas control valve. Pilot light when lit is a large yellow flame. When trying to light the pilot light, a flame also comes out of the main burner. If the tank is hot, pilot light will not light. If the tank is cold, pilot light will light, main burner lights with large yellow flame but when it reaches the set temperature, both go out and cannot be relit until the tank cools down completely. Makes no sense

Yellow flame is not good, burner lighting when lighting pilot even worse. I'd consider replacing the gas valve unless the unit is more than 10 years old, then consider a complete replacement.

Sounds like you changed 'a' gas valve already... is there more than one? What if the replacement was faulty out of the bag? Are there other gas appliances? Are they running normally? Maybe there is a pressure issue?

Well, it seems I don't have any answers for you just more questions sorry. But what would you expect from a bricklayer?

Hope my two cents count for something. Cheerio!

Good catch mudslinger, Op says the valve was replaced. If the OP comes back maybe he can tell us about other gas appliances operation.

Thanks for the complement ballengerb1. One question... don't forget I'm just a bricklayer... what do you mean by Op or OP? I could venture a few humorous guesses but I suppose you mean 'operator'?

Cheerio! Aaron

Forgot you are new OP is original poster.

First off, what brand and how old is the heater? Second, I bet the flame is coming from the compression fitting on the bottom side of the pilot, not from the main burner, which means you need to tighten that fitting. Now, it really sounds like you have a lack of combustion air due to plugged inlet filter, or plugged anti explosion screen, which lets in combustion air. But that's just a guess at this point. Really need to know the make and year of the heater. This will narrow down the trouble shooting. Take care.

Not to hijack the thread, but I actually have exactly the same problem, and can provide some more detail. HWS (Bosch Wizard 960) was running fine yesterday (we are on bottled LPG) - when one bottle ran dry & water became cold, I noticed & switched over the regulator thingy (connected to our 2 bottles) to our other full bottle. Eventually got the pilot going last night, but with large yellow flame (about hand size) continually burning - lots of soot. Couldn't fix - turned gas off overnight. Rang around Sat morn to gas fitter & a few relevant tradey mates - they all said similar story. Yellow flame = incomplete burn, not enough oxygen - likely blockage in little gauze / air vent or something stuck in pilot jet line. Opened it all up about 5 times today (safely) cleaned everything I could find - the pilot is all clean, and the little S-shaped tube below it. Gas is coming through OK - still large yellow flame, lots of soot - turn on hot water flat out, and HWS doesn't have the grunt to properly light up. Left a message in desperation this arvo with my gas-fitter - he's stuck at a kid's party! Looking like cold showers for the weekend. Any help would be appreciated, and hopefully help the OP too :)

Original poster... of course. Cheerio