This post is not really to ask a question. It is rather to share the experience of attempting to overhaul my mammoth.
The heater came with the house and its gas supply was off. When the gas technician was here to turn the water heater and whole house air heater on, he also had a look at the mammoth and told me it was an ancient machine, but well built. He refused to try to fire it up because of the amount of debris on the top (collector).
10 screws later the top was off. A rat's nest later and a fossil rat later the top looked much cleaner.
Said rodent and ancestors had chewed the insulation of the wires to the high limit switches and thermostat. I patched these up with electric tape. Moved them as far as possible to the heat. The high limit switches provide continuity and the thermostat + on/off switch worked.
The pressure switch however did not close the circuit with either high or low pump speed. Went ahead to short it. Saw a 1psi for $10 on line. Not sure if I need 1psi or more.
The pilot light lit, but the flame was coming out of the bottom, not the top. The flame was hot enough to hold the pilot, but not hot enough to let the gas valve fire things up.
I was becoming impatient -not good when dealing with a mammoth - and got a used 9v battery and jumped it over the pilot connectors, then shorted the control circuit and the mammoth fired up. Wow, you can really tell a serious amount of gas is burning in there. I kept it on for 2 seconds. Pump on.
Took the pilot assembly out, cleaned the tube, which was full of soot or whatnot. brushed rust off the chimney going sideway towards the burners. Nice, steady hot pilot fired right up.
Decided to be brave and this time ran the heater about 1 minute, pump on. Heard water heat, felt warmer water come out. But lots of banging. After reading more here, sounds like water boiling and sending high pressure in the pipes. Did see the filter pressure reach 45psi +, even though there is a check valve between filter and heater. One valve past the heater got stiff because of junk in it. Another valve started leaking.
Quite some rust/dust came out. It seems one spa port got a bit plugged with it. Water coming out was nice and warm.
Plan for future action: disassemble and check the flow control valve. It may have gotten lazy. Need to investigate what the different spring colors correspond to. Also may need to 'ream' the tubes of the exchanger. I have no idea where to buy the appropriate reaming tool. One burner line does not burn clean, need to find a way to clean.
My water chem is rather normal, but I'm dealing with 20+ of unknown history from previous owners. The pool had fish in it for a couple of years.
Old stuff is Fun stuff. I am hoping to get many hot spas from this mammoth.