Teledyne Laars Mark V 400'000 BTU heater overhaul

[blackPlaster]

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.

cheers

 

[ps0303]

Re: My Teledyne Laars Mark IV 400'000 BTU

Be prepared to possibly buy a new heater. Many times bolts and what not are rusted on these old pieces. Then the possibility of it not going back together for some silly reason. Be very careful on "reaming" the header. If the copper is too thin it will break and you will have water all over. Once again, be prepared to buy a new heater. Also good luck finding any parts if needed. Hard to come by.

 

[blackPlaster]

Re: My Teledyne Laars Mark V 400'000 BTU

It seems the flow control is shot and let most of the water always bypass the header. That explains the big bangs: water slacks in the header, boils and and pressure bursts.

The by-pass valve disc looks like it lost its outer part. In addition i saw nothing looking like a useable valve seat between the inlet and outlet ports (inside the inlet/outlet header). Instead it looks like Fukushima in there. Sounds like I need a new inlet/outlet header + by-pass valve disc. Looking at $200. That may not be the end of the story if also the header needs replacement.

So I need to take the heater out of the plumbing, to get time to work on it.

So maybe the pressure switch is not broken: it may indeed not be getting sufficient pressure, because the header is mostly bypassed.

My current understanding of what a flow control valve is supposed to do, based on the observation that the spring pushes the valve to closed from the warm side.

* heater off: pressure is lower on hot side because of head loss in the header and water is at same temp on both sides. Valve opens as is it calibrated to provide a path with less head loss than the head of the header
* heater on: pressure on hot side is increased because the water is warm in there. The additional pressure closes the valve when the water coming out is warm enough.

The hotter the water, the more the valve wants to close. The colder the water the more the valve wants to open. This regulates outlet temperature

So if I had a manual bypass, and I had a way of keeping the valve always closed, it seems the heater could function. Pro: less head loss when heater is off. Cons: more work to use heater, and probably not very safe.

Thanks Paul for your words of caution. The advantage of being a hobbyist is that I can waste any amount of time to attempt the overhaul. Buying a new heater is not an option. For now it will be the mammoth or no hot spa.

 

[blackPlaster]

Took the header apart over the week end. Hammered a lot of rust out of the in/out header and the return header. The inside of the heat exchanger tubes is perfectly clean and reaming is not needed. Brushed black soot off the exchanger fins to improve heat transfer.

Ordered a by-pass valve assembly and new header gaskets. $78 shipped.

Re-plumbed to avoid heater temporarily. Got rid of three 90 degrees and used two 45 degrees instead. Amazing how much more water gets pumped.

Got in touch with old timer on Craigslist who had a laars mark v for sale, but his in/out header was in no better shape than mine. Anyway picked up two gas valves, a pressure and high-limit switches from him for $40. It was nice to meet what appears to be the only other DIY person in town!

Happy labor day everyone.

 

[blackPlaster]

The parts I ordered at poolpartsonline were a perfect fit. re-plumbed with a valve allowing to by-pass the heater (to reduce head loss).
Took burner tray out, brushed quite a bit of rust off.
re-sealed and tightened gas pipes
pressure valve switch was defective, swapped with unit from an old timer I met on craigslist
had a leak around temperature sensor tube on intake header. Requires a special o-ring that would not have been easy to source. Made a seal using blue Permatex (stuff for engine valve covers gaskets). Seal is now tight. Everything works flawlessly, including flow control.
Enjoyed the spa friday and saturday night.
400K BTU is a heck of a lot of heat. When the heater cycles i can really feel hot water arriving in the spa. Takes about one hour to heat spa from ~65 to ~95.
Keep your Teledyne Laars Mark V, it was built to last forever.