MiniMax CH heater not working - bad thermocouple?

[merit]

Hi All,
I have an old MiniMax CH 200 pool heater. It worked intermittently last year but this year I'm not getting any heat. The pilot lights and stays lit fine. Looking around on the Internet I've seen several people talk about thermocouples going bad. I cleaned some carbon off it with sandpaper but it didn't make a difference. I've measured the voltage at the terminals where the thermocouple connects to the gas valve and I was getting about 60 mV, I have no idea what it should be though.

Any suggestions of things to look at? Should I consider replacing the thermocouple?

Thanks!

 

[merit]

I did a little more messing around. I measured the resistance of the wire with all the safety switches - (TH on the thermostat board to TH on the valve) and its about 2 ohm. When I turn off the pump it goes to disconnected, so that seems to make sense. Then I tried jumping it out - connected TH straight to T - no change, the burners still don't fire. So I guess I can rule out the hi limit, safety shut-off (whatever that is?), pressure switch, and thermal cut-off.

That seems to leave the gas valve, thermostat board, temp sensor, or the pilot generator. Anybody have an idea how I can rule those out? Is there any way to try to manually activate the gas valve to see if it's working?

Thanks

 

[danpik]

We first need to determine if you have a thermocouple or a powerpile (commonly called a milivolt generator)

This is a thermocouple...

Notice that this has a copper tube that threads into the gas valve. A thermocouple will generate a milivolt output in the 20-30 Mv range if it is good. High teens and they will sometimes work...mid to low teens and they need to be replaced. I have seen some thermocouples generate over 30 Mv but rarely. 20-25 seem like the normal good range.


A powerpile or milivolt generator looks like this...

A powerpile has a two lead wire that had attachment terminals on it to mount to the power input screws on the gas valve. The milivolt output on these should be 750 Mv plus or minus 25.

Both need to be clean and well enveloped in the pilot flame to work properly.

 

[merit]

Thanks for the reply danpik. It's a power pile, I was using the wrong word

I found the service manual for the thermostat and it's troubleshooting section says I need at least 200 mv from the pilot generator. Since I only have about 50 I think I need to replace it.

 

[danpik]

Don't worry about it, unfortunatly the terms get mixed use all the time.

Normaly milivolt generators output 750 Mv. There are some though that put out less. It probably would be best to replace it with an OEM part. either way, 50 is way too low for it to work.

Good luck

 

[merit]

For posterity and google's sake, here is the service manual for the TS8205, which is the thermostat and sensor assembly: http://vancontrols.com/pdf/hw/TS8205.pdf

This is the gas valve (well, a drop in replacement, the original doesn't seem to be available anymore): http://customer.honeywell.com/honeywell ... VS820C1100

Also, this guy has a very good rundown on troubleshooting steps for the very similar minimax 150 heater: http://www.fixya.com/support/t10286454-pentair_minimax

Hopefully this can save someone the time I've spent searching the web in the last few days.