Oh no! My H500 broke!

josephny

Well-known member
Jun 4, 2009
82
Sullivan County, NY
Don't think that I can go much further with this as I am not familiar with the 3 wire temp sensor that is used and also don't have any specs on operation of the circuit board. I can deduce that if the display circuit board/keypad has a higher setpoint temp than the actual water temp and the control module has determined that the blower vacuum switch is open, the next step would be to power the blower through one of the relays and close the vacuum pressure switch to progress through the ignition sequence. But what I can't tell you for sure is why the relay is cycling ( It is possible that the relay is for powering the blower, but for some reason the blower is either not getting power or there is a problem with the blower motor. Also not sure why the relay is cycling with no input from the display citcuit board/keypad.

I just put an ohm meter on the ignitor and it reads 16 ohms. Not sure if that's within spec.

I don't know what a fully functioning heater would do if the display board were disconnected. If it would still function, then, in my case of the relays still clicking, then we can rule out the display board.

I suspect, however, that the unit would not operate normally with the display board disconnected. If this is true, and my heater's relays cycle open-closed, then it still points to the control board or a sensor input.

I think the process is that that the ignitor is energized and heats up and then the gas valve opens. In my case, the igniter doesn't heat up (I didn't check for voltage going to it, but will), so the system is failing in some way at a very early stage in the process. Either a sensor is telling the control board not to supply current to the igniter or the sensor board itself has failed.

Does that all sound reasonable?

Thank you!
 

josephny

Well-known member
Jun 4, 2009
82
Sullivan County, NY
The heater has 2 pressure switches: One for blower vacuum and one for blower pressure, I think. In what state (open or closed) would both of those be in in order for the heater to operate? I think I have one that is open and the other is closed (electrically, that is).
 

swamprat69

Well-known member
Mar 30, 2019
578
Las Vegas, NV
The lower pressure switch is normally closed and only opens if the exhaust is blocked after the blower runs and the heater fires. The upper vacuum switch must be open before the blower runs and closed after the blower starts up. But you are not even starting the blower, so the upper switch needs to be open without the blower running and the lower pressure switch (normally closed as a safety) is not even at play until after the blower starts up.
 
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josephny

Well-known member
Jun 4, 2009
82
Sullivan County, NY
Anyone have any ideas?

What could it mean that the relay(s) on the control board clicks, display lights, but nothing else happens (fan motor never starts -- no voltage gets put on the plug to the motor) after replacing the control board, vacuum sensor and transformer?
 

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josephny

Well-known member
Jun 4, 2009
82
Sullivan County, NY
So I slept on it yet again and woke up (at dawn) re-determined.

Did a little free associating/meditation/mind-flow techniques and thought: "if the relay is closing, all else appears to work fine, but 120v doesn't get send to the blower fan, then let me check AGAIN if 120v is getting to the control board."

Sure enough, 105v was getting through the control board.

Then I found this oxidized fuse. Swapped it and all is working.

$500 and 100 hours of work later, it was a $0.10 part.

Thank you so very much! This is a really great forum.
corroded_fuse_10ad754d4ee0dc037f25dfcfe44cc9b65d76720b.jpg
 
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