heatmaker 9600 fails handoff between ignition and valve opening sequence

archibald tuttle

Heatmaker / teledyne / laars has a pretty good and knowledgable contractor support line but they did not seem to recognize the symptoms i was reporting. i filmed the ignition sequence on a working 9600 and on the malfunctioning one i'm servicing. the norm is it runs "purge" for about 15 seconds and proves airflow with a little bladder switch, then it proceeds to "ignition" which sends 120 to the hot surface ignitor for another 15 seconds. i'm not sure if the circuit tests for amperage going to the ignitor as a way of proving the ignitor is working or relies solely on flame sensor following ignition but this unit has a new ignitor with about 55 ohm resistance that is well within the 50 to 80 spec for operation and draws about 4 amps during the ignition phase. then, very briefly both the ignition and gas valve are engaged as indicated by the "valve" light on the control sequence indicator. then not even a second after the hand-off to the gas "valve" the control discontinues 120 to the ignitor (which is already hot) and places it in the flame sensor circuit to confirm flame.

in this video of the working unit:

https://www.youtube.com/watch?v=DwsAzcE3ERs

the "ignition" light goes out as the 120 goes off (although the indicator i think is in the 24V control circuit) and the "valve" light goes indicating 24V to the gas valve - briefly crossing over with both lights on as indicated. you can hear the ignition take place. the "flame" light gets a low glow and then a bright glow as the sensor/ignitor confirms flame. the valve/flame confirmation takes maybe 2 or 3 seconds. the "flame light is clearly lit through that confirmation cycle.

then, in this video of my failing unit:

https://www.youtube.com/watch?v=LRTBPFdChUo

it runs the same through the "ignition" part of the sequence and then very briefly, i.e., milliseconds, flashs to the "valve" portion of the sequence while the ignition indicator is still lit -- so quickly that i never catch voltage at the valve with my tester -- and then it drops out to a fail/retry.

I replaced the control board and got identical symptoms. from the terminals on the control board plug to the valve one wire goes straight and the other goes through a temperature sensor i think on the tube from the blower to the combustion chamber that is maybe supposed to sense back pressure if the combustion chamber is clogged. that sensor is showing open the whole way.

anyone seen anything like this. ideas. help. commiseration gladly accepted.

thanks,

brian

Ironman

1. Put a manometer in line with the pressure switch and probes from your voltage meter across its terminals. Read the pressure before and when it fires and see if it's in range of the switch. Watch your volt meter the moment it fires to see if the switch opens.
2. Make sure your flame rod is clean and the the unit has a good solid ground path all the way back to the service panel.

archibald tuttle

ironman - didn't realize we shared initials. which "pressure switch" are we talking about. do you mean the gas valve or . . . and no flame rod, these heatmakers before the endurance used the hot surface ignition as the flame sensor. the contacts are switched in the black box following the heating to ignition temp.

brian (bishop)

PS - tech at laars this morning suggested maybe something else in the 24V control circuit not in the digital portion that is only making marginal contact is dropping out at the instant that the load for the gas valve is instituted. he suggested the flow switch as one possible culprit. i'll be there in a half hour to check this . if you can tell me which sensor or component you mean by the "pressure switch" i've got manometer over there although i don't think this is making it to even opening the gas valve.

Ironman

The flue or burner pressure switch, if it has one.

JUGHNE

Whenever I have a butt kicker like this, I try some unusual tactics:
Shut off the gas supply valve, see if it does the same with no gas pressure applied.

Unplug or disconnect the wires to the gas valve, see if same shutdown. (Maybe coil faulty and stressing the board's brain)

Without a pilot burning, there must be 2-3 seconds when gas valve is open and flame sensor looks for flame before shutdown??

If you have access to the one that works, I would turn off the gas supply and see how longs it takes for the flame sensor to see nothing and shut down.
FWIW

archibald tuttle

jughne, very close on that, disconnecting the gas valve was the right idea to watch it run without any load on the 24V circuit. i had ruled out the problem being the various operating and safety limits in series in the the 24V circuit because if any of those are open the boiler won't even try to start. Although I don't think of the gas valve as a lot of load, it is 'the load' for that circuit and as soon as it would get called, the 24V safety circuit was going down. i jumped out the various safeties until i found the offending sensor.

thanks for your helpful suggestion.

and ironman, for future reference you were referring to the air pressure sensor that proves the fan?

brian

Ironman

Yes.

unclejohn

Jumper the air pressure switch wires. Some times you have to wait befor you place a jumper on them until after the fan starts. Then use a ohm meter across the switch contacts and see if the meter jumps. If it does you most likely have a bad switch.

archibald tuttle

turns out it was the hydronic operating limit which is in series with a number of safeties in the 24V control circuit. the problem in diagnosing is that normally, if anything in the chain is open, the boiler won't even try to start, so my logic ruled those out. however it must have had marginal contact for the circuit in the sensor so it would signal the control to start, but as soon as the control tried to hit the gas valve, amps on that24V circuit would rise significantly (on a relative basis, not that the gas valve is that much of a draw) and the circuit through the safeties would collapse, it would cut out to retry but it never got so bad that there wasn't a sensor connection without hte gas valve load so when i was testing i was showing the safety circuit as closed.

all better, except my frayed schedule after that set me back a day, but no use whining, who wants to listen. thanks again all.

BobC

That's a serious problem digital volt meters have, they don't readily detect sags in supply voltages that an analog meter would show.

I've seen stack computers driven beserk by a dip to 4.8v for a fraction of a second on a 5v line (a 4% dip). The fool that designed that system didn't even bother to specify a remote sensing power supply, you had to set the output to 5.25v to make sure it never dipped too low.

For some of these faults even a analog meter would be left wanting, you needed an oscilloscope to see the dip. The key is you have to have the right tool for the job.

Bob

Solid_Fuel_Man

Jumper wires and paper clips have always been in my arsenal of troubleshooting tools. Love those teasers. Divide and conquer, seems so simple on paper, in the field one has to keep their word about them.

Intermittent or poor connections were always the worst to catch back when I was an auto tech. here in the salt belt!

Glad you got it figured out!