Water heater problem

[dx]

I have an AO Smith FPS-75 series 230. This is a gas-fired 75 gal heater with spark ignition and induced draft, about 15 years old.

The problem is that it occasionally (once every few days)fails to ignite. If I reset the power, it starts up and works normally for a few days.

I can't troubleshoot it because once I turn it off and back on everything is normal. If I understand it correctly, it tries to ignite 3 times before it locks out permanently.

I had the same problem last year and found the spark gap to be too large. Closing it solved the problem. This time the spark gap is fine.

Any suggestions?

dg

[leejosepho]

Maybe a weak coil or capacitor or something, resulting in a weak spark?

[dx]

Yes, that's one of the possibilities.

What I'm asking the seasoned pros is this: if a customer had this complaint, how would you go about troubleshooting it? The AO Smith tech manual procedure does not work (obviously) unless something is failed permanently.

Replacing parts at random until it works better is not an option.

dx

[jimbo]

Well, as my auto mechanic is fond of reminding me, problems which do not manifest themselves at time of service...cannot be guaranteed to be fixed.

You mentioned this is an induced draft....the first thing I might do is check and probably replace the draft pressure switch.

[dubldare]

Off the top of my head, here goes...


Sequence of normal operation will be something like this.

A)thermostat closes, sending 120v to draft fan; fan starts
B)draft proving switch closes, sending 24v to ignition control
C)ignitor begins to spark
D)gas valve opens
E)~5 seconds for flame stabilization
F)flame rectification looks for microvolt signal from flame rod
G)unit runs until thermostat opens

Some units with pilots are slightly different:

A)thermostat closes, sending 120v to draft fan; fan starts
B)draft proving switch closes, sending 24v to ignition control
C)ignitor begins to spark
D)pilot valve opens
E)~5 seconds for flame stabilization
F)flame rectification looks for microvolt signal from flame rod
G)ignition control recieves flame rod signal, opens main burner valve
H)unit runs until thermostat opens


Cheapest thing to do first is clean the flame rod and ensure the pilot assy ground is intact and not corroded. The ground is very important as the flame rectification signal/voltage is actually the electrical difference between the pilot housing and the flame. Flame rod systems are nothing like a thermocouple or powerpile, as the later generate voltage, the flame rod is a voltage sensor. Any slight ash or corrosion will insulate it. To clean a flame rod, sanding with a crisp dollar bill is recommended, although I have used a wire brush everso carefully. Do not use sandpaper or sandcloth as any residual adhesive or sand can insulate the rod when heated.
Symptoms of a dirty flame rod would be...ignition of pilot and failure within ~5 seconds.

A restricted pilot burner orfice and poor gas supply can also be a possibility. Looking at the pilot and how it changes when the burner is on is what to look for here. If the pilot dwindles down to next to nothing when the burner is running clean/replace it.

If the burner runs a bit and then fails, chances are the inducer motor may be suspect. Motor can loose a bit of rpm performance as they age. Hot air is denser than cold, and when combined with a weak motor, burners can fail on the draft proving switch opening. A usual symptom of this is lack of capacity of hot water/lukewarm and frequent, short cycles of heater.

Thermostat... well the draft fan would never start.

Coil/s in gas valve are a possiblity for failing, although more remote of a problem.

Weak spark, replace ignition control/spark wire. You should have a pretty good gap that the spark covers. Also, if spark color is yellow/orange its weak. You want to see a white and a bit of blue. Sparking sound that is irregular in rhythm usually indicates a faulty wire.

Check all spade connectors. Make sure they are tight and mating surfaces are not corroded.

Also check venting for any backpitch/sags or blockage.


Good luck. Now you see why most guys just sell 'em a new one.

[Cass]

I have an AO Smith FPS-75 series 230. This is a gas-fired 75 gal heater with spark ignition and induced draft, about 15 years old.

The problem is that it occasionally (once every few days)fails to ignite. If I reset the power, it starts up and works normally for a few days.

I can't troubleshoot it because once I turn it off and back on everything is normal. If I understand it correctly, it tries to ignite 3 times before it locks out permanently.

I had the same problem last year and found the spark gap to be too large. Closing it solved the problem. This time the spark gap is fine.

Any suggestions?

dg

I would strongly consider replacing a 15 year old A.O. Smith.

I wouldn't be surprised if you can't find replacement parts.

[GrumpyPlumber]

I would strongly consider replacing a 15 year old A.O. Smith.

I wouldn't be surprised if you can't find replacement parts.

And heres the sad irony....you'r next A.O.Smith won't outlive that.

[Cass]

And heres the sad irony....you'r next A.O.Smith won't outlive that.

You got that right !!!!!!!!!!!!

[dx]

Thanks guys, but a new heater is not gonna happen. And yes, all replacement parts are available.

Dubldare, thanks for taking the time and effort. Your sequence of operation is same as shown in their service manual.

There is no flame rod, so that simplifies things. It's a simple ionization detector. When it's done sparking, it sends a voltage to the spark electrodes and measures the current. If the space is ionized (flame), the current is much higher then if there is no flame.

The pilot flame is nice shape/color and stays nice. Main burner flame is perfect. Burner runs perfectly until shutting off (reaching temp). Heater has great recovery and does not short cycle. With the thermostat at 130F I can run 3 showers forever while doing laundry.

Spark gap is specified at 0.090-0.150 and it's right around 0.100. I can't see the spark, but sounds fine. I'll have to get a dentist mirror or something to get a visual. Excellent idea. Thanks.

He he he, I do see why most people would sell'em a new one. I couldn't afford me if I had to pay myself for all this troubleshooting either

dx

[dubldare]

There is no flame rod, so that simplifies things. It's a simple ionization detector. When it's done sparking, it sends a voltage to the spark electrodes and measures the current. If the space is ionized (flame), the current is much higher then if there is no flame.

My bad, it's a microamp signal we're looking for here. The ignitor is also the flame rod in your case. A little googling on 'Flame rectification' can probably help you better than I have.

[dx]

Yes. Unfortunately I can't check the flame rectification current. I do have a microamp meter but I'm not about to stick it in a circuit that's going to see several kilovolts when it fires I'll take a close visual look at the spark tonight.

dx

[jimbo]

I reiterate to investigate the pressure switch.

[hj]

Maybe you will be lucky and it will start leaking tomorrow so the problem will be solved. Or at least not just after you spend a couple of hundred dollars trying to fix a ghost problem.

[dx]

hj,
Did you have a bad week bud, or are you just an unhappy fellow? You remind me of Eeyore.


Jimbo,
I did look at the pressure switch. Works fine and clean contacts. I ordered a new one anyway, they're cheap enough. Thanks.

dx

[GrumpyPlumber]

hj,
Did you have a bad week bud, or are you just an unhappy fellow? You remind me of Eeyore.

15 years old.
a Mustang you restore.
A yacht sure.
a 15 year old water heater you xcome home one day wondering why you didn't get a new one 6 months ago when the basement is flooded.
He offers you the best advice I've seen on this thread...you insult him.
Maybe you should just request a refund.