Intermittent ignition failures on Hayward 400FDN heater

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I thought about this for a couple of minutes, and remembered that earlier today when I was experiencing type 1 failures, I was watching the meter and it was drawing gas for about 3.5 seconds during a failed trial. So I don't think it is shutting off gas too soon. I wasn't watching the meter during the type 2 failure.
 
I've seen corrosion inside gas valves that produces intermittent firing problems. Only way I know to test is to replace the valve.

BUT... I'd get your gas supply issue settled first. If the burner is choking on not enough gas (maybe the regulator is affected by the moisture?) that would cause a similar issue.
 
My wife and I are currently watching a series called The Investigation on HBO Max, which is about the investigation of a notorious murder case in Demark. At one point, over dinner the lead investigator comments to his wife that every time they think they are on to something, it turns out they are not. This is exactly how I feel at this point. To summarize where I am in my own "investigation" into why my heater doesn't fire reliably when the weather is damp:
  • Started off thinking this was a problem with flame sensing. But improvements to grounding and replacing the flame sensor did not help, and further digging into the details of the ignition cycle revealed that the symptoms were not really consistent with a flame sensing problem.
  • I was able to inspect the orifices, and they all look clean. Accessing the burners themselves is not easy, and I haven't tried to tackle that yet. But I do operate the heater at least once a week for 15 minutes year round, so it doesn't sit idle for long periods of time.
  • Performed some testing on how the system was behaving during the ignition cycle, and determined that behavior matched the descriptions in the documentation. So it does not appear to be a problem with the control board.
  • During the ignition cycle, the igniter is only glowing about halfway down the rod from the tip, but according to Hayward this is normal.
  • My gas supply looked suspicious, particularly the part from where the supply pipe emerges from the ground and connects to the heater. The work was not well done, but based on discussions with the gas company and some meter testing I did when the heater fires successfully, it does not appear the flow is constricted as I had thought might be the case. The heater consumes just under 380 CFPH when it is in operation, and consumption is constant even when all other gas appliances in the house are operating.
  • I've ordered a manometer to test the pressure at the input and output to the gas valve. We'll see if it is in spec.
 
"When you have eliminated the impossible, whatever remains, however improbable, must be the truth." – Sir Arthur Conan Doyle

I'll be interested to see what the manometer tests show.

I also wouldn't be surprised if it ends up being a bad gas valve.
 
Manometer came in late this afternoon. My manifold pressure *was* out of spec -- out of spec high (around 2.4 "wc when it should be 1.8 - 2.0), which is not what I expected. I dialed it down at the heater gas valve regulator to 2.0 and did several trials to make sure the pressure was correct. Tomorrow is supposed to be rainy, so we'll see if that change made any difference.

I also did another clock test after the adjustment, consumption rate is down to 336 CFPH (from 378).
 

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No, it was getting late and I was losing light. But if the gas valve regulator could be adjusted to make the manifold pressure in spec, then is the upstream pressure all that important? The valve will tolerate a relatively broad range of input pressures (4.5 - 10.5" wc).
 
Meausured the input side this morning. Pressure before the valve opens stabilizes at around 12.6" wc. When the burners are fired the steady state is around 5" wc. That's on the lower end of the range but within spec. Rain is coming soon, we'll see if it still has firing problems.
 
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After the rain, I'm getting "type 1" ignition failures again (very brief sound of weak flame, goes into IF after three trials). I've monitored input and output pressure of the gas valve (not at the same time) during these failures, and it seems like the problem is on the input side, I think with the regulator on the supply. According to the label on the regulator, it is supposed to step down from 2 PSI to 8" wc (with an adjustable range between 6-14").

This morning when the heater was firing successfully, when I first connected the manometer before firing the heater, the input pressure started in the 50's but then gradually bled down to between 12-13 before I started the ignition sequence. Then the pressure dropped to around 5" while the heater was firing. The pressure went back up to between 12-13 when the gas valve closed.

After the rain, again the input pressure started in the 50's but dropped much more slowly than before. When the gas valve opened, it dropped way down (sometimes less than 1", sometimes between 1-2") while the gas valve was open. When the gas valve closed, it jumped back up to the 19-20" range and remained there until the next trial.

I haven't tried to make any adjustments to this regulator yet. I'll give this a try later if the rain stops this afternoon.
 
Got a respite from the rain, so went out to the heater to do some more experimentation. Hooked up the manometer to the input side of the gas valve, fired up the heater and ... it fired right up. So I took the opportunity to adjust the supply regulator to its stated output while the gas was flowing (8" wc) and did several more trials, all of which worked perfectly.

So I thought I better check the output of the gas valve to see if perhaps my adjustment on the input side had now taken it out of spec. The only changes I made were to put the cap on the supply regulator adjustor and move the manometer from the input to the output side of the gas valve. Of course, the heater no longer fired, going into type 2 failures (flame just enough for the controller to sense that the burners were lit, but then the flame goes out as soon as current is removed from the igniter).

From watching the manometer on the input side during the two failure modes, it appears the difference is that in failure mode 1, when the gas valve opens the pressure on the input side drops below 2" and its not enough to sustain any real flame; in failure mode 2 the input pressure is between 2" and 3", which appears to be enough for the igniter to partially light the burners so that a flame is sensed, but it's not enough to fully light the burner so when the igniter goes out so does the flame.

There is nothing but 1" hard pipe between the meter and the supply regulator, so I'm concluding that the supply regulator is flaky and needs to be changed out. While I'm having that done, the supply should be reworked to use hard pipe from the regulator to the gas valve, install a trap for debris/moisture, and install a union just before the gas valve so that if I need to pull the valve and manifold it will be much easier to do. Does this make sense or am I overlooking some other possibility?
 
Seems like you're on the right track. That regulator should be able to pass 1500cfm with an inlet pressure of 2psi and an outlet of 8"wc.

Are you saying that you are seeing 50"wc downstream of the regulator? That's a bit troubling. I'm guessing the gas valve is only rated up to 1/2psi.

I found a Specification link on this page:

1 in. Steel NPT Gas Regulator Valve | Gastite | Ferguson

Shop for 1 in. Steel NPT Gas Regulator Valve from Gastite At Ferguson. Ferguson is the #1 US plumbing supply company and a top distributor of HVAC parts, waterworks supplies, and MRO products.
m.ferguson.com m.ferguson.com
 
I did some more testing this morning. After attaching the manometer to the input side of the gas valve, when I first open the shutoff on the supply the pressure jumped up into the 50's, then slowly over the course of a few minutes bled down to around 23. Once it stabilized there, I did my first trial and the heater fired, with a steady state pressure of around 8. When I turned the heater off and gas valve gets closed, it quickly jumped back up to the mid 30's, then slowly bled down to about 28. I did several trials with the same results (jumped up to the 30's when the gas valve closed, eventually stabilized in the high 20's before I started the next trial). All trials fired the heater successfully.

The manometer I have is not expensive, but it has a recording feature where you can see the maximum, minimum, and average values over time. What I see is that when the gas valve opens, the pressure plummeted initially to around 1.5, then recovered to the steady state pressure. I believe what is happening when the heater does not fire is that it doesn't recover to the steady state pressure, at least not quickly enough to satisfy the ignition sequence.

I don't have experience with regulators, so don't know what to expect to see in static pressure. But it does seem pretty clear that the regulator is performing erratically at times.
 
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