gas valve short cycling

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  #1  
Old 02-12-18, 12:13 PM
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gas valve short cycling

Not sure the gas valve is the problem. While the boiler is heating the water the gas valve clicks, shuts off the gas, then in about 10-20 seconds re-energizes the hot wire ignition, re-lights the burner then repeats the shut down and re-start in about 2-3 minutes. This continues until the boiler gets up to the aqua stat upper limit of about 190 degrees. Of course this takes much longer to get up to temperature because of the interrupted heating cycle. Does this mean a bad gas valve or aqua stat or what?

The boiler is a Teledyne Laars JVH 100P induced draft (later changed to natural gas) installed in 1996. This is a high elevation derated unit for 5001-8000ft. The aqua stat (Honeywell L4006A1967) was replaced 2011, the gas valve (Laars V2001800) was replaced in 2006.

What do you suggest I do to verify which component is faulty? Any advice sincerely appreciated.
 
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Old 02-12-18, 03:25 PM
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Can no longer modify the original message so I am adding a few thoughts.

I am now wondering if it might be the pressure switch that verifies the fan (inducer) is working. I have verified the fan is good but am wondering about the pressure switch. If the switch is going bad and does not see constant pressure it could switch off the gas valve. If this pressure switch was going out this might be causing the problem. Is this reasonable? How do I check this?

Similarly, there is a flow switch that verifies the Taco circulation pump is working. Could it be the flow switch?

I am adding more questions or possibilities and am looking for advice as to what is most likely. Prefer to not throw parts at this. I can short or bypass these switches to see but am reluctant for the obvious reasons. Don't want to take chance that something goes wrong while these switches are temporarily taken out of the circuit.

Any suggestions from those who have the experience?

Thanks
 

Last edited by jeweler; 02-12-18 at 03:30 PM. Reason: clarify
  #3  
Old 02-12-18, 06:22 PM
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Since the ignition system was changed from original manufacturer.... the factory wiring diagram won't be of much help. What is that using for an ignition module ? Post the model number.

Most ignition modules run on 24vac and it comes thru the limit loop and possibly the pressure switch.
You can monitor the 24vac at the ignition module..... if that is going out and causing the burner to shut down.... you need to follow that line back and see what it passes thru.

If the 24vac to the module stays constant and the burner is shutting down.... there is a burner monitoring issue... possibly a flame sensor issue.

Gas valves don't short cycle.... their controls do.
 
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Old 02-13-18, 08:07 AM
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I was not clear. The ignition module is unchanged from the original. The boiler was originally set up for propane and changed to natural gas after about 2 years. Otherwise everything is as it came from the manufacturer.

Is it safe to temporarily bypass the pressure switch? This is the one that verifies the induced air blower for the exhaust is functioning. I figure this is the simplest to verify first. There is a approx. 1/4" hose going from the blower to a diaphragm that opens/closes a switch. If I unplug one of the wires the gas valve shuts down immediately. I am guessing this switch may be going out.
 
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Old 02-13-18, 10:37 AM
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No, do not jump anything out.
Use your meter to verify which component is opening.
If it is the pressure switch, verify correct draft with a manometer.
 
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Old 02-14-18, 10:24 AM
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If I unplug one of the wires the gas valve shuts down immediately. I am guessing this switch may be going out.
No.... that is exactly what should be happening. If you read what I wrote...anything open in the safety loop will not allow the boiler to fire.

Since you are trying to troubleshoot something electrical..... you should have a meter. Even a basic $15 analog meter from the home improvement stores will work. You set the meter to AC volts and connect the leads directly to the sensor you are testing. The probes can usually be inserted carefully into the plastic connectors while they are connected to the switch.

If the sensor/switch is normal.... you will not read any voltage.
When it fails or opens.... you will measure 24vac.

With the pressure switch. With no call for heat you will measure 24vac across the switch leads. When the inducer starts and gets up to full speed.... you should measure 0v and it should stay that way all the way thru the cycle. If you see any voltage.... the switch is opening.
 
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Old 02-19-18, 10:14 AM
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losing 24v for a second

I appreciate your help. I put a meter between the 24v transformer ground and the fan proving switch or sensor. It reads 24 volts then drops to zero and the flame goes out. Then the 24 volts comes back in one or two seconds and the hot wire starts again and the flame reignites. It reads the same 24 to 0 to 24 on either side of this fan proving switch. Also does the same on either side of the flame roll out safety switch.

I am assuming that since both switches read the same and both go to 0 volts and back to 24 volts that means the problem is elsewhere. I do not really understand this wiring diagram. Should I do the same test with the aquastat and the flow switch? Or, where do I check next? Thanks
 
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Old 02-19-18, 10:40 AM
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The wiring diagram shows an aquastat (optional) which is indeed installed, and a high limit switch. Actually there is not a high limit switch that I can find. But this is where there is a flow switch that verifies the circulation pump is functional is installed in the circuit. I assume the aquastat functions as the high limit switch.

I checked the voltage at the aquastat and the flow switch and they both show a continuous 24volts even when the other feed at the flame roll out goes to zero. So I assume that means neither of these components are bad.

So I am still assuming the voltage interruption is upstream from the fan proving switch. This is where my lack of ability to read a wiring diagram fails me. Where do I check next?
 

Last edited by jeweler; 02-19-18 at 10:42 AM. Reason: clairification
  #9  
Old 02-19-18, 11:19 AM
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j,
This is only a suggestion. I know there was a post to not try to jump anything out and I'm not a big fan of it myself. A meter or instruments are a better option to find problems but in your case where you seem to be going through a little torture it might be something to try and at least narrow down the problem area.

It's a 24V series circuit and the Flame roll out switch and the Fan proving switch are just that, switches.

When operating properly the fan proves and then goes through the roll out to the gas valve.

If you first bypass the roll out switch and you still have the problem that eliminates that switch and no damage is done.

Replace the wires to the roll out and go to the Fan switch and bypass that. If nothing changes then at least you have eliminated those 2 switches and will know that the problem is before that.

Your high limit and optional aquastat are just switches that can also be checked.

Your flow switch is most likely 24V and can also be checked. Your floswitch is activated by your pump and sensors flow. If your pump has a problem which periodically suspends flo then the switch is doing its job and it may be in the pump.

If the system runs with any one of these bypassed it would narrow down your search.

You have aquastats coming off the same relay terminal which may be something to check. I don't know how far you've gone with this. I know it's been a while.

Have you tried bypassing your t-stat to see if that is dropping off.

Just suggestions that I would try in your position.
 
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Old 02-19-18, 12:26 PM
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Ok.... I highlighted your diagram. Right click and view image or save image for full size. I used my colors.... not necessarily what is actually used.

The 24vac on yellow and red should always be present at the gas valve.
On a call for heat... the pink wire should be 24v to the circulator relay. That relay closes and outputs 24v on the blue wire. It goes thru the aquastat and high limit (or whatever) and arrives at the inducer relay. There it becomes the purple and goes thru the pressure switch, thru the flame rollout switch and then to the valve.

So to test.... put one probe on yellow wire or ground. Other probe to gas valve P (violet). I think you lost it there. Then go back to the rollout switch... check both sides. Keep working backwards in the circuit. Pressure switch is next.

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  #11  
Old 02-19-18, 01:27 PM
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Spott and Pjmaxx many thanks for explaining this so I can understand... I may have finally figured this out. It looks like I have two issues. One is the fan proving switch after all.. Don't know why I was previously getting 24v on both sides of the switch when the burner cycled off. Tried it again and am only getting 24v when the burner is on, and one side is going to zero volts intermittently when the burner goes off. So I removed the switch and jumped the two wires together. The burner continues to burn as it should, until it reaches about 190 degrees. So I am assuming this switch is bad.

Looking at the function of the switch there is a small hose going to the exhaust fan. It looks like it creates a small vacuum when the fan is on. This must close the switch and pass the 24 v. It looks as though the switch no longer works consistently. I tried applying a small amount of air pressure positive and negative. Negative closes the switch. Then no pressure opens the switch. Is that correct? Anyway, it no longer works so I ordered a new one. Keeping the wires temporarily tied together until the new switch arrives.

The second issue is a bad connection on a junction of a couple of wires. This also intermittently misbehaves. That is what was confusing me. At present everything is continuing to work.

Many thanks
 
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