Water Heaters - Ruud Ignition Failure

I have a two year old Ruud LP gas-fired 75 gallon water heater with a power vent. The model number is PVRP75P. The gas control is made by Robertshaw and I believe the part number is AP12557-2. Recently the burner periodically fails to achieve ignition and goes into safety lockout. After a power-off reset it usually fires up on the next cycle.

I took off the access covers and watched it try to light. It apparently has a piezo crystal ignitor. I could see a spark but it wasn't across the spark gap--it was from the high voltage supply wire to one of the gas supply tubes, about an inch a half from the spark gap. The high voltage wire in that area has a gray-white deposit on it, like maybe it's been arcing for a while.

The owner's manual shows that this heater has a spark ignitor, a pilot light, and a main burner. Why is a pilot light needed if there is an electronic ignitor? I assume this would not be a standing pilot since there's an ignitor--correct? So why have it?

What are the things that could cause the spark to occur in the wrong place? I checked the gap between the high voltage electrode and two nearby metal prongs and it looks to be about an eighth of an inch, which seems reasonable. Is it possible to replace just the high voltage wire (there appears to be a joint about halfway along its length but I didn't trying pulling it apart)? It looks like it's possible to unscrew the pilot tube and main tubes from the bottom of the gas control, disconnect the high voltage wire and remove the whole assembly so I can get a better look at it. Is there any reason not to do this? When I reassemble it is there anything special I need to do ensure I don't end up with a gas leak at the connections? By the way, the supply pressure was checked this winter and as I recall it was about 11" or 12" w.c.

Also, I don't see any kind of flame sensor. Is the high voltage electrode used in some fashion to sense the flame? This is my first gas water heater so any help would be much appreciated.

Hello, and welcome

It sounds like there is some kind of crack in the wire insulation and will need to be replaced.

The sparker only lights the pilot then the thermocouple/thermopile senses there is a pilot and tells the gas valve it is ok to open for the main burners. Because it is power vented you cannot have a standing pilot so it goes out each time the heater shuts down, then is re-lit on start-up.

BEFORE GOING ANY FURTHER TURN OFF THE GAS !!!
Usually you will need to pull out the pilot assembly to change the sparker and sometimes that means disconnecting and removing the whole main burner. Before removing the sparker note the position of all parts in the pilot area so you replace them exactly as you found them. If the sensor get bent or improperly placed, it may not sense the pilot and not open the main burner.

The connections you will be disassembling should not need pipe dope or anything when reassembling. Just remember to use a soap solution to test for any leaks after everything is reinstalled.

If you have any problems during removal or installation, DO NOT TURN THE GAS BACK ON AND CALL A QUALIFIED GAS REPAIR PERSON.

Is it possible to purchase high-temperature high-voltage wire separately or will I need to purchase an assembly with the electrode and wire? I did some searching online and couldn't readily find a source. Can you suggest any?

I looked in the area of the spark gap and pilot light and didn't see a flame sensor. What does the thermocouple or thermopile look like? How close are they usually mounted to the spark gap/pilot light? I'd like to identify and locate that sucker before I go mucking around and removing things. I used my trouble light and inspection mirror but didn't see it on my initial investigation.

Thanks for the cautions regarding shutting off the gas beforehand and checking for leaks after reassembly.

http://waterheating.ruud.com/content/resources/documents/use_care/Res_RU_PowerVent75Gas.pdf

Thanks for the link but I already tracked that document down and that's the reason I was asking about the flame sensor. On page 27 it shows the LP burner assembly and pilot ignitor assembly. Nowhere do I see a flame sensor indicated.

The wiring diagram on page 15 has a few components shown that I cannot identify. Both are shown schematically as switches; one is identified as TH and the other as ECO. Is one of these the flame sensor? How close would it be positioned to the pilot flame? Thanks again.

OK, Sometimes a picture can tell more, so that's why I sent the link. But even I can't quite make out everything in that picture, as you said.

Typically you will have an aluminum tube about 1/4" coming from the gas valve. That is the pilot tube. Then if you have a thermocouple, there will be a heavy copper wire also coming from the gas valve and connected to the pilot assembly, and will be positioned approx. 1/2" directly in front of the where the pilot flame exits the assembly. It must be positioned so when the pilot is lit, the sensor is in the flame to sense a pilot and tell the main gas valve to open. If you have a thermopile, then instead of the copper wire, you will just have a high temp coated wire leaving the gas valve to the sensor. In water older heaters that have a standing pilot, that is all you will have. In newer ones you may also have a igniter it light the pilot.

Your water should be pretty much the same. I think, because of the power venter, that your water heater automatically relights the pilot each time there is a call for hot water. There are also hot surface igniters in some water heaters which do not require a pilot at all.

Does this help?

I did some internet searching. It seems my water heater uses a single rod flame rectification system. The spark electrode is also used to sense the flame through flame rectification. This system uses only one wire, and that's what I have. It looks like I need to purchase a new pilot ignitor assembly.

Thanks bill, even I need to learn a little once in a while. These new water heaters are coming out with new stuff all the time, and with me doing less and less service calls...


Oh well, thanks for the info and if you could place the link here so I can read up on it.

Here's a handy link to a Honeywell video-on-demand I found that talks about flame rectification. This video oversimplifies the actual mechanism of alternating current rectification in a flame but the practical information it provides is very helpful. Enjoy!

http://www.hvactv.com/hvactv/new_site/chat/new_chat_dynamic.php?p=2268-15