Intermittent problem with Ruud gas furnace

[RichMiller]

I have a Ruud gas furnace that I am having an intermittent problem with. I cannot find any model number on the furnace but it was new in 1991 when my home was built. The furnace does not have a gas pilot light. It does have an electric igniter and does have a Robert Shaw Model HS780 – 34NL – 306A Ignition Control Unit. The part # on the Robert Shaw Unit is 100-00812.

The problem is that sometimes when the thermostat calls for heat, the electric igniter heats up and glows adjacent to the burner but the main gas valve does not open to allow gas to flow to the burner. This problem first presented itself in the spring of 2006. During the last few days of that spring when I needed heat I observed that the igniter was illuminated but the gas valve did not open, but then the need for heat was over for that spring. In the fall of 2006 I expected to have to face the problem with the furnace but when I tested the furnace, it fired up and worked flawlessly until the spring of 2007. Again, for the last few days of spring 2007 when it would have been nice to have a little heat, the previously noted problem reoccurred. Now, in the fall of 2007, I am testing the furnace for the coming heating season and the furnace has fired up twice out of perhaps eight attempts. But the ignited is still glowing brightly.

Can any one tell me what it is exactly that is supposed to open the gas valve? I expect that there is a sensor that will verify that the igniter is if fact hot enough to ignite the gas before the gas valve is allowed to open but what is the nature if that sensor? Does is need cleaning?

Just what is the function of the Robert Shaw Ignition Control Unit?

Is any diagram available, which will show how this system is supposed to work?

Thanks. Rich

[Mr Bill]

II expect that there is a sensor that will verify that the igniter is if fact hot enough to ignite the gas before the gas valve is allowed to open
Thanks. Rich
:eek: :eek:

Rich you need to call a service professional, trust me this is something that only a service
professional can resolve.

[HeyBob]

10-4 on that Bill. You need a service tech to properly diagnose your problem. Sorry but we are not allowed to dispense DIY help in this forum.

[dennis663]

replace your ignitor

[Edwards Htg & Clg]

How will changing the ingnitor going to open the gas valve?

Call a professional.

[t527ed]

replace your ignitor








WHAT !!!

[dennis663]

o.k., i'm not going to school you on how a gas valve with a glow bar ignitor works, but if you did your homework, you would know that a poor resitive value going through an ignitor will keep it from operating, if the gas valve doesn't get the right value from the ignitor, it will not energize....

[Edwards Htg & Clg]

o.k., i'm not going to school you on how a gas valve with a glow bar ignitor works, but if you did your homework, you would know that a poor resitive value going through an ignitor will keep it from operating, if the gas valve doesn't get the right value from the ignitor, it will not energize....

You might want to check into another school.........

[dennis663]

i didn't mean to offend you, i know you wasn't taught things like this back in the stone age, i've changed quit a few ignitors with this symptom, it always works, + it's about the cheapest thing you can buy to replace, if it's wrong, your not out much !!!

[Edwards Htg & Clg]

I 've changed dozens of bad HSI recently on systems with no pilot, and the valve will still open, whether the ignitor is burned into or not. How can that happen according to your post?
People taught in the stone age need more information than was given by the OP to just tell them to change a part, no matter how cheap it is.
There are techs and there are part changers, big difference.

[dennis663]

o.k., then tell me how the gas valve knows to open ? because the ignitor get's hot or because an elecrical value is put across the coil ?

yes, it's a trick question.

[Edwards Htg & Clg]

Timed on through the board, not through the HSI.

[dennis663]

VERY GOOD!!!, now what tells the board to open the gas valve ?, is it because the ignitor get's hot or because it send an electrical value to the board ? & what happens if the value the board is looking for is not strong enough ?

[Edwards Htg & Clg]

You must have missed the word "TIMED".

[dennis663]

have you not ever changed an ignitor with a weak value ? just because it glows doesn't always rule it out.

[dennis663]

there not all timed !!!!

[Edwards Htg & Clg]

Yes, but it didn't keep the valve from opening.

[karsthuntr]

VERY GOOD!!!, now what tells the board to open the gas valve ?, is it because the ignitor get's hot or because it send an electrical value to the board ? & what happens if the value the board is looking for is not strong enough ?
Dude, you have no idea what you are talking about. Edwards does know what he is talking about. I'm not trying to be smart but you may want to open up the book you are studying and read it again. If your book says what you are saying than post the book title.

[docholiday]

Dude you are missing it... Not to hijack this thread but when a W signal calls, the inducer starts for a predetermined, prepurge time. Then as time continues, (and all pressure switches have made), the ignitor is energized. the board energizes the ignitor for a set period of time and then opens the gas valve. Shortly after that, the ignitor is denergized and a few seconds is offered for flame sense. The resistance of the ignitor has nothing to do with the gas valve. You can disconect the ignitor completely and it will still go through it's trial for ignition.

To the OP, get a tech.

[dennis663]

if he knows what he's talking about, then what is his diagnosis ????

[cmajerus]

VERY GOOD!!!, now what tells the board to open the gas valve ?, is it because the ignitor get's hot or because it send an electrical value to the board ? & what happens if the value the board is looking for is not strong enough ?

sorry but no, an open ignitor will still let the valve open, ever wonder what that hiss is when the ignitor don't glow then something clicks... then hiiissssss! hmm might be the gas valve.

inducer starts, pressure switch closes, relay starts ignitor warm up, after predetermined delay(different for every manufacturer) gas valve opens for trial for ignition, if the flame sensor(or ignitor itself on some models) senses flame gas flow continues and flame stays on. no flame sense(or flame) and board shuts off valve, retries 3 times(some LP units lock out after 1st trial) and will lockout, if there is still no flame sensed.

There you go, HSI 101!!:D

[Edwards Htg & Clg]

My diagnosis? I would need more information, like I said earlier before I told anybody to start changing parts. Is there power going to the valve? He didn't say.
I could tell him to start changing parts, (cheap ones first of course) but that's not how people trained in the stone age do things.

[cmajerus]

if he knows what he's talking about, then what is his diagnosis ????

for the answer to that post a thread on the pro board and we'll school ya quick like. No DIY on here so we can't go too deep on this thread.

[dennis663]

YOUR RIGHT, I STAND CORRECTED.i was under the impression this was a smart valve!!!

[karsthuntr]

YOUR RIGHT, I STAND CORRECTED.i was under the impression this was a smart valve!!!
Rheem/Ruud are too smart to use a dumb valve. :D

[Edwards Htg & Clg]

Rheem/Ruud are too smart to use a dumb valve. :D

Don't think anybody used them in '91. Seems like they showed up in '95 or there abouts. Plus the smart valves also had a pilot.

[karsthuntr]

Don't think anybody used them in '91. Seems like they showed up in '95 or there abouts. Plus the smart valves also had a pilot.
This is true. :D

[dennis663]

yes, the earlier smart valves had a pilot , but now take the q3400 ignitor & sense assembly