Help, furnace not starting unless...

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Old 01-03-09, 06:23 PM
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Help, furnace not starting unless...

Hello,

I have a RUUD Deluxe 80 Furnace (Forced Hot Air) and for the past week I have had the following problem. The furnace was working normally and about a week ago stopped working. I poked around a bit and checked connections and such. I wound up giving the outside of the furnace a light smack where the BLOWER RELAY is located and I heard a click from the relay (I guess the circuit closed) and the furnace started and worked for a couple days. Then it dwindled to the point where it will not work at all unless I give the area where the blower relay is located a tap and will then until the temp hits 68 (where the thermostat temp is set). It may turn on and off normally 1 or 2 times after that if at all.

I thought the problem may be that the blower relay was "shot" so I replaced it and that did not solve the problem. The relay is a 40-390

I then changed the thermostat and that did not fix the problem either.

Also, over the past week there have been two occasions where I flicked the cut off switch "on-off-on" very quickly and the furnace started up. I thought there may be a problem with the switch and replaced that too but the original problem remains.

I'm wondering if anyone knows what the problem could possibly be and any possible solutions.

PS - The furnace is at least 10 years old.

Any help is greatly appreciated, thanks in advance!
 
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Old 01-03-09, 11:09 PM
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What I'd suggest is to take the cover off the burner compartment and carefully watch and listen to the sequence of events that happen when you turn the thermostat up. You'd need to do that several times until you can absord and appreciate what is happening.

Post that series of events along with the model and serial number of the equipment.
 
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Old 01-04-09, 06:01 AM
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Thanks for the reply!

There was no real series of events. I'd just give a good tap to the outside of the furnace shell where the Blower Relay is located and the furnace would engage.

I should mention that the relay is mounted directly below the pressure switch. After going through some back posts I disconnected the tubing that goes from a fan motor to the pressure switch. I left the tubing connected to the pressure switch and blew into the tube and nothing happenned, when I blew in and out into the tube a could hear a clicking. I reconnected the tube and then the furnace worked normally throughout the night with the thermostat set at 70. I turned the thermostat down to 68 and the furnace shut off but did not start again. I repeated the process of blowing in and out of the tube connected to the pressure switch and everything is working normally again.

Should I take a chance and replace the pressure switch?
 
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Old 01-04-09, 06:50 AM
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Jumping in:
FYI the pressure switch probably closes on a negative pressure so sucking on the hose would activate it.
You can check to see if the pressure switch is what is causing the furnace to not operate by do the following:
Replace the pressure switch with a toggle switch. Make sure the switch is open and then give the furnace a call for heat at the thermostat or by jumpering the "W" and "R" wires at the furnace. After the inducer blower starts up, close the switch. The furnace should light and operate normally. Repeat this several times and make sure it works every time. If it does the pressure switch signal is probably what is causing your furnace malfunction but the switch itself may not be at fault. You could have a week inducer or you could have a blockage in you heat exchanger or flue. Both are potentially dangerous situations. It is more likely it is the switch but just wanted you to know it could be something else. If you can borrow a digital manometer (one that will read inches of water column) you could check the pressure and it it's OK change the switch. Or if you can't measure the switch pressure you could change the switch and hope for the best.

PS: Sometimes even on 80% furnaces you get some condensed water in the pressure switch hose so take the hose off and make sure it is dry.
 
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Old 01-04-09, 07:18 AM
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Originally Posted by xpogi View Post
Jumping in:
FYI the pressure switch probably closes on a negative pressure so sucking on the hose would activate it.
You can check to see if the pressure switch is what is causing the furnace to not operate by do the following:
Replace the pressure switch with a toggle switch.
I don't have a toggle switch on hand, could a single throw light switch be used instead? Also, there are brown - black - and white wires connected to the pressure switch. black and white are connected to prongs on the switch and brown is screwed down. The Brown and White wires are comming from the relay. Am I correct in assuming that white is common and black is the hot wire?

Something else just came to mind. About 6 weeks back I had a heating and cooling contractor come over for basic maint on the furnace and to fix a rattling noise(turned out to be loose screws on the inducer moter) One of the things he did was change the tubing from the motor to the pressure switch because it appeared that the old tubing was actually just a bit to short and was tightly crimped at the motor. The furnace DID work normally after that though... I guess I'm thinking maybe the old tubing had the crimp as a cheap fix of some sort.
 

Last edited by rrrg1965; 01-04-09 at 07:46 AM.
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Old 01-04-09, 08:16 AM
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Originally Posted by rrrg1965 View Post
I don't have a toggle switch on hand, could a single throw light switch be used instead? Also, there are brown - black - and white wires connected to the pressure switch.
Well the fact that there are three wires going to the pressure switch complicates things. Must be a normally open, normally closed and common. In that case trying to bypass the pressure switch with a manual switch could get complicated, would need a single-pole double-throw toggle switch or a three way light switch. Sorry, without a wiring diagram I am pretty much lost. Can you post a wiring diagram? How about the make and model number of the furnace?

Sorry, my personal experience is pretty much limited to one Manufacturer's furnaces and then only to furnaces manufactured in the last ten years.
 
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Old 01-04-09, 09:01 AM
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First of all, THANKS for trying to help!

The furnace is a RUUD Deluxe 80

Here is the wiring on the pressure switch:
White is going to MC (from blower relay)
Black is going to MO
Brown is going to Common (from blower relay)
Hope this helps.

I may just change the Pressure Switch and see what happens. Also, I'm thinking maybe I should disassemble the vent assemble going from the INDUCER MOTOR to the chimney to check for blockage of any sort... would this be worthwhile????????
Lastly and most importantly - As a precaution I just went to ACE and got two new CARBON MONOXIDE detectors. The furnace is in the basement so I put one detector near the top of the basement stairs and one in my son's room.

Thanks again
 
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Old 01-04-09, 01:20 PM
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Originally Posted by rrrg1965 View Post
First of all, THANKS for trying to help!

The furnace is a RUUD Deluxe 80
I did an Internet search and couldn't come up with a diagram. I did find an 800 number that may be of some help:
Ruud Customer Support:
(800) 432-8373

Originally Posted by rrrg1965 View Post
Here is the wiring on the pressure switch:
White is going to MC (from blower relay)
Black is going to MO
Brown is going to Common (from blower relay)
It's probably NC for normally closed and NO for normally open. Before operation there there would be no continuity between NO and common and continuity between NC and common, when the switch operates the states would be reversed: Continuity between NO and common and no continuity between NC and common.

Originally Posted by rrrg1965 View Post
I may just change the Pressure Switch and see what happens. Also, I'm thinking maybe I should disassemble the vent assemble going from the INDUCER MOTOR to the chimney to check for blockage of any sort... would this be worthwhile????????
Lastly and most importantly - As a precaution I just went to ACE and got two new CARBON MONOXIDE detectors. The furnace is in the basement so I put one detector near the top of the basement stairs and one in my son's room.

Thanks again
Sounds like a good plan. Have you looked around for a wiring diagram? I know it's a 10 or 20 year old furnace but I never throw away owners manuals... maybe the past owner of your house was a pack rat too.
 
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Old 01-05-09, 05:24 AM
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I took apart the ductwork from the inducer motor to the chimney and there turned out to be a blockage at the chimney. I cleaned it out and the furnace appears to be working normally. Next order of business is to get a chimney cap!

Thanks, XPOGI, I would never have thought to look for a blockage.
 
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Old 01-05-09, 09:48 AM
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I guess that pressure switch worked as advertised.
 
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Old 01-06-09, 06:06 PM
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Originally Posted by rrrg1965 View Post
Thanks, XPOGI, I would never have thought to look for a blockage.
No, thank your furnaces pressure switch for doing it's job and letting you know something was wrong. That switch just may have saved your life.

A little secrete here: I design furnaces for a living. What I really do is design a device that, if installed, operated, and maintained properly will heat your house AND if improperly installed, not maintained or otherwise abused will not kill you!

We in the industry spend 10% of our time and resources attempting to design a reliable and efficient product and 90% designing a safe one. Something to think about if you ever see a budget furnace manufactured in China. Lead paint could be the least of your worries!
 
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