PDA

View Full Version : Furnace keeps cutting out



Myk Rian
01-28-2014, 11:09 AM
This has been going on a couple years now. The first time it happened, I called in a HVAC man, and watched what he did.
After that, I fix it myself. Here is what is going on.

In this picture, there is a rubber tube going from the induced draft fan, to the fan pressure switch. It gets moisture in it to the point that the switch loses pressure, and the flame kicks off. A minute later the switch makes, burner starts, and all is well. Couple minutes later it happens all over again.

All I have to do is remove the tube, blow the water out, and reconnect it. Lasts for a day. I've considered putting a T fitting in the low point of the loop, and connect a drip leg with a plug in it. That will let it run more than a day without having to go through the whole blow-down thing.

I can understand it is humidity condensing in the tube, but why? There is no air flow, just pressure. Unless the switch has a small leak, I just don't get why water is condensing in it. Any ideas from the collective?

280993

Matt Meiser
01-28-2014, 11:31 AM
I wonder if its just warm air meeting cold and condensing. That's sensing exhaust air which would be both warm and moist. I wonder if you just lift that hose up (mabye support with a zip tie?) if it wouldn't let any condensation run back to the fan? Not ideal to be sending water there, but it would dry quickly due to the airflow.

Lee Schierer
01-28-2014, 12:36 PM
I agree with Matt, change the hose routing so it slopes back toward the blower and the moisture should collect in the hose. Can you move the switch bracket to the right so the sag in the tube goes away?

Brian Elfert
01-28-2014, 12:38 PM
The blowers for high efficiency furnaces often have drains so water won't build up in the blower housing.

Dave Richards
01-28-2014, 12:56 PM
An alternative to moving the switch might be to find a little barbed elbow, cut the tube just enough below the switch to put the elbow in and connect from there to the blower with the shortest possible length of tube.

We run into similar issues in the ventilators on anesthesia machines and ICU ventilators. Patients exhale a lot of moisture and that rains out in the tubing and passages in the equipment. To prevent it from raining out in the wrong place the circuit is heated. Maybe you could put a small incandescent lamp near the tube and see if keeping it warm prevents the problem.

Phil Thien
01-28-2014, 1:00 PM
Shorten the tube to remove the sag. The water collects in the sag. The installer was supposed to shorten the tube.

Tom Stenzel
01-28-2014, 1:58 PM
It looks like you have the same furnace as I do made by Goodman/Janitrol. The pressure switch line on mine has a loop that droops lower than yours yet I've haven't had a problem with moisture collecting in the line. The furnace is located in the laundry room, there's a bit more humidity there than in the rest of the house and the ambient temperature is in the upper 50's. It's been like that for 12 years.

The water shouldn't be collecting like that. Make sure the line doesn't have a leak where it attaches to the switch. Or the switch does have a leak.

The easiest way to test if the line or switch diaphragm is leaking is to pinch the line shut while the furnace is running. If the furnace burner trips there's a leak somewhere. The faster the trip the bigger the leak is.

-Tom Stenzel

Phil Thien
01-28-2014, 2:01 PM
Here, read this:

http://www.doityourself.com/forum/gas-oil-home-heating-furnaces/375086-goodman-vent-pressure-switch-failure.html

I had the same problem on a new furnace at the office. The installer came out several times. I found the manual online, and it said to make sure that tube didn't have any sags in it.

In the manual it said the tube is left long at the factory, so the furnace can be mounted in different orientations. The installer is supposed to cut the line once the final orientation and installation is completed. This is foggy, BTW, all from memory. Some people are apparently able to reorient their tube so there are no sags, but I had to cut mine. Oh, and the sag in my tube was really no larger than yours.

Myk Rian
01-28-2014, 2:22 PM
I can't shorten or reroute the rubber tube because the fan is in the way. That's why I was considering a drip leg in it.
The fan tap is on the same plane as the switch connection. I can't raise the switch either.
Since it's in the firebox, there is already heat in it.

I suppose I could try mounting the switch on the other side of the furnace, horizontally. That's about the only way I can figure to get it higher than the fan. A shorter hose would then be a down-slope to the fan.

Ole Anderson
01-28-2014, 2:29 PM
I have a similar problem with my furnace, it won't run continuously to bring the house up to the set temperature in the morning. So I ran down, popped the cover on mine, hoping to see the dreaded dip in the tube. No such luck, mine tube runs vertically from the pressure switch down to the draft fan.

Myk Rian
01-28-2014, 2:48 PM
I have a similar problem with my furnace, it won't run continuously to bring the house up to the set temperature in the morning. So I ran down, popped the cover on mine, hoping to see the dreaded dip in the tube. No such luck, mine tube runs vertically from the pressure switch down to the draft fan.
When this started, I suspected a high temp switch on the plenum was causing the problem. You could have that problem.

Tom Stenzel
01-28-2014, 3:01 PM
When this started, I suspected a high temp switch on the plenum was causing the problem. You could have that problem.

If the high temp switch is tripping, check the furnace filter. A plugged filter will cause the furnace to overheat and trip. Also if you recently started using one of the super high efficiency filters it may be restricting the air flow more than the furnace can tolerate.

My system wouldn't work with any of the 3M filters when in air conditioning mode, the airflow was too low. I was forced back to using the cheap blue filters.

You can also try cleaning the flame sense rods with fine sandpaper or steel wool. It doesn't sound like that's your problem but it's cheap and easy to clean them and eliminate that as an issue

Phil Thien
01-28-2014, 3:01 PM
I can't shorten or reroute the rubber tube because the fan is in the way. That's why I was considering a drip leg in it.
The fan tap is on the same plane as the switch connection. I can't raise the switch either.
Since it's in the firebox, there is already heat in it.

I suppose I could try mounting the switch on the other side of the furnace, horizontally. That's about the only way I can figure to get it higher than the fan. A shorter hose would then be a down-slope to the fan.

My recollection was that the switch is typically relocated depending on the orientation of the install (the furnaces can apparently be used on their sides, etc.).

So look around inside and see if there is another mounting spot already drilled or tabbed for the switch that would make it easier to get the sag out of the line.

Dan Hintz
01-28-2014, 3:46 PM
If you can, reorient the pressure switch (silver can)... twist it about 45 degrees clockwise, the slightly shorten the tube. Should give you a very gentle slope downward towards the fan.

Brad Adams
01-28-2014, 4:53 PM
That is an 80 percent efficient furnace. The first thing I would check would be the temp rise across the furnace. If the fan setting is too high the exhaust can be cooler than it should be, causing condensation. The temp rise is listed on the rating plate in the furnace. Check the incoming air temp close to the furnace, and the supply temp in the duct right where it leaves the plenum.

Myk Rian
01-28-2014, 7:55 PM
I will check that, Brad. Makes sense.
I take it you mean the speed of the recirculating fan. That is adjustable on the control board.

Tom Stenzel
01-29-2014, 8:13 AM
Myk, when you get this sorted out let us know what was wrong.

This isn't the winter to be screwing around with a furnace

Myk Rian
01-29-2014, 8:42 AM
You can bet on that, Tom.

Matt Meiser
01-29-2014, 9:07 AM
This isn't the winter to be screwing around with a furnace

But after today its warming all the way up to the 20's for the next few days!

Myk Rian
01-29-2014, 9:40 AM
Rated temp rise: 45-75
Incoming air: 72
Plenum: 149
T.R.: 77
Right on the money.

Looking at the control board, the fan is not speed adjustable. Must have been something else I was thinking about.

Matt Meiser
01-29-2014, 9:56 AM
It might be selectable by using a different fan wire. The furnace I gutted as an air handler for my shop AC was that way.

Brad Adams
01-29-2014, 9:59 AM
Myk,
If your temp rise is 77, you are tripping the high limit. This would cause the furnace to kick in and out. Did you take the outgoing temp at least five feet from the heat exchanger? I usually take the temp in the duct where it leaves the plenum. Take a reading in both ducts if there are two and average them. If the temp rise is really 77, you need to either raise the fan speed on the blower, or make sure all the registers in the house are open.

Phil Thien
01-29-2014, 10:28 AM
Isn't there a flashing LED somewhere that tells you the error? Like three blinks for the pressure switch, four for high limit?