Natural gas furnace not working properly

warno

For the past month or so my forced air natural gas furnace hasn't been working properly. It will turn n on with a low thermostat reading then run for about 5 minutes or so then go into shut down cycle. As soon as the blower fan quits running the draft fan kicks back on and a new heat cycle starts. It will do this many times and still never get the thermostat up to temperature to shut down. Thermostat is set for 72 and best it will get is 71. If it does hit 72 it doesn't over shoot enough to trip the relay in the thermostat to shut down. It's been cold here in Illinois for awhile now and there's night's it will do this all night long. I've woke up to the house at 69 some mornings.

This is the first year in 9 living here that it has done this. I've cleaned the thermocouple thinking that might help but it did nothing. I've been told by a friend, ex HVAC, that it may be over heating and going into shut down mode. He said maybe adjust the gas flow alittle less and see if that helps.

Does anyone here have any ideas what's going on?

Furnace is an old Ruud 75,000 btu unit. Can't remember the date but thinking close to 15 or better years old.

ratio

Turning the gas down isn't the cure. You need to find out what's going on, then find out what caused that, then fix it; not start tweaking things until it hides the problem.

It does indeed sound like it's hitting a limit, but far more likely that the airflow has been restricted to the point where the limit is tripping, or the limit itself is failing. Is the filter clean? What is the ΔT when it's running? The nameplate should give you the acceptable range. If that's good, I'd replace the limit—they're cheap.

DZoro

When was the filter last changed? What do you mean by low thermostat?

warno

> @ratio said:
> Turning the gas down isn't the cure. You need to find out what's going on, then find out what caused that, then fix it; not start tweaking things until it hides the problem.It does indeed sound like it's hitting a limit, but far more likely that the airflow has been restricted to the point where the limit is tripping, or the limit itself is failing. Is the filter clean? What is the ΔT when it's running? The nameplate should give you the acceptable range. If that's good, I'd replace the limit—they're cheap.

Filter was replaced late December. I could grab a new one and throw it in just to see.

What's the easiest way to check delta T. I'm guessing a probe in the duct work on both sides?

Thre high limit on these things, are they usually just a snap disk switch?

> @Dennis said:
> When was the filter last changed? What do you mean by low thermostat?

Filter changed late December. But I could try a new one.

By thermostat reading low I mean the house is at a temperature that the furnace should kick on.

JUGHNE

Does turning the stat up to 90 keep the furnace running?

You could have a failed or badly adjusted heat anticipator in the thermostat if it is as old(er) as the furnace.
Take the cover off the stat and look for an adjustment lever or the word "longer" on a scale.

warno

> @JUGHNE said:
> Does turning the stat up to 90 keep the furnace running?
>
> You could have a failed or badly adjusted heat anticipator in the thermostat if it is as old(er) as the furnace.
> Take the cover off the stat and look for an adjustment lever or the word "longer" on a scale.

The thermostat is newer than the furnace. It's a digital programmable T stat. I'll try cranking it up and see what happens though.

JUGHNE

If you jumper the R and W (might be red and white wires) on the tstat and still have problems then it is not the tstat.
Jumpering those should keep the furnace on constant.

HomerJSmith

Is this a condensing furnace? I have to look at my Rheem-Rudd book. Is it possible that you have a plugged condensate trap?

ratio

Thermometer in the duct before & after the furnace. Before doesn't matter too much, I usually stick it in the filter door, but the outlet needs to be far enough down the duct that you get a good mix of the air going through the furnace. Take the difference of the two, that's the Δ. It should be in the range on the nameplate, 20-60°, 30-80°, something like that, look for "temperature rise". If it's greater than the listed numbers, you're not moving enough air. There's also a "max discharge temp" that you can't exceed, it'll be something like 140° or 180°. That's probably what's happening.

DZoro

Make sure all your supply and return air vents are free and clear of any obstructions. How dirty was your filter in December? How often do you change it? Do you have dogs and or cats? Does the main blower shut down before the flames come back on?

HomerJSmith

What's the model # of the furnace and thermostat? I would be concerned about short cycling if it because of the tripping of the Over Temperature Switch.

warno

I've taken a bunch of pictures and I think I might have found the issue.

First off a picture of what I think might be the issue. And you guys tell me if it's as scary as I think it is. This is the draft fan outlet. Should it be attached to that duct just above it?

Then pictures of the unit and thermostat

warno

A picture of the flame a forgot to post.

ratio

Shut it off immediately!

ratio

You're correct, the flue must be connected or all the products of combustion are entering the house. It's an easy way to end up dead, along with everyone else in the house.

That said, I don't think that's the cause of your issue. It might be time to call a professional.

DZoro

Agree with ratio, shut it off, don't use. Have it looked at by a professional. From the burner pic the burner on the left is suspicious looking. Let along your exhaust is beyond extremely dangerous.

JUGHNE

So this has been doing this a month or so? You must have your Angels living with you in the house.

The inducer fan is probably inhaling it's own exhaust, could wreck the inducer, probably overheating the heat exchanger tripping out the hi limit??

Yes, it is more scary than you think it is.

warno

So sickening feelings defiantly confirmed.

I pulled the flue duct down abit and threw some foil tape over the seam at the draft fan. Then worked my way up the flue duct. The elbows had enough give to close the 1/2" gap at the draft fan. But with it running again it still shuts down at the same time and does the same things as posted in the OP.

I'm texting my friend now about coming out to look at it.

warno

I took a temperature reading at the draft fan outlet. It got up 270°F. Does that sound normal?

JUGHNE

Is this a counterflow furnace.....blows the hot air down into ductwork below the floor?

warno

> @JUGHNE said:
> Is this a counterflow furnace.....blows the hot air down into ductwork below the floor?

Yes it is a down flow furnace.

JUGHNE

Some of the older ones used a special heat activated fan switch.
If so it could have gotten cooked causing big blower fan malfunctions.

You need a pro there.......

rick in Alaska

Try changing the thermostat. Wouldn't surprise me if that was the issue. The first generation Nest stats did that to me.
Rick

HomerJSmith

YES, the draft inducer housing must be connected to the vent pipe in the square vent cover. The vent pipe was evidently pulled up when the flue was constructed.

The way to secure the plastic draft inducer outlet to the metal pipe is as follows: You have to loosen the pipe from the flue. Remove the pipe and sand the inside of the pipe, about 1" where it connects to the draft inducer. Clean the plastic draft inducer outlet on the outside. Remove any silicone rubber from the plastic outlet of the draft inducer. When everything is clean, you can make the connection between the draft inducer outlet and the pipe. You use High Temp Silicone that you can buy at an auto parts store. You thinly coat the inside of the pipe about 3/4" with silicone and then put a lot of silicone on the outside flange of the draft inducer. Do not get the silicone into the inside of the draft inducer. Push the two pieces together and wait 24 hr. for it to cure. DO NOT attach the two pieces together with screws. After 24 hr. you can attach the flue to the pipe. THIS IS THE WAY IT NEEDS TO BE DONE!

HomerJSmith

The pipe and outlet of the draft inducer is 3". When the pipe exits the furnace there should be a pipe increaser from 3" to 4".

Some Rudd vent connection instructions are as follows:
VENT PIPE ATTACHING HOLES
MUST BE PREDRILLED IN THE
DRAFT INDUCER COLLAR TO
PREVENT DAMAGING THE
INDUCER. DRILL 1/8”DIAMETER
HOLES THROUGH THE VENT PIPE
AND COLLAR AND USE #8 SCREWS
TO ATTACH. SEE FIGURE 9.
FAILURE TO FOLLOW THIS
WARNING CAN CAUSE
RECIRCULATION OF FLUE
PRODUCTS CAUSING CARBON
MONOXIDE POISONING
RESULTING IN PERSONAL INJURY
OR DEATH.

I was taught in the Rudd class to attach the vent pipe with silicone and not screws and I have always done that.

warno

So would the way I "fixed" the flue pipe get me through the rest of the winter, or at least too the end of March when days get warmer? I mean it's got to be better then it was.

I programmed the thermostat down a few degrees across all hours to help prevent the furnace from running as often.

Another question now. My furnace guy can't make it over until the weekend. Other then a chilly house what problems can be caused by the furnace short cycling?

I do want to say also that I appreciate all the help.

unclejohn

A downflow has a aux. limit in the blower housing. Also clean the cooling coil. The gas valve is from 1997 the plastic top of that looks discolored by heat so it's time for a new furnace.

warno

Would others agree with unclejohn about just biting the bullet and replacing the furnace?

HomerJSmith

If you are going to rely on the duct tape, put a CO detector in the boiler room. Be aware that the CO detector sounds off at about 50 to 80 ppm, so there still could be CO contamination.

JUGHNE

IIRC, I might have put in maybe 2 of these.
That box runs right thru the blower compartment?
The filters are above it.....change from the inside of blower compartment?? PITA actually.

So you possibly have exhaust leakage potentially going into the blower compartment??

If so that is like having huge holes in the heat exchanger,
delivering exhaust gases through out the house.

DZoro

Yes replace, your playing Russian Roulette, and nobody really wins that game.

warno

> @HomerJSmith said:
> If you are going to rely on the duct tape, put a CO detector in the boiler room. Be aware that the CO detector sounds off at about 50 to 80 ppm, so there still could be CO contamination.

We have a detector near by. I did grab it and place it right in the air stream coming from the draft fan separation. It didn't go off but it still didn't make me comfortable leaving it open for the obvious reason. God knows actually how long it's been like that. Just because the shut down issue started a month ago doesnt mean the seperator issue started then. given the corrosion around the draft fan outlet I'd unfortunately say it has been this way for some time.

> @JUGHNE said:
> IIRC, I might have put in maybe 2 of these.
> That box runs right thru the blower compartment?
> The filters are above it.....change from the inside of blower compartment?? PITA actually.
>
> So you possibly have exhaust leakage potentially going into the blower compartment??
>
> If so that is like having huge holes in the heat exchanger,
> delivering exhaust gases through out the house.

Yes the filters change from above and yes it is a HUGE PITA! But that square duct that goes up through the blower cabinet kept the CO gases from entering the blower. They were actually blowing straight up and out the furnace housing into the closet the furnace is in. Not ideal either. There was a good amount still going up the flue pipe as I could see the condensation trail leaving the top of the flue outside when it's cold.

> @Dennis said:
> Yes replace, your playing Russian Roulette, and nobody really wins that game.

Unfortunately that's what I was afraid of. But if it means the safety of my family in years to come with this turd getting worse with age, it's a no brainer.

warno

little update here...

Just to see what would happen I decided to pull one of my filters out of the return duct. The filters sit on a rack and make a V shape in the duct, that's why there's 2. So I pulled one out and the furnace ran a whole cycle without shutting down. It ran the house from 70 up to thermostat setting of 72 in one run, as it should. So apparently the filters are restricting enough return flow causing an overheat issue.

Does this sound like it was my problem? I know you guys all asked about my filters early in this post, I guess just in the mix of the holidays and being busy I must have forgotten when I actually changed the filters.

My furnace guy wasn't going to come out until tomorrow but I want to ask you guys, do you think he still should?

Jamie Hall

Yes

warno

Ok I'll still have him check it out. Better safe then sorry.

It ran 3 full cycles the way it should. I went a grabbed new filters and so far it has ran 1 full cycle correctly with new filters. So I'm guessing that was it. But I'll still get him out here to take a look.

I'll update with any findings.

lchmb

wouldnt hurt to check the blades on the blower if your filters were that dirty. Have your guy check the amp draw on the motor as well to see if maybe it's failing...

warno

My buddy just left and he said everything looks good. He said the flame is probably one of the better flames he's seen given the furnace age. Alittle bit of orange once in awhile but not enough to worry about. The blower looked fine. The fix on the flue pipe should be fine until spring. Obviously I just need to pay more attention to my filters.

No going to lie, I feel like a bit of a jacka$$ for not doing a filter swap. But if all this didn't happen I would have never found the disconnected flue pipe.

Thank you everyone for the help.

SuperTech

No offense, but not changing the filter isn't what you should feel foolish about.

You should feel foolish after all the good advice that you were given here, you are trusting the opinion of a friend who judges the condition of your equipment by eyeballing the flame?

You need a professional who knows how to perform a combustion test. I wouldn't put the safety of my family at stake based on a guess, which is what took placed when he told you how wonderful your flame looks.

unclejohn

At least you knew where the filters were.

warno

> @SuperTech said:
> No offense, but not changing the filter isn't what you should feel foolish about.
>
> You should feel foolish after all the good advice that you were given here, you are trusting the opinion of a friend who judges the condition of your equipment by eyeballing the flame?
>
> You need a professional who knows how to perform a combustion test. I wouldn't put the safety of my family at stake based on a guess, which is what took placed when he told you how wonderful your flame looks.

I'll make a phone call tomorrow to see if I get a service tech to perform a combustion analysis.

SuperTech

Please do. I can't stress the importance enough. A properly trained technician with a digital combustion analyzer is the only way to ensure that your equipment is operating safely and efficiently. The technician can also test for CO Inside your home, down to one part per million.
Make sure the tech leaves a printout from his analyzer