Now I've got a furnace question
 

The controller lights the gas burner and the limit switch turns on the fan but it just blows lukewarm air. The filter is clean and there is plenty of air flow. It is 13 outside and it has run continuously set to 68 maintaining 67.

Duct leaks?

Are you sure your burners are firing? Does it seem like the fan/blower is running at full speed?

I'm guessing the question is why is it 13 degrees in St. Louis.

My guess is the gas solenoid is not opening up fully. Enough to fool the flame sensor but not enough to get your 80,000 BTUs worth of gas?
Just a guess.

1. Have you ever known it to blow 'hot' air?

Seems to me lots of modern furnaces don't actually get 'hot.' My 80% Trane blows lots of lukewarm air, more often than not.

When it got down to -5 here the other day the furnace ran all night and just made it up to 68 or so by morning.

I guess another thing is to actually watch to see that the flame stays on. If the flame sensor or high limit switch is defective the furnace will ignite and turn off shortly thereafter and keep repeating, thus no significant heat.

I can definitely see the flame through the two inspection ports and it is on continuously. The limit switch has a temperature dial on it and when it gets up to where the fan starts it stays there whereas I think it used to go up another 20 degrees after the fan started. The limit switch just controls the fan and turns off the thermostat input if it gets hotter than 160

The gas pressure thing makes sense. Looks like I will have to call someone.

If you have a digital thermometer, place one in the main duct after the first heat run. Then place in the return air duct at the furnace. Check your temperature difference and report back.
Make sure you watch a complete cycle by the way. If you're not holding temp you could be cycling off of limit.

Do this first!
He is asking what the difference in temperature is across your furnace.
Normally you should see about a 40-50 degree temperature rise across the furnace.

It doesn't take very much very cold fresh air entering the furnace return air duct to drive supply temperature down. You may have to throttle the amount of fresh air entering for colder weather.

To expand on my initial thought and Bill's as well, depending on the age of your furnace you should have a label with model number, serial number and temperature rise range. I had one recently that I had to speed up the blower on because I was in the high 70 range. I got it down into the low 70's which was acceptable for the model. You could be fighting low temperature rise which could be cold air infiltration or your blower speed could be a little too high. Slowing down the blower will reduce the amount of air moving across the heat exchanger making your supply temps higher. If your return air temp is lower than your home indoor ambient air temp I'd be checking for leaks in your return in cold places (attic or crawl space) that would cool down your return air lowering your temperature rise.

It says air temperature rise 45-85 (it isn't anywhere near even 45), and max outlet temperature 185 (with the limit switch set at the specified at 160 ??)
120,000 in 96,000 out (79%). mfg 3/87

I took the cover off and looked at the six burner tubes. Two have an irregular flame pattern (along with some yellow flames).

I got a guy coming out to look at it but he is busy till Thursday.


EDIT it is in the low 40's till Friday

I don't suppose you have outside air ducted to the return duct?

Sometimes that is done for make-up air for range hoods that pull more than 500CFM.

All the duct work is in heated areas. The combustion air is from inside also.

You have a small air compressor?

Are there movable adjusters (shutters) for air (primary air) on the inlet of the burners? If so are they all in about the same position?

Has there been a furnace cleaning company in recently?

Take and blast (when the furnace isn't firing... Lol turbo furnace!) some air down the burners tubes. Try not to move the adjusters or mark where they are.

It is possible that your gas valve isn't fully picking up too.

Is it a high efficiency furnace?

If so is the water drain line from the exhaust blower fan draining properly?

If not get out a clothes hanger and clear any gunk that has grown inside the drain tube

Whatever it is it sounds like the computer board on a modern furnace is in "limp home" mode

Old 80% furnace non-condensing.

He doesn't have even the minimum temperature rise, if I read that correctly.

Betting gas valve at this point.

I used to work for a natural gas utility. The gas meter or outside regulator could be sticking. You will need to phone an hvac guy. They can measure the manifold gas pressure at the furnace. If your furnace is natural gas it should be around 3.5-4.0 water column.
If you have other gas appliances are they working as usual?

Bill K
Do you think having two burners partially clogged would reduce the heat output enough so it won't heat when it is 15 deg? The furnace has never been cleaned since I've lived here.

I'd take the burner assembly out and clean it but I'm not confident about the gas fittings to the main valve. Air probably won't do it since there is some cooked goo dripped out of the troughs that the burners are in.

Last heating season I took the duct work off the chimney to the furnace and there were a couple dead birds in it. You don;t think one got all the way into the heat exchanger and I cooked it do you?

A few things:
1. You say it's not near the low end of temperature rise. What is the temperature rise? Again measure Supply temp at main trunk after the first duct going to a room vs return temp at furnace? It would also be nice to know what your ambient air temp is in the home versus what your return air temp is. Again this will tell if you have any cold air infiltration.
2. You mention yellow flame. Does your unit have an inducer fan that runs prior to the unit lighting? I'm guessing an 87 will have in shot burners with no air adjustment. Are you burning propane or Natural Gas? The yellow naturally indicates incomplete combustion. A small amount of buildup on the burner orifice can create this, You can usually use a flashlight and inspection mirror to see the ends of the burner orifices.
3. As for how far a bird can get, if you have an inducer/purge fan, not very. I have cleaned them out of the purge fans on several occasions.

Post some pics of your burners. I would be interested to see the cooked goo you're describing.

Also don't want to be an alarmist but yellow lazy flame can indicate a cracked/plugged heat exchanger. If you don't already have one, get a Carbon Monoxide detector in your home. Also make sure the tech puts a Combustion Analyzer on the unit and does a CO test