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Magic Chef oil furnace: very high stack temp


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10-02-16, 03:57 PM   #1  
Magic Chef oil furnace: very high stack temp

Decided to measure the stack temp on our old Magic Chef L6A112C16-1 oil fired furnace and found it to be 690 deg F! Double checked with an old lab grade thermometer and it pegged at 600 deg. I read on another thread that 550 F was rather high and was wondering if these are universal numbers or is it normal for an older unit to run hotter? I used a spot smoke tester and got a one, not sure if this makes any difference. It is a 2 zone system and both zones were calling, the dampers were wide open as were most of the registers in the rooms.

The unit is rated at 112,000 BTU with a 1.0 GPH nozzle which it has, and our house is about 2000 sq feet. I think that it is oversized but am open to opinions. We live in the NE and have an occasional zero deg night in the winter, maybe -5 deg once or twice a decade. But even zero is not that common.

I'm thinking that I should downsize the nozzle from 1 GPH to .75, and or speed up the blower which is a single speed type. The drive pulley has an adjustment to ride the belt higher or lower. It has roughly a 2 to 1 speed reduction through the pulley sizes - I can measure more carefully if that helps.

Tried out the spot smoke tester and got a 1.

Also, the temp in the duct work leaving the furnace was 145 deg F. ambient in the house was about 72.

Suggestions very much appreciated!

Here is a long thread where I've been doing repairs on the system in the past. I think that it is working properly:
http://www.doityourself.com/forum/ga...1-rumbler.html

 
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10-02-16, 04:26 PM   #2  
High stack temperature is indicative of a fouled heat exchanger. Running a 1-spot (or perhaps higher) on the smoke over the years would have contributed to the heat exchanger fouling. Running 145 degrees on the air supply to to the rooms is excessive in my opinion and can cause excessive room temperature excursions as well as short cycling of the furnace.

I would definitely consider reducing the firing rate although I think I would go only 20% at this point (go to a .80 nozzle) and see how the comfort level in the house evens out. Raising the blower speed will increase noise levels and it may cause other problems as well. Reducing the firing rate should reduce the stack temperature and it will reduce the duct air temperature. My opinion is that duct temperature should not exceed 130 degrees and 120 is not too cold. Most likely your furnace IS oversized as it was normal to oversize furnaces by at least 50% if not more when yours was installed. Any energy savings measures that have been implemented since then have only made the furnace even more oversized.

Reducing the firing rate WILL cause the furnace to run longer and it will take longer to recover from a deep temperature setback if you use a setback thermostat. On the other hand, with a properly located and calibrated thermostat the comfort level throughout the house will be higher and you may be comfortable with lowering the thermostat setting a degree or two. Your total fuel consumption will probably be the same or slightly lower.

 
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10-02-16, 06:45 PM   #3  
Thanks very much, I'll try downsizing by 20% and see what happens.

Is there anyway to fix a fouled heat exchanger?

We set the temp back manually and I'm often impressed by how fast it gets back up to temp, so I don't mind it being longer. We can adjust how far back we set and do less in very low temps.

I think it makes a lot of sense to really downsize it, how can the efficiency _not_ go up significantly since aren't these the principals behind the newer more efficient furnaces?

 
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10-02-16, 07:25 PM   #4  
Some heat exchangers are more easily cleaned than others. From what I gleaned from a quick scan of the original thread your furnace is a difficult one to properly clean.

You can only downsize a certain amount on any given size combustion chamber before reaching a point where efficiency also decreases, that is why they have different sizes of furnaces. Modern furnaces had different designs for the burners as well as the combustion chambers as well as different heat exchangers and that is how they get increased efficiency.

Do you have natural gas available at your home?

 
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10-02-16, 08:49 PM   #5  
We have natural gas at the main street and the houses on the corners are hooked up. They say that they will run down the street for free if at least 2 houses hook up. I could be one of them and I'm next door to a corner lot.

 
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10-03-16, 04:16 AM   #6  
I asked about natural gas because it is a superior fuel for home heating, at least in my opinion. Because of the age of your furnace and the fact it is probably far larger than necessary I suggest that you limp along through this winter and then seriously consider a new gas furnace installation next summer. This is not the time of year to be installing a new furnace unless it is absolutely necessary.

 
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10-03-16, 06:20 AM   #7  
The stack temp is high but not really unusual for that vintage of furnace. Before making changes, you need to measure the supply & return duct temps. Normal difference for that furnace should be in the 60-70 degree range.


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10-03-16, 10:48 AM   #8  
Pulled the Burner to Change End Cone

I'm not sure what there is to clean inside.
There is a 3" gap with some sort of sheet metal housing, there is no dust just
a very small amount of soot probably from leakage. Then further in is what I
guess is the combustion chamber that I've read is delicate.
Do I use brushes in the sheet metal area, is there air flow there?
Anyway I'm going to put the new end cone on, make the adjustment and try it
out. A friend gave me an AFG burner that looks new and will bolt right in, I'm
very tempted to try it. I just have to swap the air tubes.

I thought of a very easy way to remove the burner which is to just unbolt the
fuel pump, then the fuel lines can be left in place. If I have to do it again that's
what I'm going to do.

Thanks!

 
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10-03-16, 11:44 AM   #9  
I thought that might be the case, thanks Grady.
Ambient in the house was 72 and output was 145 so that's 73 deg difference.
I can measure it again with the temp probe actually in the duct work on the
intake side.

 
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10-03-16, 10:06 PM   #10  
Increasing the blower speed should drop the stack temp some but I would doubt the change would be significant.


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10-03-16, 10:15 PM   #11  
Removed the burner and found that the oil lines were installed with compression fittings. I remember when we had the water heater replaced they also noticed them and said that flared fittings should be used.

Were compression fittings allowed back in 1989 when it was installed?

Does this need to be fixed?

 
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10-05-16, 07:12 AM   #12  
As far as I know, there is no "law" against using compression fittings but flares have been the preferred method for forever. Yes, the compression fittings should be replaced with flares.


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10-05-16, 04:58 PM   #13  
Thanks Grady, they did replace them on the water heater so I'll do it soon.

Since I had it apart and was suspicious that there was something wrong with the Webster pump on the old burner, I put in the AFG burner that I think is from 2014 - that is the date on the ignitor. The controller is the same electronic unit as the one I recently bought, and the motor is the same newer style. I was also going to replace the ignitor so I thought why not just put in the whole burner. The beckett tech told me that the castings were the same and that parts would swap between my old AF (or was it AE?) burner on up to the AFG. The Air tube and new F3 cone head fit just fine and it worked first try once I got it primed. The set back was dead on 1 1/8" without making any adjustment.

It locked out the next morning just like the old burner did, and I think that I didn't get the inlet line all the way in so it was leaking air. Put pressure on it while tightening it and it seemed to seat further in, I think it is good now but
I will change them over to flare fittings.

It says to set at 140 PSI on the side and to use a .85 GPH nozzle which it had and is equivalent to the 1 GPH that was in there.

Now I'm getting 662 deg F stack temp, 74 deg inlet, 136 deg out for a delta of 62 deg both measured in the duct work. Still at smoke of 1, I didn't touch the tuning but plan to soon. I just got fittings so that I can set up a fuel pressure gauge that I bought on ebay.

The system is very quiet now, I sometimes have to open the basement door to listen and confirm that it is running, much better than the old rumbling.

 
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10-06-16, 07:09 AM   #14  
With a delta T of 62*, it sounds like the firing rate is right where it should be. I don't think you are going to improve on the stack temp without dropping the delta too much. A delta T of 50* would, in my opinion be the very minimum for that furnace.


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10-07-16, 01:02 PM   #15  
It is still losing prime, odd that the outlet from the oil filter T's off to the hot water heater with the pump about 6" off the floor, the furnace pump is about 2' off the floor. The hot water heater is running fine without losing prime.

What prevents air from coming back in the nozzle, is there a check valve?

 
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10-07-16, 01:20 PM   #16  
There is a shut off valve in the pump but the fact there is a tee in the fuel line is nearly always troublesome. Is there also a return line?


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10-07-16, 02:32 PM   #17  
Grady, how about using a Tiger Loop mounted at or above the level of the higher fuel unit? That would essentially give both burners a gravity head on the fuel inlet. Use tees in the TL outlet and return connection to accommodate both burners.

 
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10-07-16, 03:06 PM   #18  
Furd, I'd have to check but as memory serves me, I don't think one Tigerloop will serve two appliances due to the volume of fuel being flowed.


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10-07-16, 03:27 PM   #19  
The system has run fine, in the past for over 10 years.

Yes, there is a return line.

I just (last week) changed the oil filter and I used an all white cotton looking one, that
our service guy says he doesn't like. I'm going to try a grey one with the screen in the
middle - that's what he uses. Not sure if this matters at all.

I have a very small air compressor, should I blow out the feed line?
Disconnect at the filter and blow back to the tank, is that the best way?
Is the return usually blown out also?
Any precautions? I don't want to screw things up.
I don't have direct fittings, the plan would be to hold it and let it "rip".

The fuel is clean I should probably convert to flare fittings first, this was my plan.

I'd really rather avoid the cost of the Tiger loop since this system has worked fine
in the past and we might move to natural gas in the next few years, certainly when
the furnace fails completely.

 
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10-08-16, 08:40 AM   #20  
I spoke too soon about the hot water heater it started locking out.
The strainer was full of sludge and it is working again. It has never locked
out before:
http://www.doityourself.com/forum/bo...ml#post2569108

I noticed something working on the hot water heater that even with the inlet valve closed there was oil dribbling out of the pump and it did not seem to stop. I'm thinking that the return line is back feeding is that possible, and would it help keep the pump primed for the next start?

I'm starting to wonder if the system just doesn't work well when it gets down to 1/4 of a tank of fuel.
The furnace has had lock out issues maybe two times when I called for service and perhaps they
also filled the tank before they did the tuneup.


Last edited by Pete2010; 10-08-16 at 09:08 AM.
 
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10-08-16, 11:32 AM   #21  
I'm starting to wonder if the system just doesn't work well when it gets down to 1/4 of a tank of fuel.
Depending on how it is piped that is quite possible, especially with ferrule (compression) type fittings in the piping. If the piping comes out of the top of the tank then it MUST have an absolutely vacuum tight suction piping. Ferrule fittings have been known to leak air (destroy the suction) while still not leaking any fuel outward.

I personally would change the fittings to well-made flared joints. For any tapered (pipe) thread joints use Permatex Number 2 joint compound on the male threads only. Keep it off the first thread or two and use only enough to fill the bottom of the thread. Never use Teflon tape with fuel oil.

 
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10-08-16, 12:18 PM   #22  
I'm ready to go out and buy a few fittings needed to switch over to flares. Is there an inexpensive flare tool that is known to work well? I have a pipe cutter.

Anyway to check for air leaks, it could be the pump, valve on the inlet side, etc.?

Is there a check valve to stop the pump from leaking air back in?

I have been using a light blue pipe compound from HD, don't remember the brand.


Last edited by Pete2010; 10-08-16 at 01:55 PM.
 
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10-08-16, 02:05 PM   #23  
The firomatic valve at the pump seems to be a type that takes a compression fitting, since that is what it is connected to and it has a fine thread. I need an adapter from the fine thread to pipe thread or to replace the valve I suppose.

 
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10-08-16, 02:57 PM   #24  
Grady will have to answer concerning the Fireomatic valve as I have never used one. I don't know about any thread sealant for fuel oil other than Permatex Number 2.

I rarely buy cheap tools so I can't make any recommendation there either. My flaring tool is an Imperial Eastman Rol-Air and it was, as I recall, over twenty bucks more than forty years ago.

 
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10-09-16, 09:37 AM   #25  
Dumb mistake - when I installed the new/used burner I had to reroute the return line a bit and needed a few more fittings on the pump. I didn't have pipe joint when I did the test fit so didn't use any. When I got the pipe joint compound I only did the inlet side. Applied pipe joint to the return last night and it at least held prime overnight which it has never done since I installed the new/used burner and started several times today without locking out. The way it was with the old burner was that it would lock out at worst 2 to 4 times a month - will have to wait and see if that happens.

Did some research and now I see that usually the copper pipes go in at the top of the tank with the return only going in about one foot and the inlet to within about 6" of the bottom so as not to pick up sludge. I also notice that there are a good number of youtube repair videos covering the loss of prime, so it is not that uncommon.

Anyway, I think that the furnace burner being about 1.5' above the water heater helps it in the sense of the oil from it acting as a supply back down to the water heater. This led me to the thinking of why not run the return off of the furnace in a loop up, say 2 feet so that there is oil in that 2 feet of pipe to drain back to the burner, while it is not running, and give it a better chance of self priming on the next start. I'm going to try this when I change over to flared fittings since it is just the cost of the pipe to try it. Wondering if I have to worry about that "head" of fuel dribbling out of the nozzle?


Last edited by Pete2010; 10-09-16 at 10:53 AM.
 
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10-09-16, 10:58 AM   #26  
Repeating tips from Grady on tuning with a Smoke Tester, to remind me how to use it:
"try to get a "trace" of smoke (less than #1 but still a little) then open the air a bit to get a zero smoke."
"Allowing the furnace to run 5-10 minutes before testing is ample. On each stroke of the smoke pump, pause 2-3 seconds after pulling the handle. Test procedure calls for 10 full pump strokes."

"Open the air a bit"
Say one number on the scale, or half?


Last edited by Pete2010; 10-09-16 at 11:17 AM.
 
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10-13-16, 05:44 PM   #27  
A few corrections:
I reported a stack temp of 690F in the first post with the old burner, I measured it again after posting, and after the burner ran for an hour and it was 720F!

I hooked up a pressure gauge and while the new pump mentions a .85GPH at 140 PSI setup on the side, the pump was actually set to 100 PSI. So the .85 was actual flow and lower than the original 1 GPH which explains the reduced/current stack temp of 662F.

Oil company topped up the tank and it has been running fine without lockouts, so I might do the flared lines and or Tigerloop in the Spring. I found new ones on ebay at a very good price, model S220 if that is the correct one for our application. I like the fact that the Tigerloop provides warm oil no matter the outside temp. I assume that I can use solid copper pipe to hook up the Tigerloop?


Last edited by Pete2010; 10-13-16 at 06:32 PM.
 
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10-15-16, 11:36 AM   #28  
Do not use copper pipe. Copper tubing is fine. The pipe is too rigid & would require joints to be soldered. They sell braided stainless lines which are flexible & great for hooking up the Tigerloop.

You shouldn't have to worry about oil drip from the nozzle.


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11-27-16, 09:08 AM   #29  
Furd wrote:

" I don't know about any thread sealant for fuel oil other than Permatex Number 2."

I bought blue thread sealer in a tube from HD and it does not dry, want to try the best since I'm redoing all the joints.
Suggestions? Is Permatex Number 2 the best?

 
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11-27-16, 12:37 PM   #30  
The first guy at HD told me to use Blue Monster pipe thread compound,
and it remained soft and never dried.
Second guy said to use Hercules brush on Blue Block which I have not tried:
8 oz. Block Pipe Joint and Gasket Sealing Compound-157072 - The Home Depot
My own research led to Rectorseal #5 which I am inclined to use at this point:
You searched for no. 5 - RectorSeal
This says that it is good for fuel oil, kerosene and jet fuel - it probably will work:
http://www.rectorseal.com/pipe-thread-sealant-chart/

Just read that this one is soft set and I'm not sure if that is the best.

I could not find Permatex #2 as Furd suggested.


Last edited by Pete2010; 11-27-16 at 02:09 PM.
 
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11-27-16, 02:08 PM   #31  
I dislike the word "dried" when referring to pipe joint compound, "harden" is a better word. Most pipe joint compounds do NOT harden but stay pliable to allow for the eventual disassembly of the piping. One noticeable exception is Permatex Number 1 which gets rock hard.

I've been retired for almost twelve years now and it has been closer to fifteen (or more) years since I have done any work with fuel oil. There may be other products that are as good as Permatex Number 2, I just don't know of any, and when it comes to fuel oil I don't like having to go back and re-do the job. I have not seen Permatex Number 2 in any big box plumbing section (I don't really look for it) in the recent past but I HAVE seen it in the automotive section of department stores and I suspect they have it at complete auto parts stores as well.

 
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11-28-16, 09:18 AM   #32  
Permatex #2 should be available at any auto parts store. Note: It comes in a tube, not a can.


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11-28-16, 03:34 PM   #33  
I just looked up Permatex #2 and see that it is Form a Gasket which I'm
very familiar with from the old automotive days.

Anyway, I used Rectorseal #5 and it worked well.
I also flared the return line that seemed to be in the worst shape and learned that the nut will not go on the pipe when it is bent. I finally had to cut off 2" more back to a straight section and it went, everything else was easy.
It was taking on air again probably because the oil level is down, last time I asked them to fill it up and the priming issue went away. It came back since it has been a few months since they filled it up, but after flaring the line it is working fine again.
Turns out that only the two fittings right at the pump were compression type all the others were flares.
The inlet is bent a lot, so I'm not sure if I should try to flare it at this point, it is working fine so I could wait until Spring, then if I have to cut it a few feet back and put in some new copper tubing I wont be under pressure with the cold.

 
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11-30-16, 08:24 AM   #34  
As long as the tubing is accessible, I'd go ahead & change everything to flares. To spice the new tubing to the old, you will need a flare union & 2 flare nuts.


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12-04-16, 08:51 AM   #35  
Thanks Grady, will do when I have the time, for now it is working better than ever. It is so quiet sometimes (before the blower starts) I have to open the basement door to check if it is running. Remember it started as a loud rumbler years ago when I started posting.
Thanks for all the help everyone!

 
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12-05-16, 07:39 PM   #36  
Good to hear about it running so quietly. Yes, I remember.


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01-08-18, 08:41 PM   #37  
I have to say thanks again for all the help gentlemen.
I have to confess that I've been lazy, have not done the Tiger Loop and there
are still a few compression fittings in the system.

It has gotten very cold here 10 to 20 deg F most days the other morning it was -2 deg F.

Burner is locking out again. I always knew that this happens when the fuel level in the
tank is low. Service guy came out and said you have half of a 500 gal tank, this was news
to me, that it is losing prime at half way. He claimed that because it is a 2 pipe system it
is not necessary to use the bleeder screw, rather just let the pump self prime. That did
not work, he tried the bleeder and that did work.
He said that he wished he remembered how to put the pump into prime mode - I will
look it up to see if this controller has that mode.

He suggested that there might be a restriction or a pin hole in the feed line in the tank
to explain why more than half a tank was needed. He suggested that the higher flow with
the bleeder open might explain why it would prime that way.

He suggested putting a tank in the basement even if we just drain the outside tank and
worry about the rest of the issues when ready to sell the house. I said the plan is to go to
natural gas when we need a new furnace. I didn't think that there was an option to leave
the old tank in the new tank in the basement would certainly fix these issues.

We should get a fill up tomorrow which should hold us over for a few months.

 
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01-11-18, 12:34 PM   #38  
I found an old, very faded tag on the furnace, probably from when it was installed,
that lists the following:
Smoke 0 Draft .03
CO2 12%
Stack Temp 450 deg
Efficiency 82.5%

STACK TEMP! 450 deg

I measured it again today with a Fluke probe at 690 deg!

 
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01-11-18, 01:25 PM   #39  
You know it steady state temp. minus room temp.(most say 50 deg.) So my guess is somewhere between yours and old.

 
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01-12-18, 12:34 PM   #40  
I'm not following, are you saying stack temp is measured minus room temp?

 
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