Oil Furnace High Stack Temperature


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Old 12-01-09, 01:49 PM
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Oil Furnace High Stack Temperature

Hello from central Maryland-

The over fire draft is 0.015, stack draft is 0.025, trace of smoke, but the stack temperature is 570 F. I should have expected closer to 420F, correct (with room temp of about 70F)?

Here's what I found when I removed the vent stack and manifold:
Picasa Web Albums - onewheeltwo - Oil Furnace

Cleaned it out (took the photo halfway through) and stack temp is still high, 570F. Any suggestions? Is there more to clean than the horizontal run of the exchanger that I can see?

It's a Unicell ComfortAire Forced Air Oil Furnace by Heat Controller, model number of OBUA95 - D3.5S. Beckett AFG Burner. Nozzle was replaced with the specified Delvan 70 degrees Type B 0.85 gph just a few months ago. The draft was 0.06 when I first checked it, adjusted the side shot powered vent to lower it.

Thanks for any input!
 
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Old 12-01-09, 03:06 PM
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High Stack Temp.

I'd try downfiring to a .75 nozzle. Your furnace has the same burner air tube combination as does the 84,000 btu model.
 
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Old 12-02-09, 05:04 PM
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So it may simply be a matter of improper installation? In your experience, what would you expect the stack temperature to drop to?

Would increasing the speed of the blower fan affect the stack temp?

Thanks for the suggestion, I hope to get to it sooner rather than later
 
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Old 12-02-09, 06:51 PM
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Stack Temp

I wouldn't expect a drastic drop in stack temp. Maybe 50-75, maybe more but no where near the 150 you are shooting for. I would say the 570 is a little high but not too far from average. Before changing the fan speed you need to know the incoming & outgoing temperatures. You want to maintain around a 60-70 difference. The return temp can be measured in the cold air drop & the supply should be measured in the supply trunk close to, but not in, the supply plenum.
 
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Old 12-14-09, 03:58 AM
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Just to update, I've put in the 0.75 nozzle. Stack temp is down to 505F for a net of 440F. I guess that's as good as it gets.

The split was at 80F, and is now at 50F. That's the low limit according to the label for 119000 BTU, is it any different for 89000?

Thanks very much for your help!

(Now on to my powered vent "issue"...)
 
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Old 12-14-09, 03:00 PM
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50 Split

You might want to check the fan speed & drop it one speed if it's not already as low as it will go.
FYI on wire colors vs fan speed: Black = High, Blue = Medium,
Red = Low.
 
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Old 12-23-09, 01:55 PM
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Hi- Turns out it's a 4 speed fan, I changed it from "Medium Low" to Low. The split remained at 50F, and the net stack temp stayed the same at about 440F.

I set it back to the Medium Low speed. All other things the same, I'd prefer to have more air moving around.

Thanks!
 
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Old 12-23-09, 06:57 PM
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Fan Speed

I was hoping the reduced fan speed would increase the split temp a bit but you are running within the range spec'd by the manufacturer. We did manage to get the stack temperature down some which should be reflected in an efficiency increase. All & all I don't think we did too badly. If you are satisfied, I'm tickled to death.
 
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Old 12-26-09, 03:46 PM
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Yep. I'm not sending as much heat out of the house now, so that should lower my bills

However, now that I think about it, there is about 25' of (mixed 6" and 8") flue between the furnace and the SS1 powered vent. Currently it's not insulated. Could it have been set high so that the gases could get outside before condensing? Should I insulate the flue? It's going through a storage/crawl space - mostly about 55F.

Thanks very much for your replies.
 
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Old 12-26-09, 06:57 PM
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SideShot

I doubt the stack temp was that high intentionally. Probably simply a lack of fine tuning. Is there a barometric damper in the vent pipe?
 
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Old 01-02-10, 08:48 AM
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Yes, there is a damper. Contrary to what the powered vent instructions say, it's installed a foot or so above the furnace instead of near the side shot.

The damper opens completely when the side shot turns on.
 
 

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