Magic Chef oil furnace: very high stack temp


Old 01-12-18, 01:23 PM
Join Date: Oct 2013
Location: United States
Posts: 1,945
Yes Pete. You take your gross stack temp from the furnace, for example 500 deg. Then you minus the room temp to get your net stack temp.

WE used to deduct 50 deg. as a rule of thumb. 500-50=450
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Old 01-12-18, 02:19 PM
Thread Starter
Join Date: Oct 2010
Location: USA
Posts: 155
Thanks very much, so 690-70 = 620 F still very high.
Should the limit switch be cycling the burner?
Old 01-13-18, 11:10 AM
Join Date: Oct 2013
Location: United States
Posts: 1,945
The high limit will cycle the if the air temp reaches what the high limit is for and the stat sill isn't satisfied.

The high limit is actually a safety so the furnace doesn't overheat.

If the furnace is oversized and it makes warm air faster than it can be delivered to the registers, that is where the cycling high limit comes. Your making heat faster than the blower can deliver it.

Most furnaces have multi speed settings on the blower, which will remove the warm air faster from the plenum and could stop your cycling if it bothers you.

Your math is correct and it's how it's figured. That figure is high, especially for a furnace.

I think I would have someone come in with a combustion analyzer and check the size of your nozzle.

A chamber can only except or store so much heat. Anything beyond its capacity will work but the excess heat that it produces will go right up the chimney and it shows up in your stack reading.

When replacing your nozzle don't assume it's the right one if you have had different people working on in the past. A lot of times out of laziness if they don't have the right one in the truck, instead of going to get it they'll put in what they have on hand.

Sometime the specs will be on the burner or unit somewhere. If this is your first time, take the time to look your info on your particular model.

You will be amazed by just using the proper nozzle will bring your stack down and other readings also.

Hope this helps a little.
Old 01-18-18, 11:52 AM
Thread Starter
Join Date: Oct 2010
Location: USA
Posts: 155
I think it was Grady who looked up the unit and found that it took a 1 GPH but the unit is oversized for
our 2000 sq ft house by today's standards so I reduced it to .85 GPH.
It does not hit the limit, I was just wondering if it could be used to regulate the stack temp and if this is
normal usage.
Thanks for all the info.
I had a previous thread about rumbling from the burner which turned out to be the nozzle set too
far back in the air tube:

It is very quiet now, thanks to all the help on here.

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