Add on furnace cracking bricks (long winded)

[2broke2ride]

I have a Shenandoah AF77 add on furnace. I purchased it nos, (it was made in 1977 but was still new in the crate) it is designed to hook inline with an existing furnace and use the existing furnaces duct work. I do not have an existing furnace so I purchased a furnace blower and wired it up to the thermostatic switch and built my own ductwork. The furnace does not use a thermostat it is basically just a wood stove with a plenum like a hot blast or an englander. It has a 6" pipe and its hooked into a 7x7 clay flue 20' tall. I have a key damper in the pipe and a condar flue guard thermometer above the damper. I try to run the temp around 350 to 400. The thing cranks out heat and easily heats my 2k sq ft ranch and the basement. The problem is I was looking at it today and it already has 6 cracked fire bricks, 3 on each side. Anyone know why this might be happening?

 

[DoubleB]

Not familiar with your furnace, nor do I have a guess why they're cracking. But first thing I'd consider is finding newer firebrick to replace the old. If the new ones don't crack right away, then you might be able to convince yourself the problem was with the old firebrick and just replace it.

Maybe they picked up a lot of moisture over 40 years and fractured from boiling the moisture when you first fired it up. No idea.

 

[2broke2ride]

Yeah, kinda figured people wouldn't be too familiar with this lol. I thought the same thing about the moisture but a friend of mine bought another one of these from the same place and hooked it up in his house the same time I did mine and he hasn't had any bricks crack so idk.
We purchased these from my fathers work, apparently they where a Shenandoah dealer in the 70s and had these two units left over just sitting in the hay loft collecting dust.
Either way, I'm not gonna lose a ton of sleep over it. They are cracked but it's not like they're falling out.

 

[maple1]

If you fired hard from the get go, that's likely what did it. First few fires should be low & slow to dry the bricks.

Not familiar, is that thermometer magnetic or probe?

 

[brenndatomu]

They are cracked but it's not like they're falling out.

Firebrick most often crack due to rough handling somewhere along the way...usually after they are in use...like hitting them with wood...they are pretty brittle. If they are not falling out I wouldn't get too worried about it, but at the same time if it is the same size bricks that are common in most wood furnaces now days, you can buy new ones for a couple bucks at most any hardware or stove shop. Menards had 'em on sale a while back for $10/pack ($1 ea. IIRC)
Like was already mentioned, it could have been a moisture or age thing, but I kinda doubt it, more than likely been banged around a little. New ones should hold up fine for you, wouldn't expect further problems

 

[joes169]

Generally speaing, split firebrick (1.25" thick) aren't really that tough. As toher's have mentioned, moisture can definately cause an issue, if that's the case. Also, I wouldn't worry too much, it sounds like they're still doing their job.

 

[2broke2ride]

Thanks guys, I wasn't really too worried, just gonna change them end of season. Just weird that mines doing it and not my buddies. He fires his way harder than me too, his door is already dis colored white lol

 

[brenndatomu]

his door is already dis colored white

Whoa, he is solidly in overfire territory there...like it's gonna be warped/cracked in a season or two...

 

[2broke2ride]

Whoa, he is solidly in overfire territory there...like it's gonna be warped/cracked in a season or two...

Oh I know, but he won't listen, he's been burning a lot longer than me so he thinks he knows it all. The one saving grace is these things are all plate steel so more apt to warp than crack thankfully for him.

 

[brenndatomu]

he's been burning a lot longer than me so he thinks he knows it all

Send him here, we'll straighten him out!

 

[lexybird]

Every one selling wood furnaces on Craigslist has the notorious white discoloration around the loading door perimeter . This is a sure sign of an over fire . That's a reason alone for me to never trust it and avoid at all costs