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I am in the process of building a chimney with an 8x12 flu for my EFM 520 using chimney block to a height of 24 ft. above ground level. The building is a story and a half attached to the house. The chimney block measures 21x17. The attached photo shows my excavation. The building footer can be seen extending 8 inches to the side of the foundation block and appears to be 6-8 inches deep. I have excavated a for a chimney footer along side the building footer to a depth of 12 inches below the top of the building footer. The dimensions of my chimney footer are 42" L x 29" W and 12" deep from the top of the building footer, which is 4 inches deeper than the building footer. Our soil is Ohio clay and I am excavating previously undisturbed earth. I am unable to go wider than 29" to the side of the building footer due to underground electrical service paralleling the building.
My question is this:
Do I undercut the building footer by a few inches to create a ledge with the chimney footer and rest a portion of the chimney block on the building footer to keep it close to the building? If not, where on the chimney footer is it recommended to place the chimney. I am concerned about putting the chimney block on the edge of the chimney footer which would keep it 8 inches from the side of the building. Every add-on chimney I have seen appears to hug the house which means that a portion of it must rest on the house footer.

Drill and pin the house footer with rebar extending into your new footer,then pour the new footer to the top of the exsiting footer,then plumb up where you want the chimney to go and cut the siding away allowing for trim pieces ect., slide tar paper behind the chimney and siding because most likely you will cut thru the old weather barrier.Use wall ties to anchor the chimney to the framing and get a precast chimney cap to fit the chimney block,then stucco or paint to match existing house.Good luck and heating season is approching quickly.

Drill and dowel the old footer to the new as Dennis said but my professional opinion is that you shouldnt cut the siding away. If there is plywood or osb behind the siding or better yet a stud ,screw the block tie in the cove part of the siding,snug but not overly tight enough to buckle the vinyl. Not that you should do it but ;I had a job putting a two story addition on the gable end(brick all the way up) and had to remove all the old brick. There were, no exaggeration, SIX brick ties in the whole thing There had been no movement at all in some sixty years the house had been there. If you have no movement in the footer youll have no movement in the chimney. Fill the bottom of that hole with #3500 lb concrete to the top of the existing footer and rock on. And good for you on the masonry chimney.

creek im a mason 40 plus years.Good advice here .All I would add use 2 wall ties every block minimum or bey better yet every block .They cost pennies jack

52 years a mason I agree with all the above u can do it either way with the siding , just make sure that wall tie nails hit a wood stud, find stud and draw a plumb line up, u might only get one tie in so put one every block good luck, take ur time doer rite first time..its a bitch tearing down and re laying.. B.B.

Many thanks for the excellent advice. Good common sense solutions that I will put into practice here. One of my least favorite things is drilling concrete, but difficult problems rarely have easy solutions.
What size rebar is used for this purpose and how deep to drill the holes? How far apart should they be spaced? Also not certain if it would be necessary for the rebar to be epoxied into the holes. I am tearing off the vinyl siding anyway and replacing with fiber cement Hardi Panel. I hope to pour the foundation Thursday so that I can let the footer strengthen for at least a week before starting to lay the block. Is 5000psi Sakrete overkill?

1/2 inch bar should be fine
min. 6 inch inbeddment (IMO)
1/2 inch hole and drive 'em in.....3/4 inch hole to bed with epoxy.
...i wouldnt bother with epoxy here though...u could also keep the bar long enough an tie a rebar mat to them for the new footer as well
good luck
bag mix will suffice....just mix it WELL !!
...but the aggregate wont be as large as a footer mix

agree with drilling and re.bar will keep new chim footer from moving,the whole house will have to move..5000lb concrete more than enough,u aint building a nuclear station 3000-4000 lb would be plenty...quickcrete brand is 4000 that is what I use for small jobs (lowes) after it looks like concrete give it about 25 more strokes with the hoe...

Thanks again to all for the great advice. I am going to start drilling this evening.
How should I space the rebar holes.................every 8 inches?

keep the outside one (side) 3-4 inches inside ur pad.....then every 12 inches....more often if ur ambitious...lol
about 1/3 the way up from bottom of pad
grid some the other way too....if ya got the bar

.

I usually buy the high strength and a couple of bags of portland and add a couple of shovels of portland to each bag of sackrete . There is no reason to wait more than 24 hrs to start laying block,Ive seen masons start the same afternoon

Didn't get home 'till after sunset this evening, so I won't be making any holes tonight. I purchased some 1/2" rebar and a 1/2" bit on the way home. Hopefully, I can get started tomorrow afternoon. 5 holes shouldn't take too much time (famous last words). My projects always have several "glitches". Sounds like I may be laying a few block this weekend if I can make the pour Thursday. I thought I might have to wait longer for curing, but with my experience level, I won't get very far the first day any way. A few block per day as time allows will add up quickly.

if u can get a hammer drill it will make the drilling go a lot better and much faster...b.b.

May I introduce some of my helpers................

that must be one heavy drill...!!!