Chimney Cap Question

[ridensnow23]

Question: Do I need a flue top cap if I already have a metal cap that spans multiple flues?

We bought this house over the summer. The chimney needed immediate repointing of the top rows of bricks, and a new mortar crown. It was a big job because the chimney has 4 flues and is approximately 56" x 27". I didn't want to have to redo the crown every 10 years, so I did a 4" poured concrete crown, and then got a SS cap to cover all four flues.

I just got two wood burning fireplace inserts; one new, one used. I am going to reline the two fireplace flues. So that brings me to the question: Since I already have the big SS cap on, should I bother with putting the individual flue caps on, or should I leave them off.

 

[DAKSY]

Are the two fire places on the same floor? do they both
have OAKs on them? If they are back-to back at the same
level, you are probably good to go with your crown mount,
multiple-flue cap. However, if one of your two fire places is
on a lower floor than the other one, & below the pressure
equilibrium point of the house, you may get some smoke
being drawn in thru that lower flue, after it exits from the
higher one. This will happen when the lower one is not being
burned & the replacement combustion air is drawn in thru
that flue. If that does, happen, you will have to add a section
of Class A to your flue from the lower unit...

 

[ridensnow23]

That is definitely causing me to rethink things. The fireplaces are on different floors. One on the main floor, and the other in the basement. The fireplaces are basically stacked on top of each other. The chimney runs up through the middle of the house. Both flues are the same height at the top and are couple feet apart. The inserts are a Hampton HI300 and a Buck M81. It does not appear that either was make to work with an OAK.

I have not installed the inserts nor relined the flues yet, so I can change the plans if necessary. Running class A pipe all the way down to the basement is not an option because the flue is not straight. It angles to the right out of the smoke chamber. The flues are terracotta lined with 11" x 6.5" internal dimensions.

It's a 1950's house that I would not consider exactly air tight.

I'd like to hear some more ideas on how to manage this situation if there are any.

 

[Hogwildz]

If it were me, I would get rid of the shared cap, raise the height of one of the flues as Daksy suggested, say a foot or 18", and put separate caps on.
If your handy enough, You could leave the shared cap, and extend one of the stacks up and through the shared cap, and put a separate cap on the extended flue.
Make sure you seal where the Class A goes through the old cap though, or you may get away with a storm collar around the Class A under the old cap. If your not using the other 2 flues, cap them off for now.

 

[ridensnow23]

Cutting through the shared cap it not an ideal fix, but it had occurred to me.

As far as the other two flues go, one vents the hot water tank and is lined with a flexible aluminum kit. The other is an unused flue for an incinerator that no longer exists.

 

[ridensnow23]

Since my inserts are not compatible with any out of the box OAKs, could I just do a run of sched 40 from the outside into our finished game room for fresh air supply? I can run it between the joists since most of the basement ceiling is unfinished.

I'm thinking this will solve the problem of smoke being drawn down the basement flue if upstairs insert is burning. The downside is that it will probably make the basement colder.

 

[Hogwildz]

The OAK should draw air from outside the house. So if you want to do it from the basement to the stove, I suggest that to be fine, but I would continue it to and through an outside wall of the basement. If your connecting it to the stove somehow, use metal piping, not PVC, at least near the stove. Depending on how your rafters run, and how far from the nearest basement wall the drop from the insert would be, you could prolly run between the floor joists to the sill at the wall. Some use a cap setup that can be open & closed manually to keep drafts out when not in use.