I think the missing parameters in this otherwise extensive description are the interior dimensions of the chimney portions now used as ducts. It may be worth noting that modern oil burner configurations often make use of existing, perhaps poorly lined, chimneys by inserting stainless steel flexible ducting. I don't know the range of sizes that may be available, but this type of duct would certainly be resistant to most environmental conditions, and should be able to follow reasonable chimney bends. One question would be the duct size needed to deal with the desired flow rate at the pressure loss the system can tolerate.

An alternative would be to add interior chases to the house that reach the rooms served by the chimney.

I am a bit surprised by the idea that the chimney is essential to the structural soundness of the house. In more modern construction, the house is built around the chimney. Due to long-term micro-cracking of the mortar due to age or wind loads, it might be that the house effectively restrains the chimney, rather than the chimney supports the house. I leave this as a question rather than an assertion.

Air flow is required for heating and cooling. A restriction in air flow will not allow a system to operate properly.

You need approximately 400 CFM of air per ton of air conditioning. So if the chimney doesn't provide enough space for the required air flow it simply will not work / can and will cause continual problems and in some cases damage to the equipment.

You have a finished attic. So this cuts your options. You either let the home defeat you or come up with a plan to fix it.

This means giving up space within the structure to house the ducts (return and maybe supply) to be able to supply the necessary air flow to operate the equipment.

If this means creating a HVAC closet somewhere with chases for duct work to run thru that's what it means.

The other option is mini splits. The problem with mini splits is needing heads in each room you want to control temperature and then figuring a way to snake line set thru to each of those heads. Over the long term the mini split option will probably be more costly with time because there is more equipment to service, maintain and eventually replace.

So I think the nuts and bolts of this is going to come down to how long you plan to live in this home.

Well, to be honest, your "chimney ductwork" sounds like an inventive solution to a common problem in older homes! The size of your chimney is important in determining whether a flexible metal insert can be used, but this shouldn't be a major issue if the size is sufficient. You may have to compromise on the ideal airflow per ton and it may, indeed, shorten the life of your blower but you gotta do what you gotta do!

Get some more HVAC companies in there (bigger ones) and see if they have any solutions.

Austin Air Companie wrote: You either let the home defeat you or come up with a plan to fix it.

A great maxim for this issue and many others.

<?xml version="1.0" encoding="UTF-8"?><md>Thanks everyone! The HVAC guy I had here eyeballed the width (looking through the vent) and did not think stainless steel flexible ductwork would be possible. The chimney is split into 3 flus which makes it challenging in itself. He did say, @sktn77a that it was very inventive and done very well; so that was nice... I think when I have the “stovepiper” guy come look at things he will stick a camera down there to measure. I am only making an assumption that the chimney is important for structural integrity but don’t know it for sure! It is cool but I would love to disassemble it and have a more open floor plan... and room for a closet for real ductwork. All this sounds very pricey! And you’re right I am trying to keep dialing it back because we are not sure this is our “forever” home but excited to be here and make it a great place to live for ourselves and future occupants. I do feel that our only option could be to built in new ductwork if we plan to stay for the long term. In the meantime I would love to get the dust under control. Maybe even some max hold hairspray

Perhaps a sheet metal fabricator could manufacture a customized duct that could be inserted into the chimney, making maximum use of the available dimensions. It might need to be made in interlocking sections in order to maneuver it into place.

JM$.02W

Thanks everyone I’ll update when I know more about measurements. I appreciate your input —it’s a thinking outside the box situation. Here are some pictures of the hulking chimney, two entrance points and then bedroom vent, for those who are interested.

Other half

The carnage.

How many supply vents are associated with this chimney?

At a minimum, one could put furnace filters (even if they had to be cut down) behind the output registers to catch the particles. (Note: This will increase the pressure loss and the furnace blower may need some adjustment. Or, supplementary blowers might be applicable.)

Seeing that vent on the wall reminds me of an episode of TOH where the HVAC guy went to tour a newly updated mansion, now a museum. Apparently that style of air circulation was more common around the turn of the 19th/20th century. In the mansion, electric fan motors were not common and 'central' heating was done purely by convection. There were boilers in the basement with large heat exchanges (radiators) in the cavities of the chimneys and plaster walls where the hot air rose to the rooms exiting registers like the one you have. They still had one of the original coal-fired boilers. The rest had been replaced with modern gas boilers.

Mike_home there are 6

weedmeister yes! It used to be powered by coal furnace and neighbors who have been here a long time tell me most homes on the block were heated this way. Converted to oil furnace in the 60/70s

Kaseki to control the dust that will be my plan if we can’t line it with anything. I suppose as long as I don’t do it myself I work with somebody who knows what the pressure ought to be and how to adjust everything, that it wouldn’t be too hard.

We could possibly seal these vents off but there would be no delivery/duct work to any of the 4 bedrooms, and about 50% reduction to the main living area downstairs.

Where is the equipment located that serves this house? Is it in the basement or the finished attic or somewhere else?

How many HVAC units do you have?

How old is the current equipment and do you have any other problems except for the dust / debris you show?

Meaning is the current equipment cooling and heating the structure well?

I believe the equipment to be 5 years old, one HVAC unit located in the basement (compressor for AC outside) and it works wonderfully. The only thing is the 3rd floor has trouble coming to temperature because there are no outtake/receiving vents up there. That was also something I was looking into.

"I am sure it is integral to the structural soundness of the house."

From the vantage point of my couch, I am NOT sure the chimney is integral to the house structure.

More likely it is not... would removing it and reclaiming the space allow better solutions?

Possibly structural, but I agree, most home of this vintage have a stand alone central chimney structure. Having said that, even if this is the case, it would be an expensive remodel!

Is that black old style vent a supply or return vent?

The one that has the carnage of the dust / dirt below it.

This could be related to possible high static pressure because space for air is not big enough. So as the chimney crumbles this dust is pushed out on to the floor.

The remedy might be to somehow run a flex duct from below and seal this supply vent off from the chimney. The duct just merely uses the chimney space as a chase to run the duct and not to use the chimney as a air duct if you will. (I am not a psychic or fortune teller either one, so I don't truly know if this is possible.)

OR you somehow remove this supply vent (cap it off) and add a supply vent somewhere else close by. In order to do this you may have to build a chase to house the new duct. Then sheet rock over that new chase to hide the duct.

What floor is this supply vent on?

The off temp issue on the upper or 3rd floor might need to be resolved separately and possibly due to constraints (more than one) it might be better suited to have a separate system handle this area.

Another problem this poses is that adding more capacity to this house, will probably make the system you have over sized for the area is serves.

If there were one or two supplies, then one or two chimneys liner would be a simple solution. But it does not seem you have enough room to install enough liners to service 6 supplies.

This is going to require a unique solution, something you would see on This Old House.

"Ask This Old House"?

A couple weeks ago, I toured the personal home of a well known midwestern architect of that era. The master bedroom had a register cover on the chimney wall which looked quite similar to yours. Another old house lover and I were discussing the situation as neither of us had seen that setup before. We assumed it must have been a way to capture some of the heat which would otherwise disappear up the chimney. The heating system in that house was radiators, not certain whether they are hot water or steam. I am interested to know how you tackle the problem and I hope you will post future updates

@LyndeeLee funny i was just in Philadelphia in an old building that is now an apartment and I noticed they have the same type of vent over the chimney; cold air exiting the vent from the AC so it is tied in as a supply. I looked in there with a flashlight and it seems to be cast iron and a pipe bringing the air up from the system below (it looks like the inside of a stove). This appears to be fit into the chimney space. As @Austin Air Companie pointed out this is exactly what I am hoping to do. It appears I just have to find the right professional with this knowledge. I'll keep looking! I do plan to ask "This Old House" I was having trouble getting registered to the forum. Thanks everyone! I will keep you updated.

and @Austin Air Companie that vent pictured is indeed a supply.

I looked in there with a flashlight and it seems to be cast iron and a pipe bringing the air up from the system below (it looks like the inside of a stove). This appears to be fit into the chimney space. As @Austin Air Companie pointed out this is exactly what I am hoping to do. It appears I just have to find the right professional with this knowledge.

Well be careful in that, because what you likely have is some heating only channel in which they probably used natural heating principles to heat this home with. (Heat rises from the heating apparatus in the basement... due to age of the house this was probably not a forced air system like we have today.)

Forced air requires more room as air takes up space if that space is not there it leads to a whole assortment of various problems. Then you throw in air conditioning another modern thing that wasn't there when the house was built.

400 CFM of air per ton of air conditioning. 3 tons that means 1200 CFM. 4 ton 1600 CFM, 5 ton 2000 CFM.

Not only supply air, but also return air. Measure the entire cavity, it will tell you in quick fashion. I really doubt the cavity of the chimney is large enough to do what you will require to get this to work.

You're definitely right --I have definitely heard enough & will be this cautious of making sure the space the air will travel though a space that is plentiful enough/system can handle the amount. The pipe I saw in their chimney looked to be 4 inch diameter, maybe 5, (likely 4) BUT they probably do not have their chimney delivering air to so many rooms as we have it delivering into 6.

I gather verifying non load-bearing chimney and removing and thus claiming tons of space has been eliminated as a possibility?

Salti Dawg,

Removing the chimney if it's even possible would be the way to go if I was going to do it... but it would be very costly, time consuming, messy AND there may be asbestos or something worse in or around it in some fashion.

Even if the chimney is load bearing it could be done if you off load this onto new supports... but this adds to complexity and cost. One wrong move and the roof collapses.

To the OP: If all you have is a 4 inch pipe it's no where near enough room. Realistically in todays world of HVAC systems I would want to see two 18 inch returns for a 3 ton HVAC system. (*If I was going to do it.)

Those two (2), 18 inch returns are just return air. Then you would have supply ducts on top of that.

If the system is bigger than a 3 ton, the recommended ducts would be larger than those noted above or more of them.

If this is a straight chimney, maybe you could attack the problem from the roof. It would not be too difficult to remove the chimney cap. Then you may be able to drop down sections of duct work down towards the basement. They duct work would have openings at the right locations for the supplies.

Without removing the chimney, it seems to me that the best next option is to bite the bullet and allocate space to one or more chases.

DavidR likely the best solution for the 3rd floor rooms long-term. Thank you! I will probably put filters over the vents that are in the chimney for the time-being, to quell my worries. Chimney inspection in the near-future when we have funding for that.. I appreciate everyone's input thank you so much. I will let you all know when we progress and what we choose to do....