1950's metal ductwork: seal or replace?

Novagen_Ed

My folks just bought a 1950's rancher with rectangular metal ductwork that supplies heat and cooling. It's mounted up almost flush in the basement joist bays and it leaks like a sieve. I had thought of using something like Versa-Grip 102 or similar at the seams, then facing the 3 accessible sides with polyiso board. But it seems that the inaccessible top seams will still leak. Would you just remove and replace with something better (and insulated?) or pull it down in sections, seal, insulate and replace? It's not a huge house, probably 1500 sq. ft. at most. Thanks in advance for your suggestions and advice.

JUGHNE

Is the basement ceiling finished with something...sheetrock etc?
Does the main-largest ductwork run perpendicular to the floor joists....it usually does in a ranch.
Do you have access to the end caps of the main duct?
A few pictures would help a lot.

How old is the furnace and AC unit?

Novagen_Ed

Hi JUGHNE,
Thanks. I will be able to take pics tomorrow. All of the ducting I've seen so far is parallel to the joists and up in the joist bays. There are a few duct runs where I can access the end caps. I'll map it out along with pictures and post this weekend. Furnace is a 14 year old Rheem gas unit. Carrier condenser is 16 years old.

JUGHNE

Good legible nameplate pictures tell a lot also.

BTUH input/output of furnace.
Tonnage (BTUH cooling of AC.
Where is this located, what climate?

Novagen_Ed

Heating is rated at 100000 BTU/Hr. Cooling is 30000 BTU /Hr (2.5 ton nominal). Will get nameplate data. Location is a suburb close to Philadelphia, PA. Thanks.

JUGHNE

Are the supply registers on the inside walls and the returns grills around the outer perimeter of the house?

Novagen_Ed

....for the most part, yes. The home inspector I hired prior to settlement indicated that one or two returns were "flipped" in a bedroom, supply ducts on outside walls, etc.
Main living area takes forever to come down to temperature if system has been shut down. It's supply / return setup is "normal"( supplies on inside walls, etc.)
The cellar gets very cold, fast, and after investigating leakage from the supply runs I can understand why......

aircooled81

Gonna be a lot of work (elbow grease) to drop those ducts and seal them. If you go that route, brush and vacuum all the dust out while you are there.
Box duct is great, very pricey to get it replaced like for like on the material side.
If you are handy, draw up the dimensions and sections you have, send it over to the sheetmetal supplier and get a quote on new ducts. If you had a bit extra room in the joists, make it bigger and have it internally duct lined. If you dont have the room, internally lined ducts have smaller id so less airflow could happen.
Another slightly cheap handy man idea is to cut-open an access between two joints. Get in there with a vacuum and clean the ducts. Get in there with a rag and wipe the ducts, then use some duct sealer on the inside where you cant reach the tops. Put a larger piece of sheetmetal over the access you cut and seal that up.
Not sure about that board product you mentioned, but anything is better than nothing for insulating them ducts. And so what if the top isnt insulated, cram some f/glass into the joists and the lost heat will radiate into the floor of your house anyways.

Jmo

SWEI

Would Aeroseal work on these?

hvacfreak2

Old residential duct work is just not glamorous like the water and steam piping that is discussed on here. It's not the right size for modern systems , it's full of dust , it leaks.

Sealing joints is an issue , even with new duct. There are hinged access doors available that are good for getting to critical areas. These are also good to have in the slow moving return air ducts for future cleaning.

Indeed I have been in clammy old basements and marveled at hand formed double -seamed rectangular transition - offset tee's and 90's. Crazy nested contraptions of multiple smaller ducts that weaved up and over like rectangular snakes. I've seen 100 ft shafts of hand bolted cleated angle iron connections connected to fans that occupy 1/2 of a city block. And in every case , people were always happier when the old duct went away , lol.

Tinman

A good tinner could have new, properly sized supply and return trunk lines up in roughly a day and the branch ducts done the next day in a home that size. Another day for the sealing whether mastic or UL181 foil tape is used. In an unfinished basement, it's a fairly easy job.

If you choose to go that route, be cautious of who you hire. Ask to see pictures of their work. Are they using duct change fittings or just slamming a length of 16 X 8 into a 20 X 8 and capping off the extra 4" inches? How much experience do the installers have? Are they using trunk line starters or just connecting duct to the plenum?

JUGHNE

Most houses built in the 50's did not have AC in mind. It was an afterthought add on. Seldom was much consideration given to the return air duct system. Even today RA duct is often undersized without grills in each area. There is a good chance that that house needs more RA ducting/wall-floor cuts and grills.
Could contribute to the pressure leaks you feel in the basement supply.....air is not freely returning to the blower.
Just a rough rule of thumb is that the return ducting should be about 150% of the size of the supply ducting. Also the corresponding return grill to supply register ratio. As I said just a visual estimate to go by. I have seen a lot of large RA duct that was large enough but limited by the floor cuts inside stud spaces......big duct but limited and small openings into it from the conditioned space.

On these inside wall supply systems I would convert the high wall supply registers to RA and add additional cuts/ducting to handle the AC CFM. The exterior wall RA would be abandoned and floor registers cut in for SA. That gives the old SA grills a purpose and removal/patching is not needed. The old RA grills are usually left in place after packing insulation into the wall cavity, if remodeling and new baseboard trim is in the plan then they could be patched shut. Otherwise leave them there.
This is even better with older gravity air systems as the supplies are large and ornate and provide a lot of RA CFM. (If homeowners like that look, most do).

This is usually a complete change out for me. You could reuse existing equipment. It is just how much do they want to spend?

Pictures still excite us!

Novagen_Ed

Yes, although home inspector said several supplies / returns were "flipped", I am finding them all as you describe. Started to map out the supply distribution, ran out of time, but there are two takeoffs from the furnace supply plenum: one of those feeds a large rectangular duct with multiple branches and takeoffs from the top and side. All supplies and returns are parallel with joists.

Novagen_Ed

I will take pictures tomorrow and post. All supply and return ducts are roughly the same size. all supply and return grills are baseboard mounted. Shut down system to day to upgrade thermostat. After 90 minutes off time (Honeywell Redlink system replacing classic White Rodgers Chronotherm unit) it took 3 hours to get inside temp 11 degrees cooler than outside. Basement sure was cold, though........

Tinman

An 11 degree drop in 3 hours is really good. Is the humidity in your area low?

Novagen_Ed

As a rule, no: Delaware valley has fairly high humidity mid June through early September, though recently the temps and humidity have been moderate . I believe the humidity outdoors this afternoon was about 70%. Thanks for the perspective, I thought the drop should have been greater in that time (88 deg. outside, 77 inside)

hvacfreak2

As mentioned earlier the correct placement of the supply registers is at the outside wall. However , if that statement is correct why were so many forced air systems done the other way ? A leftover practice from gravity maybe , or is this like a " counterflow " strategy like is used with hydronic air coils ?
( Brad White , you out there ? ). Maybe economics is the only reason we supply to the outside walls nowdays ? By " front-loading " the supply air at the outside wall are we less efficient ?

I helped a friend last year change out a system. Due to economics the basement duct was replaced but the configuration of interior supply / exterior returns was not changed. He is always telling me how great the system is working in heating and cooling. The first for me doing this in 30 years , I didn't expect it to work very well.

Novagen_Ed

Just to clarify: the supplies in this house are all routed through inside walls. I wonder if this (originally heating only ) system was designed at all, and if so, designed to avoid condensation forming in warmer supply ducts through cold exterior walls.
Insulation wasn't a prominent feature in these houses. There are, however, mineral wool batts in the attic joist bays for what that's worth.

JUGHNE

Possibly the walls have the same amount of insulation, certainly not more in the walls than ceiling for the original construction. Those might be balsam wool (wood fibers) batts you can see. More heavy black paper than insulation actually. My parents put that in their 1942 war time house, they burned wood and knew that that was not free heat.
FWIW: My best guess on the location of your supply ducts is this:
early hot air furnaces had no fans/blowers (perhaps no electricity available). So to make the hot air gravity heating function, there were cold air returns on the perimeter, especially at the windows. This was a major source of cold air infiltration which would come in and fall to the floor. That ducting would carry that cold air (capture it before you could feel it) to the bottom of the furnace, (sometimes those ducts at the outside grill would be boxed in and drop straight to a tunnel under the floor which would end up under the furnace.)
The supply were large, sometimes 10 to 12" in diameter, to promote the gravity flow up into the center wall grills. Always a good place to put your ice cold clothes on in the morning. Sometimes the wall stack would extend up to the 2nd floor for heating there. The more vertical drop of cold air and rise of hot air improved the flow of heated air. Eventually some of these basement "Octopus" furnaces got a fan kit to speed up air flow, especially if converted from the wood/coal to an oil burner.
When modern fan driven furnaces came along the same placement of supply and return grills was duplicated.
Eventually it seemed more comfortable to wash the offending hot or cold exterior walls with conditioned air and return it to the center of the room. Who's right.....I'm not sure (as hvac freak mentioned above) which is the best. But the new standard has been set for the day.

AMservices

I was in a small house with very old ductwork last week. It was using only 5 supplies straight up the middle walls, 3 for the first floor and 2 for the second.
Then 3 returns, 2 on the outside side, 1 in the middle, all on the first floor and no insulation.
The furnace was 80%- 80,000 out, 15-20 years old and from what I could tell with the size of the ductwork, the system was squeezing 800-900 CFM into duct that I wouldn't try putting more then 400 CFM into.
When I asked the customer if she had cold spots or any or problems heating rooms ( because a bathroom on the first and a bedroom on the second had no vent) she said
"No everything worked and heated well"
I was only there because she wanted AC, so I explained adding AC without changing ductwork could be risky. If Im not moving enough air I can't guarantee the system will work.

The explanation was longer but you get the picture.

So I explained with her open floor plan on the first and the 2 bed rooms upstairs, I could cool her house with 3 ductless units that would also provide heat. Her furnace would only have to kick on mostly at night when the outside temperature drops under 20°F and on equally cold days.
She has friends with ductless that love them so she took that option.
Running 3 modulating zones will be much more efficient and effective then 1 with 1 speed.
But if the owners would rather stick with central because of the usual reason, being people don't want that "S" taking up wall space, then to be efficient about it, I would design a new system.

Supply ductwork was designed to give the path of least resistance to heat gravitating through a house.
With a blower and a good duct systems, you can have control over the CFM , the velocity and the balance in the home.
And insulation around tight ductwork, you will be conditioning the living space and not the inside of walls.

It's crazy to think about what people have worked with for so long and the crazy duct work I've seen that people tell me works fine, I can't explain.
It's been so bad I feel like I could just hook up a furnace in a basement, attach NO ductwork, cut holes in all the floors and give people the same satisfaction they've known all there life.

But really, that's no option. So to have an efficient heating and cooling system that can be balanced and comfortable,
Start fresh with new ductwork, or just change the furnace and go ductless.