Need some help with ducted system

[matt_i]

Looking for some advice regarding a ducted heating system. Previous owner of my house/shop was a HVAC contractor and I have added to the shop.

It has a Ruud Silhouette natural gas furnace (not sure of size yet have not found appropriate label yet) with a 20x20 vertical plenum and an 8x20 horizontal duct. There are 7 takeoffs from the horizontal duct and 3 takeoffs direct from the vertical plenum. All of the takeoffs have ducts buried in the attic insulation and then the sidewalls of the shop with a 6x10 register on the interior drywall.

What I propose (in generalities) is to tap off the 20x20 plenum going the opposite way of the existing horizontal and add in 6-7 new sidewall registers off the new ducting in my new part of the shop.

In most basic questions I'm wondering if I can run a total of 16 registers off this unit? I don't feel like I can go really far wrong, the furnace is going to put out plenty of heat and airflow, its just a matter of how fast it can achieve temp. Its very oversized for the existing space, so I know the ramp time is going to be slower for about double the square footage, but not really a problem.

Sorry if I'm rambling here, just wanted to try to get a few opinions. Thanks in advance

 

[TractorJeff]

First question everyone will ask is what size was the Shop and what size is it now?

 

[matt_i]

Original was 44x26 and I added 25 x 40 + 5 x 18, so 1144sf + 1100sf = 2244sf now.

 

[dreasoner]

You need to know what the CFM rating is on the blower. A 2000 CFM drive is typical. With 16 runs, you would have 125 CFM per outlet not taking into account for duct loss.

 

[dreasoner]

Why do you need so many takeoffs? Is the space open or divided up with walls? For reference, a typical exhaust fan is about 100 CFM. You will have problems with airflow unless you can keep the air velocity up.

 

[matt_i]

Why so many...Its a good question and I don't have a good answer. The original space is divided into a 26x26 and then a 18x26 with a common shared wall.

of the 3 takeoffs which come from the big vertical plenum, I have closed the shutters on the register for 2 of them. The reason is these 2 blow a ton of air directly into the area where the Tstat is located, thus shutting it off prematurely in sort of a short-cycle. I would rather the heat come from the outside in towards the center, in an attempt to heat more evenly. So there are really only 8 active takeoffs right now.

I suppose I could live with 4 or 6 in the new area.

Another big question I have, is how did they get the duct work past/thru the double top plate of the walls? The existing shop is 2x4 construction...I see a "wallstack" duct that's 2x10 cross section, but that would seem to be the only way.

Looking at http://www.snappyco.com/products/ for ideas on the possibilities.

 

[PWC Repair]

Should be a model tag on the inside of the furnace door. With that model I can tell you the btu and blower drive CFM. We can go from there to determine what needs done.

 

[matt_i]

Sorry, just opened it up. Thanks!

Ruud Model UGDJ-10EBRJR. Noted as 100,000 btu with 3/4 hp blower drive. Under Blower it lists 11-10 but I'm not sure if that's the powered vent or the room-side blower.

 

[DeltaWye]

Well the proper way is to pay a qualified person to design the entire system and follow that plan. I say pay because unlike electricians, HVAC people generally don't give their knowledge away for free.

If I was doing this, I'd get rid of the last takeoff that comes directly off the plenum and run another 8x20 duct the other way. You can get a sheet metal shop to make you a nice Y-shaped transition if you want. Then run your branches to the outside walls near the floor like you want to, that's the way to do it if possible. You may have to fur the walls out for that. Size the branches according to the size of the room, number of outside walls, window area, etc. You'll just have to use rules of thumb for that. You may want to put dampers in the branches for balancing. Or you can add them later as needed.

You'll also need to make sure the air has a return path. Dedicated ductwork, doors undercut or transfer ducts.

 

[PWC Repair]

That's a 100,000 80% upflow furnace with 1800-2075cfm blower drive. You need to make sure the duct system will flow out at least 1800cfm.

 

[matt_i]

Here's why or how I'm trying to simplify this into something I can work on based on my limited knowledge...
- in the extreme case where I had no duct whatsoever and just took off one side of the plenum it would pour out warm air into the shops, but distribution would be pretty poor, warm in the center and cold on outside walls where the heat is being lost. The registers that go directly into the plenum are sort of an example of this, and I will have to close them to direct airflow elsewhere.
- the existing ducts & registers put the heat on the outside walls, down low, basically "put heat where the most cold is" to improve comfort.
- the heat source is existing and very adequate for the space, and I'm not trying to "size it"...
- I have minimal concerns about temp gradients, its basically 3 big rooms all merged together.
- Max temp I've ever targeted with the thermostat is 60F. I just ramp it up to work for 4-10hrs at a time.

Trying to think of worst-case how it could potentially go wrong...
- consider the extreme case where I added 1 mile of new duct to the existing system hoping to get heat out of a register at the end, worst case would be that I'd get zero heat there and 99.99% of it would go out to existing.
- to get a handle on how extreme I am compared to above extreme case, longest existing register would be around 60 feet away from the plenum (25 feet south, 25 feet west and 10 feet down next to the floor) My proposed longest run plenum to register would be (5 feet north, 25 feet east, 25 feet north again, and 10 feet down) for a total of 65 feet of duct.

Seems like I'm mostly splitting the flow in half with definite propensity for closer registers to receive more flow hence the dampers for tuning if wanted or needed. I'm not losing any heat, its all staying within, not trying to use flow killing flex duct.