Critique Duct Work Design Forced Hot Air Furnace Proposal

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.UPDATE: Hopefully going with a Boiler instead. Seems to make more sense in my particular situation.

THANKS

My current heating system is forced hot air oil trunk and branch. I want to see if I can add a hot air furnace to my existing system. The problem is I don't have direct access to the existing oil plenum.

What I could do is create a type of radial perimeter duct system using some of the existing trunk/branch duct work creating a radial perimeter/trunk branch "hybrid". ??

My coal plenum would have (3) 12" round ducts servicing 3 areas of my home.

Area "A" is the main living space. It's an open plan and very large space. I would run a new 12" round duct to that area. It would be a short run with limited bends. Largest area of home. I'm not concerned with not getting enough heat as there is a hand-fed in the room. Northeast exposure with lots of windows also.

Area "B" is bedrooms. I would tie into the existing trunk branch system. I'd access it with a 12" round duct off the coal plenum and tie into the 8 x 16 rectangular duct. This in turn goes to the existing trunk/branch system servicing numerous registers in the various bedrooms.

Bedrooms are ok to be a little cooler so not overly concerned with heat getting here.

Area "C" third is in-law apartment. This is kind of tricky. I would come off the coal plenum with 12" round and access the same 8 x 16 rectangular duct going the other direction. The supply air would need to pass through the oil plenum and then enter the trunk/branch system which services the apartment. All registers and any ducts not feeding the apartment would be closed off so all air going to that side of the house would only enter the apartment.

** This is the area that I would like to ensure warm air was being supplied. Basement apartment needs heat.

There are concerns for friction and air turbulence as the oil plenum is obviously not made to perform in this manner. I don't know how much that would affect air flow to that area. The apartment area is my greatest concern in getting heat.

BTW, the house was designed so there is a 3 foot difference in the flooring between the sections of the house.

Blower for furnace would be 1,800 cfm.
The 12" rounds are rated for 600 cfm and the 8 x 16 rectangular duct is rated for 700 cfm.
Oil burner would be properly blocked off.
There are plenty of returns in each living area.

Any thoughts?

more pics

looks like a good start to me. don't forget return air.

plumberman wrote:looks like a good start to me. don't forget return air.

I second the return air, just as important and in many cases return air ducts are bigger than supply for a reason.

Also, have you considered a coal boiler with a water to air exchanger in the oil furnace plenum. Kill 2 birds with one stone with endless hot water.

I second the HWB and exchanger. a lot less work. Bobo K

bobok wrote:I second the HWB and exchanger. a lot less work. Bobo K

Any suggestions on where to learn more about this? Is there a sticky somewhere on this board? I'd like to read up on this.

That system is likely not going to work. With out a cold air return the thing doesnt work. If you could interrupt the cold air return for the existing furnace with a bypass through the new furnace it might limp along... maybe. If you want to add BTU's from coal a heat exchanger coil in your existing ductwork is by far your best option...Dont get stubborn and try to make it work youll hate yourself in the morning

skobydog wrote:

bobok wrote:I second the HWB and exchanger. a lot less work. Bobo K

Any suggestions on where to learn more about this? Is there a sticky somewhere on this board? I'd like to read up on this.

I will put a third vote in for the boiler. Not only is it easier, you can heat your domestic hot water with it to. My system uses a liquid to air heat exchanger mounted in the ductwork. Here are some of my threads about it. The first one has the heat exchanger install, page 24 is where the heat exchanger gets installed.

Yellow Flame, A Christmas Stoker Boiler
Gentleman Janitor GJ-51??? Project
The Bairmatic - Van Wert Project

If you just run the boiler into the heatexchanger like I started out with, it's very simple. Set the boiler somewhere close to a chimney, mount the heatexchanger, plumb some sort of insulated pipe to the heatexchanger. Make sure the pipe is bigger than 1" ID otherwise it will be complicated getting more than 70,000 BTU's to the heat exchanger.

-Don

StokerDon wrote: I will put a third vote in for the boiler. Not only is it easier, you can heat your domestic hot water with it to. My system uses a liquid to air heat exchanger mounted in the ductwork. Here are some of my threads about it. The first one has the heat exchanger install, page 24 is where the heat exchanger gets installed.

Yellow Flame, A Christmas Stoker Boiler
Gentleman Janitor GJ-51??? Project
The Bairmatic - Van Wert Project

If you just run the boiler into the heatexchanger like I started out with, it's very simple. Set the boiler somewhere close to a chimney, mount the heatexchanger, plumb some sort of insulated pipe to the heatexchanger. Make sure the pipe is bigger than 1" ID otherwise it will be complicated getting more than 70,000 BTU's to the heat exchanger.

-Don

The more I'm looking into this the more it makes sense to go this route. Lots of learning ahead Thanks Don

Two small pex tubes can be run and deliver much heat with great ease compared to doing the same with ducts.
Zoning w/ separate/additional t-stats and additional fan/coils can be possible where otherwise having only one t-stat can be at best a compromise.

McGiever wrote:Two small pex tubes can be run and deliver much heat with great ease compared to doing the same with ducts.
Zoning w/ separate/additional t-stats and additional fan/coils can be possible where otherwise having only one t-stat can be at best a compromise.

I am definitely leaning towards this way now.

skobydog wrote:

McGiever wrote:Two small pex tubes can be run and deliver much heat with great ease compared to doing the same with ducts.
Zoning w/ separate/additional t-stats and additional fan/coils can be possible where otherwise having only one t-stat can be at best a compromise.

I am definitely leaning towards this way now.

I don't know about the PEX tubes being small. I would go at least 1.25" ID for the supply and return.

Some notes on pump, heatexchanger and pipe sizing.

I would go as big as you possibly can on the heat exchanger. You don't want the heat exchanger to be a limiting factor, better to have more capacity in the heatexchanger than the boiler can output.

My heatexchanger is as big as the plenum it's mounted in, 20"x18". CoalKirk actualy put a kick out in his plenum to mount an even bigger heatexchanger.

Heatexchanger output is calculated by surface area of the heatexchanger, CFM of air passing through it and the water temp being pumped through it. I didn't do any calculating to size my heatexchanger, I just went with what would fit in the plenum.

The idea of running an over capacity heatexchanger is so you don't run low on heat and more importantly, being able to run the boiler temp under 180 degrees and still have plenty of heat. The lower you can run the boiler temp to heat your house, the lower you coal usage will be.

The pump is easy to figure out, you just have to do the math. Follow the instructions in the "Taco Radiant Made Easy Application Guide". If I was doing it over, I would select 150 degrees for the water temp instead of 180. I would also have selected a larger than 1" diameter pipe. You can use PEX but remember, 1" PEX is actually 7/8" ID and all the 1" PEX fittings are only 3/4" ID, a lot of restriction there. Only use oxygen barrier or PEX/AL/PEX. You don't want oxygen getting into your hydronic system.

Although PEX is rated for well over 200 degrees, it degrades quickly if run over 180 degrees. This is another good reason to run the boiler at a lower temp.

Another part of the pipe sizing/water temp puzzle is the pump. You want to be able to transfer this 120KBTU to the heatexchanger without having to run a big expensive pump. This is another reason you would want to select piping bigger than 1". A bigger pump also uses more electricity. Ideally you would want to design your system, piping/heatexchanger to run with a Taco 007. The 007 is the cheapest, most reliable pump out there.

A radiator or two in the coldest part of the house is also a good idea.

-Don

Sorry for the brain dump above, this thread just got me to write down all of the pit falls I ran into installing my first boiler.

You will want to keep an eye out for one of these;
Van Wert VA600 **Broken Link(S) Removed**

$1500 is a bit to much in my opinion, $500 to $900 would be a good deal for this one. It is an older one with the very small round door, great for efficiency but a pain in the but to get your arm in there. It looks like they cut the bin auger when they removed it so it will need at least that part. The Van Wert boilers are the most efficient and the stoker is more heavy duty than most. It's deep pot design will hold a fire for hours.

-Don

Not many boilers currently listed anywhere. Gives me more time to research and learn.