# First Furnace Install - Need Some Help Getting Started



## hartkem (Oct 4, 2008)

Hello all and thanks for taking the time to help other people. I want to start off by saying this is my first furnace install. I am however very mechanically inclined and have a lot of experience in Auto mechanics, household electrical, woodworking, welding, and general fabrication. I usually have no problems figuring out how to do things after I ask some questions and do enough research. I am doing this myself because I love the challenge and I always learn so much. I know this usually isn't a do it yourself job but I want to attempt it anyway. So to make sure I do this right I am seeking advice here.

I just purchased a Trane XR90 60K BTU Furnace for my 1400 sf house in southern California. I am not installing a coil for air. As for temperature, It does get somewhat cold here in december and january but nothing compared to the midwest. (I am originally from Illinois). I will be installing the furnace in the attic of the house. I have throughly read the instruction manual and I understand this furnace works best with a direct air supply and vent (two pipe) setup. I understand most everything explained in the manual but am confused about some of the things that I guess are common sense to most HVAC guys that are not in the manual. I understand that this furnace would put out about 1000-1200 cfm of air. I used a chart online to figure what size duct I need. The chart lists a round size and two square sizes. Now my first question is. Can you use either round or square for the duct coming right out of the furnace? I guess you call this the supply plenum. Also If you use round how do you connect it to a square box. My main problems are how the duct system is built. Do most HVAC guys custom bend the metal ducts and rivet them together. I just need some good pictures/explaination of how the ducts are assembled. Here is what I am thinking at this point. You build a metal box the size of the furnace box and somehow connect it to the furnace?. This box would be considered the supply plenum correct and is custom fabricated? From here you would make this box a desired length to allow enough room to connect enough of the runners to it that you would need. I am planning to use 6" flex for my runners off the plenum to each diffuser. If I got the correct info, 6 inch flex should flow about 100cfm each so I will need a minimum of 10 runners. Please correct me if I am totally off on this. I am going to go to the library and try to find some books on duct working. I can handle the wiring, gas line, venting, but am just a little unsure of the rest. Thanks for all help. 

Mike


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## hvaclover (Oct 2, 2008)

Weeeelllll... where to star? Did you do a man J heat load calc? The design temp for S. Cal seems to make your furnace over sized. If you are using 6" flex you are going to run into over heat problems. The online duct sizer you used seems like it was geared for smooth metal not high resistance flex. Otherwise you would have larger than 6" flex.

Duct work is as much an art as it is a science and I don't think given your skill set I would attempt this project based on the question you are asking. This is not a learn as you go project. if you make one bad mistake in the duct system or venting you could very well kill your family.

I won't sugar coat this as it is a very serious project you are attempting that deals with gas, electric and metal.

And did you know that Trane voids the warranty on their products if not professionally installed?

No true pro who cares about life, limb, and property would try to walk you thru this install. it would be like trying talk someone thru a surgical procedure over the inter net.

I know this will anger you but having seen the results (gas explosion that leveled the home or worse) of several guys who did a DIY install has convinced to me disuade anybody who is considering to take on the job with out the proper training or expierence.


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## 8 Ball (May 17, 2008)

Yes the "box" on the supply and return are the plenums, and extend out from the furnace as a "trunk line" to allow air distribution to the "branches". The "branches" are the smaller, typicaly round ducts, that direct air to the diffusers in the living spaces. Where they connect to the trunk line a "takeoff" is cut in to adapt from square to round. 

A trunk line is used to direct the air in a general direction to allow picking up the branches in the shortest and most efficient distance. As branches are taken off the trunk line, the size of the trunk gets smaller to allow consistent pressure in the system. 

It is not a good idea to cut all the takeoffs into the plenum directly off the furnace. If you do, you will have created an "octopus"...not good.

Limit your flex duct runs to under 10ft and use hard pipe for long runs.

The return duct by contrast does not reduce in size with distance, but again, limit your flex to under 10ft runs.

Any local sheetmetal shop can make up the trunklines for you and show you how to hammer a "pittsburg". From there most of the connections to the furnace will be with sheetmetal, or "zip" screws.

Enjoy your project Mike, and dont be afraid to ask for help.


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## 1610 CUB (Jul 19, 2008)

It sounds like you have a good grasp of how to do it. Things to consider 1. Leave room for an A/C coil, you may want one later and 2. Place the furnace above a catch pan that is at least 2" bigger around and 3 or 4" deep with a kill switch in it, make sure its big enough to catch h2o from the future A/C. 3. use larger runs for the return ducts, using larger ducts will slow down the air and it will filter better, it will also keep th grill from whistling's! 4. Be sure to install a service disconnect at the furnace. And every furnace comes with an installers guide follow it.


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## Marvin Gardens (Sep 30, 2008)

Boy. where do I start.

First off build a plenum for all the pipes to come out of. The plenum is not just a box on top of the furnace. It is also meant to equalize pressure coming out of the fire box.

You have to determine how many CFM each room would need. If you have a 20x20 room with vaulted ceilings then a 6 inch will be way short of what you need. If you have a half bath than a 6 inch would be over kill.

If you don't balance out your system you will have some very hot rooms and some very cold rooms. The key to venting is to put the correct amount of cfm's into each room.

Most home have a mixture of pipes. Larger rooms have several 8 inch feeds while small bathrooms have 5 in feeds. I just installed a system where we had an 18 inch main line with 5 8 inch lines going to the main room.

You always go from bigger pipe to smaller pipe. You can never get back the loss in flow by having a bigger pipe added on later.

The ratings on pipe are as follows.

8 inch is about 300 cfm good for about 2000 cu ft of room.
7 inch is about 210 cfm good for about 1200 cu ft of room.
6 inch is about 140 cfm good for about 800 cu ft of room.

Each boot has to be matched with the size of pipe. Can't have an 8 inch pipe going to a 4x6 boot. Unless of course you WANT to restrict the flow of air and like the high pitched noises of air trying to get through a small opening.

For cooling if the room is facing north then you can get by with a smaller pipe. If it is facing south you need a bigger one. If it facing south and has lots of windows then more vents are needed. If it is facing south and has lots of single pane windows and no insulation then you need a furnace for just that room.

For heating it is all about quality of windows and insulation and sealing from wind.

To go from a plenum to a round pile you need a starter collar.

To go from a round pipe to a smaller round pipe form the side you need a saddle. From the end you need a reducer.

Friction loss is a killer. If you run a 6 inch pipe for 40 feet you will get less flow that if you run a 7 inch pipe for 30 feet and then reduce it down to a 6 inch pipe. Flex will give you even less flow. Metal is always best for long runs.

To go from one round pipe to 2 other pipes you need a Y. Going from one pipe to two pipes should always be from a larger pipe to a smaller pipe. Like an 8 inch to 2 six inches. Going from an 8 inch to 2 7 inches is a waste since one 8 inch pipe will not feed to 7 inch pipes. I use the rule that you have to drop 2 inches at a Y.

If you are in an unheated space all pipes. plenum and furnace need to be insulated.

All joints need to be screwed and sealed. Use 1/2 inch sheet metal screws with 1/4 nut heads. For sealing use mastic tape or mastic paint. No duct tape allowed and no foil tape for sealing joints. Foil tape is only for sealing duct insulation.

Make sure you read the instructions carefully on the venting. Get the right size for the run you are making and don't forget to have the 1/4 inch drop per foot or you will get CO back in the house and lots of water in the pipe. Consider 90's since you can only put in so many in a long run before you have to go to a larger size pipe.

Do you know about the valves and traps in the gas line and where they go? High efficiency furnaces to not like impurities in the gas. And the size of the pipe to get enough gas to your furnace? And what kind of return size do you need?

Learn about S lock and hammer/drive lock. Different types of connections for sheet metal in the HVAC setting.

Also get some good leather gloves. Unless of course your tetanus shot is up to date and you don't mind seeing your own blood.


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## hvaclover (Oct 2, 2008)

:no:

Good luck. You guys. My liabilty insurance does not cover me when I give advice on the net.


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## hartkem (Oct 4, 2008)

Thanks for the help guys. I found some good info on duct sytems last night. I am going to take another week of research before I start on the project to gather as much info as I need. As far as the gas line goes I know I need to have a gas valve shut off and a drip line before the furnace. I will be running 3/4 black pipe almost straight from the meter. I still am somewhat confused about the plenum and trunk line(supply line). Are they the same thing or do you make a square box on top of the furnace and then calculate your supply trunk line size and adapt the plenum to the supply line? Does anyone have some pictures of different duct arrangments? I really appreciate your willingness to help. I know many are concerned with my safety but I am taking my time and taking proper safety precautions.


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## hvaclover (Oct 2, 2008)

hartkem said:


> Thanks for the help guys. I found some good info on duct sytems last night. I am going to take another week of research before I start on the project to gather as much info as I need. As far as the gas line goes I know I need to have a gas valve shut off and a drip line before the furnace. I will be running 3/4 black pipe almost straight from the meter. I still am somewhat confused about the plenum and trunk line(supply line). Are they the same thing or do you make a square box on top of the furnace and then calculate your supply trunk line size and adapt the plenum to the supply line? Does anyone have some pictures of different duct arrangments? I really appreciate your willingness to help. I know many are concerned with my safety but I am taking my time and taking proper safety precautions.


 
Not wishing you mis-forntune of any type. You had some very accommadating assistance. But the HVAC biz is like any other science or trade: Ask three questions you get three diff answers.


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## Marvin Gardens (Sep 30, 2008)

hvaclover said:


> Not wishing you mis-forntune of any type. You had some very accommadating assistance. But the HVAC biz is like any other science or trade: Ask three questions you get three diff answers.


What's really bad is if you ask 2 guys the same question and get 4 different answers.


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## Marvin Gardens (Sep 30, 2008)

hartkem said:


> Thanks for the help guys. I found some good info on duct sytems last night. I am going to take another week of research before I start on the project to gather as much info as I need. As far as the gas line goes I know I need to have a gas valve shut off and a drip line before the furnace. I will be running 3/4 black pipe almost straight from the meter. I still am somewhat confused about the plenum and trunk line(supply line). Are they the same thing or do you make a square box on top of the furnace and then calculate your supply trunk line size and adapt the plenum to the supply line? Does anyone have some pictures of different duct arrangments? I really appreciate your willingness to help. I know many are concerned with my safety but I am taking my time and taking proper safety precautions.


Plenum on the furnace (depending if it is an updraft or downdraft). Off the plenum with large ducts, most are square in the range or 8x20, 8x16 or so depending on the flow needed. Sometimes they need to have one duct go one way and the other duct another way.

Off the ducts there pipes going to the boots, or maybe pipes going to wall stacks (ducts in the walls). Or pipes going to flex pipe.

As hvaclover said there are many different way to get the same job done.

Moving air is like moving water and electricity. You have to know the rules and science behind it. Once you get that done then you put it together with accepted construction methods to get it done.


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## 1610 CUB (Jul 19, 2008)

Marvin is correct there are more ways to do your job as their tinner's. Sit down and draw us up your ideas then we will see what flies and what don't.


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## hvaclover (Oct 2, 2008)

Marvin Gardens said:


> What's really bad is if you ask 2 guys the same question and get 4 different answers.


Depends on how many on-screen names he uses. :thumbup:

I use only one.


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## 1610 CUB (Jul 19, 2008)

Well I think "Hartkem" can do it so to day I went out and bought stock in _*"**Band-Aid". *_Just kidding. You can do it!


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## biggles (Jan 1, 2008)

if you GOOGLE attic furnace installs there is loads of info and go to images for some good pix of attic installs.i didn't tell you this but if you go to www.hvac-talk.com and there are 86,000 HVAC techs arguing over HVAC applications/installations and who has the best tools and who knows what.they have a "residential" section and a "wall of pride"..."wall of shame" all pictures of installs soup to nuts.if you go over there you can look and read but if you need to search you will have to registerthey do not allow DIYers to ask questions .so if you keep it down to searches and viewings you will be :thumbup: you can search from what type duct,screws,gas piping,stats,registers...etc.WARNING...if you ask a question the residential techs will pick you off in half a heart beat......this is them<< <<<<<<<<<......your taking food out of there mouth doing it yourself


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## hvaclover (Oct 2, 2008)

biggles said:


> if you GOOGLE attic furnace installs there is loads of info and go to images for some good pix of attic installs.i didn't tell you this but if you go to www.hvac-talk.com and there are 86,000 HVAC techs arguing over HVAC applications/installations and who has the best tools and who knows what.they have a "residential" section and a "wall of pride"..."wall of shame" all pictures of installs soup to nuts.if you go over there you can look and read but if you need to search you will have to registerthey do not allow DIYers to ask questions .so if you keep it down to searches and viewings you will be :thumbup: you can search from what type duct,screws,gas piping,stats,registers...etc.WARNING...if you ask a question the residential techs will pick you off in half a heart beat......this is them<< <<<<<<<<<......your taking food out of there mouth doing it yourself


 
I'll email the moderators and tell them to expect him:thumbsup:

IP noted:laughing:


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## HeyBob (Oct 5, 2008)

hvaclover said:


> I'll email the moderators and tell them to expect him:thumbsup:
> 
> IP noted:laughing:


 

ROTFLMAO! Fancy meeting you here!

Oh yea, we're ready and waiting! LOL


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## 8 Ball (May 17, 2008)

A plenum is usualy the first piece of metal on the outlet of the furnace, the trunkline is any ductwork that follows.

If you are using a horizontal built up air handler, you may not need a plenum, the main trunk screws directly on the air handler discharge opening. 

Establishing a consistent pressure and smooth, uniform flow is what needs to be focused on, plenums are sized for and used to establish both, so that distribution of the air can be predictable.


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## beenthere (Oct 11, 2008)

6" flex, will give you a high static pressure, if you try to move 100 CFM through it.

You may be looking a duct sizers that are intended for use for sheet metal.

You may be confusing static pressure and friction rate.

They are not the same thing, and that misconception, has harmed more furnaces then not.


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