Sizing ductwork- residential

[shaden0000]

I am a commercial plumber and have been for 15 years. I was a commercial pipefitter for about 5 of those years and really, truly understand the heating and cooling ends of HVAC. Its ductwork I dont understand. Im not looking for a handout or anything like that. Im a tradesman as much as most of you are. Here is my problem... I just bought a cape cod that is 2 years old, the cape(upstairs) wasnt finished. I am finishing it now and have absolutely no clue on what size duct to run up to the cape part of the house. I have been looking for 2 weeks on this darn internet trying to find "the answer". If someone could give me a website....any clue as how to size this damn ductline I'd appreciate it.

[framehvac]

It is hard to tell with out looking.Call a hvac company let them look & lay it out for you.If you wont to do the work just see if you can pay for them laying it out but I would let them do the work so you will know it is right.

[tinner73]

why don't you ask a tin-knocker on one of your jobsites.

[troyorr]

Ask one of the other trades people for some help. Maybe even work out a barter if you like. Not a lot of ducting to go to the upper level of a Cape. The biggest problem is not having a chase or raceway for the ducts to go from under to over.

[Green Mountain]

When you are a tradesman it is always better to get into a barter trade as previously stated.

But just so you get a mental picture. Air is a substance just like water. You wouldn't run a 1/4"water line to feed a residence or would you run a 2" line to feed a residence.

First thing you have to figure out is how many cfm of air that your furnace handles. Just for example say it is 1600 cfm. Now you figure out what percentage of the upstairs is in relation to your house. Say it is 25% of the total. Then take 25% of the total cfms that your have to work with.(1600). This would mean you need 400 cfm for upstairs. Then go to the supply house and get a ductsizer. The ductsizer will tell you the different size ducts that you can use which will handle 400 cfm. This is a very rough explanation of how to size ductwork. ( don't beat me up other HVAC guys.)

[paddyhvac]

Its been my experiance that putting a second system or zoning your current system would serve you better in the long run

[md master]

Paddy is right. Add a new zone to your home with a seperate system for the upstairs only. You won't regret it. Have a reputable pro size (properly with a man-j) and install it.

[shaden0000]

Thanks for the help guys. I talked to a tin-knocker today and he suggested I run the largest supply and return that I can fit in the chase. The chase is about 3 foot x 3 foot from basement to the 2nd floor. He recommend the largest duct then balance at the reggisters. He even hooked me up with a sheet metal company and told me of a place to get registers and grilles from a knowledgable salesman that could help me out. So this all sounds good to me so far. Im getting more comfortable about this project. Thanks for the help. If you have anymore suggestions feel free, I'd really appreciate it.

[dash]

Get it sized correctly,the largest size that will fit is not the way to go.

[spotts]

talk to a higher up tin knocker. Not th $8. an hour variety....... One who knows whats going on. "Put in the biggest duct you can fit" is the stupidest advice I've ever heard.

Hey Beenie...... Its called a Ducky-lator.

[dec]

I dont think the largest size you can get in the chase is much of an answer but then we dont know what the tinner they turned you on to will say. If he says the same thing then I would run. If he sizes it to the cfm you need then that sounds better.

[shaden0000]

Man you guys are brutal! Ok, I know what a ductulator is and have used one in the past. I guess I need to research what equiptment I'm running and go from there. I need the manual j, that would answer alot of questions.

[Green Mountain]

Okay, I guess you are not going to act like one of those hit and miss HO that we get here. They ask a question and then you never hear from them again.

So tell us something we can use. What kind of furance do you have? Model and serial#? How many square ft. is the area that you are trying to heat? Is cooling involved in this system? Since you have a duct chase I am guessing that this is one of those modular or preframed homes. Which might mean that the furnace was sized for this unfinished area.

A ductulator is the same as a ductsizer that I mentioned earlier. So you are going in the right direction. Putting in the biggest duct that will fit is bad advice. We do have to consider velocity too.

We need the whole story then we can help you out here.

[speaking]

First and utmost,before you waste any time,is to determine if your blower has sufficient CFM for the addition.If it was sized properly when installed you probably don't have enough air to go around.Remember cool air is hard to push upstairs and if the system was comfy you may just be robbing Peter to pay Paul.

[shaden0000]

At this moment I cant get the information on the unit. But i know the sizes of the rooms and the windows and the exterior walls. The rooms (bedrooms) are 12x12. There are 4 of them. There is going to be a 3/4 bath on the same floor, no windows in the bath. The total heating/cooling space is close to 800-900 sq ft. Each bedroom has 3 windows. 2 of them are 33x47, the third is 33x57(to meet egress). All windows are low-e, argon filled, double pane windows. The exterior walls are 2X6 with R-19 insulation. The interior walls are 2X4 and will be insulated. I hope this will help any of the professionals on this site help me. Hey, if you got a plumbing question...ask me.

[Green Mountain]

shaden, why don't you drop me an email with the rest of the information that I asked in my last post. I'll see if I can't size this up for you. Just click on the credentiials button below and it has my email address.

[spotts]

I hired a "Backhoe guy" to install a water line from 1 home to another in my yard 120 ft of sch 40 pvc. apart. Paid him to put in 1". He didn't have a 1½" hammer drill bit so he reduced the pipe to ¾" right before he went into the building because he did have a 1-1/8" bit. [all underground is still 1"] He says it should flow just like a 1" pipe, I say he's an idiot and because of the ¾" reducer after the first building, I only have the equivalent of a ¾" pipe with less friction loss since it's piped in 1" after that [until the next reducer before it goes into the next building]. I can barely run a hose from this setup. How much less volume of water do I get from this set-up than if he'd have put in 1" like I asked for?

[shaden0000]

The guy is an idiot. Restricting the flow is like a a spray nozzle on a hose. The more you pull the trigger the more water comes out because of the bigger opening...the less you pull the trigger, the less water. A 3/4" inch pipe has an area of .4415 square inches. A 1" pipe has an area of .785 square inches. Almost double. Length of the run and the supply pressure will also determine pressure drop. Hope this answered your question.

[tinner73]

i love seeing tradesman help each other out. i'm getting all misty-eyed...........who's got a tissue?

[trane]

Originally posted by paddyhvac
Its been my experiance that putting a second system or zoning your current system would serve you better in the long run

If you don't follow this advise the upstairs will most likely heat but you will have problems with cooling. The problem you will have is you need more airflow upstairs for cooling and less for heat. Unless you have someway to make sure the needed air is going upstairs(zoning) it won't. If you don't have a good plan you are wasting your money and time and will find when you get done there is hardly any air coming out of the upstairs ducts.