new furnace and crappy ductwork

[friesenk51]

I bought a house in May '04, One of the great aspects we thought at the time was a new furnace. Since we have moved in and have been through a cooling and part of a heating system, we have noticed that the furnace maybe be undersized and the ductwork does not properly disribute the air.

I have two major questions. I have determine the CFM that my furnace puts out based on calcualtions from my sis in laws boyfriend that is HVAC major at a tech school. Now that I know what the CFM is, can I base the duct size on sq ft instead of a heat gain/loss calculation? The reason I ask is that the house was built in 1931, I have insulated the attic and walls that are accessible via the attic, however I cannot get all areas of the walls, thus a loss/gain calcuation would not be a true measurement.

Second major question. I have looked at local and chain stores only to realize that everything I need is available to special order/purchase. Since my relatvie with HVAC experience lives 7 hours away, is there a computer program or aterials via internat that can help me determine the best overall design as well as the do's and don'ts of ductwork? Also do warehouses sell to private individuals?

I had a local contractor out to quote me a price, however he did not take measurements but rather jsut "looked" at the system and we talked about what I wanted done. He said it was possible but for $2000 dollars and told me that I should hire my sis in law boyfriend to design and install the new duct work.

any help or suggestions would be greatly appreciated!!

[imeduc]

it all goes back to the CFM in the duct A ton needs 400cfm so for 3 ton AC you have 1200 cfm. Try www.lllbuildingproducts,com

https://www.heatcoolparts.com

https://www.snipsmag.com

Ed

My mistakes dont define me they inform me.

[LazyPup]

When determining the CFM for any given room you should compute for at least two or perhaps three air exchanges per hour. Thus you compute the length width and height of the room to find the total cubic feet, then multiply that by the number of desire air exchanges per hour and divide the result by 60 to get CFM.

EXAMPLE,, a room 10 x 10 x10 would be a total of 1000 cubic feet.
for two air exchanges it is 1000 x 2 = 2000 CF/hr
2000cf/hr divided by 60 = 33.33CFM.

In the trade they often use a simple cardboard circular slide rule called a "Ductolator". (run a search on you web browser for "Ductolator" and you will find many sources. It sells for about $12.95) To use it you compute the room length width and height then Set it for the required CFM then select the appropriate duct size to meet the need.

When designing a duct system you have to make allowances for the fittings too. The internal resistance of the duct is determined by the overal length of the run, but offsets and elbows add additional resistance. There are tables that tell how many feet to add to your total developed length formula for each type of fitting. By example, a 90deg elbow may have the same resistance as a ten foot section of straigh duct.

Most HVAC supply houses do offer over the counter sales of both parts and materials, with the exception of Refrigerants or refrigerant handling tools and equipment because federal law prohibits possession, transportation or distribution of refrigerants by anyone who does not possess a currently valid EPA certified "Refrigerant Handlers Certificate."

[HayZee518]

Try to find a supplier that sells "DUC-PAC" This product is made in East Longmeadow Mass and is snap together sheet metal parts. Elbows, transistion fittings offsets etc.

[Hube]

Friesenk51; Allow Me to point out that "air change calculations apply in many cases of ventilation and exhaust systems, but however DO NOT have any thing to do with the calculation of duct sizing for an forced air htg/cool system. This designing is almost a trade of its own,as your sister's friend will find out. Also, its not learned overnight .
A heat loss/gain sheet should be done first to see what the Btu requirements to the house are.
The needed amount of heat/cooling is calculated separately for each room.
The outlets are then sized, and the duct size calculated accordingly.
The equipment neede to do the job is selected taking into consideration the systems resistance, static pressure calculation,etc,ec,.
Duct size calculations are then designed to have the same equal friction loss thru out the entire system. This will also provide a constant level of adequate volume and velocity thru out the system
I would suggest to you for a properly designed layout that you either contact a reliable contractor in your area and get proper input. Or you could wait until your sister's friend passes his course and does the calculations for you. It may end up costing you a lot less too.

[friesenk51]

hube,

I agree that designing a forced air heating delivery/return system is very much a trade and something that I can not learn over night. The biggest drive behind my wanting to balance ductowrk is the fact that the house has "cold/hot" areas. More info behind the house, the new furnace replaced the original I am assuming, this was a gravity feed furance. The previous owner was a single lady that did not have the $$ to do the job right. The new system vents in the old ducts on the interior walls of the house. The hvac tech did not even really pay attention but rather just "installed" the furnace. He tied into the old cold air returns which are 4 ft wide and do not work with a high efficiency forced air furnace. the main level has returns but they do not have much suction to them except for one run. The local guy did what he was supposed to do, "install" it. He however did not really think about the "system".

I am pretty certain that the furnace does not put out the BTU and CFM that my 1500 sq ft house needs. In fact since it is going to get -15 below tonight the furnace has all it can to keep the house about 67 degrees.

I know what CFM and BTU the furnace is putting out however, I do not have access to a spreadsheet and data tables that can assist me in a heat loss/gain calculation. Do you know of any resources on the web that I can obtain these from? Also if my furnace is too small can you trade in a furnace or is that not usually possible?

I guess I am stuck paying for a bad job by the previous owner!! i am a huge do myself kind of guy, so I wish this was easier for me to get information and learn about as I would really like to say that I installed and designed the system and look how well it works.

[LazyPup]

From what you are describing the ductwork is not near as bad as you make it out to be. Given that all appreciable heat loss is through the outside walls, the proper place to locate heat registers or baseboard radiation is on the outside walls.

While the tables do determin the minimum size of the return ducts, having oversized returns is no problem, as the air will return at the rate of supply because nature abhors a vacuum. In fact,There should never be any "suction" in a return duct. if you can sense the motion of air in a return duct, the duct is tooo small.

In fact, if ducting is sized correctly there should be no real appreachable feeling of air flowing from supply or return. Forcing air into a room in a high velocity manner only tends to cause drafts and convection currents that cause serious comfort problems.

[friesenk51]

Lazypup,

let me explain more and see what your opinion is. The living room is the coldest as it has only a 6" supply on the inside wall. There is a 7'x7' entry way that does not have a full basement under attached to it. This room does not have a supply to it. From the entry there are stairs that go to a second level. The second level was added later and only has supply runs but no returns. Sitting in my recliner I can feel the cold air go by me that comes down the stairs from the second level to the return in the living room. I should add that the LR also has the most window space in the house with 2 5'x5'windows, 1 faces south and the other west

Obviously with this scenario the second level stays the nicest during the heating and least nicest in the cooling. The professional thatcome out suggested that I put two runs to my living room, one into my entryway, and add returns on the second level. I plan on doing this when I can determine the right size of neccessary duct work etc...

Is it advisable to switch the supply lines to the outside wall from the inside and use the inside vents as the returns? The local HVAC guy suggested this however I am not sure how "professional" he was.

[LazyPup]

Sounds to me like your HVAC guy is pretty much right on the money.

The supply should be placed on the outside walls, and normally concentrating the supply registers, baseboard or radiators under the windows, which is normally the area of greatest heat loss.

Unless a house is divided into zones their is no appreciable advantage to insulating inside walls except for sound proofing. The idea is to supply heat around the perimiter which places a barrier of heat between the living space and the cold outside. Quite often there is no return ducts in residential. This is especially true on one level structures on slab where the furnace is customarily installed in a closet near the center of the structure. When return ducting is installed, such as in the case of a multilevel with the furnace in the basement such as yours, the returns are placed near the center of the structure, which would help form a natural convection from the outer perimeter toward the center which helps insure an even distribution of heat across the rooms.

In your case, since there is no returns from the second floor, the stairway is acting as a duct between floors. The problem with using a stairway, or even a doorway to direct the airflow is that air cannot efficiently go two ways through there at the same time. The laws of nature dictate that warm air rises, so the heat on the first floor will naturally rise to the ceiling, then up the open stairway to the second floor, leaving the downstairs area colder. This is even worse if the thermostate happens to be on the ground floor as the furnace will continue running in an attempt to heat the lower floor to the set point, but the heat is rising up to the second floor. End result, overheating the second floor, cold ground floor, and high heating bills.

Years ago when they had infloor furnaces or gravity systems they often placed the furnace or a rather large return air grille right at the base of the stairs. This caused a slight negative air pressure at the base of the stairs which tended to retard the rate of heat loss from the ground floor to the upper floor. The theory was that the colder air on the second floor was heavier, which would naturally fall down the stairs, once the rate of heat rising from the ground floor was slowed down.

Ceilng heights have a tremendious effect on room comfort tooo. Years ago when people had no AC and fired coal or wood, which was relatively cheap, the idea was to build houses with high ceilings. In summer the hot air would rise and concentrate near the ceilings, leaving the comfort zone below 6 feet cooler. With the advent of Air Conditioning it became more efficient to lower the ceiling heights and duct the hot air back to the air handler. By reducing ceiling heights you also dramatically decrease the overall cubic footage of the structure. Less cubic footage of air means smaller units could handle the comfort zone more efficiently.

As has been mentioned by others, designing a duct system involves a whole science unto itself. Not only do you have to consider the actual cubic footage of the room in question, but you have to consider the natural air flow patterns within that room and how they relate to adjacent rooms. The trick is to design a system that will enhance upon the natural airflow characterics rather than try to work against them. Try to desing a duct system that goes against the natural air flow and your simply shoveling against the tide, so to speak.

Most people are aware of the concepts of supply and return air, but few would grasp the third part of the process that is commonly used in large structures such as schools, hospitala or office bldgs, that being Transfer Air Ducting. Rather than keep generating more and more heat, and pushing bigger supplies, you can use the method of transfer air to improve upon the comfort zone with the heat you already have.

A few years ago I lived in a huge victorian in western massachusetts that had an oil fired boiler and steam radiators. My living room alone was 14 x 38 feet with 13 foot ceilings. No matter where you sat in that room during winter months you were cold and drafty. Out of curiosity i set up an indoor outdoor thermometer with the outdoor sensor near the ceiling. Just as i suspected, when the comfort zone, 6ft and below was only 67 deg, the ceiling was nearly 80 Deg and their was a blast of hot air rising up the front stairway. After sitting there in sweaters and wrapped in a blanket for a month or so, i got a silly idea. I had the heat, just not where i needed it. As an experiment i rigged up a piece of 6" duct with an old computer whisper fan in the bottom set to downdraft air, and stood it in the corner. Turned the fan on and voila', i was now taking the hot air from the ceiling and routing it to the floof, and almost instantly the room became comfortable.

The point i am trying to make is that while i certainly would not say that installing s duct system is beyond the ability of an intermediate DIY'er, designing a duct system is not only beyond most DIY'ers, but it eludes many ppl who actually work in the trade.

Most large HVAC duct suppliers have a staff of designers who take your floorplan and work out these details infinitely. Work up a detailed floor plan of your houe, including window placements, outside wall construction details, (thickness, insulation, type of sheating, siding etc) and take it to your local commercial HVAC duct supplier, and usually for a nominal fee they will design you a duct system to match your needs. You will probably find that if you buy your materials through them, they will have everything to install the system in the exact manner they lay it out, and in many cases the bottom line will be less than trying to create a similar system with the off the shelf look-a-like parts at the big box stores.

[friesenk51]

thanks for your info lazypup!

It sure is nice to talk with somebody about ideas. I am near Minneapolis/St. Paul MN. I am going to look into seeing if there is a large wholesaler that would be willing to draw up the plans for me.

Thanks again!!!

[LazyPup]

I just had an afterthought,,,,if your not in a big hurry to get the results try locating a technical school in your area that offers an HVAC course. Part of the course is "Estimating". You may be able to contact the instructor and convince him to use your floor plan as a class project. You could end up with some very innovative solutions with state of the art ideas and it probably wont cost you anything.

CORRECTION: "you might be able to contact the instructor and convince him or her to use your floor plan"........My appology to the ladies, I was reminded by my female apprentice that the instructor could be either male or female, and she is absolutely correct.

We live in a new world today, and there are many ladies working the trades on an equal with their male counterparts. Given that i have never seen a gender specific task on a construction site, i normally do not think in terms of male or female, but rather, both the men and women are "co-workers" and with that in mind, my use of the term "him" is generic.

[friesenk51]

Has anybody ever heard or used this persons service before? You send your house floor plans, he sends you a detailed list of what you need to design it yourself!

Here is the link http://www.aircomfortdesigns.com/index.html

[Speaking]

What Hube said about starting with a heat loss/gain and proceeding through the registers size,duct size and ultimately at the furnace was right on the money.Your HVAC student friend should have access to Manual J short course(heat loss/gain) and Manual N(I may be wrong on the letter)(duct sizing)which he can borrow for you.You'll need to spend some time and read what is pertinent to your scenario but you will REALLY understand enough to resolve your problem.You seem like your involved enough to tackle the task.Most homeowner couldn't find the CFM and BTU rating on their furnace if their life depended on it.I'm not an engineer but I do heat loss/gain and duct sizing daily for a large wholesaler and what I've read so far won't steer you wrong.