I am adding on a second floor to a one story house. The addition will be 816 square feet, 3 bedrooms and 2 baths. The contractor recommends attic installation with one return duct in the hall ceiling and cutting the door 1 inch from the bottom. Will this work? Not only the dedicated return ducts will cost more, but also, the contractor says, the hall return duct will be humidified by the first floor air. The existing system has a humidifier. With the hall return, can the supply grill be on the ceiling for heating? Should the grill be located at the opposite end from the door? Also, what tonnage air conditioner do you recommend? Is the “hall return” a workable solution in the real world or is it an easy but not effective heating system? Thank you all and I hope I included enough information for general suggestion. I plan to do the heat/AC load calculations, but a supplier told me that furnace heating calculation is not exact. I plan for 40K Btu furnace. Can I specify the blower size for the contractor or does the blower come with the furnace and I have to adjust the supply with the duct size? How are my guesses? Again thank you and excuse my guesses. They are not even educated. I saw them as logic exercizes. All interesting.
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You can use a central, single return. It will work, but not as well as returns in each bedroom. As you suspected, it is an easy and inexpensive installation. As far a the downstairs humidifier taking care of the upstairs, that is marginal. It may or may not work very well. Try it and see. It will work equally well with whatever return system you employ. I would not recommend installation of a humidifier on a furnace located in the attic. For that matter, I would not install a furnace in my attic. It will work, just not my preference. See if you can't find a corner of the addition somewhere where you can afford to lose 12 square feet and put in a 3x4 furnace closet. With the ductwork in the attic, your greatest concern is to seal and insulate the ductwork. I would recommend mastic on all joints and 3" of rigid, sealed and foil-faced insulation.
The supplier is partially right, in that load calculations are not exact due to a variety of variables, but it is more exact than any other method. Without knowing where this house is located and any details of contruction, any guess for the size of the unit will be only a guess. The smallest available that I am familiar with is 1.5 ton. With decent insulation and windows in most of the country, that will be more than enough.
If you go with a single return in the hall, supplies in the ceiling will work. Have them located in front of widows and against exterior walls. If you decide to put a return in each bedroom, pan the stud space (or use a wall stack is code requires) and take it near the floor, opposite the supply.
As far as the blower size is concerned, most furnaces (all but the cheapest) have four speeds to pick from, where there are taps for the blower controller to connect to blower motor. Typically, high or medium high for cooling and low or medium low for heating. You are limited in flexibility here. 400 cfm/ton at 0.5" esp, is nominal blower rating, funaces typically come with 50, 75, 100 or 125 MBH input (1000 btu/hour). Different manufacturers will vary. At 90% you will get 45, 67.5, 90 & 112.5 out. Do the loads, but I believe you will be in the smallest size.
The ductwork and registers should be sized for the maximum required cooling air flow, independent of the air handler. Be sure to have volume dampers installed in each and every branch.
Tim
"400 cfm/ton at 0.5" esp, is nominal blower rating,"
What is the nomial relation between blowers and furnace (heat only) ratings.
What is the big deal with adding multiple returns? If the second floor is utilizing the attic, I would imagine the only concern is costs for adding one-return per bedroom/bath (meaning four) instead of one. Sounds like the builder is trying to 'simplify' his costs.
Edited 4/10/2002 1:04:24 PM ET by kartman0
Bill,
No such direct relation. Furnace heating is usually in total capacity in MBH. Air flow rate is not inconsequential, but not as critical as that for cooling. For instance, a 90% efficient, 50MBH input furnace, with 70 degree return will deliver 121 degree supply air at 800 cfm. Same furnace will deliver 139 degree supply at 600 cfm. Still get the same 45,000 btuh into the space. On the other hand, 600 cfm at 55 degrees will provide about 1.5 tons of cooling. 800 cfm at 55 degrees will provide about 2 tons. PROPERLY sized an AC (evaporator/condenser combo) will provide about 55 degree supply air. Due to economics, it is not practical for a reasonably priced AC system to deliver cooler air and much warmer than that, there will be problems with humidity. Since air flow rate is more important for proper cooling, that is what dictates the ratings and the design. A design or contractor will pick a unit to meet the heating requirements but specify flow based on cooling.
Thanks for the info.
What is ever better you gave me the reasoning behind the numbers.
Thank you and the others for the valuable suggestions. I have no room for furnace room but I have about 6 inches on opposite sides of the chimney. Instinctively, I also would feel better to have the furnace in the basement. The heat gain calculation shows that I need about 11,000 Btuh for the addition. The calculation program suggests that I need to deliver the air at 400cfm. It recommends the air speed of 900 which I again assume can be adjusted at the furnace. The duct recommended is rigid 14x5. The distance this duct will span will be 24 feet from basement floor to attic floor (attic will be unused). The question is, based on this cfm and the air speed, can I supply and return with 14x5 duct from basement to attic, tee off with same size ducts to take the air closer to the room locations and then branch off to the rooms with appropriately sized ducts. What is the effect of drag on the air and the effect of using flexible round ducts as the head on the tee? Supply grills will be in the ceiling close to the windows and return will be in the inner walls opposite supply. Can I assume that cfm, air speed and the duct sizes control the air volume delivered and I can fine-tune it with the volumn dampers? If not, what does the dampers do? Is this same as grills with moveable louvers? By the way, I got the heat gain/loss calc program this morning and spent the whole day inputting the numbers. At the end, I got all the numbers I needed but also a big headache. Now I feel a little sorry for the Enron accountants.
So now that you have the heating (and cooling?) loads, the rest of the output from the program can pretty much be ignored. I am going to assume that you wish to keep costs as reasonable as possible and that we're only going to consider "off the shelf" equipment and materials.
In terms of a furnace, that means a 50,000 btuh INPUT furnace (gas). For most manufacturers that is compatable with 1-1/2 to 2-1/2 tons of cooling and correspondingly 600-1000 cfm of air flow and a 0.25 hp blower.
For 600 cfm at 0.1 inches of static pressure loss per 100' (the upper limit that I would recommend) of duct, you would need a 14x8 "trunk line". Standard sizes for ducts (residential) 4 to 30 " (even sizes only) by 8" deep or 10" deep. Round ducts come in 4, 5, 6, 7, 8 and even sizes larger than 8. Wall stacks (ducts made to fit between wall studs) are either 2-1/4" or 3-1/4" deep x 10, 12 or 14" wide. Sheet metal duct can be made in any dimensions required by most contractors but that will cost more.
Why volume dampers are recommended is because, you could design a system that was perfectly balanced with custom ductwork, but its not worth it. Transitioning trunks after every take off costs too much as well. So ducts are slightly oversized (hopefully) and the system is then "balanced using volume dampers. Fine tuning and changes due to user preference, changing uses, etc., is done at the registers (grilles with integral dampers are called registers).
In your case (I believe that you said 3BR and a bath) I would run the 14 x 8 (or 18 x 8 if space permits) to the attic branch as necessary (2 equal branches would be 10 x 8's) and put 150 cfm through a 7" duct (with volume damper near the trunk line) to each bedroom with a 12 x 6 2-way ceiling register (like Hart and Cooley model 652) and return low opposite side of the room with a 12 x 6 return register near the floor and supply 50 cfm to bathroom (with exhaust only no return) and put the rest (100 cfm) in the hall/corridor at one end, return from the opposite end with the thermostat near the corridor return.
Thank you again!