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Discussion Starter #1
I am in the early stages of hiring someone to install an a/c - heating unit in my two story house. I currently only have forced heating ducts in my house. My 1965 house was not built with an air conditioner. Only a heating unit. Overtime I have changed about 30% of the ducts using a thicker gauge duct and sealing all ducts with silver backed adhesive tape and topping that with cloth tape with a pasty glue on all joints meaning it will never leak or come apart. I used to be an insulator so I have some experience in doing this. I am thinking the size ducts I will actually need will probably be dependant on the size unit I buy for my house. My neighbor just had a 4 ton unit installed in his attic and he has 1400 sq ft. I have 1950 sq ft and I am thinking 5 ton just because of increased sq footage. I still have a lot to learn. My ducting goes from a 24" plenum attached above the 1st floor heating unit to 8" horizontal ducts reducing to 6" ducts for 1st floor rooms. There are two 8" ducts coming out of plenum and one is used for 1st floor rooms farther away from unit and the other 8" duct is sending air to the vertical reducer headed upstairs which reduces to a 6" duct for the two rooms upstairs. The current heater I have says its input is 100,000 btu/hr and the bonnet capacity is 80,000 btu/hr. I am guessing this would have helped determine size of the duct back then. Is it possible to do the size layout I need by myself,maybe getting it verified by someone in this field and then get a good installer to finish the job? I am retired, remodeled most of my house over the last 25 years so I think this is something I could do. I have been in my attic many times insulating ,running new ducts,etc.... I would appreciate any input. Thanks
 

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Doubt your neighbor needs 4 tons, and doubt you need 5 tons.
Might want to do a load calc...
Pawl... don't start multiple threads to discuss the same issues.

Mostly the same people are reading them all and they will mostly the same advice will be given... but details will tend to be lost by dividing the flow.
 

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Your neighbor would have probably been better off in the 2 to 2.5 ton range. Please make sure the contractor you hire performas a load calculation. You say you've added insualtion and replaced windows? That will help quite a bit in reducing the tonnage needed.
 

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Pawl, as another homeowner, let me tell you my experience. My home was built in 98 and soon after we moved in. It was an energy star rated home by the local utility, which was suppose to mean it had a lot of sealing, double pane windows and insulation, etc. I only say this because I know yours is not and to point this out. I live in Houston Texas.

I have 3600 square feet with 1800 up and down. The load calc done on this home said it needed 2 3 ton units. (unofficial rule of thumb is 400 sq feet per ton) While the 3 ton downstairs was ok, it certianally was not for the upstairs. So we argued and fought with the builder and the AC company and after letting them do balancing and checking over and over, with no good results, we gave them a choice: replace the units with 4 tons, or go to court.

They replaced both units with 4 ton units. Yes my electric bill went up some, but we accomplished the comfort levels and operations that we wanted from the units, especially upstairs.

So my point to you is as one homeowner to another is this: you are in a very old home, in contrast to mine which has a lot of insulation and energy efficient stuff. If you want comfort on the days when it gets tough outside, you probably will need to go bigger than what the load calc suggests. I base this on the old age of your home and poor insulation and sealing in walls and attic.

So based on the 400 per square foot unofficial theory and poor insulation and sealing, 5 tons could be what you want. Would the experts here agree, probably not because they prefer to build based on a load cals. But I have seen more and more where some experts have said that sometimes they need to re-evaluate the load calc and listen more to the customer.

In my case, the smaller 3 ton unit upstairs would run and run on extremely hot days. I know that a unit needs to run a ceertian amount of time in order to remove humidity. Cycling too fast is not good. Once we put the 4 ton in, not only would it cycle on the challenging days, but it felt more comfortable.

One other thing that you need to consider for comfort that also assisted us was the addition of more return airs, especially upstairs. This assisted in comfort, in my opinion, but adding these alone would not have resolved the long run times on hot days.

I am not an expert, there a lot of great ones here for those opinions! I can only tell you that from a homeowners standpoint, you may have to be ready to put your foot down on what gets installed. I think load calcs are great on new built tight and sealed homes.

JMO.
 

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Load calc doesn't have ability to account for heat rising from first floor to second floor. Your first floor unit probably should have stayed a 3 ton, as the upstairs cool air also falls down to the first floor. If you had kept the first floor 3 tons. It would probably have a much better humidity level.

The second floor probably would have been fine with a 3.5 ton. Splitting hairs? maybe, but often a 1/2 ton increase in size will reduce temp by 6 to 8 degrees, and keep the humidity in much better check.

Your also forgetting your location compared to other posters. Your area is a hotter area.
 

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beenthere said:
Load calc doesn't have ability to account for heat rising from first floor to second floor. Your first floor unit probably should have stayed a 3 ton, as the upstairs cool air also falls down to the first floor. If you had kept the first floor 3 tons. It would probably have a much better humidity level.

The second floor probably would have been fine with a 3.5 ton. Splitting hairs? maybe, but often a 1/2 ton increase in size will reduce temp by 6 to 8 degrees, and keep the humidity in much better check.

Your also forgetting your location compared to other posters. Your area is a hotter area.
Thanks and you are probably right about the 3 .5 .

Thanks for the info on heat calc, see after many years I have learned again how wrong they were in what they told me!

Thanks
 

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Discussion Starter #8
I am going to be talking about a lot of things on here because I value everyone's opinion so be patient. I am all over the place trying to get questions answered. I am looking into the suggestion for a do it yourself $49 load calculation as we speak. I feel I can do the load calculations myself and maybe a more thorough job because its my house and I have more time.

I have popped a piece of drywall out on the 2nd floor where the original ducting in the 1st floor attic changes in size from 8" round to rectangular (5'h x 14"w x 3"d) to fit into the vertical wall going up and comes back out in the 2nd floor attic 6" round in diameter. And my hunch was right --->>>. After cutting away the drywall I could see the original installer did not seal all joints properly so heat has been leaking out of this rectangular ducting in the wall since the house was built in 1965.

I needed to open the wall anyway because I am going to have to run return air ducts from the 2nd floor ceilings to the 1st floor attic.There are no return air ducts because the house has never had air conditioning. I will just position the return air ducting in between the wall studs right next to the existing rectangular duct.

Is it true that each room should have the ceiling register where air is coming out of positioned in each room closer to the area where the heat is the highest? Say for example near a window that brings in more heat from the sun as opposed to an interior wall in the same room. I just figured the middle of the room.

After determining where to put the ceiling registers where or how far away does the return air vents go in the same room. On the 2nd floor would I include return air ducts in the bathroom and the small area at the top of the stairs with the walkway to each bedroom?
 

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Bathrooms don't get returns. Don't want all the odor spread through out the house.

Return in the ceiling of the second floor landing will help a lot in cooling season.

The supplies should throw the air toward the highest heat gain/loss area. Returns should not be in any direct line of throw from the supply register. So if the supply is in the middle of the room. The return should then be near the inside wall.
 

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Discussion Starter #10
1.) Does the area at the top of the stairs need a return or supply duct? The square footage of floor area on the 2nd floor landing is 51 sq.ft

2.) Do you feel that using rigid duct is the best way to go? Original duct is rigid,pretty dirty inside and 48 years old so I am going to change it.

3.) Just for my education today is their a rule of thumb when talking about return ducts. Are they the same size as supply or 75%,50% or 25%.

4.) I am going to run new ducting so I can move supply registers closer to heat gain/loss (window). Are there ceiling register configurations you feel are better than others? It sounds like a rectangular one placed in the ceiling dispursing air four ways is the way to go.

5.) Upstairs there is a total of 381sq.ft in both bedrooms plus 40 sq.ft in bathroom. Is it worthwhile or even possible to try and isolate the 2nd floor when using only one ac/heating unit for the whole house?

6.) Is it possible to put something in the duct system that gives me a choice to direct air only to first floor? I rarely go upstairs. Most of my time I spend downstairs. I saw this clip on the internet where this family would shut down certain areas of the house but it was causing a whistle in the floor registers from too much pressure in one zone area so they placed a weighted damper in the supply duct routing the additional pressure back into the return duct.
 

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hope this helps...

1. supply us with a drawing, including windows and all here can help you locate the supplies and returns for you.

2. I believe that the best choice for duct work is rigid steel. Note that old ducts can be cleaned, so dirt is not the issue. Air leaks (sealing), izing and rust are issues that could be corrected with new ductwork.

3. Standards for returns are tied to the size of the supply ductwork size and air handler sizes. Supply ductwork sizes are tied to the room size and calculated hot and cold expectations based on windows, insulation, air tightness, air handler size, throw of air from the register, noise and orientation of the room to the sun. static pressure within the system is the form of measurement you need to test, along with FPM air flow at the air filter. There is lots and lots of math and science that goes into design of the ductwork. In order to get it right you have to have a real pro design it or do the homework information gathering on the internet. Dont mean to scare you away from doing your own work, it is possible!!!!! (I did it !!)

4. drawings of the floor plan would help the forum tell you the best placement of supplies and returns!! even a napkin drawing would be good!!

5. zoning is what you are thinking of.... yes, zoning is good and possible, but it has some drawbacks. its my understanding that a correctly built zone system can do what you want, but with anything, that extra ablility to control things comes at a cost. Time, money, complexity and maintiance. But it can be done!!!!!!!!!

6. it is possible... but the supply to return pressure bleed off sure seems like a hack job to me.. I think what you are looking for is a single hvac unit and two duct (supply and return) systems tied to the single unit, with the ability to switch between the two, given the need. This is great and would be really neat if it was all automatic and all designed to provide the correct air flow into the two zones in all circumstances. given the proper controls and sizing everything is possible.. but cost and complexity enter into the idea... as well as effency.. as I would guess there is no insulation and air sealing between your floors so keeping the heat or cool between the floors might be tough. but it could be done...

again, this is just one guys opinions and self taught knowledge.
 

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Discussion Starter #12
I did the $49 residential load calculation myself. Under the Whole House totals it showed The Total Heat Loss BTUH was 39,758 & Total Heat Gain BTUH was 17,631. The Latent Heat Gain BTUH was 1,495 and Sensible Gain BTUH was 16,136. Below the Total Heat Gain column it listed (1.5 tons). The square footage totaled 1626.
Maybe someone can give an explanation of these numbers to better educate me. Thanks
 

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Discussion Starter #14 (Edited)
These are the Design Conditions that the program used for my area in Cypress,Calif.
Indoor: Summer Temperature: 75 - Winter Temperature: 72 - Relative Humidity: 50
Outdoor: Summer Temperature: 80- Winter Temperature: 43 - Summer grains of moisture: 84 - Daily Temperature Range: Medium

1.) Under Total Heat Gain (BTUH) they listed 19,238 (1.5 tons) - Are they saying I need a 1.5 ton A/C unit under these Design Conditions listed at the top to maintain those temperatures?
 

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These are the Design Conditions that the program used for my area in Cypress,Calif.
Indoor: Summer Temperature: 75 - Winter Temperature: 72 - Relative Humidity: 50
Outdoor: Summer Temperature: 80- Winter Temperature: 43 - Summer grains of moisture: 84 - Daily Temperature Range: Medium

1.) Under Total Heat Gain (BTUH) they listed 19,238 (1.5 tons) - Are they saying I need a 1.5 ton A/C unit under these Design Conditions listed at the top to maintain those temperatures?
Yes, that is what they are saying.
 

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Discussion Starter #16
I was watching the sun today relating it to my house. The rear faces east. Sun hits the clear glass patio door & windows around 7am to 11am. Neighbor behind me and over one has a two story house blocking the sunrise a little longer. Southside has no windows/doors. Northside gets no sun. West side (front) starts seeing sunlight on windows around 2 pm to sunset. Entry door is set back 2 ft in so door gets no sun exposure. No wonder my house is so cold when the heater is not on. Duh?

Attached garage is on northwest side blocking half the west side sun exposure. Eaves all the way around 1st and 2nd story are 3' deep with 2x8 facia so it blocks some sun. Two small 14x33 windows at the top of stairs are blocked from the sun partially by facia. I have a 95x33 window on southside 2nd story that gets sun all day. My point to all of this is that for a residential house I think it is positioned pretty good to ward off sunlight.

I have to figure out my next move to get this rolling. I have to find someone to work with me designing a ducting system,where to place supply/returns and an A/C unit. Just randomly calling a contractor is not going to work because most want to come in, do it all in a few days and be gone. I am trying to do as they say," Measure twice and cut once".

I just cant believe my neighbors have a 3 bedroom house with a 3 ton unit, one has a two story with a 4 ton unit and the last one is a two story with a 5 ton unit. Mine calculates out at 1.5 tons?
 

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1.5 tons.... yup,,, its hard to believe...

but its all about the target temps and run times at the target temps.


so lets look at the hottest day and what your hvac system will be doing.

lets say that between 2pm and 4pm it is 80 degrees outside (your summer outdoor target temp) and 75 degrees inside. Your system is designed to run 100 percent of the time with these conditions (in this case between 2 and 4 pm). so you are saying... I dont want my ac running all the time!!!! well, thats the misconception. HVAC systems do not become effecent until at least 10 minutes has passed while they are running. The initial start up is a huge rush of current, but after that the pressures etc equalize and the amount of energy levels off at a fairly reasonable rate. so yes, your ac system will be running a lot at your target temp. but you will be saving the start and stop costs that others are paying.

The next idea to get your head around is when the outside temp is not at 80, but lets say 78. At this point, you should start to hear the system cycle, still more on then off, but the run times should still be fairly long. with an oversized system you will be running very short times, never long enough to get into the economy zone and believe it or not that start stop process is hard on the equipment.

the closer you get to your indoor and outdoor target temps matching, the shorter your run times will get until you dont need the hvac (temps indoors and outdoors the same)... .

so what if the outside temp is 82 (2 degrees above your design temp)..well your system will not be able to keep the house at 75 degrees for the time period that it is 82 degrees outside. it may only keep the inside 76 degrees. +-


so now lets look at your neighbor with the 4 ton system. His system, at the 80 degree outdoor target temp and 75 indoor temp, will run a very short time. chances are it will never get past the 10 minutes of hard running. His bill will be higher. The energy to start it and then stop it and then start it etc will be more then yours which ran for 2 hours in the heat of the day. His system will not last as long as yours will.

so now comes a bit of the magic and your input into the design.... the question becomes, is 75 degrees enough to keep you cool?? if not, change that number in your calculations... maybe 78 is cool enough for you... you could put that numnber in.. also think about other heat loads... do you have massive parties in the afternoons with kids and friends running in and out... well, your hvac system may not be able to keep the house at 75 degrees.. but do you want to oversize your system for that occasion??? there are lots of pieces to this, but its my understanding that the manual j is very well done and will work for you!!!

now lets look at my system

2500 sq feet, 115 outdoor design temp, 75 degree inside temp, design calcs say 3.5 to 4 tons.. I have 5 tons and the longest run I have is about 30 minutes when its 119 out. so my system is oversized (I joke to my family that our ac system will keep the house cool when its 146 degrees outside as per the manual J!!)it also costs me more to run then it should.. I found all this information after I had a new system put in. We went from a seer 10 to 14 and the utilities increased!!!

Have fun and remember this is not rocket science, you will have to make compromises and in the end, you will sleep well at night being an informed consumer.
 

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Discussion Starter #18
Thanks for that explanation bobinphx! Trying to understand something here. When I did the Manual J calcs I just inserted the name of my city where I live and the program gave me the Design Conditions.

1.) So are these the recommended temperatures to keep the inside of my house comfortable and to have a 1.5 ton unit run the most efficient?

And as far as having fun I am like you. I have a mechanical brain. For instance with my house I kept wondering where I could run return ducts because this house has never had air. So the old heating duct that transitioned up to the 2nd floor attic is set between the vertical studs so the return can just go up between the studs right next to it. I had a feeling that the existing vertical duct probably was leaking just because work that cant be seen is usually hurried and the drywall covers it. Sure enough the duct was never sealed from day one so heated air has been coming out since 1965 when it was built. I wonder how much. The ducts are 48 years old. Time to throw them out.

For me personally the challenge is more fun than the end result but in this case I will come out of this very comfortable during the hot weather.

2.) If you have any recommendations on what brands a/c - heater units to buy I am all ears. I dont know if you have an extensive background in this area but it sure sounds like it. I cant wait to tell someone I need a 1.5 ton for my two story house.

3.) I intend on using rigid supply duct again. Is it also better to run the rigid return duct?

I also learned from the Manual J calcs I can add some batts in my attics to go from R 11 to R 19. Every little bit helps. I cant believe your summer temps in Arizona.
 

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Discussion Starter #20
2nd story floor plan - "W" stands for windows - "S" is for existing heating supply registers. 1st floor attic entry is small entrance door in right upstairs bedroom. I was thinking of putting new rectangular supply registers closer to windows which seem to get the most heat coming through the glass.The bedroom on the right gets full sun exposure all day long being its on the south side. East window in the other room gets full sun from sunrise until 11 am. Two small windows side by side on west side get very little direct sun but exposure is from about 2pm until sunset.
 

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