Some A/C questions

[calivette2003]

Hello all,

Retired engineer (USN) here, I have some experience in AC&R. Retired and hoping I can do my own work as I rely solely on my retirement income.

Just moved into 2100sq ft, 4bdrm. 2 story home built in 1951. Has central A/C and natural gas furnace, both Rheem units, A/C is Rheem RAKA-048JAZ, 4-Ton, manufactured August 1996. Exception is the master bedroom, which was an addition added in the 70's and uses combination of large attic fan and ceiling fan to pull A/C up from downstairs. We had two days of 98-108 weather. House took 5+ hours to cool down from 96-83. Tried both methods of cooling, master bedroom attic fan on and master bedroom isolated just cooling downstairs, with same result. A/C air is cold but lacks blower flow (force) aside from the 2 shortest ducting runs from the blower plenum, under the house. All ducting is 8", looks brand new, extremely well insulated, with new registers. Above head registers are not as cold as they run through the attic which is 110+ degrees even with 4 squirrel cages on the roof. Both A/C and natural gas furnace/ blower look in great (as new) condition.
Yesterday I cleaned/ replaced the inlet filters, cleaned the condenser. Combed the fins, cleaned the fan blade, oiled the condenser fan motor, and insulated all of the ducting connections at the plenum and registers.

I believe following is happening;
8" ducting is too large, i.e. large volume, long runs thus less air pressure, one of the ducting runs is 40+ ft.
The Freon charge is unknown but I say sufficient as the air coming out of the floor registers is ice cold.

My ideas are:

1. Put restrictors in the ducting to increase air flow, I'd hate to spend lots of money, nasty dirty job, replacing almost new ducting, with 6" or 4" duct.
2. Replace condenser fan motor with newer, more efficient 3 phase motor, i.e. lower electrical load for same speed.
3. Replace condenser fan motor blade with new blade with deeper pitch (if anyone makes that). More air flow for same speed. Unit is really loud; blade seems to be balanced with no noticeable bends but is really loud compared to my last Tran unit.
4. Replace A/C/ furnace plenum motor with newer, more efficient 3 phase motor, i.e. lower electrical load for same speed.
5. Replace A/C/ furnace plenum motor blade with new blade with deeper pitch (if anyone makes that). More air flow for same speed.

As far as the master bedroom, even with the attic fan, and ceiling fan running full blast with the room isolated and all windows shut it was still 95 degrees when the rest of the house had cooled off to 83. Believe even if I ran, 100 ft of new duct from the central A/C to the master bedroom it would not have sufficient air flow and involves three roof penetrations so a simple self contained window sill or wall mounted A/C unit in conjunction with the pre-existing attic fan would be a better economical choice.

Any advice would be helpful.

[paul52446m]

Hello all,

Retired engineer (USN) here, I have some experience in AC&R. Retired and hoping I can do my own work as I rely solely on my retirement income.

Just moved into 2100sq ft, 4bdrm. 2 story home built in 1951. Has central A/C and natural gas furnace, both Rheem units, A/C is Rheem RAKA-048JAZ, 4-Ton, manufactured August 1996. Exception is the master bedroom, which was an addition added in the 70's and uses combination of large attic fan and ceiling fan to pull A/C up from downstairs. We had two days of 98-108 weather. House took 5+ hours to cool down from 96-83. Tried both methods of cooling, master bedroom attic fan on and master bedroom isolated just cooling downstairs, with same result. A/C air is cold but lacks blower flow (force) aside from the 2 shortest ducting runs from the blower plenum, under the house. All ducting is 8", looks brand new, extremely well insulated, with new registers. Above head registers are not as cold as they run through the attic which is 110+ degrees even with 4 squirrel cages on the roof. Both A/C and natural gas furnace/ blower look in great (as new) condition.
Yesterday I cleaned/ replaced the inlet filters, cleaned the condenser. Combed the fins, cleaned the fan blade, oiled the condenser fan motor, and insulated all of the ducting connections at the plenum and registers.

I believe following is happening;
8" ducting is too large, i.e. large volume, long runs thus less air pressure, one of the ducting runs is 40+ ft.
The Freon charge is unknown but I say sufficient as the air coming out of the floor registers is ice cold.

My ideas are:

1. Put restrictors in the ducting to increase air flow, I'd hate to spend lots of money, nasty dirty job, replacing almost new ducting, with 6" or 4" duct.
2. Replace condenser fan motor with newer, more efficient 3 phase motor, i.e. lower electrical load for same speed.
3. Replace condenser fan motor blade with new blade with deeper pitch (if anyone makes that). More air flow for same speed. Unit is really loud; blade seems to be balanced with no noticeable bends but is really loud compared to my last Tran unit.
4. Replace A/C/ furnace plenum motor with newer, more efficient 3 phase motor, i.e. lower electrical load for same speed.
5. Replace A/C/ furnace plenum motor blade with new blade with deeper pitch (if anyone makes that). More air flow for same speed.

As far as the master bedroom, even with the attic fan, and ceiling fan running full blast with the room isolated and all windows shut it was still 95 degrees when the rest of the house had cooled off to 83. Believe even if I ran, 100 ft of new duct from the central A/C to the master bedroom it would not have sufficient air flow and involves three roof penetrations so a simple self contained window sill or wall mounted A/C unit in conjunction with the pre-existing attic fan would be a better economical choice.

Any advice would be helpful.

Attic fans have to get air to move air so you would pull a vacuum on the house, and you would be pulling hot air from outside where ever it could get in. Do not use attic fan when trying to cool a house with air con.
How many 8" rounds do you have? and are they coming off the hot air plenum? How much return air do you have and what size is it?
Is you blower on hi speed, it should have 3 or 4 speeds Paul

[paul52446m]

Hello all,

Retired engineer (USN) here, I have some experience in AC&R. Retired and hoping I can do my own work as I rely solely on my retirement income.

Just moved into 2100sq ft, 4bdrm. 2 story home built in 1951. Has central A/C and natural gas furnace, both Rheem units, A/C is Rheem RAKA-048JAZ, 4-Ton, manufactured August 1996. Exception is the master bedroom, which was an addition added in the 70's and uses combination of large attic fan and ceiling fan to pull A/C up from downstairs. We had two days of 98-108 weather. House took 5+ hours to cool down from 96-83. Tried both methods of cooling, master bedroom attic fan on and master bedroom isolated just cooling downstairs, with same result. A/C air is cold but lacks blower flow (force) aside from the 2 shortest ducting runs from the blower plenum, under the house. All ducting is 8", looks brand new, extremely well insulated, with new registers. Above head registers are not as cold as they run through the attic which is 110+ degrees even with 4 squirrel cages on the roof. Both A/C and natural gas furnace/ blower look in great (as new) condition.
Yesterday I cleaned/ replaced the inlet filters, cleaned the condenser. Combed the fins, cleaned the fan blade, oiled the condenser fan motor, and insulated all of the ducting connections at the plenum and registers.

I believe following is happening;
8" ducting is too large, i.e. large volume, long runs thus less air pressure, one of the ducting runs is 40+ ft.
The Freon charge is unknown but I say sufficient as the air coming out of the floor registers is ice cold.

My ideas are:

1. Put restrictors in the ducting to increase air flow, I'd hate to spend lots of money, nasty dirty job, replacing almost new ducting, with 6" or 4" duct.
2. Replace condenser fan motor with newer, more efficient 3 phase motor, i.e. lower electrical load for same speed.
3. Replace condenser fan motor blade with new blade with deeper pitch (if anyone makes that). More air flow for same speed. Unit is really loud; blade seems to be balanced with no noticeable bends but is really loud compared to my last Tran unit.
4. Replace A/C/ furnace plenum motor with newer, more efficient 3 phase motor, i.e. lower electrical load for same speed.
5. Replace A/C/ furnace plenum motor blade with new blade with deeper pitch (if anyone makes that). More air flow for same speed.

As far as the master bedroom, even with the attic fan, and ceiling fan running full blast with the room isolated and all windows shut it was still 95 degrees when the rest of the house had cooled off to 83. Believe even if I ran, 100 ft of new duct from the central A/C to the master bedroom it would not have sufficient air flow and involves three roof penetrations so a simple self contained window sill or wall mounted A/C unit in conjunction with the pre-existing attic fan would be a better economical choice.

Any advice would be helpful.

as far as replacing condenser motor blades, that air coming out of the unit should be about 13 degrees hotter than the air going into the condenser. So you do not want it to cool the gas any more than that.. What is the deference of the temp. going in and out of the indoor coil?
Air con should be set at a temp that you want to keep the home, like 74 degrees, do this in the morning. Do not let the home get up to 90 degrees and then ask your air con. to bring it down. Unless you have a way over sized unit
it will not work that way.
I would use a wall or window unit to cool the upper bedroom.
A system has to be laid out right to cool the upper story. You would need
hot air opening in the floor and return in the ceiling or hi in the side walls.
The reason i asked you about duct sizing is the blower you have should move at least 1600 CFM of air if it is on Hi speed. Later Paul

[HayZee518]

do you have three phase coming into the building electrical service? if not do you have a phase converter to get three phase power from 240 volt single phase?

[calivette2003]

do you have three phase coming into the building electrical service? if not do you have a phase converter to get three phase power from 240 volt single phase?

I do have 3-phase coming into the main junction box. I had a A/C Tech reply to another forum post on DIY. He said the effiecency difference between single and three phase motors would not justify the cost of replacement. I made a home-made diffuser/ restricter in one of the ducts and it did increase the flow strength from 2 feet to about 5 feet. So I might make some more diffusers. I found a casement window mount A/C for my master bedroom, Kenmore 11,500 BTU Slider/Casement Air Conditioner (this site will not allow me to post the URL

[calivette2003]

Attic fans have to get air to move air so you would pull a vacuum on the house, and you would be pulling hot air from outside where ever it could get in. Do not use attic fan when trying to cool a house with air con.
How many 8" rounds do you have? and are they coming off the hot air plenum? How much return air do you have and what size is it?
Is you blower on hi speed, it should have 3 or 4 speeds Paul

paul, the attic fan was installed by the previous owner. I have six 8" rounds coming from the plenum. Believe the blower is on high speed, how can I find out?

David

[paul52446m]

paul, the attic fan was installed by the previous owner. I have six 8" rounds coming from the plenum. Believe the blower is on high speed, how can I find out?

David

The black wire coming from the motor will be your Hi speed wire, so make sure on your board that the black wire is on cool. Under normal static pressure a 8"
round pipe will run about 225 CFM of air if it is not too restricted. So 6- 8" would be about 1350 CFM of air. your blower should be able to move at least 1600 CFM
of air. It is important to know that a blower can push air easier than it can pull it. So that means that the return air has to be larger than the hot air ducts. That is why i asked you the amount and size of both hot and return ducts. If a blower is not getting enough free air to it, it will just bypass air even if you try to run the motor faster. Later Paul

[simonzhan]

thanks for the post,i learn much about the topic