Home Repair forum - Sizing A/C and Furnace

Author

Sizing A/C and Furnace

Walter R.

2005-07-24, 9:12 pm

I will replace my A/C, which is 25 years old and probably has a SEER around
6, with a new A/C with a SEER 10 rating. I only use A/C 200 hours a year
(San Diego).

The old one is 3 ton and cools just right. Since the new A/C will be
considerably more efficient, can I buy a smaller A/C, let's say with a 2.5
ton rating?

Or, do I need to retain the 3 ton capacity but will merely see lower
operating costs due to higher efficiency?

The same logic would apply to my furnace: Can I reduce the size from 80,000
BTU to 65,000 BTU, because the new furnace will have 20% higher efficiency?
Or will I merely save money operating a new 80,000 BTU furnace?

My logic is a bit challenged due to my lack of knowledge.

Thanks for your help
--
Walter
www.rationality.net
-

SQLit

2005-07-24, 9:12 pm

"Walter R." <wer25@example.com> wrote in message
news:3sUEe.1715$vf.88@tornado.socal.rr.com...
> I will replace my A/C, which is 25 years old and probably has a SEER
around
> 6, with a new A/C with a SEER 10 rating. I only use A/C 200 hours a year
> (San Diego).
>
> The old one is 3 ton and cools just right. Since the new A/C will be
> considerably more efficient, can I buy a smaller A/C, let's say with a 2.5
> ton rating?
>
> Or, do I need to retain the 3 ton capacity but will merely see lower
> operating costs due to higher efficiency?
>
> The same logic would apply to my furnace: Can I reduce the size from
80,000
> BTU to 65,000 BTU, because the new furnace will have 20% higher
efficiency?
> Or will I merely save money operating a new 80,000 BTU furnace?
>
> My logic is a bit challenged due to my lack of knowledge.
>
> Thanks for your help
> --
> Walter
> www.rationality.net

If you have a 3 ton load then a 3 ton it needs to be. To small and it runs
to much. To big and it will not cool effectively. Get your contractor to do
a Manual J calculation on the home then you will know that the unit is
properly sized. Price difference between the sizes is not worth the risk.

Same applies to the furnace.

Yes the more efficient the less it costs to operate,,,,,,,, usually.

IMO installing anything less than a 12 is a mistake. Next year the minimum
Seer for new construction will be 13.
I base my thoughts on living in Phoenix, and cooling is ~ 2500 hours a year.
For sure I would check to see what the cost difference is between your
target and a 12 or 13. If your looking at the long term then check this
site.

http://198.147.238.24/ac_calc/default.asp

udarrell

2005-07-24, 9:21 pm

Walter R. wrote:

>I will replace my A/C, which is 25 years old and probably has a SEER around
>6, with a new A/C with a SEER 10 rating. I only use A/C 200 hours a year
>(San Diego).
>
>The old one is 3 ton and cools just right. Since the new A/C will be
>considerably more efficient, can I buy a smaller A/C, let's say with a 2.5
>ton rating?
>
>Or, do I need to retain the 3 ton capacity but will merely see lower
>operating costs due to higher efficiency?
>
>The same logic would apply to my furnace: Can I reduce the size from 80,000
>BTU to 65,000 BTU, because the new furnace will have 20% higher efficiency?
>Or will I merely save money operating a new 80,000 BTU furnace?
>
>
You would size according to the furnace's OUTPUT Btu/hr; ignore the INPUT.

>My logic is a bit challenged due to my lack of knowledge.
>
>Thanks for your help
>
>
I lived in San Diego for over 3 years and wonder why you need that big
an A/C and furnace.
The weather was rather mild the year round.

I would rather slightly undersize than oversize on the A/C unit.
How big is your home? High ceilings?
I have a little half ton 6,000-Btu/hr window unit cooling the entire
first floor; over 920 sq ft.
Today the HEAT INDEX is 112-F; 92-F with 70% Relative Humidity, and the
little A/C has it perfectly comfortable in here.
A floor fan circulates the air from the units evaporator coil. The floor
area is 77-F and 58% RH and very comfortable all day.

http://www.udarrell.com/airconditio...h_charting.html

Have someone with credentials run the numbers so you know what size A/C
and furnace should be installed.
(Don't guess!)

You won't get the SEER or furnace efficiency if you oversize.
http://www.udarrell.com/air-conditi...atent-heat.html

- udarrell

--
Optimizing Air-Conditioner Efficiency
http://www.udarrell.com/air-conditi...rator-coil.html

Walter R.

2005-07-24, 10:21 pm

I live about 10 miles east of San Diego. Temperatures are 10 degrees higher
here than in the city. My house is 1650 sqft. I was only wondering, if I get
a more efficient A/C, should get a smaller A/C unit?

--
Walter
www.rationality.net
-
"udarrell" <anonymous@anonymous.com> wrote in message
news:42e41eae_3@newspeer2.tds.net...
> Walter R. wrote:
>
> You would size according to the furnace's OUTPUT Btu/hr; ignore the INPUT.
>
> I lived in San Diego for over 3 years and wonder why you need that big an
> A/C and furnace.
> The weather was rather mild the year round.
>
> I would rather slightly undersize than oversize on the A/C unit.
> How big is your home? High ceilings?
> I have a little half ton 6,000-Btu/hr window unit cooling the entire first
> floor; over 920 sq ft.
> Today the HEAT INDEX is 112-F; 92-F with 70% Relative Humidity, and the
> little A/C has it perfectly comfortable in here.
> A floor fan circulates the air from the units evaporator coil. The floor
> area is 77-F and 58% RH and very comfortable all day.
>
> http://www.udarrell.com/airconditio...h_charting.html
>
> Have someone with credentials run the numbers so you know what size A/C
> and furnace should be installed.
> (Don't guess!)
>
> You won't get the SEER or furnace efficiency if you oversize.
> http://www.udarrell.com/air-conditi...atent-heat.html
>
> - udarrell
>
> --
> Optimizing Air-Conditioner Efficiency
> http://www.udarrell.com/air-conditi...rator-coil.html

Wayne Whitney

2005-07-24, 10:21 pm

On 2005-07-25, Walter R. <wer25@example.com> wrote:

> I was only wondering, if I get a more efficient A/C, should get a
> smaller A/C unit?

No. The size you need is independent of the efficiency; a more
efficient unit will simply cost less to run.

Wayne

Joseph Meehan

2005-07-24, 11:21 pm

Walter R. wrote:
>I will replace my A/C, which is 25 years old and probably has a SEER
> around 6, with a new A/C with a SEER 10 rating. I only use A/C 200
> hours a year (San Diego).
>
> The old one is 3 ton and cools just right. Since the new A/C will be
> considerably more efficient, can I buy a smaller A/C, let's say with
> a 2.5 ton rating?
>
> Or, do I need to retain the 3 ton capacity but will merely see lower
> operating costs due to higher efficiency?
>
> The same logic would apply to my furnace: Can I reduce the size from
> 80,000 BTU to 65,000 BTU, because the new furnace will have 20%
> higher efficiency? Or will I merely save money operating a new 80,000
> BTU furnace?
> My logic is a bit challenged due to my lack of knowledge.
>
> Thanks for your help

Tons measures the amount of cooling and increased efficiency only means
it takes less energy to do the cooling. If you are happy with the size you
have, I would generally not change it. That said too big or too small is
not good, but if it is really comfortable for you, I would stick with what
you have.

--
Joseph Meehan

Dia duit

Edwin Pawlowski

2005-07-25, 12:21 am

"udarrell" <anonymous@anonymous.com> wrote in message
> I lived in San Diego for over 3 years and wonder why you need that big an
> A/C and furnace.
> The weather was rather mild the year round.

Maybe he has an 8,000 square foot house. Maybe he is just outside of SD
where the temperature can be a bit more heated in summer, say closer to El
Cajone where it can be 100 when the coast is 72.

>
> Have someone with credentials run the numbers so you know what size A/C
> and furnace should be installed.
> (Don't guess!)

I agree with that but since the 3 ton cools just right, that seems to be
what is needed. Worth checking as the house may have changed over the years
with additions or added insulation.

The original question is "can I get away with less capacity if it is more
efficient?" and the answer is no. A Btu is a Btu no matter how efficient
the delivery is.

udarrell

2005-07-25, 11:21 am

Walter R. wrote:

>I live about 10 miles east of San Diego. Temperatures are 10 degrees higher
>here than in the city. My house is 1650 sqft. I was only wondering, if I get
>a more efficient A/C, should get a smaller A/C unit?
>
>
Absolutely, NO!
By all means have a manual J done and see what tonnage it calls for.

If you oversize with short run cycles the unit will not deliver anywhere
near the rated 10-SEER!
You may end up getting only 8 SEER or less due to short cycling.
Why not get the SEER you paid for by sizing it right?
In fact the comfort levels should be better due to correct sizing.
http://www.udarrell.com/air-conditi...atent-heat.html

The summer outdoor design for San Diego is 80 dry bulb and 69 wet bulb
for around 57% relative humidity.
If you are further from the ocean with average of 10 degrees higher
temps then the humidity would be less than SD's.

You probably have a lot of days when the humidity is a problem with
rather moderate outdoor temps; that calls for more run time to achieve
the comfort zone with efficiency!

Don't oversize the system; 3-ton is 36,000-Btu/hr. I don't run the 5,000
Btu/hr upstairs window unit during the daytime and the hallway stairwell
is wide open; therefore the little half-ton 6,000 unit also handles
considerable latent load (humidity) and some heat from upstairs.
I know it makes considerable difference ion the latent load on the
downstairs unit, because when I run the other unit the Relative humidity
will drop 2 to 4% in a short period of time. Without the upstairs unit
running it will normally keep the humidity at 50% or lower, and we have
a lot of humidity here.

I would also ask what you could do to decrease the heatload with
insulation, etc.

Do it right! - udarrell

--
Optimizing Air-Conditioner Efficiency
http://www.udarrell.com/air-conditi...rator-coil.html

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