dan sw fl- you wanted to know

[elkhvac]

QUOTE]Originally posted by dan sw fl

Originally posted by elkhvac
We are doing a new custom home that will require one ton per 1250 sqft!

Could you provide some building details, features, and location that enables such a low cooling load?

I believe that
windows area must be very small And
infiltration an absolute minimum
to achieve > 800 Sq. Ft. per ton.


Elkhvac,

Would you start another thread to address how to develop energy efficinet houses?

Thanks. [/QUOTE]



Walls-R-21 w/ R-6 board insulation below stucco
Windows-577 Sqft-SHGC .29 with 3’ min projection over all but north windows
Roof-Light, Vented with R-50
Auto close shades on all skylights
All ductwork is inside the envelope
Every seam in the envelope sealed

The builder wanted to build “The Best Home”.

Being in Denver helped 825 cooling hrs.

We installed a Unico system (cooling only). Two zones 1st and 2nd floor

[txborn]

Being in Denver helps indeed. 2,700 cooling days in Dallas makes for a different scenerio. But with the foam I am going to use the "rule of thumb" ;) is 750 - 1,000 sq ft assuming minimal west window area (9 sq ft in my case) and a non vented attic, LED and fluorescent lighting among many other considerations.

You putting all the HVAC equipment in the sealed envelope?

I was fortunate enough to find a VERY competent HVAC contractor who has experience with these types of houses. Should be getting the results of the Man J, D and S calcs this week. 2 story, 3,250 sq ft and he originally said (again, just a calculated guess), one unit, 4 tons - 5 tons max zoned. Of course won't know until the calcs come in. He even was able to discuss the difference between Home Depot/HVAC supply companies cheap supply registers that actually restrict airflow vs. quality registers that can do a much better job, but of course co$$$t more.

Guess you get what you pay for.

[dan sw fl]

Originally posted by elkhvac
QUOTE]Originally posted by dan sw fl

Originally posted by elkhvac
We are doing a new custom home that will require one ton per 1250 sqft! Could you provide some building details, features, and location that enables such a low cooling load?


Walls-R-21 w/ R-6 board insulation below stucco
Windows-577 Sqft-SHGC .29 with 3’ min projection over all but north windows. Roof-Light, Vented with R-50
All ductwork is inside the envelope
Every seam in the envelope sealed

Being in Denver helped 825 cooling hrs.

We installed a Unico system (cooling only).
Two zones 1st and 2nd floor [/QUOTE]

I appreciate the details on the DENVER residential design.

Window area is more than I anticipated.
The 0.29 SHGC significantly reduces the heat gains.

[Panama]

Two conditions really help reduce the AC equipment needs in Denver, but increase the risk of the indoor coil freezing up.

First, the latent load of a typical home in the Denver area is negative. To get full cooling capacity and efficiency -- and to reduce the risk of coil freezing -- you need to run about 600 cfm per ton. Such high air flow increases the sensible capacity from the typical 70% to nearly 100% of total rated capacity. Higher cooling output permits smaller equipment.

Second, the daily temperature range is close to 30F. But the cool evenings and nights can quickly cause a coil to freeze up if the air flow is inadequate.

A good rule of thumb in the Denver area is to make sure your blower is at least one ton larger than the condensing unit. That of course means you should never see a 5-ton condensing unit here; but for some reason there are many of them. Most homebuilder installs have same-sized condensing units, blowers, and coils. Many installers who match equipment that way install freezestats as standard equipment.

Unicos can be very problematic in high, dry climates. Head pressure controls are definitely required, but often are not installed.

[slice]

txborn, I am building 20 miles inland from gulfcoast, My calcs came back at 1250 per ton, 3000 sq ft two 1.5 ton units one of them will be a strong 1.5 ton. variable speed AHU both heat pumps. one unit pulls a combined 9 amps running....

ICF walls 11" finish stucco

icnenye roof and garage

[txborn]

slice, so is your garage attic in the sealed envelope? I was debating whether to just use batts for garage walls (icynene for common garage/living area wall) and batts above garage ceiling; do soffit vents there and ridge vent above garage roof only.

Sounds like you will have pretty low electric bills. :)Anxious to see what the calcs for my house are going to be.

[Carnak]

Originally posted by Panama
Two conditions really help reduce the AC equipment needs in Denver, but increase the risk of the indoor coil freezing up.

First, the latent load of a typical home in the Denver area is negative. To get full cooling capacity and efficiency -- and to reduce the risk of coil freezing -- you need to run about 600 cfm per ton. Such high air flow increases the sensible capacity from the typical 70% to nearly 100% of total rated capacity. Higher cooling output permits smaller equipment.

Second, the daily temperature range is close to 30F. But the cool evenings and nights can quickly cause a coil to freeze up if the air flow is inadequate.

A good rule of thumb in the Denver area is to make sure your blower is at least one ton larger than the condensing unit. That of course means you should never see a 5-ton condensing unit here; but for some reason there are many of them. Most homebuilder installs have same-sized condensing units, blowers, and coils. Many installers who match equipment that way install freezestats as standard equipment.

Unicos can be very problematic in high, dry climates. Head pressure controls are definitely required, but often are not installed.

Intereting, nice to hear if from the local perspective

[slice]

garage attic is sealed . Since I like to spend time out there it will be part of it, and I am installing a damperr off one of the units to dump a little cool air out there while i am working on those humid days, wont cool the whole place but will make it nicer... My quotes for icnenye were worth it to do the attics and garage exterior walls plus two interior walls... I dont see energy prices coming down like gas prices. here we pay 16 cents KW and climbing.....

[Carnak]

Bad idea, its like running a cooled exhaust fan. House gets negative and sucks humidity into your home, or stuff right back from the garage. Shut the car off :)

[Edited by Carnak on 09-05-2006 at 06:10 PM]

[dash]

Originally posted by Carnak
Bad idea, its like running a cooled exhaust fan. House gets negative and sucks humidity into your home, or stuff right back from the garage. Shut the car off :)

[Edited by Carnak on 09-05-2006 at 06:10 PM]


I agree ,get a mini-split for the garage.

[Carnak]

Originally posted by Panama
Two conditions really help reduce the AC equipment needs in Denver, but increase the risk of the indoor coil freezing up.

First, the latent load of a typical home in the Denver area is negative. To get full cooling capacity and efficiency -- and to reduce the risk of coil freezing -- you need to run about 600 cfm per ton. Such high air flow increases the sensible capacity from the typical 70% to nearly 100% of total rated capacity. Higher cooling output permits smaller equipment.

Second, the daily temperature range is close to 30F. But the cool evenings and nights can quickly cause a coil to freeze up if the air flow is inadequate.

A good rule of thumb in the Denver area is to make sure your blower is at least one ton larger than the condensing unit. That of course means you should never see a 5-ton condensing unit here; but for some reason there are many of them. Most homebuilder installs have same-sized condensing units, blowers, and coils. Many installers who match equipment that way install freezestats as standard equipment.

Unicos can be very problematic in high, dry climates. Head pressure controls are definitely required, but often are not installed.


Was just looking at your post a second time Panama, it sure is amazing the differences climates make.

Here latent load is very positive, takes a lot of cooling to pull fresh air down to a 55 dewpoint here.

I would almost think the thin air would compound the fact that your load is mainly sensible. Higher flow per ton becuse it is all sensible and even more flow as the air is thinner?

[Panama]

Originally posted by Carnak
[QUOTE]Originally posted by Panama
[B] ....

Carnak's comment:

I would almost think the thin air would compound the fact that your load is mainly sensible. Higher flow per ton becuse it is all sensible and even more flow as the air is thinner?

You are 100% correct. The actual air flow must be increased about 21% (more in mountain communities) to get the equivalent of sea level mass flow.

The idea is to get the leaving air close to 55F for a DT of 20F. At an altitude of 5,300 feet, nominal atmospheric pressure is 12.1 psi. So one nominal ton of AC running with no latent load would require 675 cfm to attain a DT of 20F: (12,000/20/1.08)(14.7/12.1) = 675.

The capacity of condensing units falls about 5% at this altitude according to ACCA Manual S.

Unfortunately the equipment manufacturers ignore the realities of high, dry air and continue to preach "400 CFM PER TON."

[Carnak]

The limit on what defines 100% RH, is based on how vapour pressure of water competes against atmospheric pressure.

At high altitudes then, more moisture can be held per pound of dry air.

[Panama]

True. I call it aggressive air: it sucks the moisture right out of you.