Nathan9999

[bornriding]

Please let us know how your new unit works when you get it.
I'm sure that some of us are very interested in the outcome and whether or not that you will be satisfied with the 5 ton system on 3190 st ft, with 10 ft ceilings.
Thanks
Richard

[nathan9999]

I'm either really confident in my calculations, or I'm starting to get worried.

However it comes out I'll post my experience and my satisfaction, or lack thereof.

I forgot to mention that my contractor actually was okay with a 4 ton solution. The HVAC-Calc program indicated a 4 ton when window coverings were included. And keep in mind I changed summer outside design temp to 110, about 6-7 degrees higher than program default.

Have tentatively decided on zoned Trane XL19i with Trane integrated zoning system. Wanted XL16i (R410A and scroll) but Trane zoning system does not support the XL16i.

Ceiling insulation R38, walls R19, low e windows but no film treatment, north south orientation, fully covered front and back patios, standard (24"?) eaves.

By the way, there have been many posts about studies that purport to show that Manual J actually overstates load requirements.

What is the experience of the professional participants on this forum? Do you think Manual J overstates actual requirements?

[Edited by nathan9999 on 07-18-2005 at 06:36 PM]

[jacob perkins]

Originally posted by nathan9999
And keep in mind I changed summer outside design temp to 110, about 6-7 degrees higher than program default.

What is the experience of the professional participants on this forum? Do you think Manual J overstates actual requirements?

I dont think so.
Maybe the newer editions have errors.I dont really know.

Sounds like n9999 overstates actual requirements!

[docholiday]

No. The reason you hear that is because the full load is often between sizes and you most often dont go down.

[Panama]

How did you specify your window glass in your Manual J calculation? Did you enter it as "tinted" or just "low e?" I ask because Manual J, 7th Edition does not handle modern windows very well.

Did you measure the window sizes by measuring the area that opens (per Manual J) or did you simply use the nominal sizes? This can make a difference of 15% or more in the heat gain from windows.

Two-foot eaves? Did you "shade" portions of the windows when you did the calculations? That can easily make a 10% or more difference in the load calculation. HVAC-Calc may not handle shading calculations, but the procedure is in Manual J itself.

I hope you have specified cellulose insulation, as it is far superior to fiberglass. There is a lot of evidence supporting this.

Kicking the design temperature up 6-7 degrees is not a good idea. It takes a long time for temperatures even 10 °F above design to raise the temperature inside a building more than a couple degrees. People and lights do a much better job of that. If an unusually hot day is predicted, just set the thermostat a little lower the night before.

The most comfortable home day in and day out is one with a slightly undersized system, as it runs continuously for hours on end. When it runs like that the air in the house mixes better and temperatures are more even throughout the house. Also, when the air is moving you feel cooler.

Did you use Manual S and Trane "Cooling Performance Data" to select your equipment? Most "shortcut" Manual J systems assume a 77% sensible heat ratio by having you add 30% to your sensible load; your house in Bakersfield probably will have a 90-95% sensible heat ratio. This makes selection of the blower, evaporator, and condesing unit very different.

I suspect you are going to be substantially oversized. If you're not, then there will be something wrong with the design, installation, or commissioning. It's criminal how many systems don't cool well simply because the are incorrectly charged.

[bornriding]

In my, very humble, opinion, manual 'j' sizes for energy efficiency, and not comfort.
At least in my area of the south, I have seen many (including my own home), that were sized for manual 'j' that after a few years would not keep up.
I bought my 1950 sq ft home with 2.5 ton HP when it was ten years old. Would not cool below 78 degrees if outdoor was 85 or over.
Manual 'j sizes for 93 OAT, 75 IAT, here. Ok if you like it 75 or above. But after a few years, when system not new anymore, and home is not as tight as it was when new, units tend to not be able to cool as easy.
I have been in business 15 years, and I size units for customer comfort, and I have 'never' had to go back to one because the customer wasn't happy with the system...never.
I've not had but one customer complain about electric bill, and he upgraded his system to larger unit, and wife kept thermostat on 70.(would not get there with old system) Have had several customers let us know that their electric bills had gone down, and were very happy with that.
Have replaced unit less than ten yrs old, due to not keeping temp during hot summer days. I assumed it was installed per manual 'j'.
So for comfort issues, I do not have faith in manual 'j'.

[Gunnery Sergeant Hartman]

I don't know how anyone could want their temps much lower than 75?
I keep mine at 74 and we have to sit under blankets at night while watching tv.
My RH is 40% also...

[dash]

Yes ,Man. J ,oversizes ,but it's not a lot,and it varies ,5 to 10% maximum,without user input error.
The Author states it so I'd say it's a fact.

[docholiday]

Born,

With all due respect a unit should not lose capacity after a few years. If a homeowner fails to maintain their system, you cannot go back and blame the load calc.

If he designs his system for 107 out and 70 in, he'll need a 30 ton chiller.

[Panama]

One problem with a 5-ton system in a dry climate is that you can't move enough air with a typical 5-ton blower to get good efficiency out of the system. It is costly to run a system at a 0.8 sensible heat ratio when all you need is 0.9.

It makes better sense to use a 4-ton condensing unit with a 5-ton blower and coil. Set the sucker on high speed and you will be cool, comfortable, and economical.

[ruud-man]

Regarding your post about 74dF and the blankets. I'm curious why run the A/C at 74dF if it is too cool for comfort?

The reason I ask is I prefer a dry 78dF and my wife wants the temp lower, but then grabs a blanket or sweater. Me, I just watch dollars fly off to the utility company.

Best regards...

[nathan9999]

Responses as best as I can.

Jacob, I think you're right. I am overstating the requirements by increasing the outside design temp.

Panama,

The windows were measure using their inside wall area dimensions.

I did not shade the windows in the program

I'll check on insulation. I'm sure contractor specified standard for this area.

I appreciate your comments on sizing. I'll discuss with my contractor. I have not used Manual S. I believe the contractor said he would use a "Ductalator" (can't be sure of spelling). I'll ask contractor about Trane Cooling Performance Data.

How would using a dual compressor system and VS alleviate oversizing issues, if at all?

Panama, I'll discuss your recommendation on condensor and blower with contractor.

Thanks again for the comments. I'll post again when a final configuration has been selected. It'll probably be next summer before I will know if I made the right decision.

[Gunnery Sergeant Hartman]

Originally posted by ruud-man
Regarding your post about 74dF and the blankets. I'm curious why run the A/C at 74dF if it is too cool for comfort?

The reason I ask is I prefer a dry 78dF and my wife wants the temp lower, but then grabs a blanket or sweater. Me, I just watch dollars fly off to the utility company.

Best regards...

I'm just saying from my personal experience we keep our stst at 74 and it feels pretty cool.
I'm always in shock when I hear of folks wanting it to be 68!

[ruud-man]

I'm with you on that one,68dF seems pretty cold. As I understand the psych chart, the lower the temp the higher the relative humidity. Seems to be self defeating after a certain point.

I always felt, in the long run, it is harder on the body to go from a really cool house to a very hot work place.

Anyway, that's my story and I'm sticking to it.

Best regards...