Manual J Calculations for a zoned A/C System: Peak Cooling Load Adjustments


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Old 01-06-11, 01:55 PM
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Manual J Calculations for a zoned A/C System: Peak Cooling Load Adjustments

Hey all. I have a 30 year old home with 2350 square feet. Basically, I have 1100sf of living space in an open floor plan ranch configuration with a two car garage at the west end. I have another 800sf of bedroom space in a wing added on behind the garage. And then I have another 450sf of bedroom space upstairs, 2/3 above the main living space and half above the garage.

The house came with a 5 ton unit downstairs and a 1 1/2 ton unit upstairs, both systems are close to 15 years old. The 5 ton system is matched with a 115 kBtu gas furnace and a 2000 cfm blower. The house has had many energy efficient improvements done over the years and these systems are now oversized in my (non-HVAC professional) opinion. I have spoken to 4 HVAC contractors and all of them are telling me I would be stupid to downsize the units and that I should replace them with the same. None of them would run Manual J calcs. My brother has worked for several HVAC contractors and has never heard of manual j calcs. Convinced no HVAC contractor in New Orleans knows what Manual J is, I bought some software myself (HVAC-Calc), ran the numbers and came up with a 3 ton unit for the entire house! What a difference! Its such a difference I may rerun the numbers to confirm them.

Finally my question. I would like to zone the system for the main living area, the lower bedroom area and the upper bedroom area. In doing some internet research, the 8th edition of manual J includes some adjustments for a zoned residential system to account for "Peak Loads". I read this in two locations in the ACCA Manual RS. Does anyone know what "Peak Load" this entails that is different from the traditional Manual J design?

Secondly, my heating needs are only 39,000 BTUís. However a 46/32 kBtu gas furnace would only provide 1200 cfm airflow. While this matches the needs of a 3-ton AC unit, my house registers are all 10x10 and I am concerned the smaller air flow would not give me enough mixing in each room. I donít want to take out my registers to put in smaller ones. My question is can I use a larger furnace and a larger airflow with the 3 ton AC coils? Say I go with a 92/64 kBtu furnace to maintain the 2000 cfm in the system. I know under sizing the blower will cause the coils to freeze. What is the drawback to over sizing the blower? Also my gas furnace would also be oversized by 65% at the low stage; what would this drawback be? I assume I can set it so that the high stage is disabled.

Thanks.
 
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Old 01-09-11, 05:14 AM
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Originally Posted by clsanchez77
I have spoken to 4 HVAC contractors and all of them are telling me I would be stupid to downsize the units and that I should replace them with the same. None of them would run Manual J calcs.
Guess keep on looking for a dealer who will do it.

Does anyone know what "Peak Load" this entails that is different from the traditional Manual J design?
I honestly don't know since I'm not in the field anymore.
Secondly, my heating needs are only 39,000 BTUís. However a 46/32 kBtu gas furnace would only provide 1200 cfm airflow.
My question is can I use a larger furnace and a larger airflow with the 3 ton AC coils? Say I go with a 92/64 kBtu furnace to maintain the 2000 cfm in the system.
Where are you getting this 2,000 cfm air flow?

Rule of thumb is 400 cfm per ton. So, if you got a 3 ton system, it will move 1,200 cfm. And for very humid area, goes down to 350 cfm per ton.

my house registers are all 10x10 and I am concerned the smaller air flow would not give me enough mixing in each room.
Are they in the ceiling or the floor?

What is the drawback to over sizing the blower?
For cooling, you are going to have a "clamy" feeling, and will end up lowering the temps to fee better, and it will have poor humidity removal.

Also my gas furnace would also be oversized by 65% at the low stage; what would this drawback be? I assume I can set it so that the high stage is disabled.
Spent extra money on a system that you be using, unless it get very cold like we do up here. :-) (right now it's -10˚)

Why are you looking at two stage furnace?
 
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Old 01-09-11, 07:19 AM
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Guess keep on looking for a dealer who will do it.
I intend to, but in the mean time, I plan to do the calcs myself so that I know what questions to ask and can spot the guy who actually does this correctly.

Where are you getting this 2,000 cfm air flow?
That is the capacity of existing system. The air circulation feels good at this rate. I am concerned a lower rate will greatly reduce flow and comfort, requiring all the registers to be replaced as well.

Are they in the ceiling or the floor?
They are ceiling registers, the aluminum, 4-way, curved blade type.

Spent extra money on a system that you be using, unless it get very cold like we do up here. :-) (right now it's -10˚)
In New Orleans, we hit freezing three or four times a year, usually about a week at a time. Normal winter temps are between 35 and 45. Im concerned over sizing the furnace may lead to short cycling. I am not sure what the disadvantage to short cycling a gas furnace is.

Why are you looking at two stage furnace?
I am under the impression all the current high efficiency gas equipment includes two stage heat. Three stage is not needed here. Besides, if I do indent with a three ton unit, I will need 1200 cfm, the 1200 cfm furnaces are generally 45-kbtu. If I indent up with a 4 ton A/C, thats 1600 cfm and 60-kbtu. Both of these furnaces are more than what I need. But the two stage units allow a lower heating rate, seems to match the Manual J numbers better.

Thanks.
Chris
 
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Old 01-09-11, 01:58 PM
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Originally Posted by clsanchez77
I intend to, but in the mean time, I plan to do the calcs myself so that I know what questions to ask and can spot the guy who actually does this correctly.
You are doing the right thing.. Glad to see that, other wise if you did put in a new system, you wouldn't be saving any money on an oversized A/C.

That is the capacity of existing system. The air circulation feels good at this rate. I am concerned a lower rate will greatly reduce flow and comfort, requiring all the registers to be replaced as well.
With the vents in the ceiling, the air will be fine.

They are ceiling registers, the aluminum, 4-way, curved blade type.
If you have what I am thinking, worst comes to the worst, close half of it down.

In New Orleans, we hit freezing three or four times a year, usually about a week at a time. Normal winter temps are between 35 and 45. Im concerned over sizing the furnace may lead to short cycling. I am not sure what the disadvantage to short cycling a gas furnace is.
Higher gas bill, shorter life and poor comfort. System don't get a chance to "warm" up.


I am under the impression all the current high efficiency gas equipment includes two stage heat. Three stage is not needed here.
Are you looking at 80% or 90% furnace?
 
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Old 01-10-11, 07:47 AM
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Higher gas bill, shorter life and poor comfort. System don't get a chance to "warm" up.
This is what I am seeing now with a 112 kBtu furnace. If I do end up with a 3-ton unit, it would match with 46/32 furnace which would be pefect. But if I do end up with a 4 ton unit, I would end up with a 69/48 unit as the matched set, which would be a little more than I need.

Are you looking at 80% or 90% furnace?
I was actually looking at the 95% units. I just believe in spending the money upfront to save over the life of the equipment. Even if the savings dont cover the difference, I also believe that higher efficiency is often better built and that the equipment will last longer.

Thanks for your help. I did order a hard copy of Manual J as there just is not any information on the internet dealing with the specifics. The concepts seem to be really easy to understand and I am starting to wonder if this is a service I can offer to the local HVAC industry since it seems to be absent.

Chris
 
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Old 01-10-11, 08:38 AM
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Glad to help out. Let us know if you need anything else.
 
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Old 01-10-11, 09:32 AM
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One other question

The HVAC-Calc software I used provides sensible, latent and total heat gains. From what I have read, units should be matched by either sensible or total heat gain, which ever controls. I was looking at some Goodman A/C unit specs and it appears that I may have a case where latent heat gain controls. This, while I know is not normal, I believe is due primarily to high air infiltration and our humid summer weather conditions. I am looking to resolve the infiltration and hopefully will substaintialy improve prior to having to replace my A/C.

But if I end up needing to replace the A/C before I resolve the infiltration issues and latent heat controls, should I size the unit based on total heat gain, or latent heat gain.

Again, Thanks
 
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Old 01-10-11, 07:39 PM
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I'd do the total since you are going to seal up the house better.
 
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Old 01-18-11, 08:01 PM
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a 16 seer system with variable speed fan and two stage compressor controlled by a communicating user interface will serve to smooth out all the edges and will be smart enough to fine tune itself. i don't mean to make the newer technology sound overly simplistic, but it's where the industry is evolving. these systems are nearly plug and play. that said, you are doing your design homework and with a properly sized and designed system, including ample return air, a cutting edge system will accomplish your goals, to automatically adjust airflow and staging, constantly monitoring conditions of inside and outside temp and humidity. at face value, 3 tons sounds a little short, but the manual j will tell the story. the advantage of a 2 stage system is that you have some flexibility to handle those peak loads and throttle down when the load diminishes. when the load is light, the compressor will run half power. the fan will be adjusted to a lower speed during high humidity conditions. although heating season is short and light for us in the south, the 2 stage furnace will use half the gas, and there are options in the interface to tweak algorythms in cooling and heating mode. as you pointed out, most of these high end systems are matched with high efficiency furnaces. the boards in the condensing unit and furnace receive commands via 4 wire dc bus system rather than traditional 24 volt ac signals. until very recently, this level of control was not available. with the right equipment, and the right zoning system, the handling of load changes can be seamless and comfort can be mastered.

i would recommend you compare 16 seer and above among the top manufacturers and consult with some of the experts from each . i'm sure your research will pay off.
 
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Old 01-18-11, 08:47 PM
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Thanks nomadpeo -

We are thinking along the same lines. I am actually considering ponying up for the SEER 18 equipment. Since the cooling season is almost 8 months long hear, I dont need to be convinced of the payoff.

I am a little skeptical of the SEER 21 as it usually involves proprietary equipment all the way to the thermostat and it greatly reduces my installers to too few. This also knocks out the DC communicating thermostats. Since of all the professional trades, I trust A/C the least, I dont want to corner myself to just a Carrier and a Trane dealer. While they are undoubtedly great equipment, AC contractors in my area are less professional. I dont want to end up sole sourcing my setup to only one guy who can sell, install and then service the equipment. In my experience, you cant keep a guy honest when he is the only one and he knows it.

Also, I plan on using a specific thermostat that is compatible with my home controller. With that, I can custom command the system based on multiple interior thermostats and exterior temperature and humidity conditions, lock out the second stages on the heat and cool and so on.

And lastly, the zone controller I am considering will let me lock out the second stage heat based on exterior temps. I plan on setting it to 40. This way, I dont cycle the second stage heat when its only 45 or 50 outside.

I will also set the Fan On setting on the thermostats with my alarm, so they are on when we are home, off when we are away and on only the two bedroom areas when the alarm is in sleep mode. I have always been a fan of keeping the A/C fan set to on and with the variable speed, it makes even more sense as the fan speed greatly reduces when the A/C and heat are off.

But yes, SEER 18, variable speed fan and two-stage furnace. You and I are on the same page.
 
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Old 01-19-11, 03:57 AM
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FYI, I don't suggest running the fan "ON" mode in A/C mode. It throws the humidity right back into the house from the coil. I tried it on mine for a bit, an it went right back to "AUTO". Only time I run fan "ON" is in heating mode.
 
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Old 01-19-11, 08:39 AM
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Wink Interesting...

I can see that in concept. It will give me something to play with this summer on a rainy day
 
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Old 01-19-11, 11:18 AM
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I never done it in the past, but when I got my new variable speed blower, figured "Ok, it's quiet, I can run the fan all the time.. " First summer I had it, my humidity was staying around mid to upper 50%, and I can't stand humidity, so I switched it back to "AUTO", and humidity dropped down to lower/mid 40%.

I also have the IAQ stat, so it can control the blower speed in cooling, if the humidity is over 50%, the fan slows down to help remove the humidity, and if there is no/low cooling load (rainy day) and the A/C hasn't run for the while, 55%, the IAQ will force the A/C on till humidity is below 50%, or temps is 3˚ below set point then it shuts off.
 
 

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