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Manual J vs. Rule of Thumb Sizing


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By Jeff in Henderson, NV on 11/29/2004


Hi All,

I completed a Manual J calculation on my house plans today using the HVAC-Calc software and there is a large discrepancy between what the Manual J and "rule of thumb" sizing I am told is commonly used. I am in Las Vegas, NV so the A/C load is really the most important part.

My construction consultant (UBuildIt) says their usual HVAC sub generally calculates HVAC tonnage by figuring one ton per 400 sq. ft. of living space. My house is a simple one story 2,200 sq. ft. ranch style house. He said they would probably go with two 2.5 or three-ton units on these plans.

My Manual J calculations are calculating a total heat gain of 32,228 BTUH of which 27,858 is sensible and the remainder is latent. I read that a typical A/C unit is designed for 70% sensible/ 30% latent load, but even calculating that into the mix (27,858 / .7) is figuring a total load for the whole house of ~3.3 tons, which is considerably less than 5 or 6 total tons. I've heard they over-size units often, but that seems ridiculous. According to the HVAC-Calc website they recommend not using a fudge factor with the A/C load, rather only with the heating load.

I am just wondering if I am crazy or if this is common. I am going to have a hard time convincing a sub to work off a Manual J if the sizing is that far off from the rule of thumb. Additionally, the house isn't anything super insulated. It is R-19 in the 2x6 walls and R-38 in the ceilings with low-E vinyl windows.

-Jeff


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By Kenneth in Lees Summit, MO on 11/30/2004


I would absolutely go with the Manual J over some "rule of thumb." I am building an ICF house, and the Manual J shows I need less than three-tons of cooling, yet a typical rule of thumb around here would suggest twice that based on house size. This is a unique condition (ICF construction, R-49 roof insulation instead of R-30). I found "rule of thumb" is usually based on some worst case scenario, and not real world conditions. I had one contractor tell me I should size my AC using his rule of thumb, and that it would work well at 110 degrees and keep my house in the low 70's. In this locale, we very rarely get over mid 90's, and my concern is not at 100+ but how well I can remove humidity at 80 to 85 degrees, which with an unit that over sized would be near impossible due to short cycling.

Manual J will take in to account all factors such as number, size, U-value, and orientation of windows (low-E and insulated glass help), wall insulation, attic insulation, ceiling heights, exterior temperature, interior temperature you wish to achieve, etc. Manual J will also help you determine air flow from room to room, and should not just be used to size the overall units (having the capacity with poorly balanced airflow is no good either).

What I am doing because the Manual J and the rule of thumb is so far off is to size ductwork for a larger unit, size the A-coil for a larger unit, put manual dampers in the branch lines (these can later be converted to servo motor operated for zoning). This way if Manual J is so wrong, the only change necessary is to replace the compressor, everything else is sized for a larger a/c - this is a compromise that doesn't cost much to install now but completely replacing ductwork after construction can get very expensive. The upside to this is that the larger A-coil will allow my AC to operate at higher efficiency, the downside is that the larger A-coil will reduce humidity removal, a big factor here in the Midwest and any green grass climate. As a side note, we had to size the furnace not based on heating load (ICF, R-49, and good windows reduced this tremendously), but based on airflow required for cooling. As a result the furnace is way over sized, but I addressed this by using a two-stage variable speed unit.

Find a good AC contractor to help you work through this. Alternatively find a mechanical engineer that understands HVAC and ask them to help. My mechanical engineer charged $300 to do the complete analysis, including an analysis of return on investment of upgrading to more efficient units (AC comes in SEER of 10, 12, 14, 15, 16, 18, which one is most cost effective?). The AC contractors tend to push the higher dollar, higher SEER units (more profit?), when the payback for decreased energy usage would dictate they are not worth the extra cost. The $300 gets me an unbiased opinion, as the mechanical engineer does not sell or install HVAC and has no vested interest in recommending a smaller, larger, more efficient, less efficient, or anything else. He got his fee, he has no way to make additional profit from my job.


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By John in Erie, CO on 11/30/2004


Follow Ken's advice -- it is VERY common for HVAC installers to oversize equipment. Why not? Bigger equipment is more money for them, and will err on the side of not getting a call back. Plus, then they don't have to do a real calculation.

The fact that your UBuildIt guy was using rules of thumb is scary too - They ought to know better if you are really trying to build a quality house.

I had some HVAC guys try to sell me 11 tons of heating and cooling based on my square footage. Real load calcs showed I only needed five tons of heating and 3.5 tons of cooling.

If the HVAC contractors can't show you calcs, then you might try other guys. I was going to go with a guy that had a good price on a five-ton unit, but my gut told me I'd be sorry. His price was good, but when I worked with a better contractor, he did the load calc, whose results matched mine, and the smaller unit that was all that was really required allowed me to get a "brand name' unit as opposed to the larger "builder grade" unit.



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By Jeff in Henderson, NV on 11/30/2004


The idea of up-sizing the ducting sounds like a good idea. Does this affect the balance of airflow to the rooms negatively or do you just up-size everything equally? I'll have to talk to the A/C sub when the time comes about this. Las Vegas isn't terribly humid so I'm thinking it's common to oversize a unit here because if it runs shorter periods of time from being too large a unit it will remove less humidity, but that is probably less noticeable here vs. Texas or some place very humid.

I'm going to have to find a HVAC sub who will run a Manual J and Manual D and show me the numbers. I found out the HVAC-Calc software I am using isn't 100% correct due to the fact it doesn't take into account duct sizes or run lengths, which obviously affects heat loss/gain; however, it is probably still considerably more accurate than "rule of thumb" sizing.

I wish I had the time to learn to just do all the trades myself :)

-Jeff


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By Michael in Cave Creek, AZ on 11/30/2004


I had a similar experience with my house. One-story 2,550 SF in Phoenix. Contractor estimates ranged from 6 to 8 tons. I used HVAC-Calc and got just shy of five tons using the exact sizes of windows and doors and making allowances for all the shaded overhangs. I concluded that I could get by with a single five-ton unit, which is not what the contractors wanted to sell me.

So, I investigated doing my own flex ductwork and buying a unit on the Internet. It would probably have cut my HVAC budget in half had I followed this approach, but I was in the middle of construction at the time and decided that I did not have time to learn the HVAC and tin bending trades while organizing the other trades and performing my own electrical.

I went with the contractor who offered the smallest size units and they are working fine. My furnace is also a two-stage and is sized for HVAC airflow rather than heating.


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By Jeff in Henderson, NV on 12/1/2004


So how big was the unit that they ended up installing? Was it a six-ton?

BTW, I am also considering doing my own electrical. How did that go? Did you hire somebody to hook up your main panel to the grid or how does that work? Any suggestions when doing your own electrical?

Thanks,

Jeff


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By Brad in Sahuarita, AZ on 1/8/2005


Jeff -

Definitely go with the Manual J calculation. We bought an existing home here in Arizona last May and found the system was oversized. The builder told me the standard method in the industry for sizing is one ton for every 450 square feet -- this is totally false. If you use this method for newer homes you will always oversize as newer homes are built tighter, have better insulation.

Our problem was half the home was 7-10 degrees warmer in the summer and the humidity was 45-50% at all times with AC running. Found the system short cycling and running only four minutes on and 6 minutes off, thus the coils weren't getting cold enough to cause the interior moisture to condensate and drain out.

Ended up spending $9,000 to replace the entire heating/cooling system on a home that was just built in late 1999. We went from a five-ton to a four-ton and within one day the humidity went from 45-50% to 32% and the entire home became much more even temp. Electric bill in summer went from $190/month to $104/month.


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By Andrew in Corpus Christi, TX on 1/9/2005


Has anyone used the services of EnergyWise Systems?

They claim to perform a computerized analysis of your plans along with recommendations to achieve a least cost solution to dramatically reduce energy consumption and improve environmental quality. Some interesting recommendations are U/V Lights for HVAC Air Handlers, and Temperature/Humidity/CO2 Sensors hooked up to a fresh air exchange/heat exchange device.

There are additional recommendations but I will let you read them for yourself. EnergyWise claims additionally to be on the verge of getting an EPA certification standard somewhat similar to Energy Star. So my question is has anyone availed themselves of this service, do you feel that you received good value for money spent, and is this service really any different from that which others are offering?


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By Kenneth in Lees Summit, MO on 1/10/2005


PM me with your email address and I will give you what I know. I don't want to post it on the public forum. Short answer, find a local knowledgeable contractor.
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By Heidi in New Orleans, LA on 12/28/2006


Hi Kenneth,

We meet again. This time I am interested in using EnergyWise Services to design my HVAC system. Did you use them? Do you have any insight as to the actual necessity of a service like this? Their service was recommended to me by the bio-based insulation company here in New Orleans. 

Heidi

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By Jeff in Provo, UT on 9/12/2007


It has been a while, but if you are interested in taking the EPA certification test, you can study and take the exam online with escoinst.com
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By michael in miami, FL on 10/8/2008


Need load calculation for victorian home in New Orleans. May need two units. Any recommendations on people in my area who do this?
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By Grant in Blacksburg, VA on 10/8/2008


I have a friend in Ruston, Louisiana who is a retired professor. His specialty is humidity control and energy efficiency in hot humid environments. He has completed several research projects for the US DOE over the years and really knows his stuff. He's a serious academic "egghead," but I can't help but like the man. 

I don't know if he'd be interested, but he'd be exceptionally thorough for the specific conditions of your house. He'd likely be considerably more accurate than a Manual J calculation. [He could also probably do a "blower door" test for you and help you ensure the house is adequately sealed and insulated.] If he's not interested, he'd know qualified experts in Louisiana who would be.

If you are interested in contacting him, let me know and I will send you his information privately.

Regards,

Grant


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By Christian in SLC, UT on 10/19/2011


I wasn't aware of there needing to be so much regulation for HVAC! I also didn't know that Georgia requires a blower door test for any new homes. Have you found any other requirements that are new for new homes? I want to make sure everything is done correctly!

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