HVAC Sizing

Welcome to AcronymLand!

Are the "dedicated truss chases" actually within the "conditioned spaces"? Is Dallas design dry bulb really only 100°F? Etc.?

Yes, all hvac equipment and ducts are in the conditioned space; ducts are all metal and insulated w/ R6. Design temps are correct.

That's weird; more usually the equipment and ductwork are not in the conditioned space. I found a source that agrees with your design temperatures, so that is no longer an issue.

This is a high level green building house, where all equipment is in the conditioned space. We are expecting to certify this house under 4 major green building programs at the Gold level. I've been doing this for a long time, and surprisingly, HVAC contractors are the hardest folks to bring on board with new technology...

Well, I hope the condenser doesn't exhaust into the conditioned space....

I don't know what you are talking about, and I don't think you do, either.

The terms "high level green" and "Gold level" sound like marketing hype.

HVAC folks might get on board better if the "new technology" (whatever it is) is actually explained. Please proceed.

Tornado,

Well, this discussion is getting off track, but, I did not try to be arrogant in any way with my comment about high level green building or gold level, I was just making the point that today in most of the USA, green building is becoming the way to build, to the point that all this new ways are in the 2009 IRC, 2009 IECC and its becoming more stringent in the future codes. Green building is not a marketing hype but the only way to build homes in the near future if not now, as today many states and cities are regulating them.

You can find all the information you want on high performing buildings on the web, here are some you can start with:

1. http://www.buildingscience.com/documents/information-sheets/information-sheet-ducts-in-conditioned-space

2. http://resourcecenter.pnl.gov/cocoon/morf/ResourceCenter/article/1404

3. http://www.easytitle24.com/do.php?page=faq/detail&fid=1106

4. http://www.energy.wsu.edu/documents/AHT_ACEEE%20Ducts%20inside%5B1%5D.pdf

The compressor (condenser) is of course exterior (unconditioned) I believe he is referring to the air handler, which of course should be in conditioned space, along with all ductwork.

If all the information is available at the sites you list why are you asking here?

If you have the information try reading it and make your own conclusions.

As for not wanting to influence any discussion, I think several people will be able to discuss the subject fully for a price.

Well I can tell you standard rule of thumb, 1 ton for every 450 squre feet to be cooled.

There is no such a thing as a rule of thumb in AC sizing any more. By code, Manuals J,S,D & T need to be performed. The issue is whether you add more fluff than the 20% built-in the MJ, like must contractors use to upsize the equipment.

Most high performing houses are using 1 ton per 1,000 to 1,500 sq ft. A lot of the Passivhaus guidelines use even less. Read some of the links I gave before, it explains the process.

Who did the load calcs./ What was the sensible and latent load for each story?

You should use the actual sensible and latent loads to select the equipment,comparing to the detailed cooling capacities of the equipment ,rather then assuming a SHR,check the actual capacities of the equipment.

In my question you can tell the sensible and latent loads for each story. I also give you what the MJ calls for tonnage; it's not a guess. The SHR is not assumed, it's there written in B&W. No rule of thumbs, no guessing, no equipment outside of the conditioned space, 1ACH50 house, 2009+ Code house.

I just happen to have a difference of opinion on how to use the information from the manual J, and the size of the equipment the HVAC contractor wants to install.

Thee actual Man J loads have a sensible and latent requirement for the home.

All I saw was at total with a SHR of 70%,which would be assumed,if you used an ANSI approved method ,you could answer the question,unless I'm missing something in this thread.

PS , your tone is not acceptable for someone seeking help/advice,for what it's worth! I'm new to this site but if that's the norm,I'm out of here.

I received several differing size quotes for a replacement furnace so I did my own heat study to confirm.

See HVAC-Calc at

http://www.hvaccomputer.com/index.asp

Norm

Thanks Norm, I have the same program and my cooling loads are very close, but my heating loads are about 60%-70% of theirs. However, the minimum 96%AFUE furnace I can get is the same for both figures, which is a 37,000 Btuh furnace.... It's a shame that the Heating equipment has not caught up with new technology.

I really don't quite understand this statement. 96% is pretty good, you can get maybe a percent or two more,, but still that's pretty good technology.

Anyway, that being said the rest of the sizing seems about right. I have 3400 sqft, not so hot but much colder and use geothermal heat pump,, that provides 42000Btu/hr heating and 52000btu/hr cooling.,, works just fine

Martin

HVAC-Cac is very old software ,and not applicable to modern Green Homes, not ANSI approved,but is okay,not great, for older homes.

You are correct. It really doesn't deal with today's green homes. I've already made the comments to Don and he said he was working on an upgrade. However, it does help in getting a general idea. The HVAC contractor did his own MJ with Right-Suite (ACCA Approved)

I don't know where you live, but in most of the HVAC contractors around the country grossly upsize equipment, in part because houses are so leaky and part because of knowledge.

See here: http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/38069/Oversized-AC-Screwed-up-Manual-J-ENERGY-STAR-HVAC-Tirade

If they actually did it, it's Wright- Suite, I'm in Florida with thousands of load calcs and installs since 1975.

Your cooling loads are both between 1.5 and 2 ton, I do not see where you get the 2.2 and 2.3 ton cooling.

From your summary, I would install a 45,000 btu/h furnace, 2 stage (modulating gas prefered), variable speed fan for the 1st floor matched with a 16 SEER 2 ton cooling system, preferably variable compressor.

The second floor would possibly be the same as I am not sure of a manuacture that makes a smaller gas fired forced air furnace.

Quote: I am not sure of a manuacture that makes a smaller gas fired forced air furnace.

My furnace is a 40,000 btu by Carrier.

Carrier Infinity 96 is a two stage 96.6% variable speed blower.

Norm

Thanks for your input. The 2.2 and 2.3 @ 70SHR is given by the MJ software. We are installing 16 SEERs and 96 AFUE furnaces, variable speed fans for both floors. There are 96 furnaces down to 37,000 Btuh in the market. So far we are all in agreement. Now what we disagree is with their recommendation for the size of units they want to install. They want too much, as usually done in the market, and we want the size called for by the analysis with no up-sizing.

In responding to the OP; it sounds like you have an opinion that you are very strong in. You're paying the bill, do whatever you want. I have a quote on my whiteboard here in my office and it says, "learn of the skillful; he that teaches himself only, has a fool for a teacher." Benjamin Franklin. You come to CR4 for advice/extra information, then you argue with almost everyone who seeks to provide input.

Green building and all that other stuff is a bunch of marketing hype! It is simply political correctness and sensitivity to the tree huggers run amuck. Finding ways to be more efficient in the stewardship of our natural resources has always been around; calling it "green" nowadays is just fashionable in the environmentalist wachos crowd. An example of this is the high efficiency furnaces/air conditioning, 2- stage compressors/fans, etc. that were developed by people who wanted to make better use of resources and cut down operating costs long before "green this and green that" ever came along. Calling a short 4 legged aninmal with a curly tail and that wallows around in the mud a cow doesn't make it anything different that what it is, a pig.

I'm glad that you want to be as efficient as possible and get the proper equipment for what you are trying to accomplish, but you came here to CR4 for input, so be teachable.

"Genuine ignorance is... profitable because it is likely to be accompanied by humility, curiosity, and open mindedness; whereas ability to repeat catch-phrases, cant terms, familiar propositions, gives the conceit of learning and coats the mind with varnish waterproof to new ideas" ~John Dewey

I don't waste my time arguing about green building, the fact is whether anyone likes it or not, all building codes are changing to apply all those methods, you may want to learn that as a Bldg Mgr. In few years, there will be no more "green building" but just good building practices, and yes, many people have been doing so without calling it green, and yes, fortunately several groups like Energy Star, USGBC and NAHB are using "green" education and marketing to make us more energy efficient, less wasteful and, hopefully someday, energy independent. (Not a bad thing for our country, isn't?)

I'm new to this website and came in my search for HVAC and/or Mech. Engineers that TRULLY understand the proper way to design HVAC systems. My bad to think that in a Mechanical Engineer blog I would find that. As a Facilities Mgr, you may want to make the time to read the links I gave before, by chance may learn something. By the way, I don't have all answers, but I've been doing this for a long time AND I do teach this stuff, including Architects, Engineers and HVAC folks.

I'm siding with the HVAC contractor.

Rule of thumb, 400 sq feet per ton of cooling, match heat rise the same, duct as required for proper flows and balance.

You didn't read the entire thread. The structure is more of a passive design.

Agreed, I jumped the gun. But still stand by the "rule of thumb". If after that you can reduce fenestration losses or gains the adjustment can be made by very simple calculations.

As I am sure you have been doing (based on your posted location) you too have been meeting these requirements for some time here in Cali.

So Work with the Architect, develop the Energy envelope then size accordingly. most of the calculation will be derived from that initial engineering.

Have you read California, Title 24? There is a link up above... you can't do "rule of thumb" in your state anymore.

Yes and have built by ASHRAE title 24 since 2007.

And the rule of thumb still gets you to a starting point and engineer from there.

Square feet, people loads latent heat from energy consuming equipment have not changed that much in the last 30 years. The only relative changes are the envelope that contains that space. Once you confirm those needs you will find that you are close to the rule of thumb.

BTW never design to the minimum required, be sure to add the proverbial 20% somebody might open the door!!!!!

You're wasting your time on this guy, he already knows everything. He's just seeking validation. His design parameters are to 100F with in interior of 75F. Dallas has a record of 112F and Summer humidity that's always 80+. This design isn't about sizing for everyday of the week. It isn't being sized to account for 40 or 50 people over for a party or wedding. The idea is to provide year round comfort (most of the time) while using the least amount of energy as possible. He's arguing with his HVAC contractor over an amp or 2 draw on a slightly larger piece of equipment. I have no idea who the contractor is but I'm sure he's trying to keep the place cool when it's really under a load. Or has been offline and is trying to cool down the entire space when its 100F or more inside. The current design will run 24 hours a day more than once--count on it.

YEP!!!!

But to opine when there is little knowledge of the subject, is regretful.

The fact that you keep seeking validation, and have thrown out many sites to go and validate your self at leads me to one conclusion, you are not sure, unclear and have no faith in your own calculations.

For that I must differ you to two previous posts from some of the wise ones that visit this site to help people. Post 9 and 34 please go back and read them they are enlightening.

you have obviously decided to be closed minded to opinions based on experience and knowledge from those that have built many building to the standards of which you speak.

I am sorry for you.

Good Day!!! I said Good DAY!!!!

Not, I was actually hoping to have a pier-to-pier conversation on advanced residential mechanical systems in this website and blog. Apparently, most comments are coming from good folks with good intentions but with outdated and empirical information. It's my mistake to assume such and I'm done here.

Thank you.

I just know that the first thing the gentleman that put on a 3 day seminar on Wright Suite said was that if you use the ACCA design temps you will be getting a lot of calls from angry, hot customers. He said to use your experience and make a good business decision. Do all the name calling you want, but those comments from a teacher who was presented as an expert make me wonder why we are doing all those calculation, just to throw them out and make a good business decision. I also understood that manual J allows for misc additions to the calculation for contingencies. -- JHF

In my opinion,having done thousands of new homes ,in Florida,the guy from Wright J is totally off base. Manual J works correctly if used correctly,however equipment selection must be made using Manual S,and the ducts designed using Manual D. Without all three you might as well just guess!!

Interesting opinions and comments on this one.

"Pier-to-pier", what, we're going to stand on wharves and shout at each other?

Comments are coming from people with experience,, no house is ever built exactly as specified, and the blasted weather does not always do as predicted. When there are extremes, and there will be, when your builder or his crew didn't finish something the way it should be,,, that's when that excess capacity comes into play.

A bit too big is way better than marginal.

In general HVAC units should not be oversized. Although I twitted the OP over acronyms, I agree on this, and thought it was fairly well known. Yes, once in a while the 99.4% or 99% or 98% design conditions may be exceeded; but customers should be educated to expect this in return for greater efficiency most of the time, and less original cost.

I don't much favor mandating this by so-called "green" codes, however.

10 years ago I would have wholeheartedly agreed with you. In a world of variable speed scroll compressors, continuously variable circulation fans and powerful microprocessor controls things are different. A system can easily be larger than the 98% condition and still operate with very good efficiency at capacities less than 100%. As for cost, I would submit that it is negligible, when compared to the amount of money being spent on making the structure "green".

Martin

Agreed. It is interesting to watch shifting codes vs. technologies over the years. It used to be that HVACR designers used a few empirical rules of thumb such as 400 ft²/ton, and then threw in a 20% safety factor for contingencies. But if I recall correctly, there was a California energy provision (mid 1980's?) that limited overdesign factors to 10%.

Then, as better part-load efficiencies came along, by the methods you mention, this could negate the 10% limitation. I'm not in HVAC, and thus may be out of date. The constant "churning" of code provisions is certainly a nuisance, along with "greenwashing" hype. (Although there is some good stuff in there.)

You are all very right.

Codes in California efficiency guides have changed, however it is the customer that will sell you short in their explanation of use that usually leads to finger pointing. Case in point, Server room on the fifth floor, small room no windows no real load issues other than equipment again "rule of thumb" it is generally about a ton a rack in server rooms after calculating KW's. The customer told me that the 3 racks in the room were all he was ever going to put in there so 3 ton system 68 degrees easy and in. My gut told me go 5 tons (and I priced it for him but the customer said too much) for growth, but was assured no growth and he had a limited budget.

Two months later......the call.....its hot in here! Arrive at the location the customer managed to get an additional 3 1/2 racks in the room he's running 85 degrees. With the 5 ton system he probably would have been ok but on the edge.

Point is that most of our customers do not look to the Wedding day as Fredski mentioned when you have 25-30 people in your home and thats when the contractor that throws out the 10 - 20 percent is the hero. There are many ways today with technology to reduce operating costs with out sacrificing personal comfort.

Moral.....I guess is Don't be cheap up front, you will blame the contractor later!!!!

If you add enough tonnage for a wedding party,in a humid climate like Florida,there will be likely be humidity issues,and you won't be the hero. Some oversizing could be negated by the use of two stage systems,but best to size it correctly.

I agree with Fredski. The contractor is opting to be safe rather than sorry. He is aware that if anything goes wrong - only he stands responsible.