I'm looking for some insight into why HVAC contractors don't use the ACCA process for sizing and selecting HVAC and Distribution systems. I know part of it is the generational "I've done it this way for twenty years, and I'm not changing now" attitude, and some folks are apprehensive to anything related to a computer. But are there other factors that are at play that keep contractors from using the ACCA process? I'm wondering if it's something as basic as not understanding the value of properly sizing a unit and not undestanding how to explain that value to the customer. Many guys in my area think that running a Manual J takes far longer than it actually does. Any other insights would be appreciated.

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I use a manual J and N load program and manual D based sizing for every single new installation and replacement project I perform.
Not all contractors are interested in short cuts and keeping their heads buried in t the sand as it relates to innovation and new technologies. A good load program can provide me with an accurate load calc in less than an hour for multi family homes and less than a half hour for smaller dwellings. Commercial load calcs and duct design are also easily taken care of with a good load program and a solid understanding of duct design.
I have little regard for any contractor who still uses rule of thumb "calculations" to deliver jobs to customers who expect us to know our business.

If you think load calcs are all BS how should HVAC contractors determine the equipment size? There's the traditional way of sizing per sq ft but thats even worse for accuracy. If the consumer has copies of the utility bills they could be onto something, especially if their house has smartmeter technology and  hour by hour usage reports can be obtained. If equipment cycles often on design days it's probably due for downsizing for COMFORT reasons. We have a 88,000 BTU furnace in out 1600sq ft house, when it powers up you get a short blast of hot air followed by 20 of off time. Can't justify replacing working equipment, but when it dies it will be replaced with a furnace 1/2 that size.

lol...love it!

LOL yeah. I just have to laugh when I read all this babble. Load calculations are not only essential to occupant comfort and energy management, they are required by a number of states. To suggest that contractors are performing load calcs for "theater" is laughable. If I want theater, I'll pull out my Martin and sing the customer a song.
Air changes are given in estimated fashion on ANY load calculation program. For example, if I have an old house with "loose" construction or a new home with "tight" construction, I am given a set of values to consider. Duct loss values are also given in that format. It is up to us to use the values we think fit each scenario. The load "changes" you are referring to are covered by load averaging, which is another way of saying "We want to cover both latent and sensible heating loads with our best possible guess at what size equipment will do that. If a system short cycles or runs continuously, it most certainly DOES affect energy management and conservation. To suggest that performing a load calculation is a "lie"is of no consequence to me, nor am I offended by the suggestion. I think it's a reckless statement to make, but because it is required by law, by my licensing authority, I do it. It's also important to note that many of us have had no real comfort issue callbacks using manuals, j, d and n. I didn't create the "bullshit", Phil. I'm simply protecting my contractors license by doing what the state requires and what you insist is BS. Cheers!

Hi Al,

Actually that's no longer correct, more and more programs allow for a specific CFM50 leakage number.  Some programs will even allow for actual duct leakage (to indoors and to outdoors).  

I think you are taking Phil's comment too literally.  What we need is a load range that we match to equipment range rather than a fixed point.  If you look at his software, they are starting down that path and I suspect future versions will offer charting that shows seasonal matching of equipment and sliders for air leakage targets as well. 

Instead of using software to come up with a pat answer, software will allow us to say "what if".  Often, when you start asking "what if" you stumble upon amazing solutions.  

We all need to start thinking "what if" more, rather than accepting the first answer we are given as if it's written in stone.  

Yes, I know they do. My load program gives specific numbers as well as averaging for types of construction. I'm all for software giving us better and more accurate load considerations. Still, I have to note that I'm not in the business of second guessing those responsible for what we have presently or what will be the standard in the future. I'm simply following the rule of law mandated by the state where I do business. I'm acutely aware of the contradictions that exist in my business, but I have absolutely no problem furnishing a load calculation to an inspector who chucks it aside.

In new construction, in this day and age, not providing a full set of plans, design envelope, energy and HVAC analysis, third party inspections and testing may constitute fraud on the part of contractors and subcontractors, IMHO. Today, there is no reason for any homeowner to buy a new house with a half-a$$ envelope and HVAC system.

When it comes to existing construction, before anyone suggests replacing to a $15-20K+ HVAC system, they should recommend a blower-door, envelope and energy test. We all know the least expensive options to improve energy consumption is to seal the envelope. Now, would an HVAC contractor cut their own business to do the right thing? That’s the $6MM question.

An example to add to Phil's comment -

Even WITH a complete audit including blower door our load came in at 80mbtu for a 3500 sf house on a windy hill in Upstate NY.  We installed a 60mbtu Mod and held our breaths.  

That thing doesn't go above 45mbtu output at 2f outdoors.  

Homeowner sent pictures of his thermostat.  I was amazed.  In fact, I'm continuously amazed how little horsepower is required to maintain.    

But, if he wanted to recover within some arbitrarily defined acceptable time frame we might need to install twice the furnace (which in turn would require duct replacement).   So homeowner abandoned aggressive setback and is saving over 30% on total energy costs. 

So, do you want throttle control or comfort and efficiency?  I don't think you can have both.  

tedkidd, have you EVER seen a fossil fuel system not able to keep up during design cold weather? It simply doesn't happen in the south, maybe up north it's more of an issue? Unless a building has serious leakage/insulation issues even a small furnace should be able to keep up in all but the coldest climates. Yet very few 45k furnaces are actually sold/installed even in mild climates.

45k to keep a 3500sqft house warm when it's 30 below freezing is evidence of that, yet day in and day out HVAC contractors are installing 90k furnaces in houses 1/2 that size with design temps in the 20's. I was concerned about cutting my HVAC system in HALF when it comes time to replace, but after reading your post, I no longer have concerns about it keeping up.

Using EnergyPro at work, I generally use its load calcs as a substitute for Manual J.

As for Manual D, my problem with it is that the distribution of heating capacity to each room is generally different that the distribution of the cooling capacity. If you have a constant volume system and if, in a hypothetical two room case, Room 1 has 75% of the peak heating load but only 25% of the peak cooling load, the procedure leaves you giving 50% of the CFM to each of the rooms, thus guaranteeing that one room will be underheated in the winter and the other undercooled in the summer.

I also HATE the interface for ACCA's spreadsheet, so much so that I ended up using their charts to estimate the logarithmic equation for pressure drop vs velocity and creating a spreadsheet of my own where I can set a duct diameter for a particular segment and have the pressure drop for that segment, and just list each discrete segment and add the pressure drops, arriving at the terminal total pressure drop from the supply plenum to each register and from each return grille to the return plenum. As long as the largest pressure drop on each side is less than the available, I figure I'm good.

Still though, I do see the sense of using Manual D, though in our office I think it will need a different order of work. My mechanical engineers are used to using a simple boilerplate chart of diameter as a function of CFM. I'm going to have to get them used to just sticking out the schematic and then letting me tell them what diameters to write in.

The funny part is this, and I want to ask some of you this. What do you do when you have a room (usually a powder or laundry or small bathroom) which has little to no exposed surfaces and which therefore has essentially zero peak heating load and a cooling load based on fractional occupancy, and which therefore calculates out to demanding 10 or 5 or (yes, I've seen it) 3 CFM of supply???? Are the air balancers going to just have a big belly laugh out of this? Or suggest using a transfer grille instead?

Differing loads are usually associated with different floors - it is the height difference and the buoyancy of the air that makes it easier to heat the second floor and cool the first floor of a 2 story house. There are always rooms over garages and many windows in a sunroom besides this, but this is basically what we have to deal with in the mid-Atlantic.  It turns out we need about 60% of the heat to the first floor and 60% of the A/C to the second floor.

We have trouble selling this idea because it costs a few hundred dollars, but it works every time: instead of one trunk across the basement with all branch ducts coming off of it, we recommend 2 trunks side by side, one feeding the first floor and one feeding the second floor.  Each trunk is slightly oversized for its half of the load, and back at the air handler there are large volume dampers on each trunk.  It is a very easy adjustment in the spring and fall to throw more air to the floor that needs it without having to install motorized dampers.

And I would not install a duct if you needed only a couple of CFM

Ed Minch


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