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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That sounds like a nice way to design duct, for all kinds of reasons.

I'd add that this is another example of the hvac guy suffering blame for crappy envelope and coming up with creative, albeit capital and energy intensive solutions for shortcomings of others.

The buoyancy of air image I think implies air temperature delta due to thermal loss, I think that places emphasis where it does not belong.

Leaky shell/Stack losses are where I see blame belonging for this problem as air sealing houses (and ductwork) seems to cure heating and cooling imbalance between floors to surprising measure. So what is happening is cold air is always coming in low feeding stack loss, making downstairs always cold. Air higher has visited the furnace more, therefore is warmer, and just before leaking out upper stories leakage is predominantly out rather than in, that air has very little mixing from outside and thus is warmest. (reverse this image for summer)

Some people seem to be curing imbalance and humidity problems in cooling simply by having duct aerosealed. Consider the implications of THAT! Seal duct and basement is no longer an icebox and upper floor no longer an oven.

Another nice improvement is dramatic downsizing of equipment, which makes thermostat satisfaction take considerably longer. Longer cycles allow duct losses to be more balanced - long runs get warm and thus deliver heat rather than cold, and far rooms get conditioned.

Unfortunately, even with multi stage equipment, people size minimum output to maximum load for fear of fielding dozens of calls complaining "house isn't heating UP fast enought," or worse, "my equipment isn't shutting off" (lol!).

If people expect to "heat up" their houses, achieving balance is almost always going to be problemmatic, and expectation that properly sized equipment will be installed is absurd.


"The buoyancy of air image I think implies air temperature delta due to thermal loss, I think that places emphasis where it does not belong."

I think you need to reconsider this.  The buoyancy that I spoke of is not that of the air already in the house, but that of the conditioned air in the ducts.  It is relatively harder for the system fan to deliver cold air to the second floor because that air weighs more, and relatively easier in the winter because that air weighs less - so we have to allow for that floor-to-floor difference.  

Take the extreme case where I have NO delivery to the second floor - reagardless of the shell quality, the second floor won't cool in the time the thermostat needs to cool the first floor.  What I am talking about is just a way to recognize the Manual J differences in the heating and cooling needs of various levels of the house.

I don't like to use anecdotes, but I will use the case of my own house to demonstrate.  It is three years old, 2850 ft2 2 story on a full basement in the mid-Atlantic.  It has R-26 walls, R-50 ceilings, and reads 550 CFM50 on the blower door.  My high monthly bill over 3 years for an all electric house with an 18 SEER, 2 speed heat pump is $190 at 14¢ kWh.  I leave the thermostat at 69° all winter and 75° all summer and do not set back.  The house has a 46 HERS rating.  The shell losses are under control.

Every fall my second floor gets hot because in the time it takes the system to reach the set point on the first floor, the second floor has received too much warm air.  So I cut back a bit on the second floor trunk with just one handle, and the rooms on the second floor are within 1° of the rooms on the first floor.

Every spring my second floor gets hot because in the time it takes the system to reach the set point on the first floor, the second floor has not yet received enough cool air.  So I open up a bit on the second floor trunk with just one handle, and the rooms on the second floor are again within 1° of the rooms on the first floor.

You are right in saying that this technique will mask a bad shell, just like motorized dampers will.  But you are wrong in saying that this is the only case where it has value.

Just another tool

Ed Minch

Ed, I think you misunderstood the ideas I was attempting to convey. Also, I think you overestimate the weight of air, and underestimate the power of mechanical fans.

In any event, you do indicate your equipment is grossly oversized since it quickly overheats the areas close to the furnace without providing conditioning elsewhere.

I have encountered this often, and corrected it either with smaller equipment. Often modulating equipment, by its ability to run gently and continuously will keep duct work warm or cold, and deliver conditioning to far and near equally.

The house you describe is amazing. It probably needs 30,000 worst case heating and, if near NY, 1.5-2 tons cooling. I bet you have 4 tons, right? I don't know how you wouldn't have balance issues without modulating equipment. It does not work as anicdotal evidence as it is a outlier. I have never seen a house that size with even double that leakage.

Also, I believe I said I can think of lots of reasons your duct design has merit. Maybe retread my post and see if you misinterpreted.


I believe I understand, and I agree that this method would work for hiding shell problems.  The only reason I used my outlier house as an example was to demonstrate that it can happen even if the shell is not an issue.

I have a 2-1/2 - 4 ton 2 speed unit.  On a design day (14°) the 4 ton will run once in a while, other wise it is on the 2-1/2 ton mode.  If it is freezing out, the unit will run for 10-15 minutes at a shot.  

If you live in NY you may not air condition.  My brother is an architect in the Saugerties/Woodstock area and his clients only put A/C in about 1/2 the time.  In the mid-Atlantic we have exended periods in the high 90's with very high humidity.  

The best indicator of what I am saying is that in houses built in the 60/70's (here) that did not have A/C installed when new, it is hard to get second floors to A/C because the ducts are not designed to get the heavier cold air up to the second floor in the time it takes to satisfy the thermostat on the first floor. Yes there are some shell problems, but I believe I have demonstrated that the situation really exists in  my area.  And ranch houses do not have this problem.  Put one of your houses in my location and check you Manual J.

 I am frankly surprised that this is in any way controversial to you.

Ed Minch

They are heating houses almost your size in Mass with mini splits, one per floor. a bit better insulated but no ductwork at all. The strategy is to leave doors open during the day allowing heate to disperse. Overnight bedrooms will drop a few degrees from having doors closed.

I think the way we think about heating and cooling needs a complete overhaul. If we were to think about it like a flywheel, something with momentum that, kept moving, needs much less effort than the effort of breaking enertia, the comfort and energy results are amazing.

The idea of "satisfying the thermostat" is critically flawed. Design so the thermostat never satisfies. Get equipment that runs continuously, erase startup and shutdown losses, match load - provide only the heating or cooling lost rather than "heating up", and the whole house becomes comfortable. I'm seeing clients saving 30-70%, and they no longer spend summers and winters suffering, their whole houses are comfortable, all the time, and they experience amazing savings.

I am not surprised you have to jury rig your ductwork if equipments longest run on the coldest day is 15 minutes. If that's 15 minutes per hour, your load - the energy your house is losing - is 1 ton, right? 1 ton is your WORST CASE.

IMHO 2 floors should simply have 2 systems.

Bob, to me that's using a sledge to hammer a brad, or possibly to install a drywall screw. Unnecessary brute force, with very possibly unpleasant results. in modern construction how big must a home be for even the smallest of splits to not be grossly oversized if installing two?

Ed's duct solution, to my mind, is much more elegant, less expensive, less maintenance, and allows for expensively upgrading to my favorite method of zoning, communicating zone control.

Now, if we install equipment that can run continuously, delivering at the rate of loss, burner, fan, duct elegantly ramping up and down to match loss, efficiency on all parts of the delivery chain goes through the roof with surprising comfort results.

And guess what, in the last decade technology HAS brought us this ability. Modulating communicating equipment that tracks temperature, humidity, airflow and static pressures, and matches load running low and slow is a dramatic shift from a single stage blowtorch.

Now we need to get people understanding that the old brute force fixes, once the only option, are obsolete. Technology has changed the way we can solve problems, we need to understand this and reorient how we look at solving these problems.

Mini splits are the way of the future for high performance homes IMHO. Having one for each floor is the ultimate in comfort/cost ratio. It's much cheaper than a full commercial zoning system yet provides many of the same comfort benefits.

In an ideal world we would have fully modulating heating equipment. We would have A/C units that automatically adjust the CFM per ton based on indoor humidity conditions. With today's humidity sensing thermostats and variable speed blowers this shouldn't be hard to implement, but is rarely done. Add in variable speed compressors for the ultimate in comfort.

Unfortunately the typical consumer isn't willing to pay extra for latest and greatest.  Ask any HVAC contractor how much of each type he sells, I'll bet modulating and other advanced equipment gets installed less than 10% of the time in new construction. Most consumers buying replacement equipment go with "one up from the bottom". They normally don't want the cheapest, but will spend a little extra to move 1 up the line. Maybe get the 14SEER A/C instead of 13. Anything above 14SEER and the payback time becomes too long, especially in a high performance home where heat loss/gain is minimal to start with.

You clearly get it.  And the catch 22 of better efficiency and "Payback".  

And you get that in an era of heavily subsidized fossil fuel, with nearly complete lack of energy consumption (and thus savings) transparency, it becomes very challenging to convince people that the incremental cost will be more than made up from energy savings - not to mention comfort.  The total cost vs upfront cheap conflict.  

But the clear winner, upfront cheap, is losing ground.  After 4 years I've still sold nothing but heat pumps, mostly to people on natural gas.  Very expensive hybrids to people who thought they just wanted cheap AC.  People will invest, it just takes a significant amount of education and relationship building, an investment most cannot take the risk of making, yet.  

The more of us out there willing to take the time, the more the missed opportunity will become apparent.  It just need more of us to create groundswell.  I track peoples energy savings and believe eventually this will be a significant competitive advantage.  I want the biggest drop I can get, and be able to show that energy savings pays for a significant portion of the improvement.  Who doesn't want to make their home more comfortable and more valuable with money they are already committed to spending?  

Another angle; I always offer the best, that way if less is selected the ramifications of not having features that may later turn out to be critical will not rest on my shoulders.  If you kick yourself every morning from September to April because you didn't want to spend the extra 3.2 cents a day for heated seats, don't blame me.  I recommended it. 

At least I don't have to hear "why didn't you tell me how critical that feature was?" because I did.  I didn't presume "they can't afford that" and recommend what I thought I had them best chance of selling.  I look at total cost and attempt to educate.  

"You pay for it whether you own it or not" applies to more than just air sealing and insulation.Let's face it, over 15-20 years people are paying for the better equipment whether they buy it or not.    

Try to avoid payback discussion.  What do you mean you want an improvement that makes your house more valuable to also payback in 4 years?  That's 25% after tax rate or return.   Very few track, so payback is smoke.  People think about paying for this stuff up front, yet they don't do that with the energy bill.  This is mental disorientation that is our job to correct.  With 1% at the bank, anything faster than 100 years is competitive.  Reality check here folks, even Buffet doesn't achieve that.  

Because heated seats don't have payback on their own, you myopically deselected it?  What was not apparent was the success of the overall solution rested on that one feature.  That's what happens when people deselect modulating or inverter driven equipment, they have to live with frustration and disappointment.  The reason for better isn't payback, it's that all these other problems get solved.  The additional savings is simply icing on the cake. 


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