Can you trust your Load Calc Software?

Attention Wrightsoft Users!
  Have you ever completed a load calculation and it just seems like it was short of what you had expected?  Particularly when you are first starting out with a tool, contractors can make mistakes. After all, we tend to learn "the hard way", right?  In an effort to make software work, the companies tend to start with some default values that require adjustment.  For instance, most have the duct system in the conditioned space calculating zero gains or losses.  If the ducts were in the attic, this could be as much as a one ton load that is missed! Internal gains tend to start at zero, and the foreground for windows is something between crushed rock and green grass.  Most contractors find these values, over time and as they gain experience with the program.  One still overlooked setting in Wrightsoft, a feature that no other ACCA approved software employs, is the Rate Swing Multiplier (RSM).  This can cause you to lose as much as 11% of your sensible gains in an otherwise perfect load calculation, maybe more!

  This adjustment, the Rate Swing Multiplier (RSM), is used as a makeshift way of selecting cooling equipment by using AHRI data.  Those familiar with ACCA Manual S, Equipment Selection in the residential design procedure, and International/Local codes, know this makeshift procedure is not proper design and will likely contribute to over sized equipment.  What many users do not realize is this RSM de-rates your sensible gains on your Summary Report!  If you then use what you thought was the correct numbers, you may install a system that is too small, or set your fan speed too low due to the decreased sensible gains.

If you do not change the default Sensible Heat Ratio (SHR) from the .70 in your equipment selection, then the RSM remains at .89.  If you use suggested values from local utilities (SHR .85), carefully calculated to reduce selection of over sized equipment, this RSM raises to .93.  If you know how to calculate the SHR of the home, a fairly simple process, the RSM will fall anywhere between those values already discussed.  The example images do not reflect the same load calculation and are for RSM & SHR reference only.

  Great news!  You can just turn this sabotage of a design feature 'OFF', as you should!  If I was a building inspector, I would fail any report that showed up with a RSM other than 1.0.  To do so, go to 'Options' in top menu, then uncheck 'Adjust Load for AHRI Standard Rating'.  Easy as that, and now you will not be short changing yourself on those accurate, aggressive load calculations.  Of course, this alone will probably not change your equipment size, but combined with another small misstep and it very well might!

Do you make this mistake on Maintenance?

     Cleaning the condenser coil.  This has always been the number one task that Service Technicians avoid at all costs when maintaining residential equipment.  Maybe it is the nature of the fast paced business when the weather gets hot?  Certainly could not be the lack of water to rinse the coil as there is almost always is a water spigot on either side of the home.  I cannot figure out why,  and this may be one of the largest aspects of seasonal maintenance that impacts the efficiency of the unit.  Sure, the system will still operate if not cleaned, for a while.  Lets explore just how the capacity and EER are impacted...
A/C Expanded Cooling Data
     For an air-conditioner operating at design load, lets estimate 85F for simplicity, the condenser can deliver close to the nominal capacity.  Remember, nominal capacity is in tons, from 1.5 to 5 tons, in half-ton increments for residential equipment.  There are 12k btu/hr per ton.  In my example equipment, the unit is 36K btu/hr or a nominal 3 tons.  At design operating conditions, my unit will deliver 35,400 Btu/hr which is very close to the nominal rating.  When a condenser becomes dirty or fouled, the unit cannot easily reject heat from the refrigerant.  This in turn drives up your liquid line pressure and the system artificially operates at what can be determined a higher outdoor ambient than actual.  For a R-410A system with a dirty condenser, the liquid line pressure could be as high as 60# higher than normal.  This equates to approximately 20F.  So, instead of rejecting heat and operating at a capacity for 85F, the condenser think it is more like 105F.  This reduces the capacity of the system by 3,700 btu/hr.  Guess, what?  The condenser never gets cleaner without someone doing so!  Therefor, the capacity losses only get greater throughout the season.  To put this in perspective regarding efficiency, the system was operating at an EER of 13.8.  With the dirty coil, the system is operating at 10.1 EER!  When I said you were losing 3,700 btu/hr you probably thought, So What!  But at 105F, the condenser has to work much harder to deliver less capacity!

Heat Pump Heating Data
     For a heat pump, the penalty on your electric bill might be more severe - in a cold climate like New England.  As the condenser coil gets dirty, the heat pump operating in heating mode will begin to decrease in capacity just like an A/C.  In my example data for a comparable heat pump in capacity to the A/C, the unit will deliver close to the nominal rating at 47F (35,400 Btu/hr).  If the coil is dirty, it is not out of the realm of possibility to have a system operating 17F less than the actual outdoor ambient.  In this example, that drops our capacity down to 27,500 Btu/hr, a loss of 7,900 Btu/hr.  More important, the decrease in EER from 13.6 down to 11.3.  What this does not take into account is the increase in defrost cycles and use of supplemental heat since the heat pump will not be able to keep up in just the mildest of weather!
     One phenomenon I have yet to fully understand is the Heat Pump Mini-split.  Don't get me wrong, I have installed hundreds of these systems over the past 10 years.  But, the heat pump tends to keep itself clean in New England!  I think it has to do with the micro channel coils and long heating seasons.  They almost wash themselves off every Winter, and stay clean enough through the Summer.  Kind of kills the chance of making money for maintenance on these.  Except for the A/C only versions, if you do not clean the coil eventually it will shut itself down!

4 Things a Quality Contractor Would Not Say

1.  If you sign today, I will take $X thousand off!
  This is a sales tactic often taken by a used car salesman.  A Quality HVAC Contractor will price their installation with confidence, after finding a solution to fit the needs of the customer, and correctly sizing such equipment.  I have no problem with "ballpark" numbers over the phone, as this can help sift through customers that are not interested in quality, so contractors do not waste time on what tends to be free sales calls.  Do not let a salesman pressure you into signing a document before he leaves, unless you need to replace a system that is no longer operable.  Otherwise, this should raise some suspicion as to the urgency.

2.  How many square feet is your house?
  This is only one factor when sizing a heating or cooling system.  If this question is not preceded or followed up with questions like: How is the insulation?  What type of windows are installed?  As well as a quick survey of the home to gather the remainder of the details, then the system could not possibly be sized right.  "I've been installing these systems for 20 years, and never had a call-back!" is not a suitable replacement for the International Residential Code and State building code requirements of using ACCA Manual J, version 8.  In order to complete this calculation, software is required.  Although, most heat loss calculations could still be completed long hand.  Any sort of short forms or calculations not having the ACCA Manual J Approved logo are not complete and do not meet the minimum required by law.

3.  The payment is only $X a month!
  This is another sales tactic that can often be used to mask the total cost of the job.  Don't get me wrong, being able to offer financing is important for a Quality HVAC Contractor as they tend to install and quote higher efficient equipment.  This equipment undoubtedly costs more compared to their less efficient alternatives.  A homeowner should be concerned with the total cost of the job, as well as the monthly payment.  This is how cell phone companies hook customers into long term contracts, by offering low cost or free phones at the highest possible monthly payment.  This can be hard not to see past as a homeowner since the energy savings of the new system, combined with the monthly payment, will most likely be lower than the current monthly utility bill.   Remember, a lower total cost will equate to a lower monthly payment.  Get more than a single quote, particularly if you do not have an established relationship with the Contractor.  Also, remember that the lowest price is not always the best price!

4.  I will beat the lowest price!
  A Quality Contractor will be less concerned with the prices you have or will receive from other companies, since they know what it takes to stay in business.  This means a fair price for the high quality of work and equipment, combined with a satisfied customer.  If a company always "beats the lowest price", I doubt they can employ the best installers and technicians, and I would worry if they will still be in business when you need them.  Even the latest and greatest system breaks down now and then, requires service, or even needs to be replaced in the next 10 to 15 years.
  On the other extreme, a high price can be perceived as high quality.   This could be true, if the Contractor can prove to you the reason for such a cost.  Otherwise, it could be price gouging.  Unless you have something to compare, how could one ever know?