Site Survey: Area-Weighted Average R-Value

R-Value Weighted Average Worksheet

  Due to the recent plunge into Spring (or more like Summer!) weather, most Residential HVAC Companies in New England have seen a fair amount of early season sales calls for air-conditioning.  This quick changeover tends to spark some very interesting conversations around Load Calculations and Site Survey.  One in particular tends to come up every year: Attic Insulation.  Attic Insulation can be a tough R-Value to identify when out on a quick sales call, so it is important to take notes as to what you see.  This way the Area-Weighted Average R-Value can be calculated prior to entering such details into your software.  The first image is a blank worksheet to calculate this average R-Value, which is always more affected if you have a well insulated attic.  I will walk through (2) examples to examine a properly insulated attic and one with poorly installed insulation.

Example #1

  In my first example, I have a 920 Sq. ft. attic with 12" of blown-in fiberglass - a value of R-30. This value must be discounted for all of the other areas  in that attic without the 12" of insulation. For instance, an uninsulated 2' x 2' access hatch.  This wood has an R-2 value.  The 2"x 6" rafters that have 6" of insulation above them have an effective R-17 (6" of insulation = R-15;Wood = R-2).  As well as the (10) 12" x 12" supply and return registers/grilles, and 24" x 24" hall return I am about to cut-in and turn this ceiling into swiss cheese!  Remember, we insulate our ducts to R-8 outside the building envelope for code (this attic is outside).  When completing this worksheet, I need to figure the areas (in feet) of the devices being deducted from the entire ceiling in order accurately estimate the remaining ceiling area.  In the first example, with perfectly installed R-30 insulation, we reduced the average value of this ceiling to an R-25!  Doesn't sound tremendous, but this can affect your heat loss as well as your heat gain calculations.

Example #2

  The second example uses poorly installed insulation.  Sure, there was still 12" in some locations.  But, did you know that just 2% voids in insulation can reduce that R-30 to only R-15?  No offense to all the homeowners and "DIYers" out there, but this is why we hire licensed contractors - to get what you pay for!  Since the starting value is so low, when weighting out the average and adjusting for the access hatch, rafters, and registers, the value is only reduced to R-14.5.  When entering this into software, it will need to be R-15 anyhow.

  So, finding the R-Value weighted average does not need to be done for every ceiling/attic, only the ones that were well insulated!  If the insulation was not installed professionally, I would take an extra moment to probe around and discount the starting R-Value accordingly.  Just don't take it for granted, see it with your own eyes - this means a trip to the attic on all of those sales calls involving a load calculation!  Otherwise, you are doing the customer and your company a disservice!

TXV Hunting Season

     If you didn't call out sick today from Spring Fever, you may be working on some air-conditioners sooner than later!  It looks like it may actually reach 70F in New England and Service Managers will be looking to push the envelope when it comes to Maintenance.  I used to try and fit in some early season cleanings and always had trouble in the mornings.  You see, Thermostatic Expansion Valve (TXV) Hunting Season just opened and it is everyone's best guess if these units will work correctly come July.
     I found out some interesting information about R-410A TXV's a few days back, and as always I am willing to share.  When there was the big switch to 13 SEER and R-410A a few years ago, manufacturers actually adapted their TXV technology to work for the new pressure, rather than redesign their valves.  Danfoss claims, based on independent studies, a TXV will save 15-26% when replacing a fixed metering device.  When this changeover happened, hundreds of thousands of non-optimized valves were installed.  These systems work just swell under a load, but when we are completing maintenance under less than desirable conditions, the TXV's tend to "hunt" and never let the air-conditioner reach a steady state.  This can create some serious issues if liquid makes it back to the compressor, and I am sure we have all heard that wonderful sound on more than one occasion.  Anyhow, if the valve does close entirely under low load, the system operates like a fixed metering device is installed - which we know are very load dependent.  Emerson actually figured this out quite quickly, and in 2008 began to manufacture R-410A optimized valves called their C-Series.  This valve made the unit reach steady state under low loads (little to no hunting), and actually requires less refrigerant. In turn, this lets OEM's create smaller coils.  This valve actually made systems operate .5 SEER higher than ones without optimized TXV's.  The biggest influence is early season maintenance and installations since technicians can actually get the refrigerant charge correct.  When charging under low load, it is very simple for a technician to overcharge a system.  The other day I was on a job site with a valve like this, and it was only 64F outdoors.  At first, the TXV was hunting and we figured it was due to the low load, remember: TXV hunting season!  When I saw the valve, I immediately recommended that the technician adjust and tighten the sensing bulb.  Sure enough, the Superheat pegged at 8F, and now the subcooling was around 22F!  This was actually due to a short line set, he knew better than to charge under such low load - even though he could have with these new valves.
     There was a study completed two years ago by some physical engineers at Purdue that quantified a few points for undercharged units.  Apparently, SEER and COP (for heat pumps) are not decreased until the units reach 70% or less of their recommended charge (Kim, Braun, Purdue University, 2010).  This is the point when the TXV actually shuts down and acts like a fixed orifice.  These units that reach 25% undercharged see a decrease in SEER by 16%, and on average will use $100 per ton, per year more than a correctly charged unit.  But be careful here, don't overcharge a system and create problems come July.  Verify the TXV bulb is tight, to the point you cannot move it by hand.  Also, the bulb needs to be correctly installed and insulated, even if installed at the factory. Just because old TXV's used to hunt under low loads doesn't mean all of them will.  Take the time to correctly diagnose the TXV operation before you add or remove refrigerant! 

MA/RI Code: Interior Design Conditions

     Besides proper indoor air quality, comfort, and system sizing, there is one other important reason to size your heating and air conditioning equipment to the proper indoor design conditions: State Code!  This is always a tough conversation with the homeowner.  After all, the customer is always right, right?  When I start to talk about state codes, I like to explain to those less concerned that this is the worst case allowed by laws in our state.  Luckily for homeowners and quality contractors, these days that minimum is getting better.  The 2006 and 2009 International Energy Conservation Code sets the minimum indoor design temperatures in cooling and the maximum indoor design temperature in heating.  Notice the years?  This is not new information, it is just finally being enforced.  For the most part, as soon as something is enforced in this state, that is when contractors "embrace". 

2009 IECC Section 302.1: Interior Design Conditions
 -  The interior design temperatures used for heating and cooling load calculations shall be a maximum of 72° (22°C) for heating and a minimum of 75°F (24°C) for cooling.

ASHRAE Comfort Zone

      Why these temperatures you might ask?  Please refer to the ASHRAE Psychrometric Comfort Zone.  If you notice, there is no place in the Summer when sizing to 70F and any humidity level is comfortable.  If your system was to actually reach 70F in a home during the summer, your customer will feel cold and clammy.  You should always design to the ACCA Manual J cooling indoor design conditions of 75F and 50% Relative Humidity (RH).  This will put you in the middle of that comfort zone with some room for "drift" on those above design days in mid August.  If the unit cannot keep up, it will operate constantly, and it will still be comfortable in the conditioned space.  Humans are much more sensitive to the relative humidity than the sensible temperature on the thermostat.  ASHRAE was nice enough to publish some charts regarding their reasoning for 50%RH with respect to indoor air quality as well.  Lo and behold the least amount of viruses, bacteria, mold, etc. grow between 30%RH and 50%RH! This is why we add humidifiers to forced-air furnaces in the Winter.
     So, customer education to establish expectations is a must in today's equipment sales process.  If you install a system to code, the very minimum required by state laws, it will not run the same as the old unit.  We found it is much more efficient to properly size systems for homes, not homeowner lifestyles.  Sure, you can give the customer what they want - but I am sure they would rather be comfortable, with a unit installed to code!