Duct Gains: Why do we hate R-8?

  Ever since the adoption of the 2009 International Residential Energy Conservation Code (IECC), duct sealing efforts have been tested using the total leakage or leakage to outdoors methods.  It really is amazing how much duct gains effect the operation of your equipment and the comfort of your customer.  The missing part of "duct testing" appears to be how well the ducts are insulated.  Sure, you are required to use R-8 insulation on supply ductwork when located outside the building envelope, but how do we know it was installed correctly?  We're not required to test like the duct leakage codes, so is a check in the box sufficient?  How about we use some simple math to figure the gains, or lost capacities in cooling, and hold everyone to a standard - check out the equation below.

Sensible Btu/hr = CFM x Delta T x 1.08

   So, based on this math, one can take a few simple temperature measurements and calculate the lost sensible capacities due to duct leakage and insulation.   You should measure the temp of the air entering the return grille and at the unit to figure the return gain in sensible temperature.  I would also recommend averaging the temp readings at the furthest supply register, an average supply register, and a close one to figure the delta T from the air exiting the coil or air handler.  Do the same for an extensive return system.

  When you start measuring, it is very scary what you start to find.  As you can see, like the industry accepted national average, this system is losing 30% of the 3-ton capacity through it's well sealed, but poorly insulated duct system.  Unfortunately, the method is not perfect since a hotter day in the attic could cause greater gains.  But, that would be the time to sell duct sealing and insulation to the homeowner!  Any of you energy auditors out there could probably comment on how much attic ducts effect home infiltration as well.  So, why do we hate R-8 insulation?  Is it really just the hassle of wrapping the equivalent of a blanket around the ducts?  I don't recommend retrofitting a distribution system in an attic in the middle of July, but maybe it makes sense to offer to come back in the Fall?  Please, don't just leave it be!  Doing so makes it hard for this tree hugger to sleep at night!

Site Survey: Windows & SHGC

  When completing your site survey for Manual J Load Calculations, significant time can (or should) be spent identifying window values.  Did you know that most windows these days come with an NFRC rating, a tag located in the window jam that you can reference on NFRC.org? This identification number can tell you important details like window coatings, u-value, light transmittance, air leakage, and solar heat gain coefficients (SHGC).  Not identifying the correct values can sway your load calculation thousands of Btu's per hour in both directions, in heating and cooling.  Of course, the SHGC impacts the cooling load more heavily. 
  There are a few things that can influence the solar heat gain through a window or glass door.  You can manage these gains by installing awnings and/or exterior shades, but the easiest way is to use windows with low SHGC values.  Rated on a scale of zero to one, the lower the value than the less btu/hr gains from the suns rays make it into your home.
  Using a leading load calculation software program, I was able to create a few scenarios to display the importance of spending the extra few moments at the customer's home during the sales process and site survey.  When adding a 4'x6' bay style, facing south, vinyl frame, double-pane, and clear window, the cooling load was increased by a total btu/hr of 1,045.  If spending the time to include the shading default of medium color blinds at 45 degrees, and an outdoor insect screen (50%), then the load of the window can be minimized to only 790 btu/hr.
  If the contractor spent the extra few moments to identify the NFRC rating of the window, they may have been able to establish there is a low-e coating on this particular window.  When including only the coating, not the blinds and insect screen, the cooling load of the window was minimized to just 688 bu/hr.  You can see how missing this feature, multiplied by all of the windows in the home can contribute to improperly over-sizing equipment and IAQ issues like high humidity, etc.  Also, this can change the system design process when selecting equipment and designing ductwork in the ACCA Manuals S and D.  Or, maybe the window faces another direction, and this creates a spike in the "adequate exposure diversity" that could otherwise require zoning.
  On the bright side, no pun intended, you can see how installing blinds and insect screens could deflect the sun's rays just enough to possibly make an uncomfortable room bearable under high loads.  When a homeowner is uncomfortable in particular rooms, it always turns into an HVAC Contractor's nightmare to appease them.  Instead of blaming the airflow, ductwork, or even system sizing, maybe you should look to those wide open skylights and bay windows.  Sure, I like the sun, but sometimes you have to compromise in order to feel "comfortable" when it is 100F and sunny outside. 

Heat Pump Break-Even COP's

  How do you know if  should upgrade your air-conditioner to a heat-pump and create a dual-source application during replacement this Summer?  I personally always prefer the option of multiple fuel sources, particularly since energy prices have been all over the place during the last decade.  Fortunately, based on average energy prices, there is some simple math to figure out if an aggressive assessment should be made for a dual-source heat pump application, based on equations from ACCA's Manual H: "Heat Pump Systems: Principles and Applications".  First, you will need the average energy costs for the selected fuels, and then plug in the information into the equations below.  This will provide your "break-even COP", or the point where operating the heat pump will cost the same as the furnace.  You can then take the calculated Coefficient of Performance (COP) and see what temperature the heat pump will be operating at; the lower the better!

I did a little research for you, so lets insert the average annual prices into the Natural Gas, Oil, and Propane equations to see if the investment in a heat pump will make sense during replacement.

As you can see, if installing a 96% Natural Gas furnace, the Break-Even COP would be 6.8.  Based on the Heating Performance Data for Goodman's most efficient DSZC18 Heat Pump, it would need to be above 65F for the cost to operate the heat pump to be cheaper than the Natural Gas Furnace (see data at bottom).  Even if I installed an 80% furnace, the break even COP is still at 5.7, much too high to realize a savings.

Not all homes in MA are lucky to have access to a Natural Gas supply.  There are more than enough Oil Tanks out there to keep the hundreds of delivery companies busy during most New England winters.  As you can see in the equation for oil, a resulting break-even COP of 1.17 indicates a significant savings can be realized. Based on installing an extremely efficient 87% oil furnace and the same heat pump performance data, the heat pump would still be cheaper to operate as low as -10F.  Of course, you must worry about the output of the heat pump at that low ambient, and in order to feel comfortable you will need to calculate the Thermal Balance Point.

If you decide to install a Propane tank you can realize the same efficiencies as the natural gas furnaces out there, but the increased costs in fuel/delivery could be even higher in than oil, even after including recent oil surges.  As you can see, the break even COP for installing a heat pump add-on above a 96% Propane Furnace is only 1.07.  This is even lower than the Oil application, proving the recommendation of a more thorough calculation into the Thermal Balance Point and investment costs for going to Dual Source.

  With the recent technological advancements in the HVAC industry in controls and conventionally ducted VRF's like the Carrier GreenSpeed, break even COP's and Thermal Balance Points can be driven even lower.  This makes Dual Source Heat pump applications more attracting to New England homeowners.  Some contractors are still installing electric supplemental heat, hopefully in stages, for low-ambient operation.  Although not any more efficient than the heat strips it replaced, the new heat pumps could save more than enough above the temperature of the defrost cycle to still be worth it.  I would still prefer dual-source, you know these energy companies will not be leaving any money on the table over the long run!