HVAC Sizing for Eco-Panels Homes

If you really look at the philosophy behind building a super-energy efficient home, it's that the interior spaces are very much protected from outside conditions, and it only takes a small amount of energy to affect change to the interior environment.  "Heat it with a match-stick, cool it with an ice-cube" would be our mantra if we had one.  And what's better is that when you have a super insulated structure that can be affected by small amounts of energy, then you can realize savings that a stick framed home could only dream of.  We have homes set on concrete slabs and they are able to realize and leverage the passive cooling energy of that slab.  And we have tiny homes that use incandescent lightbulbs to provide 100% of the heat for a small home when it is below zero celsius outside.  We provided the roof and walls of a 15,000 square foot retail space that is cooled in the summer largely by its concrete slab (HVAC is still needed for dehumidifying) and in the winter the heat is provided by its retail lighting.

REMEMBER- the energy associated with the heating and cooling of your home represents the single largest expense of living in that home (outside of your mortgage), typically accounting for 40-60% of your total energy bill (water heating is usually about 20%, the rest is known as "plug load").  In fact, the heating and cooling of buildings is about the largest singe use of energy in the United States according to the USEIA.  By significantly reducing your energy consumption associated with the heating and cooling of your home, we can help you save real money over the life of your home - and this saved money goes straight to your pocket - not the energy company!

Most homes will have a more convential HVAC system for heating and cooling the home, and here is where you need to be careful.  DO NOT LET YOUR HVAC GUY USE A TRADITIONAL "RULE OF THUMB" that might mean he is going to size the HVAC based on 1 ton for every 600 or 700 square feet of home.  Now local building codes very often require a "Manual J" calculation which takes a more in-depth look at the construction, fenestration and orientation of your home for HVAC sizing, and even this more advanced method almost always gets it wrong with our panels.  Through experience, we have found that generally speaking a home of approx 4,500 sqft in size may need a 2.5 ton HVAC system.  Buy a variable speed HVAC system if you can - these used to not exist but with the increased presence of mini-splits we are seeing it more and more.  A 3,500 to 4,000 sqft home might need a 2 ton system.  A home 1,500 to 2,500 sqft can typically get by with a 1.5 ton system.  So if we had to say we had a rule of thumb, it would be approximately 1 ton of HVAC for every 1,500 to 2,000 sqft of house.  Always use at least Energy Star rated windows (and the rating needs to be appropriate for your climate zone), and Cardinal Glass is also a great choice (odds are your window mfgr offers it).  We do like mini-split systems as these can be great for smaller homes (they have been long used in Europe and Asia where smaller homes are more the norm).

ASHRAE tells us that an HVAC system needs to run for at least 15 minutes or so before it starts to achieve optimal operating efficiency, and in a super-insulated home we have heard past clients of ours brag that "we know our home is super-energy efficient because the air conditioning only runs for 5 or 6 minutes at a time before it turns back off".  We slap our foreheads in frustration because we know that the HVAC company sold them an oversized unit - sometimes more interested in a bigger sale than a "right-sized" sale.  Their unit may indeed be cooling the house, but it is not "conditioning" the house, which is eliminating the high humidity levels that any summer months may realize.  And all of the short-term cycling of the equipment will certainly shorten its useful life, again increasing your long term operating costs.  In reality you want a system that will run "low and slow" in the background, cooling and dehumidifying but at the same time using a lower amount of energy - this is why a variable speed system like a mini-split or one of the new TRANE variable speed units is attractive to us - they are made to run longer on lower power.  You don't want it to run all the time, for sure, but I think 30-40 minutes or so is just fine.

HERE is a nice article on "right-sizing" an HVAC system for a super-insulated home from a blog that we appreciate (and we think they give great advice most of the time).  If your HVAC contractor is not familiar with this sizing concept and always thinks that "bigger is better" then RUN AWAY - they are not the company for you.