building simulation-part1

Typical House Energy Flow (click to enlarge) courtesy of Arron Acosta and Cy Hoadley-Kilbourne, MIT

If you’re contemplating an energy upgrade to your house or commercial building, perhaps someone has mentioned the concept of a building simulation, or doing an energy model.  Such a process has been part of the commercial HVAC world for some time now.
As the name suggests, a building simulation involves the use of a computer to simulate the building’s energy performance.  The goal of performing a simulation is not only to predict how much energy a building will use over the course of a year, but also to compare alterrnative designs. We can also look at some measures of comfort within the building.

With all this data, it’s clear that a simulation can be very powerful for helping make decisions. As you might guess, authorities that regulate energy compliance as well as those that give out rebates want to see this kind of analysis done to make sure they are spending our money wisely.

The ‘sankey’ diagram above illustrates the complexity of the energy flows into and out of a typical house.  Based upon a description of the building input by the user, the building simulation software calculates all the energy flows into and out of the building. Not only does the software model the building envelope, but it models the interactions of people, infiltration, and even exotic things like trombe walls. After doing all this, the software has to predict how the building equipment will perform. Everything from furnace efficiency, daylighting controls, through daylighting controls is simulated on a 24hour/365day basis.

The pay-off is not to predict the exact energy use of any one particular house, but instead to look at alternate ways of building, different equipment, and even alternate control strategies.

Over the next blog postings, I’ll keep you in the loop on our progress.

Executive Summary:

Financial analyst is to Spreadsheet AS   Engineer is to Energy Modelling

Solar Loves Whole House Fans

There are a couple of trends that will affect solar photovoltaic installations:

  • Newer electrical rate structures encourage conservation (tiered rates)
  • ‘Energy Security’ is a powerful motivator
  • Photovoltaic rebates require house efficiency check-ups

We think  solar energy gathering is great. The idea of  getting electricity essentially for  free energy has certainly captivated our imaginations. Even though the cost of photovoltaic panels is dropping very quickly, the investment is still considerable.  Our crystal ball tells us that electrical energy is very, very versatile and will be in demand as electrical cars come on stream and coal fired electrical power plants start to show their ugly side. (coal burning)

As the demand for clean electrical energy increases, its value will certainly not go down.   In fact, if the validity of predicting cheap energy such as the famous line is any guide “too cheap to meter”,then we should get prepared for high prices.

By combining whole house fans solar contractors and  homeowners  can:

  • Decrease the initial investment in PV panels
  • AND/ OR
  • Increase the  Return on Investment for the entire system

If you are considering a heat pump

This article by Henry Gifford, an energy saving specialist discusses why heat pumps are no panacea.  His points, well made, bring home the messages that building envelope efficiency is critical and the importance of examining the entire energy chain –  from power house to your house.  Not that readers of this blog need reminding, but Henry also points out that over half of this country’s electricity derives from coal burning . (BTW clean coal = oxymoron du jour)

Heat Flow

We get a lot of questions about the ‘R value’ of our damper doors, so we felt that it’s worth going over the basics.

The so called ‘R value’ is denominated in  the units of (square-feet x degree x hour/ BTU). Sounds complicated, but all you really need to know is:

Heat Flow  Q (in BTU per hour) = Area x (temperature differential) / R-value

[For you techies, this formula is valid for steady state, and assumes an infinite plane area.]

Let’s do the math for our whole house fan dampers.  We’ll assume VERY cold conditions in the attic (0° F) and 70° F inside.

Model length width Area (sq. feet) R value Q (BTUH)
1.0WHF 22.5 14.5 2.27 14 11.33
1.7WHF 22.5 14.5 2.27 7 22.66
2.5, 2.5eWHF 22.5 14.5 2.27 10 15.86
4.5WHF 26.5 22.5 4.14 10 28.98

Bottom line:  There is not much heat loss through a well insulated whole house fan damper door.  The question that you SHOULD be asking is: “How well sealed is the whole house fan damper door?”

Most residential windows will have R values of 1 to 3. Another question that we get all the time is : “What is a BTU ?”

BTU stands for British Thermal Unit.

1 BTU is approximately the energy released by burning 1 paper match.

100,000 BTU’s make up one therm – that’s how you purchase natural gas, and right now that costs about $1.50

A gallon of gas has about 114,000 BTU.