Passive House Principles: Efficient Mechanical/Electrical Systems

The Passive House standard is about maximizing the performance of a building in many ways in order to minimize system requirements of the building.  The idea is to pack as much thermal efficiency into the building so as to take advantage of simpler, smaller heating/cooling systems, simpler mechanical systems, and simpler electrical systems.  Ultimately the extra cost of the thermal efficiency would be offset by the cost difference in the simpler systems.

A pie chart below shows comparisons between a conventional home and a passive house.  They are assuming that 59% of the total energy consumed by a conventional building is used for heating.  If the building was a passive house, the total savings would equate to 53% of the total energy consumed by a conventional building.  This equates to a 90% savings in heating cost.
However, domestic hot water (DHW) heating is still a problem.  In fact, DHW requirements will be similar to a conventional house making the energy usage associated with DHW the highest contributor to your energy bill.  So it makes sense to work on minimizing the energy usage associated with DHW.  I think there are three elements to dealing with DHW energy usage:

1. Conservation
2. Efficient Layout
3. An energy efficient method for water heating.

To promote efficiency, plumbing systems should be central to bathrooms and kitchens.  This seems like common sense but is often missed in most houses.  Using these principles will minimize the use of materials and ensures minimal heat losses in hot water as water travels down the pipe to its final destimation, whether that be a sink, dishwasher or shower.  All hot water lines should be insulated.  It will minimize line losses and provide hot water more quickly.  Have you ever turned on the hot water, then waited....and waited some more and finally started washing your hands before the hot water reached the tap?

I have!  I have one tap in my house connected to a 1/2" line that runs about 30'.    That line holds almost 1/3 of a gallon of water that needs to be drained before I get hot water at the tap.  It's actually even more inefficient that because when the tap is turned on, water from the hot water tank, enters a 3/4" line where it mixes with water that has already cooled down, then it moves along slowly mixing and heating the line contributing to line losses.  Once it's all said and done, my 1/3 gallon of water easily becomes another 1/3 gallon.  Then I turn the tap off and another 1/3 gallon of hot water sits in the branch line, where it cools down to around 20 Celcius...and, the water in the tank from the draw now needs to be re-heated!    I have estimated that every time I use that hot water line I waste almost 0.4 kWh.  That inefficiency wastes almost 300 kWh of energy every year, equating to almost 7% of my total hot water bill for the year!  Most houses have several branches like this.  It lacks efficiency and it wastes water.

It has been fairly well documented that these homes only need a couple of point source heaters. People have experimented with placing base board heaters and thermostats in bedrooms only to find that they are never active.  There is enough thermal output from a person to keep the rooms warm and enough circulation in the house to move heat round from a couple of heaters.  So no need for electrical baseboards or hot water radiant heaters in bedrooms, complicated pump systems, etc.  "The simpler, the better" philosophy minimizes electrical and plumbing costs, as well as ongoing maintenance and will maximize efficiency by design or lack thereof.

I could keep going on and on....but the take home message is efficiency leads to lower initial costs, lower maintenance costs and lower ongoing energy costs.  I'll end it here and say that I will be concentrating on minimizing my hot water energy usage since it seems like the next most obvious choice in terms of total energy used by a building.  A decision on the system is still up in the air but there are many methods; some are more technological and some are going back to the days of boiling water with fire....using a bowl made from birch bark....I won't go that far! It is 2016 after all!!


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