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Temperature Monitoring to Save Energy

I bought an investment/vacation house in Colorado in October 2010. This is a high alpine environment, the house is at 2800 Meters (9200 Feet). It gets very cold, especially at night. During the day it can be somewhat warm and VERY sunny.

My first full months energy bill (electric and gas, but mostly electric) bill was US$200. Decembers bill was $300. Both of these months the basement was occupied with a tenant, but the main floor and loft were not. I was a bit worried as the coldest months are January and February.

My energy use is about 90% heating, with the remaining 10% split between hot water, lights and appliances. There is no need for cooling. So, I am going to focus on heating.

In order to understand the problem I installed 9 Digi Xbee Temperature sensors throughout the house. Using a DigI ConnectPort X2 gateway and the iDigi Device Cloud I am able to capture the data and monitor it from wherever I am in the world. With this information, I was able to get a pretty good idea of how heat is distributed around the house.

Acting on this information, I have been able to cut my bill significantly, by half for November 2011! The biggest savings are when I am not there, as I am able to run the temperatures much colder knowing that I can keep an eye on it.

Background:

The house is built on the side of a mountain, and has three levels:

  • The Lower Level ("LL" in the graphs) is a walkout basement. This is occupied by a tenant 12 months of the year.

  • The Main Floor ("MF" in the graphs) is my area with 2 bedrooms, a bathroom and Living Room.

  • The loft is above, and open to, the Living Room.

The main floor and loft are not occupied much of the time. I am up there a lot in January-April, and occasionally friends from Denver use the home. Heat is provided primarily by baseboard electric, with 11 separate thermostats. Freezing water pipes is a big concern, so I have to be careful in certain rooms.

After doing some basics (weather sealing doors, etc.) I did some assessments and guessing, and came up with a set of theories on where the energy wasters were and some schemes to implement them without risking frozen pipes. All of this required temperature sensors to measure it and validate my theories before making any large changes, as well as measuring the success of any changes I do make.

These are the areas I am most closely monitoring. I will go into each item in more detail below.

  • Stratification: Stratification is quite simply, the result of heat rising. My perception was that the loft was trapping all the heat and that I needed to put in a ventilation duct and duct fan to move the heat back down. I was very surprised by some of the results after monitoring this for a while!

  • Zone the heat: Isolate rooms that do not have plumbing in the walls and run them very cold when the upper floors are not occupied. As each room has separate heaters, this can be done fairly easily.

  • Monitor the critical rooms: I am able to monitor the rooms that have plumbing so that my risk of pipes freezing is much lower.

  • Monitor the electric meter: I installed a Digi ERT Gateway that tracks the electric meter. That allows me to identify any issues with electric usage, such as a door or window open, oven left on, etc. that might not get picked up the temp sensors. Otherwise, I would not notice until I received my electric bill, which would be way too late.

Stratification

Stratification is the effect created by heat rising. Heat gets trapped near the ceiling and in lofts, while the occupants are cold as they are closer to the floor. This is considered one of the biggest wastes of energy, as the heating system must run more to keep the lower levels of the room warm.

I had the perception that my loft was trapping heat, as it can get pretty hot up there. My thought was to install a ventilation duct and fan to drive the heat down to the main floor bath and kitchen (directly underneath the loft). That would likely cost around $1500. A ceiling fan could help too, but would not be as effective.

When I analyzed the data, I was quite surprised to find that the problem was not as significant as I thought. Here is a graph of the past week showing the temperatures of my loft and the living room.

Notes:

  • The middle letter of the day designator is lined up right under noon.
  • Red "area" shows when the loft temp is WARMER than the Living Room temp.
  • A red line shows in the blue area when the loft temp is COLDER than the Living Room temp.
  • What I found is that the only times there was a notable difference was on sunny days, and a bit when the living room was above 10C (50F). The sun was the major influencer. Even on those days, the difference was about 1-2 degrees Celsius, 3 at most.

    I made a few conclusions from this:

    • It is certainly not a priority to install ventilation ducts. An inexpensive fan should do the trick for now, preferably controlled by the temperature differential. This should be a good application for the Digi gateway's python capabilities and a Digi Smart Plug.

    • There is probably a significant thermal leak. I really think there should be more heat trapped in the loft. I have purchased an infrared temp sensor to look for cold spots next time I am out there. My suspicion is a few windows that have lost their vacuum seal.

    Zoning

    With individual heat and thermostats in each room, and some rooms that do not have any plumbing, zoning is an obvious thing to do. With sensors in place I can run this much tighter. For example, I have had guests and a cleaning crew leave the bedroom doors open, which defeats the zoning. I am able to monitor that, and have my tenant go up and close the doors.

    I created this graph to monitor the zones I want to avoid heating:

    Making sure I do not freeze water pipes

    A single water pipe freezing and bursting would be a major problem, all the energy savings for several years could be wiped out. Being able to watch the temperatures remotely allows me to be much more aggressive at keeping the house temperatures low when I am not there, while removing the risk of pipes freezing.

    The most susceptible to freezing pipes are in the ceiling the basement kitchen, and I also want to keep an eye on the bathrooms. I created this graph to monitor the zones I am most concerned about.

    Keep an eye on the electric meter

    I monitor the electric meter using a Digi ERT Gateway. This way I can watch the electric usage and detect any anomalies (an oven left on for example) well before the bill comes in.

    I created this graph to monitor electric usage.

    To Dos

    • Document a clear "close down" procedure for guests. This will make it more likely that thermostats are set to the most efficient levels, the bedroom doors are closed, etc.

    • Check the loft for cold spots to see if there is a thermal leak there. I am suspicious there is. I bought a simple infrared thermometer to do the measurements.

    • Put a box fan in the loft to push the warm air down. I have already written the code (real easy using Digi ESP) to have a Digi Smart Plug turn on the fan when the temperature in the loft is more than 1 degree warmer than the living room

    • Longer term, install a vent duct and duct fan to take the warm air from the loft and move it down to the main floor kitchen and bath. I will likely do this in conjunction with redecorating to avoid patching and painting the walls twice. Low priority until I find and fix any potential thermal leaks.

    • Get an outdoor temp sensor so that I can calculate Heating Degree Days (HDD). Then, create a graph to compare HDD to electric consumption. This will allow me to more easily identify anomalies as these two numbers should track pretty closely. It should also help me measure my success by tracking the ratio (kWhours/HDD).

    • Convert the water heating to a gas hot water on-demand system. The current 40 gallon electric water heater is a complete waste. It is keeping too much water hot most of the time, and cannot provide enough hot water when the house is fully occupied.

    • Install a load control relay on the large heating element in the living room. That will allow me to use a standard Smart Energy Zigbee thermostat to control the heat remotely via iDigi and Digi's Energy Daytrader Android app.

    • Install heat tape on the susceptible to freezing water pipes. That will allow me to keep it even colder when the house is unoccupied. This will involve opening the ceiling, so will do this in conjunction with redecorating also.