I recently had my forced air electric furnace replaced with a geothermal system. My furnace was disconnected for the first two weeks of November. After a recent energy audit, I had been doing some sealing of the envelope. We drew the blinds at night and opened them during the day.

We found that the temperature was mostly going down to 0C outside at night and was below 10C during the day. We did not experience any problems with heat in the house. Solar gain, use of ligths, cooking, occupants, and appliances seemed to supply all the heat that was needed. The house was perhaps a degree cooler in the morning and a degree or two hotter in the early everning.

When our time without a furnace ended, the outdoor temperatures remained in the same area for a week or so. I found that by watching the outside temperature and listening for the air handler, I could correlate outdoor temperature to furnace activation.

I observed that we had lived on the border. At zero our heating almost never comes on. Perhaps once briefly in the predawn night and perhaps another time briefly in the morning, before the sun hits the southeast windows.

Is this a new challenge? We are not looking at ACH or R values, nor other strategies, but instead the results.

The challenge is to get a house to the lowest sustained outside temperature possible without requiring actually heating, by keeping the indoor temperature at 20C +/- 2C during the 24 hour period, with only moderate household energy usage supplying heat.

Say you set the thermostat to 20C and check the outside temperature, and see at what outside temperature the furnace starts coming on fairly regulary, say more than 2 times a day, for more than fifteen minutes at a time.

I notice a sharp divide. As soon as the temperature outside drops below 0C the furnace becomes more active.

It seems to me that this is an easy measurement that anyone can work with. You seal and weatherstrip, and insulate, or change windows, etc, and then you can measure any real effect that has been acheived by seeing if the house can be cooled further than before, without engaging the heating system.

I find myself now challenged to reach -5C without heating.

Is something like this already used?

I think that the contribution from appliances and people is the uninteresting part and should be subtracted out. In other words, turn on enough appliances and anyone's furnace will stay off, even at -10C. On the other hand, measure (without appliances or people) the btus to hold a steady state temperature + the outside temp and you have some useful data about the building's thermal performance.

The biggest challenge I see is with the solar gain. On bright, sunny days you're going to gain a lot more heat than on overcast days, days when it's raining, etc..

If you are trying to design a building without a heating system, you could end up chilly on those gloomy days..

The most efficient furnace is the one that does not come on.

The most effcient home is the home that keeps the furnace from coming on.

A home that heats itself from daily occupancy and appliances will not see it's energy usage change during heating season except longer lighting hours.          

That is ideal, but there should be some target that says the most efficient homes should have the furnace come on regularly only at say -10C outside temperature, during otherwise normal daily energy use, and occupancy.

That way anyone can tell how well their home is performing.

The cloudy days are often warmer, which compensates for the reduced solar gain.

If you leave your home unoccupied you turn down the thermostat.

during otherwise normal daily energy use, and occupancy

Since these and solar gain vary so much, one cannot use such a method for comparison.

Yes, but a standard appliance, lighting, and occupancy bundle could be used for exact testing and simulation.

Ballparking is good enough for the average home owner. All you need is an outside thermometer, set your thermostat to 20C, and then monitor how often heating comes on. After several weeks of cold season you should get a ball park figure of at what temperature your heating comes on. You should be able to see the difference between a cloudy day and a sunny day. you can get values for both. You can say that the heat comes on more often at -1C on a cloudy day, and -2C on a sunny day for example. If it is very windy you may get a number for that too. It can also be of interest to know how the weather other than the temperature, and some houshold activities affect heating. 

Perhaps cooking a turkey in the oven for hours is enough to keep the heating from coming on at -3C. The temperature at which the furnace comes on more regularly can be a very useful tool in seeing how different usage and weather factors affect you particular house and ball park, by how much. What happens when you run the fireplace or wood stove, for example.

So I`ve given some figures for my 27 year old house, with a drafty chimney, which still requires some envelope sealing work.

I designed our house myself sticking to codes and practices of the time. I designed in some features for energy eficiency. This what I am getting at the moment.

So, what cha got

How are your structures working out.