Initial Home Energy Efficiency Analysis

2011 January 5
by Otto

Well, the calendar has closed the books on the first year of the house. Although we only finished the house in August, and then promptly spent very little time in the house other than Thanksgiving and Christmas (but with people staying there in between), I did gather some data on the house that’s worth sharing. Here’s the schematic of the energy systems and both the current and accumulated performance:

Home energy system overview on the evening of December 31

Some noteworthy facts:

–We used 3,024 KBTUs of energy to heat domestic hot water (DHW); 2,402 of those, or about 80%, came from the solar heating system.

–We used 23,935 KBTUs of energy for home heating; of those, 1,554 (6.5%) came from the sun.

While those percentages are impressive, and in line with what had been predicted, it is also worth taking a momentary detour to examine the financial benefit of the $13,000 (retail) solar hot water system. The system produced 4,518 KBTUs of energy, which is about $45 worth of heat. Of that, the house used only about 3,950 KBTUs (~$39). Would I have saved more than $39 if I had taken the money and invested in more insulation? One thing worth noting is that we used almost 10x the energy to heat the home than we used for solar hot water. This is why perhaps I shouldn’t have oversized the system in an attempt to partially heat the home. Having a small system to heat the DHW makes sense, since hot water usage is fairly constant throughout the year. But oversizing the system in the hopes of providing partial space heating, as I did, might not make as much sense since you need another order of magnitude of energy, and the system produces the least just when you need the energy the most (ie on cold dark winter days).

On the plus side, the house only cost about $240 to heat so far this winter. Not bad!

–The underground air tube seems to be working great. At the moment I took the above snapshot, the outside temperature was -9F, and the tube pre-heated it to 27.7F. I should note that the next temperature meter, located just a foot or two further down the pipe, measured 36.2F, and I’m not sure I can completely justify why the temperature would have risen anther 8 degrees in such a short distance. Yes, that stretch of pipe is in the mechanical room, and therefore at room temperature, but 8 degrees in such a short distance seems odd. But the key is that the outside temperature is well below 0F but the fresh air is above freezing by the time it reaches the HRV. Despite the fact that we had a number of fairly extreme cold snaps, with extended periods of -20F, the HRV electric pre-heater has not had to turn on once.

–The HRV system is working great. The snapshot shows 100% efficiency, which can’t be completely right, but I’ve noticed it around 95% efficient at various times. My system is an Ultimate Air Energy Recovery Ventilator, and from this performance perspective I am very happy. However, the thing is noisy. There is quite a pronounced hum at low volumes, and when it runs at full tilt it sounds like there’s a truck idling outside. (It’s worse in my case because the laundry chute carries the noise from the basement right up to the master bedroom.) I had Mike McPherson come back and connect flexible hosing to the system, in the hopes that it would dampen the sound, but it didn’t help much. I also called the company but apparently they don’t really know what to do either. I could spend hundreds covering the box with sound-deadening material, but at some point I think I would just recommend a different system, even at the cost of a little efficiency. Since I am getting so much benefit from the underground air tube, perhaps we can sacrifice a little efficiency here in the name of quiet.

The system is now reset for the beginning of the year. Let’s see how the rest of the winter shapes up. In the meantime, a big shout-out to Jon Schafer of PowerHouse and Todd Hoitsma of Liquid Solar for installing and maintaining this great monitoring system!

To see the current status of the house, check it out here:

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