Follow the construction of Tom's super-insulated home in Port Townsend, WA

Tuesday, January 31, 2012

So How is the House Working?

The goal of building a passive house was to decrease my carbon footprint, and it's time to look at the data. Historically, January is one of the coldest and therefore most energy demanding months of the year. Historically, the average temperature is 41F and the house has been held at 68F day and night. I experimented with decreasing the temperature at night, but the thermal inertia of the house is large, and the heating power is small. That combination is not well suited to dropping the temperature at night. An 2.5 kW electrical heater in the duct leaving the energy recovery ventilator was installed to provide heat. However, to get that much heat out, the ERV has to be run at fully capacity, namely 200 cfm, which is 3 times what is needed to provide healthy indoor air. At that volume, the ERV would need ~ 270 W for the blower - a waste of energy. For that reason, I've been heating the ~1500 sq ft  house with a 1500 W electrical radiator.

It provides enough power as long as the outside temperature is above ~32F. When it gets colder at night, which it did a number of times in January,  I used a second electric radiator operated at 600W to give me a boost. Because the radiator is on intermittently, I need a power meter to tell me how much electricity is being used on average. I like the Belkin Conserve, pictured above. The thermostat on the radiator isn't very good, so I used a Lux plug-in thermostat to regulate the temperature.

For the month of January, I needed 590 kW hr for heating, which translates into a average continuous power of 820 W and a cost of ~$59/month. Given that $300 monthly heating bills are not uncommon in Port Townsend, this is a good result! At this rate, my heating cost for the entire year should be less than $400. [A Seattle friend emailed me on reading this part that his bill for natural gas heating a poorly insulated older house is only $600 because natural gas in Seattle is much cheaper than electricity in Port Townsend.  He uses five times as much energy in a cold month and pays only 50% more than I do. The important number to note in evaluating the performance of the house is the kW hr for heating, not the $ cost. It may be that the only way to encourage energy conservation is to increase energy prices through taxes that are used to bring renewable energy to market.  There is a reason why hybrid cars weren't developed a decade or two earlier. Staring climate change in the face, we have begun to pay a high price for our access to cheap energy.]

So is the ERV performing? It has 2 functions: to bring fresh air into an otherwise airtight house and to extract heat from the outgoing air. I measured the efficiency of my ERV, which is defined as (Texit-Tcold)/(Tin - Tcold). Tcold is the temperature of the air entering the house from outside. Tin is the temperature of the inside air that is flowing into the ERV to have its heat extracted, and Texit is the temperature of the incoming cold air after extracted heat has been added. On a cold day, I got 0.985, and on a warmer day, 0.927. For the higher efficiency, 98.5% of the heat in the exhaust air was extracted, and the corresponding number for the warmer day was 92.7%. These numbers are higher than I expected, which I attribute to latent heat transfer. I think that the exhausted air is cooled sufficiently to condense some water and that the enthalpy of vaporization that is liberated is transferred to the incoming air.

Another function of the ERV is to get rid of moisture. Only the dryer is vented directly to the outside; there are no kitchen and bath exhaust fans. Instead, air from these rooms is sucked out by the ERV and moisture is vented to the outside in parallel with heat extraction. Although the bathrooms and kitchen have a booster switch  to bump up the ERV to its full capacity of 200 cfm to remove water vapor, we have never used this capability. Just the normal air circulation is sufficient to dry out the shower, remove water from boiling in the kitchen, etc. The Ultimate Air Recouperator is great!

My total electrical energy use for January is 1062 kW hr.The only other energy source is ~ 5 gallons/month of propane for cooking and baking. I used 472 kW hr in January for hot water, lighting, refrigeration, computers, other plug loads and my panini press. I'll track down these uses in the next blog.