Living in an Old House Remodeled to Passive House Standard

November 01, 2015
November/December 2015
A version of this article appears in the November/December 2015 issue of Home Energy Magazine.
Click here to read more articles about Retrofit

My husband, Kurt Hurley, and I are homeowners with passion for green building, and we've managed to reduce the energy consumption of our 93-year-old house by 80% through a very detailed renovation following the Passive House (PH) standard. Our home, the Midori Haus, is a bungalow in Santa Cruz, California, that was built in 1922 and was remodeled to the PH standard in 2012 and certified by Passivhaus Institute (PHI) in 2014. We kept the foundation, floor, roof, most of the framing, and the built-in furniture, and we reused many of the interior accents to honor the look and feel of the Craftsman-style home. Graham Irwin of Essential Habitat did the architectural design and PH modeling. Taylor Darling of Santa Cruz Green Builders built the house.

Midori Haus Energy Performance

During the first year of living in Midori Haus (March 2013–February 2014), we had a total site energy usage of 4,334 kWh (2,869 kWh electricity; 50 therms gas). We were delighted that our PH performed as advertised, and we told everyone that our 92-year-old house was using 80% less energy than before the remodel, all without installing PV panels on the roof to offset our electricity use. All appliances within the conditioned space are electric. Natural gas is used for the barbecue on the kitchen deck, and for the boiler, which serves as the backup water heater. Hot water from the solar-thermal tank provides space heating via a hydronic coil that is coupled with the heat recovery ventilator (HRV). The house has a treated floor area of 1,569 square feet, so the 50 therms of natural gas usage represents less than 1 kWh per square foot per year for space heating.

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The front of the Midori Haus after it was remodeled to PH standards. (Kurt Hurley)

The front side before the remodel. (Chie Kawahara)

Glycol drained from the solar-thermal loop over 18 months was in good shape, but there wasn't enough of it. (Chie Kawahara)

Midori Haus Annual Energy Usage

Midori Haus Annual Energy Usage
Figure 1. Annual energy use preremodel and for the first two years after the Midori Haus was rebuilt to PH standards. (Chie Kawahara)

HRV filters after several months of use (black is regular; red is MERV 13). (Chie Kawahara)

The dirty charcoal filter placed in the HRV intake, after one year of use. (Chie Kawahara)

The pocket door separating the kitchen and dining room is used daily to isolate cooking odors in the kitchen. (Chie Kawahara)

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The deck facing south, covered with our low-budget shade cloth. (Chie Kawahara)

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The south side of the Midori Haus after the remodel. (Graham Irwin)

Energy use should have remained low, since the building envelope is constructed to endure and Kurt and I remained motivated. So how much energy did we use during the second year of living in Midori Haus?

In fact, we used more energy—305 kWh more—in the second year than we had used the first year. In the days before the retrofit, that would have been a blip on the radar. But living as we were in a Passive House, 305 kWh was an alarming amount—it was more than the energy we used during an entire month in the summer. How did this come about?

Natural-Gas Mystery

What stands out when you look at the data is the spike in June 2014 showing double the energy we had used the previous year (see Figure 1). Was there a glitch in the smart meter? Nope. Did the occupant behavior change? Well, slightly. Our neighbor’s apricot tree had a bumper crop of fruit, surpassing the flavor of that found in organic food stores. So we baked a lot of apricot tarts during the month of June. Still, that couldn’t have doubled our energy usage. We hadn’t used the barbecue more than we did the previous year. What then? It was something that happened very slowly over a period of time and didn’t show up as a problem until we noticed a higher than usual gas usage.

But why did we use the same amount of gas during the warm month of June 2014 as we did during the cold, rainy month of February 2014? We used 13 therms of natural gas in both months. On rainy days, the domestic hot water (DHW) is heated by the gas boiler, not by the sun. February 2014 was a cold, rainy month. But why did the boiler fire up in June, when the sun was shining every day?

To investigate, we looked inside our mechanical room. At first glance, everything looked to be operating properly. It was midmorning and sunny, and we heard the glycol pump sending the heat exchange fluid up to the solar-thermal panels—everything was humming along. What was odd was that the gauges measuring the glycol temperatures were low—about 90oF, when normally they would be between 130oF and 160oF. When Kurt climbed onto the roof to check the temperature at the pressure release vent on the corner of the solar-thermal panel, he found it to be 140oF. It didn’t make sense, so we called our plumber, Duane from Wilson Hydronics.

The first thing Duane did was to drain the heat exchange fluid circulating through the closed loop. The color of the fluid was good, but the volume was only about half of what it should have been. Visual inspection of the copper pipe going up from the bottom of the indirect tank to the 90o turn at the top of the mechanical room showed a trace of a leak. There were dried water marks on the pipe insulation. Air temperature in this mechanical room can reach close to 120o on really hot days, so any slow leak would have dried up before it could pool on the floor, making the leak less obvious.

The slow leak persisted while the solar-powered pump continued to send glycol up to the solar-thermal panels on the roof. Over time, the fluid volume was reduced to the point where only glycol vapor was in the solar-thermal panel, trapped and unable to deliver the heat down to the tank. So when the sun-heated glycol stopped coming back down into the indirect tank, it stopped behaving like a solar-thermal storage tank and started behaving like a regular water heater tank. This meant that the standby loss would eventually trigger the boiler to heat the closed-loop water in the jacket of the indirect tank until the aquastat suspended in the middle of the 105-gallon DHW tank was satisfied.

We agonized over the 13 therms of natural gas (equivalent to 381 kWh) that we used in June 2014. If we had been alerted sooner, and had had the leak repaired quickly, we could have avoided this. Perhaps if we had installed the equipment in an attached garage (ours is detached), and had walked past it regularly, we would have noticed the leak sooner. Now we take a peek into the mechanical room from time to time to see if there is anything unusual going on. Frustrating as the glycol incident was, it was a good learning experience.

Figure 1 shows that total energy use during January and February was lower in year 2 than it was in year 1. The mean temperature and the average high and low temperature were about the same in these two years. But in year 2 these months included fewer rainy days. In fact, in January 2015 there was no rain at all. (The last time this happened was in 1893.) In February 2014 there were 16 rainy days; in February 2015 there were only 4. We know that consecutive rainy days trigger the backup gas boiler, and in 2014 this happened much more often than it did in 2015. So we saved energy from drier winter weather during our second year.

If we were to do this over again we would still install a solar-thermal system for water heating. Why? Because we have a soaking tub with a capacity of 155 gallons, and the solar-thermal system has been working well for meeting the intermittent large hot-water draw without using energy from the utility. If we didn’t have the soaking tub, we would probably have installed a heat pump water heater and PV panels.

Indoor Air Quality

There are aspects of operating a PH that can only be learned by living in one. Take indoor air quality (IAQ), for example. We looked forward to having good IAQ from our Zehnder ComfoAir 350 HRV, and we even upgraded the filters to MERV 13. We are quite pleased with the overall result. What we didn’t anticipate was being bothered by the ultrafine particles coming into the house from outdoors that we could smell. The nasty smell is from the frightened skunks. When they happen to loiter near the HRV intake side of the house and are alarmed by the neighbor’s dog barking at them, we notice their presence through the HRV ducts. When this happens, we unplug the HRV and open the windows on the other side of the house. Another smell we notice inside our house is that of smoke from neighbors’ fireplaces, barbecues, and fire pits. Again, when the smell exceeds our tolerance, we unplug the HRV. The location of the HRV intake port was carefully chosen to minimize pollution from the road, and if we were to do it over again, we would keep it in the same location. The lesson learned here is that we can’t control the quality of the air outside, but we can control the HRV plug. Because we live in a coastal community not far from redwood forests and enjoy pretty clean air most of the time, we might be a bit more sensitive to smell than we would be if, for example, we lived in a densely packed city on a busy intersection.

When we shared this experience with others in the PH community, someone said, “Why don’t you guys install a charcoal filter at the HRV intake?” Great idea! We ordered the $500 kit from Zehnder and had it installed in the attic near the HRV intake spot on the exterior wall. This filter performs, but it doesn’t trap all of the ultrafine particles. So this didn’t entirely solve our problem—we still smell smoke and skunk inside the house. When we had our deck and arbor restained last year, we were bothered by the off-gassing smell from the stain. So what do we do? We still unplug the HRV. I don’t mean to criticize the HRV. We love our Zehnder unit and would recommend it to others. But there are many tiny particles out there that the filter doesn’t catch.

Over time, we noticed that the fireplace smoke from the neighborhood was most concentrated in the early morning and the early evening. So rather than unplugging the HRV on an incident basis, we decided to use a timer on our HRV. Our old-fashioned timing device plugs into the 220V outlet and has two sets of on/off settings. This allows us to maintain a regular routine of turning off the HRV between 6 am and 9:30 am and between 4 pm and 8 pm. The timing is based on commute hours, since we live just a half block away from the business route of Highway 1, where trucks and cars reach their peak during those hours. So far it’s serving us well.

Managing IAQ is not limited to controlling outdoor pollutants. We can attest to the statement made by Chris Stratton in a recent article that using a recirculating range hood and relying on the HRV/ERV return are both inadequate strategies (see “Kitchen Ventilation,” HE Mar/Apr ’15, p. 24). During the construction phase, we looked forward to making meals on the induction cooktop with the recirculating range hood, partly because it’s faster than cooking with gas and eliminates combustion by-products in the house, but mostly because we could retire the old routine of opening windows to avoid backdrafting when we turned on the vented range hood at our previous residence. We formed this routine when we learned that the vented range hood created negative pressure in the house, and this negative pressure sucked air into the house from the wall gas furnace and other undesirable places. When we started cooking in our tightly sealed house, we noticed that the food odor lingered a lot longer. In theory, the entire air volume of the house would turn over within two to three hours with the HRV on the medium setting. With the kitchen taking up about one-seventh of the house volume, the food odor should be removed from the house within 20–30 minutes—right? Well, not exactly. The once-delicious smell of aloo gobi or fried bacon seemed to waft around the house and cleverly avoid evacuation by the HRV exhaust for hours, way past the point of triggering olfactory pleasure.

How do we manage IAQ? By relying on the original design of the house, which was built in 1922. Like many homes built 90-plus years ago, this house has walls and doors partitioning off each space. We kept this layout rather than rearranging the walls to have an open great-room layout, because we liked how the dining room felt with the doors closed. Now we can close the pocket door between the kitchen and the dining room. Sometimes we close the French door between the dining room and the living room for extra containment. When the cooking odors are contained in the kitchen, the HRV can remove them effectively. To flush out the smell quickly, we sometimes open the kitchen windows and door.

Then there are a few days in the year when we set our HRV to the lowest setting, A for absent, rather than unplugging it. These are the heat wave days, which occur typically in May and in October. When the San Francisco Bay Area gets warm, the cool air from the ocean is drawn inland, shrouding the coastal area with fog at night and early morning. Overnight temperatures, even on the hottest days, range from 55oF to 59oF. This allows us to do night flushing to reset the internal temperature to about 68oF. When a heat wave is forecast, we flush the house by opening the windows early in the morning and then lowering the blinds and lowering the HRV setting to A. It works pretty well. Even our contractor, Taylor Darling, was surprised when he came by during one of the heat wave days. He said, “Wow, your house is cold,” when the exterior temperature was 100oF and the thermostat in our hallway read 73oF.

learn more

Follow the progress of the Midori Haus, including continuing energy use data.

See a diagram and an explanation of our solar-thermal system.

Cool Shading

Strategic shading on the arbor keeps the sun out during the hot season. Initially we intended to grow some edible vines, like grape or kiwi, on the arbor to provide seasonal shading. But we needed a temporary shade cloth on the arbor to get through the first summer season. So we made up a low-budget version by buying a large canvas painter’s drop cloth, cutting it to size, and installing grommets on the edges to tie it down. The side benefit of this was that it diffuses light coming in through the window and makes the experience of sitting in front of the window more pleasant. Once we realized that the removable shade cloth allowed us to control the sun more precisely than the grapevine or the kiwi would have done, and that we didn’t need to worry about cleaning up the leaves and fruits on the deck, we decided to keep it as a permanent seasonal solution.

We’ve learned a lot since our remodel—and so have many others. Over 500 people have toured the house so far, and we continue to share our experience as enthusiastic homeowners.

Chie Kawahara is co-owner of the Midori Haus in Santa Cruz, California.

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