Sunday, May 11, 2014

Week 18 - Driveway complete, a little bit about the heat pump

The driveway is done, and little bits of progress have been made here and there on the roof trim, the building wrap, the upstairs ceiling. The piping for the heat pump was installed, and I’ll tell you a little bit about the amazing science behind the heat pump below.

Driveway is done, almost

The most expensive part of the entire project – the essentially unnecessary driveway – is almost done. Normally, you want to complete the driveway first, before embarking on construction, but our project got off to a late start and the winter prevented us from doing driveway work until now.

Above, a sheet of fabric is put down to separate the bottom layer of dirt from the top layer of “gravel” – that’s what the guys call it even though it looks like dirt to me. I suppose keeping the two layers separate helps stabilize the driveway.

The tiniest steamroller in the world is compacting the gravel — it’s adorable. After construction is complete, a final layer of “TRG” (I don’t know what it stands for) will be placed on top of the dirt. The TRG consists of small stones and sand that compacts well and can be plowed in the winter.

Now that the dirt is compacted around the home, delivery trucks don’t have to worry about getting stuck in the mud. My only concern is that I want to have a vegetable garden in front of the house and I’m worried that these vehicles compacting the soil will make the ground less amenable to cultivation. The driveway guys didn’t seem to think it was an issue.

Work on the roof trim

I’m not sure how he got up there, but somehow Don got started on the roof trim – see the white boards along the edge of the roof – without scaffolding or a ladder. When I asked him how he did it, he said: “I’m a monkey.” Next week, the metal seam roof will go up and Don is eager to borrow the scaffolding that the roofers will put up.

The R100 vaulted ceiling

You might remember that the ceiling upstairs will serve as the air and moisture barrier, with an astonishing 26 inches of blown-in cellulose as insulation on top of the ceiling (in the roof truss) giving an out-of-this-world R value of R100! The ZIP sheathing (in green) is placed face down on the underside of the roof truss. On the lower right hand side of the image, you can just barely see how the taped seal runs under the roof eave and attaches to the underside of the ceiling ZIP.

The building wrap

A delicate process, reminiscent of wrapping a Christmas present, is required to seal the moisture barrier around the window openings. First, the air barrier is taped at the edge between the window frame sheathing and the SIP OSB, shown above.

Second, the building wrap is taped to the window frame sheathing.

Getting the edges right is always the tricky part. An extra piece of tape goes right at the corner, running horizontally along the inside crease of the window frame.

The heat pump

I’ll talk more about the heat pump later when it actually gets installed, but for now, here’s a quick overview.

The heat pump provides both heating and air conditioning. We’ve chosen a mini-split, dual-zone, ductless model, meaning that there is an outdoor unit (a condenser) and two wall mounted indoor units (the evaporator cassettes) connected by two refrigerant lines (a gas line and a liquid line). This is different from central air conditioning or forced air heating where a central unit in the basement blows air through ducts to each room.

How does a heat pump work?

It’s a complicated process — a miracle of science — and you need to know a certain amount of thermodynamics to really understand it, but here’s how a heat pump (as well as your air conditioner and your refrigerator) works, in a nut shell.

The outdoor condenser, which houses a big noisy fan and a noisy compressor, serves to turn the refrigerant from a liquid to a gas (when in heating mode). The refrigerant is piped inside to each wall mounted indoor unit which simply blows air over the refrigerant — turning the refrigerant back to a liquid — and in the process extracts hot air. The cycle runs in reverse in air conditioning mode, turning the refrigerant from a gas to a liquid outside and extracting cool air when turning the refrigerant back to a gas. The process can move heat around at a seemingly impossible efficiency of almost 400%, way better than a normal furnace which might be 70% efficient.

The refrigerant lines

Here’s where the outdoor unit will go once the siding is finished. Those bunches of tubing puncturing the wall are the refrigerant lines.

Inside, the tubing snakes through the walls to the evaporator unit. A water line allows condensed water to drain away. Notice how much simpler it is to install these lines than it would be to install all of the duct work for a central heating or air conditioning unit.

Why don’t we need heat in every room?

The home is so well insulated and so tightly sealed that the temperature will be essentially even throughout the entire house. You just need to add a bit of hot air at one location and the heat will eventually propagate everywhere. We have one cassette upstairs for cooling in the summer and one unit downstairs for heating in the winter. The thermostat will be set to a given temperature window and a minimal amount of energy will be required to keep the temperature constant. A great feature of these heat pumps is that they can run at extremely low settings, quietly trickling in the tiniest amount of heat without having to noisily cycle on and off like a traditional system.