Archive for July, 2011

Air source heat pump installed, part 1

Yesterday our heating contractor installed the exterior unit of the air source heat pump (ASHP) system. Our system is part of the Mitsubishi Mr. Slim product line. It is referred to as a mini-split system, because the compressor and the air blower are two separate units. The compressor sits outside the house envelope and the air blower indoors. They are connected by a small refrigerant tube.

Our particular model, the MUZ-FE18NA is rated to produce a maximum of 21,600 btu/hr at an outdoor temperature of 5° F. Our maximum heating load was calculated to be 12,200 btu/hr, so we have some wiggle room. Typically these units are sized to meet the maximum load at the minimum expected temperature. This unit will continue to operate at -13° F. When temperatures reach well below that level, the electric resistance heat kicks in as a backup if needed.

What’s amazing about these units is that they are incredibly efficient for their cost. The efficiency at which a system converts electricity to heat is called the Coefficient of Performance (COP). Electric resistance has a COP of 1. Meaning if you put in 100 watts of electricity, you get 100 watts of heat. Our ASHP operates at a COP of 4.11 at an outdoor temperature of 47° F and an indoor temperature of 70° F. That means it is 4.11 times more efficient than electric resistance. We put in 100 watts of electricity and we get 411 watts of heat. Pretty cool! Err, I mean hot.

The COP changes as the temperature outside goes down and the temperature inside goes up. At 17° F, the COP drops to 2.77. That’s why ground source heat pumps (GSHP) are much more efficient, typically 3-6 COP at low temperatures. Ground temperatures are much warmer than air temperatures in the winter months. But GSHP’s can cost a lot more than ASHP’s. Our ASHP, including installation will cost less than $5,000. I’m guessing a GSHP would cost at least double that, and possibly more due to the drilling required.

Apparently our setup is a little different than most. Typically the indoor unit is installed on an exterior wall, so it’s easy to run the refrigerant pipe directly out the wall where it can then snake around the house to the external unit. Our unit is installed on an interior wall. Phil had to run the pipe through the interior wall down to the first stair landing then out. You can snake the tubing around the outside or the inside but not both, or at least not easily. This is one of the reasons why the external unit is placed where it it. It allows the tubing to exit the wall and go directly into the unit.

The last component is the drain. When running the unit in cooling mode, just like an air-conditioner, moisture in air will condense on the coils of the interior unit. Typically the condensate is drained to the outdoors near the outdoor unit. But I was concerned this would provide a air leakage point, so we’re draining the unit into the clothes washer and dryer drain, which is near the interior unit.  This required ordering a condensate pump to ensure we get proper drainage.

We won’t be ready to install the inside unit (part 2) until we have sheet rocked the first floor. In the meantime, back to the porch framing and siding.

Porch framing begins

Despite the several downpours today, they were able to start framing the west side porch. All framing is pressure treated. The deck will be Douglas Fir. We’re using Penofin Verde to seal the deck. It is the best ‘green’ sealer we could find locally. Later in the year we will trim out the pressure treated. For now, we’re just going with the bare minimum till we have a better idea of the final budget and move-in date.

All things siding

Jill and I took several days of vacation last week to help Warren with the siding. In five days, we finished siding and painting the north side of the house and we installed the roofets / sun shades for the 1st floor south facing windows (‘awning’ sounds so 60’s no?).

The roofets will get a metal roof covering like the main roof. We’ll finish the roofets when we install the metal roof over the porch.

Next up, porch framing.

Thousand Home Challenge

I was reading The Right Target, a post on Marc Rosenbaum’s blog a few weeks ago and realized that we too might be able to qualify for the Thousand Home Challenge.

The goal of the Thousand Home Challenge is to reduce the energy usage of 1,000 homes by 70 to 90%. It outlines two paths to meet the challenge. Option A is a specific reduction from current usage. We’re going to ignore that since we’re building a new house.

Option B seems to blend aspects of both passivhaus and net-zero. It is based on a low energy target and verified using actual energy usage data over a year. Renewables like our solar PV array, count as credits toward meeting our target. (I think the net-zero house movement would benefit from having a low energy target to avoid the ‘6,000 sf house with a 20 kW array’ problem.)

The target is based on a combination of climate, size, number of occupants, heat source and whether it is attached or detached. They have a handy spreadsheet you can download at the thousand home challenge website. Just punch in your numbers and it will tell you your target.

Here’s the run down on our inputs.

  • Climate – The closest weather station data was determined to be North Adams, Massachusetts, but I chose Glen Falls, New York, since this is what we used in the original energy calculations.
  • The home’s finished floor area is 1,200 sf.
  • Two occupants, not counting critters.
  • Detached single home.

Based on that scant information, the threshold calculator produced a target value of 5,619 kWh/year (site energy) if using electricity for heating. This works out to roughly 19.2 million BTUs/year.

If you remember back to our earlier energy posts, our energy consultants estimated our yearly energy use at 5,995 kWh/year, or 20.4 million BTUs/year.

So if we ignore the solar PV array, we would be roughly 376 kWh short of meeting the Thousand Home Challenge.

I like a challenge, so I’ve signed us up for more information and to start the application process. I’ll post more info as we progress through the process.

First MEGAWATT Hour

Today at approximately 11:30 am we produced our first megawatt hour. In 36 days we averaged 27.7 kWh per day.

I had originally thought we might be able to generate our first megawatt by July 4th, but we miss it by a few days. Turns out I did my math wrong. It took us 18 days to generate 500 kWh, so it makes sense that it would take a similar amount of time to generate the second half megawatt.

But hey, who’s counting anyway?

Plumbing

We had the plumbing venting roughed in a few weeks ago. Last week our plumber completed the wet pipes, hot and cold. We’re using Pex and a home run layout. Thought it looked very patriotic for a July 4th post.


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