Archive for the 'Heating' Category

January performance snapshot

Living in a super-insulated house, I get very excited when extreme cold weather arrives. It is the only way to see how well the house performs in low temperatures. This January was the coldest month that we’ve lived in the house.

We recorded 1380 HDD for January, 16% colder than January 2013. Our coldest day was January 3rd, with 69.7 HDD. Temperatures on this day ranged from a high of 2.8°F to a low of -8.5°F. Indoor temps ranged from 60°F at night to 70.5°F during the day.

I was particularly interested in how our air-source heat pump (ASHP) would perform in these long stretches of cold weather. Our unit (Mitsubishi MSZ/MUZ-FE18NA) is rated to keep producing heat down to -15°F. According to the specifications, the unit can produce 10,300 btu/hr at 5°F.

In order to determine how well the house performed thru the month I decided to keep the backup heat (electric resistance) off. This meant we relied only on our ASHP and the sun to heat the house for the month.

Looking at the data for January, it looks like our heat pump starts to fall behind demand when temperatures slip below zero. This appears to happen mainly at night. During the day, a small amount of sunlight can raise indoor temps even in the extreme cold. January 1-3 offer a good example.

I normally turn the heat down at night or off. I did this the night of January 1st. Night temps were not particularly cold, but continued to drop throughout the next day. By 7am Jan 2nd, the inside temp got down to 60°F and the outside temp had reached zero. By the time I turned on the heat again in the morning, the ASHP was not able to make up the difference for most of the day. A little sun helped get the inside temp up to a high of 64°F. On Jan 3rd, I considered turning on the backup heat, but I held out because the forecast said clear skies. By the afternoon inside temps were back up to 70 while outside temps hovered around 2°F. By Jan 4th, outdoor temps were well over the zero mark.

The January 2nd experience and a fortuitous conversation with Mike Duclos that day, convinced me to leave the thermostat set to 68°F when temperatures are forecast to be in single to negative digits overnight. This makes it easier for the ASHP to keep up with the demand and lowers the temperature differential it has to make up.

In total, our ASHP used 529 kWh in January. If we paid for electricity, our heat would have cost us just under $80.

2014-01-02-daychart 2014-01-03-daychart 2014-01-04-daychart

Check out the interactive version of these charts on my other site, Netplusdesign.

Estimating heat energy for 2012 – Revised

Now that we have January – March 2013 circuit-level usage values, I thought I’d go back and revisit my original 2012 heat estimate using a different method.

I had estimated January – March 2012 heat energy based on a linear regression analysis of our April – December heat values. There are a number of problems with this approach. Mainly that heat pumps use more energy the colder it gets outside, and secondly the amount of passive heat we gain from the sun can significantly reduce the amount of energy required for heat.

This time I used a less formulaic approach to estimate heat energy usage. I simply calculated the kWh/HDD per month for 2012 and 2013, and compared the values.

First lets look at the first 3 months of 2013. We recorded 3,239 HDD, a 20% increase from 2012 to 2013. We used 746 kWh for those 3 months. If we divide 746 kWh by 3,239 HDD we get 0.230 kWh per HDD.

Now let’s try the same for the first 3 months of 2012. We recorded 2,107 HDD and I estimated 327 kWh for heat energy. 327 / 2,107 = 0.121 kWh per HDD. That is a 128% difference from 2013. Something is clearly off.

Since 2013 was colder and less sunny, I would expect our 2013 efficiency to be less because heat pumps become less efficient at lower temperatures. So I manually adjusted the 2012 kWh values so that the kWh/HDD percentage was similar to the 2013 values, then I lowered it a bit to take into account the warmer temperatures and increase sun in 2012. Did I mention this wasn’t very scientific?

What we get is closer to 620 kWh for heat energy for the first 3 months of 2012. This is roughly a 90% increase from my earlier estimate. It also means that a 20% increase in colder weather roughly equals 20% more heat energy usage.

Q1 2012-2013 heat energy comparison

Looking at Q1 performance again, that means out of the 445 kWh increase in 2013, 65% of that increase was due to heat energy, 33% was water heating and everything else was 2%. That sounds a little more realistic.

Estimating heat energy for 2012

April 7 Update: Using a different method I estimated  that we used 903 kWh for heat energy in 2012, that’s 16% of our total energy use. That would have cost us about $117 (using $0.13 per kWh).

We don’t have a full 12 months of data for our heat, but using heating degree days (HDD) and performance thus far we can estimate that we used 591 kWh (+/-20%) for heat in 2012. That’s 11% of our total energy use for the year.

Here’s the math…

Our estimate relies on heating degree day measurements. This discussion assumes you are comfortable with HDD. If not, we recommend this excellent article.

First we had to determine the optimal base temperature for our HDD. After some trial and error using a quick prototype, we determined that 50 degrees (F) most accurately predicted heat energy usage. This results in the following formula to calculate kWh.

kWh = 0.2261 * HDD + 0.756

To start, we know the ASHP used 283 kWh during the time period March 16-May, and September-December. We just need to estimate January through mid-March.

We started recording temperatures in February so let’s start there. There were 522 HDD (base 50F) in February.

119 kWh = 0.2261 * 522 HDD February + 0.756

For comparison, there were 518 HDD (base 50F) in December. We used 108 kWh in December for heat. That’s about 208 Wh/HDD. We could assume that we would have used about the same amount of energy in February, although we generated twice the amount of electricity in February, which means we would have used a lot less energy for heat because we were getting heat from the sun. But let’s use the more conservative estimate for now, 119 kWh.

We don’t have our own temperature data for January but we can use to find the HDD at any base temperature for January in our area. Albany International Airport has the closest matching temperatures. Using a base temperature of 50F degreedays tells us there were 654 HDD (base 50).

149 kWh = 0.2261 * 654 HDD January + 0.756

For comparison we could multiply the 208 Wh/HDD measure from December above times 654 HDD to get 138 kWh. This number is lower but since the temperatures were cooler in January than December we know the ASHP would have to work harder to make heat, so the higher number makes sense. January was about 23% cooler than December. If we add 23% to 138 kWh, we get 170 kWh. Not very scientific but if gives a sense of margin of error.

As for March, we only have the second half of the month’s data for the ASHP, which used 19 kWh. There were 90 HDD from March 16 through the end of the month. That’s 211 Wh/HDD. As a check we can plug 90 HDD into the formula to get the estimated kWh.

21 kWh = 0.2261 * 90 HDD + 0.756

It’s pretty close.

There were 174 HDD from March 1 to March 15.

40 kWh = 0.2261 * 174 HDD + 0.756

Again for comparison, 211 Wh/HDD (from above) * 174 HDD = 37 kWh. Pretty close, it was twice as cold and about twice the amount of energy use.

40 (3/1-3/15) + 19 (3/16-31) = 59 kWh total for March.

Now we can estimate our total heat energy for 2012.

283 (3/16-12/31) + 149 (Jan) + 119 (Feb) + 40 (for the missing part of March) = 591 kWh for 2012. That works out to $77 for heat, not counting delivery charges. 591 kWh represents 11% of our total energy use for the year. That seems about right since the ASHP represents 9% of our total energy use for the time period we have circuit level data.

There is at least a 20% margin of error for the estimate and that’s not even taking into account space heat energy contributed by the sun.

Chart showing HDD vs ASHP kWh use.

Polynomial curve fit analysis

I’ve also been looking at a polynomial fitted curve to better estimate kWh based on HDD. This makes some sense because heat pumps are more efficient at higher temperatures and less efficient at lower temperatures. A linear regression analysis would not be able to capture that type of operating behavior.

* Note: Electricity supply cost calculated using the last bill we payed for electricity, May 2011.

Heat pump install complete

Dee’s Electric came out yesterday to complete the installation of our heat pump. They will have to come back to complete the ERV wiring. I’ll explain a bit more later. I just wanted to post a few pics of the installation.

Siding, gutters, wood flooring, stone wall and tile grout oh my!

It’s busy time at Uphill House these days. Warren finished the siding yesterday (except for one tiny section where the east entry is going). We started siding back in June, so it was a big milestone for us. The gutters are almost finished too.

The brown maple wood flooring arrived today from Vermont Plank Flooring. It is beautiful. It has to sit for a couple of weeks to acclimate before we start to install it.

Jill was busy grouted the bathroom floor tile today while Warren and I built a small stone retaining wall that will form the parking area next to the house.

Meanwhile Howie is finishing the taping this weekend and we should be able to start priming the first floor Monday night.

Our HVAC guys are scheduled to install the indoor unit of our heat pump on Thursday.

And we’re hoping to take a trip to Ikea to purchase our Kitchen cabinets next weekend.

We’re going to need a vacation very soon.

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.

Mechanicals (ASHP & ERV)

Click image for 3 page PDF (80k)

With little going on at the house due to the weather, I’ve spent some time considering the systems and ducting required for the house. I put together an updated set of plans and specifications to start talking to a few heating contractors.

Click image for 4 page PDF (1Mb)

Our energy consultants recommended a mini-split air source heat pump and two approaches to selecting a model. We could either go with a model that is less efficient at lower temperatures and supplement the heat with electric resistance when temperatures fall below 5 degrees. Or go with a larger unit that will meet all our heating needs at lower temperatures but is less efficient overall. I am inclined to go with the smaller more efficient unit that will meet our needs for the bulk of the heating season.

I only wish the makers of the mini-split units would hire a real designer. Their units are u-g-l-y. I’ve tried to minimize the visual impact of the indoor unit, but it’s still a big plastic box on the wall. Thankfully the mini-split units only require a small refrigerant line connecting the indoor and outdoor units and no ducting.

We also discussed the ERV versus HRV question with the energy guys. It was actually quite easy. From everything I’ve read, the most important factor is low energy use and efficiency, not which type of system. Right now that still seems to be the UltimateAir RecoupAerator ERV. In our cold climate it will nice to recover some humidity in the winter. The unit will be off and the windows open for most of the summer.

But ventilation requires ducting. This was the main reason we decided to use open-web trusses, to make it easier to install all the ducting inside the conditioned envelope. It has also been a challenge to keep all the ducts out of exterior walls.

I attempted to model the ducting as realistically as I could (see 2nd PDF above), taking into account the location of walls, floors, web trusses and plumbing. I have not found many good resources online for designing duct runs, nor have I found a good online catalog of various types and sizes of ducts for small residential ventilation systems.

I’m sure any mechanical contractor eye’s would glaze over looking at these models, but it was a good exercise for me. I’m now armed with lots of questions when interviewing heating contractors, and it will be interesting to see if I’m even close.

There are a few difficult areas: 1. ) Getting the unit in the basement vented to the outdoors without a lot of 90 degree bends. 2.) One of the bathroom walls is located above a truss making it difficult to get ducts into the wall. 3.) A return duct for one of the bedrooms must either cross over a supply duct or go up and over the stair in a chase that is still inside the air barrier. 4.) About the time I started wrapping this up into a post, I read that it will soon be required to place the in/out-take vents at least 4 feet above grade. (See The Energy Star Homes Program Raises the Bar with Version 3, by Martin Holladay at It’s a good idea considering the amount of snow we’ve received this year.

I will revise the drawings after we choose a heating and ventilation contractor.

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