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.

9 Responses to “Mechanicals (ASHP & ERV)”

  1. 1 Jim Merrithew 7-March-2011 at 9:29 am

    Hello Larry,

    Your drawings are extremely easy for the layman to follow and understand.
    What software are you using for your designs?

    I agree with your comments on the intake/exhaust vents. About 4 years ago, I installed a high efficiency propane furnace in my old house. This is a closed loop unit with a dedicated ABS duct for fresh combustion air and an exhaust duct. I followed the recommendations to place the vents at least two feet above the ground. The next winter, it snowed and snowed. We had almost broke the historical record for snowfall. Due to drifting snow, I had to dig these vents out four times. If I was to do this again, I would follow the guidelines you refer to.
    Thanks, Jim

    • 2 Larry 7-March-2011 at 10:38 am

      Thanks Jim, I’m using Google SketchUp. It’s free for modeling. If you want to output pretty pictures or print to scale then you will have to buy a Pro license. It’s great for figuring out geometry but requires a bit of creative effort to be useful for construction drawings.

  2. 3 Jon Leeth 24-March-2011 at 8:47 am

    in your ERV evaluation, did you consider Conserv by Dais? I hadn’t heard of the Ultimate Air unit, and went digging to learn about their air/humidity transfer media/mechanism and found what I suspected, an enthalpy wheel. A moving part. Something to consider in the maintenance of the equipment. I do like what I see in the anti-frost feature, the controlled features of night-time cooling and CO2 monitoring. Very cool.
    Incidentally, I was curious about your predicted annual energy usage, and compared to my own. My house is a tighter-than-usual, but contractor-compromised single story, brick exterior, by all appearances regular house. With gshp, all electric (except for a propane stove top – I refuse to go back to electric cook top)I used 10289 kwh last year (2010.) I’m fortunate enough to be served by a CO-OP that has a telemetric grid that I can log into a monitoring site and get hour-by-hour usage information.
    just found your blog, but I plan to spend to spend lots more time reviewing.

    • 4 Larry 25-March-2011 at 6:33 am

      Hi Jon,
      The ERV was recommended by my energy consultants. They have had a good experience with the UltimateAir. There’s a good discussion of ERVs on GBH you might want to check out, Most energy-efficient ERV?. It’s an old thread, but a good starting point.
      As for yearly energy use, it depends on your climate, size of your house, insulation levels and number of people living in the house. I think the energy they estimated for us is a little high, but we won’t know till we finish the house and live in it for a year.

    • 5 Larry 25-March-2011 at 7:30 am

      Martin Holladay at GBA just posted a product review, A Roundup of New Green Building Products, including an HRV from Venmar. Check it out.

  3. 6 climbguy1 1-April-2011 at 10:58 am

    Hey Larry,

    I have looked everywhere for a Sketchup model of open web floor joists like the ones you have used here. Can you share the model with me?

    I would really appreciate it.

    You have an amazing blog here and have inspired me to start in on the process of writing my own during our owner-build craziness.

    Thanks so much for sharing everything.


    • 7 Larry 1-April-2011 at 11:58 am

      Hi Ryan,

      Thanks, I’m glad you’re enjoying our blog. Glad to see you have started your own.

      I’ll send you a sample truss from my model, but your truss dimensions will likely be different. They are usually engineered to the specific requirements of your situation. But if you’re just looking for a visual approximation it may work for you. I modeled mine based on the dimensions from the truss engineers.

      Good luck on your own home building project,


  1. 1 March Update « Up Hill House Trackback on 31-March-2011 at 6:42 pm
  2. 2 Mechanicals rough-in « Up Hill House Trackback on 17-May-2011 at 7:30 am

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