Moving Heat in TerraHaus with a Heat Pump and HRV

October 2012 (Photo credit: Jonah Gula)

4/14/2012 Update. First winter results are being analyzed. Looks  good! Students report that the house is comfortable, cozy, and fun to live in even with the interruption of tour groups)! 

The last two green building components of TerraHaus I want to add to the blog are the Heat Recovery Ventilator and the Heat Pump.

Our Daikon Heat Pump

The name of the Passive House standard is derived from the emphasis on passive heating. Passive heating means that the space heating requirements are met largely through passive means—the natural sunlight coming through the windows, the heat generated by appliances in the course of routine daily activities, and even the warmth given off by the residents.

The rest of the heat needed to keep the building at 70 degrees is generated by small electric baseboards and by a heat pump. Heat pumps are more common in the southern US than in the north. They have the advantage that they can be used to cool air in the summer and heat it in the winter. They are more efficient than electric heat or combustion heat because they are not creating heat but rather are moving it from one body of air to another.

A heat pump is, in a sense, a type of solar energy because it is the heat from air warmed by the sun that is moved into the home for space heating:

…outside air is heated by the sun (even what we consider cold, winter air contains heat energy given to it by the sun).

…the air is drawn through an evaporator where it warms a refrigerant like freon into a gas even at low temperatures. The refrigerant in a gaseous state is compressible.

…as a compressor reduces the volume of gas, the temperature goes up.

…the gas is transferred to a unit in the house where it condenses and releases its heat.

The heat pump circulation can be reversed to cool the house in the summer.

Given that the heat pump is not creating heat but simply moving the heat energy around, it is very efficient. One kWh of electricity for the fan and compressor results in 2.74 kWh of heat. (As our Sustainable Energy students on campus can explain, COP is the Coefficient of Performance and is calculated as the Btu of output produced divided by the Btus of electricity used. Our COP = 2.74)

In addition to the heat pump, individual bedrooms have small sections of electric baseboard which the occupants can set themselves.

HRV—The Magic Box

Once the heat is captured in the house, from passive means or from the heat pump, we want to hold onto that heat. Air infiltration is one of the major forms of heat loss in most homes, but we benefit from air infiltration too because it ventilates our homes. TerraHaus is so tight that we rely on mechanical ventilation. In order to hold onto the heat though we want to extract the heat and use it to warm the incoming air.

To do this we use heat recovery ventilation (HRV). The stale air passes through an aluminum plenum which absorbs its heat. Air entering from the outside passes through separate passages in the plenum and picks up heat.

The Magic Box

Our HRV unit, a Zehnder CA 550, is rated as over 88% efficient meaning that if it is zero degrees outside and 70 degrees inside, the incoming air is warmed to a temperature of 62 degrees before it enters our rooms. No wonder Alan Gibson of GO Logic estimates that 10% of the 90% energy savings of TerraHaus comes from the HRV! (This also explains why he refers to the HRV ventilator as “the magic box.”) The ventilator and flexible tubing ductwork is housed in a space between the ceiling and the bottom of the scissors truss system. The trusses are sealed on the bottom with Zip sheathing. This puts the HRV within the thermal envelope with just two perforations of that envelope for intake and exhaust.

The whole house is ventilated by this system, but the duct work and vents are set up to pull stale exhaust air from bathrooms and the kitchen while supplying fresh air through the bedrooms and living areas. The system is designed to assure that TerraHaus meets the ASHRAE standard of 35% air exchange per hour or 15 cubic feet per minute per occupant. Most homes in Maine, regardless of their age, meet this requirement, but most do so through natural leakage of heated air through the upper portions of our homes and intake of cold outside air from leaks in the lower portions of the building.

Douglas Fox, Director, Center for Sustainability and Global Change, Unity College


Filed under Uncategorized

8 responses to “Moving Heat in TerraHaus with a Heat Pump and HRV

  1. Charles Kennedy

    The statement, “Given that the [Daikin] heat pump is not creating heat but simply moving the heat energy around,” doesn’t appear correct as described in the article. ……compressing a gas isn’t a passive form of heating. It’s a form of work that imparts temperature energy to the gas through increased pressure. The latent heat is then removed through condensation. It’s a form of active heating that relies on an engine….. not planetary energy that is already present. The compressor creates heat according to the Ideal Gas Law PV=nRT
    According to Charles, pressure and temperature are proportional in an ideal gas

    The heat pump appears to be “semi-passive” with a compressor assist. In addition to drawing in outdoor air heat through evaporation, the compressor will probably need to increase the temperature of the refrigerant gas to some point above the ambient indoor temperature for there to be net heat energy flow into the indoor space. I think maybe the COP will decrease in the cold Maine winters.
    The only passive heat described in the article appears to be the sunlight coming through the windows. Electric appliances and electric baseboard heat are usually not passive as they usually rely on electricity generating engines. I’d argue that solar, wind, wave, tidal, ocean thermal or geothermal produced electricity are all relatively passive planetary energies. I suppose you can argue body heat is passive planetary energy, but there is usually a long supply chain of GHG emissions that support it! 🙂

    The HRV is pretty cool though!

    • Charles,
      The word “passive” in passive house generates a lot of discussion. I agree that the heat pump and electric baseboards are definitely not passive sources of heat. In “Passivhaus” circles, however, “waste heat” from appliances that are used for purposes other than space heating–radio, tv, blender, lighting, laptops, hot water from the shower, etc.–can be considered passive space heating sources. Most homes lose this heat so quickly that accounting for it is not worthwhile. In a Passivhaus, the superinsulation, lack of thermal bridging, compact form, and superior air sealing reduce the heat load requirement so much that these sources account for a significant portion of the heat needed to keep the house comfortable. Combined with passive solar gain, reliance on active heat sources such as the heat pump and electric baseboard is minimal.

  2. Charles Kennedy

    I agree. Towards the end of my post, I was being a bit loose with the technical definition of passive energy. I guess I was suggesting that the definition of passive energy be more closely linked to GHG emissions whether an engine is involved or not. Passive energy (to me) is more about using planetary energy that is already present without further mining of carbon fuels.
    I actually don’t know much about heat pumps but what little I’ve read in wikipedia suggests that on a typical Maine (cold) winter night the COP of the heat pump will begin to approach 1.0, which won’t be much better than running the electric baseboard. Waste heat sources (radio, tv, blender, lighting, laptops, hot water from the shower) will be minimal or absent during the night time, when heat is needed the most. Because we are naturally a solutions oriented culture, I think sometimes in the chase for sustainability, we have a tendency to overlook or downplay its inherent limitations or its dark side, which is unsustainability. I’ve yet to see adaptation integrated into sustainability.
    That HRV is cool though – reminds me of a heat exchanger I saw once – the liquid cooling jacket of four natural gas fired microturbines was piped across a heat exchanging plate which transferred heat into a community swimming pool (pool heating was augmented by NG fired boilers). Due to the heat exchanger arrangement, the municipality wasn’t able to accurately calculate the GHGs associated with pool heating from microturbine waste heat.

  3. This is the first time i am hearing about Heat pumps. I was wondering what way should we use to reduce the heat especially during winter days. Thanks a lot for this article.

  4. Hi, read this with great interest. We have 2 products that you may consider for the future 1) is the Magic thermodynamic box and 2) the magic heating box. We are the worldwide patent holders to these 2 products and you can see them at I invented the hot water one. The magic heating box will save you 30 to 50% of your heating bill and does away with air to water systems which at high temp output are actually worse off than running your existing boiler. Please have a look and any feedback on the products would be welcomed whether it be positive or negative. I launch both products here in the uk 6 weeks ago

  5. runuphill

    Hi, I searched long and hard for the flexible ducting that you show in the HRV image, can you tell me what it is and where you bought it? Thanks!

  6. Hi Charles, I would also like to comment on what Scott has said with regard to the two products he mentions. They are very unique and Innovative and we have customers here in the UK that are delighted with just how simple the installation was and the savings they can see they are going to be making. Although the magic heating box does only heat and therefore cannot be used as a dual purpose such as the Daikon Heat Pump can with reference to cooling, it would be interesting to see what the results would be if someone had the magic heating box fitted in a country that actually gets hot summers (wouldn`t need to use heating) and cold winters need heating,

    Probably for this type of climate I would say the heat pump would possibly out perform the magic heating box in regards to cost savings as it would be able to be used for both heating and cooling. However with regards to the thermodynamic magic box it can be used all yr round as most people require hot water. We also sell on other system that can do most of your heating and can really reduce heating bills details of this system can be found at

    Again like Scott said it would be interesting to hear your thoughts and opinions. Will keep checking back to see if you reply to this post.

  7. Natalie Hinebaugh

    This is a beautiful thing!!
    If more Americans knew about these great wonderful energy saving systems I’m sure that they would be in more demand. The saving in electric bills alone I’m sure makes up for initial costs! With all the different ways of social media what is stopping every new house from utilizing this phenomena?
    Kudos to you!

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s