Heating & Cooling
An experiment of HVAC integration
Heating & Cooling
An experiment of HVAC integration
Passive design approach
Working with nature
The design of the house leaned heavily on passive techniques. First with the orientation, window specs, and overhangs. Then with thick, insulated walls and tight air sealing. And finally with the thermal mass and operable clerestory windows to take advantage of natural cooling.
Window Overhangs
Each window was specified according to its location, and the external overhangs on the South side were optimized to provide free heat and light in the winter, and shading in the summer.
Double stud walls / Insulation
The double-stud walls use 24” spacing of standard 2x4s that are gapped to create a 9 ¼” cavity. This allows for generous cellulose insulation, as well as a thermal break at each stud, except of course where fire blocking is required.
Triple pane windows
Typical windows have only a fraction of the insulation properties of the wall, but triple-pane windows are available from several manufacturers. Ours used a plastic center pane to create two separate chambers. And the casement style improves air sealing and noise performance.
Custom doors with locking hardware
We expected that our custom, insulated doors would struggle to seal tightly because of their height. So we used a three-point locking system to pull them tight. Although the sealing performance was improved, the extra effort was not universally appreciated by the occupants.
Aerosol-sealed building
After large gaps were filled using traditional sealants, we then used an aerosolized material to fill the multitude of small cracks remaining in the air-control layer of our house. What was prototype at the time is now available nationally. Photo credit: Paul Fortunato
Project Learning: Our paint absorbed quite a lot of heat, and the cladding of the South wall got hot on many Spring days. A better design would reduce that heat gain, and then minimize its transmission into the air cavity behind.
Radiant heating and cooling
Design to the mechanisms of comfort
The human body feels thermal comfort according to several factors; we all know of air temperature, but often overlooked is the mean radiant temperature around us. To improve comfort with less energy, one of our biggest experiments was to use radiant cooling, with hydronic tubes under the first floor slab and behind the second floor ceiling. The results were mixed though, and ultimately, we were not satisfied with its overall performance, especially the response times.
Radiant Geothermal Heating & Cooling
An experiment in energy saving
Project Learning: The radiant heating
provided gentle warmth in the winter, but the cooling response times were
too long in the summer, and some people felt the slab to be too chilly underfoot.
Borehole System
A cheaper way to harness energy
Geothermal heat pumps can be very efficient because they exchange heat with the relatively-constant earth, instead of with the outside air which may be hot or cold. For an efficient design, geothermal heat exchangers are typically several hundred feet deep, which can be very expensive. The Smart Home experimented with a new style of exchanger, using 250' of tubing, but coiled into a helix only 20' tall inside of a wide borehole. We installed eight of them for a fraction of the cost of typical wells. As a further experiment, four of our boreholes were wetted. They worked well, but we recommend the dry style.
3-Mode Heat Pump
Capturing Waste Heat
The same high-efficiency heat pump used to heat and cool the house is also used to heat the hot water tank. One of our favorite features of this integrated system is that during the summer, the waste heat pulled out of the house can be used to make hot water for free.
Ventilation Cooling
Semi-Passive Cooling
To improve cooling efficiency, whenever the nighttime air cools down, we automatically pull that cool air into the house through a filtered whole-house fan, then out through the four bath fans.
Actuators can be tricky
Our original strategy for vent cooling was to open the clerestory windows. But the aftermarket window control motors were finicky, so we switched to using bath fans.
Balancing pressures
Mechanical ventilation is important, but you want to avoid using only exhaust fans since they will pull makeup air in through cracks and leaks. We ran several experiments to optimize the balance, using the whole house fan to push fresh cool air into the house while the bath fans pulled the warm air out.
Cool air is nice, cold air is not
We had to reduce our nighttime ventilation cooling because the incoming air blows straight into the living room, and several occupants found the air to be too cold on many nights as they tried to watch late TV.
Controls
A necessary complication
From a user viewpoint, our goal was for the house to work seamlessly in
the background, regardless of the complexity of the controls. Our user
interface tried to simplify the myriad options into an easy experience.
Energy modeling
A surprise not simulated
Our engineering team ran dozens of simulations using sophisticated energy models to optimize our initial house design. Those models make assumptions about efficiencies, temperatures, setpoints, etc.. Since we already knew that different setpoints would increase or decrease the energy use, we didn’t simulate those effects. Big mistake. Our system was designed to coast up to setpoint most days, so a small decrease in cooling setpoint had a huge impact on energy use.
First year HVAC modeling improvements
| Original Model | Fully Calibrated Model (assumed thermostat setpoints) |
Fully Calibrated Model (actual thermostat setpoints) |
|
| Heating (kWh) | 616 | 470 | 397 |
| Cooling (kWh) | 174 | 790 | 1,457 |
| Vent Fans1(kWh) | 151 | 220 | 234 |
Project Learning: Our original calculations
assumed very warm setpoint settings in the summer (since the radiant surfaces
would feel cool). In that scenario the house coasts all day, cools at night,
and rarely runs mechanical cooling. However, if the user turns the set point
down a few degrees that coasting concept fails and the cooling energy use doubles.
Technical Resources
The Smart Home project was about generating insights for others to build upon. As you develop your next home, please use any of our specs, drawings and data to help create a more sustainable future. See All Downloads
Mechanical and Plumbing Plans
See how we originally designed the integrated system, the heat pump circuits and the boreholes.
Everything We Learned about HVAC Report
Through the years we learned a lot. Dig into the details in this report.
Take a tour of what made the Smart Home “Smart”
Prepare your home for a sustainable future
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