What mechanical systems and fuel choices make sense for homes in a warming world? Alistair Jackson, LEED for Homes Rater Responds

With recent record high temperatures in North America and Russia and massive flooding in Pakistan as I write this, the evidence that climate destabilization is real – whoever or whatever is causing it – continues to pile up.

If we can’t persuade people that they have the power to avert that change – perhaps we need to start talking about adaptability and resilience instead of focusing only on reducing fossil fuel dependence and carbon emissions. This doesn’t mean we’re giving up. Rather, we’re acknowledging our human fallibility. We’re adding suspenders to back up our belts by designing buildings that consume and release less carbon AND that can still keep us comfortable in a destabilized climate.

The first step to this is a climate-responsive envelope. In most climates, this typically means as thermally efficient and air-tight as possible with design details that manage solar gain and natural ventilation to best advantage.

By choosing air-tight, we are already committing to some kind of mechanical system to ensure the indoor air quality is healthy for both occupants and the building itself. On one end of the spectrum, some would argue that an envelope that meets the local energy code is as thermally efficient as you should go – based on cost/benefit and diminishing returns. On the other end, some advocate for thermal performance and air-tightness along the lines of the Passive House US standard or something similar.

Where you stand on that spectrum and why you choose to stand there will determine the direction of your next steps - your mechanical system and fuel choices.

Here in Western Washington, where O’Brien & Company is based, we are blessed with a climate where a well-designed, well-built house should only have a few days of the year when mechanical cooling might be desirable, so we tend to think in terms of heating systems only. In fact, some would say installing mechanical cooling in a residence is anathema to sustainable design. But will that be true in 10 years? Or 20 years when it comes time to replace that aging HVAC system? What if in 2030, we have 30 days when cooling is desirable? Or 40 or 50 days? Cooling could become essential in those conditions. The challenge is to choose systems that can, if necessary, adapt to these changes efficiently. Let’s look at options from both ends of the spectrum for a builder in our home town – Seattle, WA.

Code Compliant Envelope

The code-compliant builder is weighing energy efficiency benefits against current market costs for construction, mechanical systems and fuel. The truly cost-conscious will use electric resistance throughout, but this is not an efficient choice when you look at the percentage of energy in the source fuel that is delivered as heat into the home. In a code-compliant envelope, this means high fuel costs. A central heat source, such as a gas stove, with supplemental electric resistance heat in remote spaces will lower overall energy costs and fuel consumption/carbon emissions. But both these solutions lack no alternate distribution options if you have to change fuel source and/or add cooling.

• For small homes, use a ductless mini-split air source heat pump with an HSPF of 10 to 12 as your primary source. Ventilate with continuous exhaust fan and trickle vents. The new generation of ductless “mini-split” air-source heat pumps offers a great alternative to that gas stove – electric fuel but very efficient and with cooling built in when you need it. If only the utilities would add some new construction rebates to make it more economically attractive.
• For bigger homes, if you’ve got gas, go forced air with the entire system inside conditioned space. Leave room for a DX coil at the air handler and wait for central air heat pump technology to catch up. Air distribution is more adaptable for adding cooling, but less adaptable to changing fuel sources so you’re gambling that natural gas will be available as a cost-effective fuel for buildings for the long term. Also, there’s a duct system in place for filtering and distributing ventilation air to where it is most needed. Use and HRV, ECM fan and smart cycler for best results.
• Switching to hydronic distribution (small water lines instead of large ducts) generally costs a little more up front, but gives you more efficient heat delivery and more fuel source options – combustion, electric resistance, heat pump, and solar thermal all work well with water. But adding a cooling option is trickier due to the complications of condensation – who wants sweaty radiators or floors? And you still have to add fans, and trickle vents or ducts for ventilation.

High Performance Envelope

The high performance builder is focused on the long game, weighing benefit against lifecycle cost rather than first cost. Because the focus is on a super efficient envelope, energy gains and losses are more modest, making “comfort” easier to achieve, but HVAC sizing harder to do. There just aren’t systems small enough for these low load homes. Because the envelope is tight, a balanced ventilation system is necessary, most likely with energy recovery, so air distribution (small diameter ducts) is “built in.” Whatever the house size, heating (and cooling) can be delivered in the central living space, and distributed to the rest of the house via energy recovery ventilation (ERV) system. This works because the envelope leakage losses are so small and the inner surfaces of walls and windows are warm in summer, cool in winter.

• Put a hydronic coil in your ERV:
• • Either plumb it to a gas or electric tank water heater for now;  
  • Or, install a ductless mini-split heat pump for the central living space and wait for the next generation of ductless air-to-water heat pumps from Europe and Asia. 

Choosing a Fuel Source

Above, we’ve suggested both gas and electric solutions for both house types. Is there a preference? We think so. Most of our private utilities say natural gas is clean and efficient and has a relatively low carbon footprint compared to the other fossil alternatives or electricity. But is it really so? The latest ductless mini-split heat pumps offer coefficients of performance (COPs) of 3.5 and getting better. That’s 3.5kWh of heating (or cooling) for every 1 kWh of electricity consumed. That more than offsets the generation and distribution losses of burning gas in a central plant to make electricity.

These systems do cost a little more to install than most of the other “small” systems and they do need a little more maintenance for best performance. But the upside is significant – that’s why they need more support in the marketplace. Many of the utilities continue to favor rebates for high-efficiency gas systems; they would rather you burn gas at the point of use than at the generating plant. They are subsidizing the development of housing stock that’s dependent on a fuel that makes it less adaptable and resilient to a future climate and fuel market that is less than predictable.

We suspect that eventually, energy- and molecule-dense fossil fuels will become too valuable as transportation fuels and chemical feedstock to be used to heat houses, making electricity a home fuel of choice. With PV prices tumbling and incentives improving, an all-electric, low-load, heat pump home might affordably go “near-zero” right now. As an example, look at the Sunnyland Sun House constructed by Living Laboratoriesin Bellingham, WA. The home has achieved high performance on a cost-conscious budget (appealing to both ends of our spectrum!) with a tight efficient envelope, a mini-split heat pump, exhaust-only ventilation and 1.4kW of Silicon Energy’s Washington-made PV. Although this all-electric "net zero-ready" home will collect an unimpressive $140 in rebates from its local utility, it has earned an impressive Home Energy Rating System (HERS) index of 57.

Alistair Jackson, LEED AP, CSBA is Principal in Charge of O’Brien & Company’s Residential Technical Services, and is a LEED for Homes Rater, LEED for Homes Faculty, Energy Star Verifier and Performance Tester, an ARCSA Accredited Professional, and a Built Green Verifier.  He also was a major contributor to The Northwest Green Home Primer on the subjects of energy, fuel futureproofing, and mechanical system choices.

Did you enjoy this article?  You might also like these Building Capacity Blog articles: 
Tips for Optimizing Forced-Air Heating and Cooling Systems Part 1
Cracking the Energy Code: What Will It Cost?
Gut Rehab: Groll Residence Turns 1918 Home from “Leaky” to Green
Type of Construction for Passive House System
Creating Effective Energy Efficiency & Conservation Strategies Part 1
Balancing Energy Efficient Adaptive Reuse with Historic Preservation

Comments

[Mike Nelson]

If the PV system is a Silicon Energy made in WA product it should be eligible for 54 cents per KWH produced annually for the next 10 years. In Western WA it should produce about 1400 kwh per year. so it should earn about 750 dollar a year and is eligible for for a 30 percent upfront federal tax credit. A made in Washington Solar electric generation project is one of the best revenue generators that can be included in a new home.

[Alistair Jackson]

Thanks for this observation, Mike. You are absolutely right about the production incentive the house will collect for its Silicon Energy PV system. But wouldn't it make more sense to use incentive dollars to reward builders for measures that reduce loads and conserve energy BEFORE they start adding renewable generation capacity. In this case, the builder built a low-load house because he knew it was the right thing to do. Not all builders are so conscious of the priorities.