Our gas furnace just died. Kaputt. Middle of Winter, -20ºC at night. Time to replace it – before the pipes freeze! The first thing we did was call the company whose sticker was on our ductwork: Custom Comfort. Now you’d think a big company like that, in an emergency ‘no heat’ situation, would only be interested in selling you whatever equipment they have in the back of their truck. Not so. Their patience with my barrage of questions was more than what I would expect from even a smaller, green engineering outfit. Being a zero-carbon zealot myself who drives an electric car and has built a career on designing zero carbon architecture for others, I would be a complete hypocrite if I simply replaced a gas furnace with a gas furnace, right? So making this phone call ran parallel with opening about 50 browser tabs and several late nights researching the latest and greatest on ASHPs or Air Source Heat Pumps, because the space heating system of the future is zero carbon. Let’s dig in…
Envelope First (Unless Your Furnace Dies First)
You may know that the best strategy for achieving net zero and zero carbon buildings is to first reduce the wasted energy that escapes through poorly insulated windows, walls and roofs. With passive design strategies, these thermal loads can be reduced as much as 90% in an advanced deep energy retrofit. Then, you replace your mechanical, heating and air conditioning systems with hyper-efficient equipment like heat pumps. But there is one glaring problem; you cannot do envelope improvements in a single day and they are expensive. Replacing our windows is an $80k job. Upgrading our roofing and insulation is a $40k job. That said, deep energy retrofits are key to reducing our climate impact since buildings represent 40% of global GHG emissions (per World Resources Institute), and since existing buildings will still constitute 80% of the building stock in 2030, we need to find ways to get them to zero carbon also.
There is nothing quite like having a furnace die on you to force your hand. The strategic order of envelope first, equipment second is out the window – there is no choice. Equipment replacement jumps to the front of the cue – but replacement with what? On reviewing reams of research in the space of a few short days, and with the luxury of ready access to professional engineers and HVAC consultants to advise us, we have finally arrived at the best options for a replacement system. If you find yourself in a similar predicament, hopefully the info we have distilled here can help you make the best choices for your own home. If you wish to read no further the Short answer is: get a gas/electric hybrid ASHP system, and if you can’t afford that, just get the gas furnace that can support the ASHP in future.
These are the respective pieces to a typical residential forced air system with AC:
- Gas or Oil Furnace
- Exterior Air Conditioner (a heat pump!) with coils in the furnace supply ducting
- A ventilation system (often just exhaust fans) or HRV/ERV
This is how you can prepare to swap these components out for for a net zero upgrade:
- *Replace Oil or Gas furnace with a unit that can support an ASHP system, this could be a) a new gas furnace, b) a new electric furnace or c) just an air handler with a secondary source of heating ie. woodstove
- Replace your exterior AC unit with an ASHP that can both do cooling AND heating
- Add the most efficient HRV/ERV with filtration
*Note: Without substantial insulation/window upgrades your electrical panel may need an upgrade for increased capacity to support b) or c).
Electric or Gas – A False Choice.
The first thing I should mention is that we were initially under the assumption that we had to make an all-or-nothing choice: basically a) go 100% electric to get to zero carbon, or b) replace a gas furnace with a new gas furnace and lock in to a carbon-emitting heating system for the next 20 years – the estimated life of the system (Warranties are for 10 yrs).
That’s a false choice. You can in fact get the best of both worlds. Most modern gas furnaces with ECM motors are controlled by a board/computer and this lets the system ‘modulate’ to match heat loss of the building. So if you replace your windows for better triple glazed units, the system simply modulates down to the lower load, to match your new and improved, lower heat loss. This also lets the air handler of the furnace receive a coil from your existing air conditioner (it’s probably there already), which, when it dies on you, can easily be replaced by a coil from a future ASHP.
The cost difference between simply replacing a gas furnace with installing a 100% electric, complete ASHP is extreme – it’s literally 3x the capital cost – $12,000 to $15,000, when just a Gas Furnace alone costs $4,000 to $5,000. The ASHP will more than likely also result in a net increase to my your total utility spend of about 20%. If those were your only options, yes, it would hurt to go green. But fortunately we can do it in baby steps by splitting the system – which is literally why they are called ‘split’ systems, and we can start building this new system on the foundation of a newer-model gas furnace to ease the pain.
The Heat Pump portion (shown on the right above) lives outside – just like a conventional central air-conditioner unit. The Air-handler and coils that connect to your ducts lives inside where your furnace does – that is what comprises the 2 parts – or the split. The Air Handler can have a furnace with a gas burner and heat exchanger, or just coils from your AC or Heat-pump – or both, what is called a hybrid system. Most of the major brands package their systems this way now. Coleman calls theirs the Echelon Series, Carrier calls theirs the Infinity series. They all cost roughly the same, and part by part, they are roughly equivalent with similar warranties. From the various engineers and installers I have spoken to, Coleman, York, Lennox, Keep-Rite, Luxe-Air are all basically different badges on the same groups of components, so it’s more important to look for the highest levels of efficiency, and to the flexibility of the system to convert from a 100% gas solution, to a 100% electric solution when you are ready for that (like after you upgrade your older windows and lower your heat loss). Simple right? Just don’t expect a simple answer from your HVAC sales reps, most of these folks, well meaning as they may be, seem narrowly focused on the details of equipment to solve a particular problem, not the bigger picture of supporting a house with aims to eventually become net zero or zero carbon. That is why we had to figure it all out for ourselves, and dear reader, for you also!
The magic of an ASHP is, unlike a 100% electric furnace, for every unit of electricity you put in, the system wrings out 3 units of equivalent thermal energy from the outside air at around 10ºC and about 2 units at -10ºC . The systems keep getting more and more efficient as the technology advances over the decades. While the marketing literature emphasizes the SEER – of up to 20 – this is cooling season performance only. What you should pay more attention to in Canada – with our predominantly heating climate is the HSPF or Heating Season Performance Factor. The system we are looking at has an HSPF of 11. If you would like to learn more about heat pumps in general, here is an excellent guide published by NRCan, although its latest revision was in 2004. What has changed since 2004 is the SEER and HSPF have steadily increased since that time, with compressor and coil improvements resulting in a SEER of 20+ and HSPF of 11+.
When we initially plugged in the data from our proposed system, a 100% electric ASHP with no gas backup – we were surprised to see an actual INCREASE of our carbon footprint. This is because we withdrew the REC offset for Bullfrog green natural gas from the second scenario with the proposed ASHP electric system. Even the relatively clean energy mix in Ontario does have a carbon footprint – due to natural gas-fired electricity generation, contributions that represent 5% of the mix as of now, but that is set to increase as much as 400% due to the short -sightedness of Ontario Power Generation). So while we shift from using about 23,000 eKwh of Gas (about 2,225m3), we then start using an equivalent amount of electrical energy (reduced by an efficiency factor due to the heat pump of 2-3x averaged over the heating season). This extremely counterintuitive result means that:
- we would need to continue paying for Bullfrog REC’s even after the switch in order to keep our carbon footprint under zero and
- that the REC does more to lower our carbon footprint than switching to a 100% electric ASHP.
Therefore the best interim solution to keep our carbon footprint under zero includes replacement with a hybrid system, and maintaining Bullfrog RECs for a green natural gas ‘offset’.
If Ontario can finally ‘get its poop in a group’ and OPG decides it is worth taking climate targets seriously, it could clean up the electricity grid – at which point we could switch off our gas meter with no impact on our heating system. A hybrid system therefore lets you anticipate the changing carbon intensity and pricing of the provincial energy mix. Stay flexible, go hybrid.
A Note on Energy Balance
Passive solar gains adds to the supply side of an energy balance diagram – this is energy that comes from the environment that a building can be designed to capture, like capturing a school of photon fish in a thermal net, using windows as the opening and opaque walls and insulation in the glass itself as the net. Similarly, energy squeezed from outside air in the case of an ASHP, is energy gained from the environment, and that is a net that takes little effort to cast and doubles your ‘catch’ – 2-3x more units of energy comes in to the house that was expended to collect it. In that sense, internal gains, solar gains and heat pump gains all add up to your supply side in an energy balance diagram, which is balanced out by the thermal losses through your envelope. What is often missing from energy balance diagrams is this component of ‘free energy’ from geothermal or air-source heat pumps. We note this ‘free energy’ advantaged by the ASHP in the red box on the graph below.
…an ASHP system should ideally be designed for 125% of the peak cooling load – which is probably going to cover 70-80% of your total heating load, but some form of supplemental heating is required, like a wood insert, pellet stove, or electric resistance heaters to make up the difference… or you install a hybrid gas/ASHP system… an ASHRAE Certified HVAC Designer
Why So Much Forced Air in Canada?
In Canada – the dominant form of space heating is a ducted, forced air systems. Since we have these extremes of climate, hot humid Summers and wicked cold Winters, and because we do not build thermally massive buildings like they do in Europe – forced air has been a cheap and effective way to deliver volumes of warm or cool air to spaces quickly – with a common set of ductwork for heating and cooling. But if you have an older, leaky, relatively inefficient home like we do, don’t think that you can just jump over to a 100% electric ASHP system – that was our first line of inquiry. Your electrical panel may not be able to handle the total peak load, and a system should ideally be designed for 125% of the peak cooling load – which is probably not sufficient to cover your heating load. If you have some form of supplemental heating system, say a gas or wood fireplace – then that may be all you need. But for us, it just made sense to install a hybrid system, starting with a replacement high-efficiency gas furnace with integral air-handler, that can also be packaged with a matching ASHP unit at a later date. In our case we opted for a Coleman CP9 furnace, and we have our eye on the Coleman Echelon series ASHP – for the day when our A/C unit cacks out the same way our furnace just did.
Not the Only Solution
Finally, I would add that in designing new homes, with a focus on better architecture, insulation, windows and passive solar design for heating and cooling – centralized split systems are simply overkill. We can reduce total cooling and thermal loads by up to 90% with just Passive measures – and in those buildings we can heat them ‘with a fart and a candle’. In those instances we can forego conventional heating and cooling systems almost entirely, or we can look at a wide range of other advanced systems, such as ‘Mini-Split’ systems, radiant heating and cooling, or hybrid radiant+passive solar as with our Quonset projects. But for the purpose of transitioning to zero carbon, without being beholden to the fossil fuel industry, we can be clever about updating systems when they reach the end of their life, by replacing them with systems that can support the transition to 100% electric operation at any point in the future.
We will be writing a followup to this article on our new ERV/Ventilation strategy also, which was part of the current installation, but we’re waiting until we have completed testing CO2 levels and optimizing the controls to connect our IAQ sensors with both the ERV and the Furnace blower controls – which is a bit complicated and few have trailblazed this approach yet – at least not that we have seen!
- Green Building Advisor on heating-historic-houses-with-heat-pumps