@Jim, when you have a heat
@Jim, when you have a heat pump with electric supplemental heat, the only reason to ever switch off the heat pump is if it were to fail. This is accomplished by switching the thermostat to ’emergency heat’ mode. This disables the heat pump and operates the electric strips as first stage.

Keep in mind that at colder outdoor temperatures, even though the heat pump may no longer be able to handle the full load, the heat it does provide is much more efficient than electric strips. For example, at 0F, a typical heat pump produces around 40% of its full capacity at twice the efficiency of electric strip heat. So during normal operations, you want the electric strips to supplement, not replace, the heat pump.

If by chance you have a dual fuel system (heat pump + gas furnace), things work somewhat differently. In that case, the heat pump MUST shut down when the heat pump requires supplemental heat. That’s because with a furnace, the refrigeration coil is mounted downstream from the furnace (i.e., at the supply outlet) to protect the heat exchanger from condensation in cooling mode. If the heat pump were allowed to run while the furnace is firing, then the heated supply air would prevent the refrigerant from condensing. Bottom line, with a dual fuel system, the furnace must handle 100% of the load below the balance point.

Older dual fuel systems rely on an outdoor thermostat to control the switch-over point, typically set to 30F or 35F. Much better is to use a dual fuel compatible stat that energizes the furnace based on demand, as a 2nd stage. Either way, you should never need to concern yourself with supply air temps to manage supplemental heat calls.

I’ve read that article and
I’ve read that article and love. I just don’t think it has spread to the architects yet. And in PassiveHouse forums I still keep seeing people wanting to design Passive Solar and get free energy.

Using the MJ8 load and my
Using the MJ8 load and my heat pump’s expanded performance data to plot the balance point, the curves cross around 37F. But I should point out that the home’s empirical load curve is only 42% of the MJ load, which is not at all typical (there are several reasons for this). Here’s a link to the graph: https://bit.ly/2aEogRH

The dip in the heat pump’s performance curve between 40F and 35F indicates defrost ops.

OTOH, Martin’s premise that ‘superinsulation won out over passive solar’ is only correct because that is in fact what happened. Passive solar adds very little cost, so why did it die out? It happened because of a lack of knowledge among practitioners, a lack of trainers, and a lack of modeling tools. Moreover, many passive solar homes built back in the day were DIY endeavors. As Martin points out, there are plenty of horror stories of passive solar homes with severe temperature imbalance and overheating, often with funky architecture that has little mass market appeal. In short, passive solar got a bad rep.

The truth is, passive solar design is a wonderful thing when done correctly, and the home doesn’t have to look that much different than otherwise. When we combine passive solar elements with beyond-code insulation and air sealing, annual heating loads can be ratcheted down to ‘Passive House’ levels, without the high cost.

I routinely advise clients how to incorporate passive solar design elements during design phase. But the first step is to buy a lot with optimal orientation. Typically that means facing north. For turnkey passive solar design, I recommend architect Debra Rucker Coleman @ sunplans.com.

Bill, that’s a good topic.  I

Bill, that’s a good topic.  I’ve mentioned it a bit here and there but I agree with Martin Holladay on two points:

1. Superinsulation won out over passive solar back in the 1980s.
2. Solar thermal doesn’t make sense economically anymore.

Here’s a link to Martin’s take on the superinsulation vs. solar:

GBA Prime Sneak Peek: Reassessing Passive Solar Design Principles

And here are Martin’s articles on solar thermal for anyone who wants to check them out:  “…Martin Holladay at Green Building Advisor wrote a couple of articles earlier this decade about how even “solar thermal is dead” (2012) and “really, really dead” (2014).”

Architects love glass walls and will accept any excuse to justify the excessive glass. Customers love the word free. I see HVAC systems getting oversized because of this glass.

All good points, David, as

All good points, David, as usual.  Do you happen to know how your 18 °F heat pump balance point compares to an analysis done as I’ve described in the article?

Dana, thanks for raising the

Dana, thanks for raising the issue of mini-splits.  I’m going to come back to the balance point and supplemental heat for that type of heat pump in a future article.

Curt, yes, you’re absolutely

Curt, yes, you’re absolutely right.  It’s definitely more nuanced for the reasons you mentioned plus what Dana Dorsett and David Butler have added below.  I mentioned actual balance points being different and will go into more in a future article.