Yea, I guess that maximum
Yea, I guess that maximum heating loads are typically lat at night or early morning with everything turned off and people at their lowest metabolic rate. The biggest complaint that I hear about MJ is that everyone makes “conservative” assumptions and rounding so that it always over estimates design loads.

MJ excludes internal loads
MJ excludes internal loads for heat load calcs, whereas internal loads are included for cooling calcs. There’s logic behind those assumptions, but the MJ procedure begins to break down as we ratchet down envelope loads to the point that internal loads becomes relatively large in comparison.

Bill,

Bill,

I put those links in a comment in my last article.  Here they are 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).”

Fortunately, these articles are old enough that they’re not behind the pay wall like Martin’s new articles (and mine at GBA).

I am guessing that Holladay
I am guessing that Holladay looked at the typical high-priced complicated solar thermal water heaters that are available on the U.S. market today. I agree that those are probably not cost competitive. I am talking about solar thermosiphon systems with the tank mounted as an integral component at the top of the collector. I did some recent web-searching and can’t find any available in the U.S. Alibaba has them for sale for about around $300, but you have to pay shipping from China. If you visit China or Japan, you will see these units everywhere on the roofs of houses and apartments. When you look at the overall concept, you can see that these are inherently cheaper than PV’s coupled to heat pump water heaters. It is just that no one has pursued this market in the U.S.

Think about it. With direct solar thermal conversion using a thermosiphon system, you are converting solar radiation directly to thermal energy with no mechanical power requirements. With PV-HPWH, you are converting solar radiation to DC electricity, then to AC electricity, then to motor shaft power, then to thermal energy via a vapor-compression system. The Asians don’t use solar thermosiphon collectors for hot water because they like to spend more money.

Martin Holladay wrote an
Martin Holladay wrote an article in 2012 called “Solar Thermal is Dead”. Then a follow up article in 2014 “Solar Thermal is Really, Really Dead”. He went thru his cost estimates and showed how much cheaper a PV system with HP water heater is. The PV system size is about 1/3 less with a HP water heater than a cheap electric water heater.
House ‘A’ with solar thermal system costs $9,000 with $1,200 for a back-up water heater.
House ‘B’ with a normal electric water heater $1,200 and PV system $6,395.
House ‘C’ with a HP water heater $3,000 and PV system $2,132.

Bill, I agree with your
Bill, I agree with your summary about solar thermal with the possible exception of solar domestic hot water. I have seen a lot of solar thermal domestic water heaters in China and Japan which use thermosiphon systems with the water storage tank above the collector on the roof. There are no pumps or controls with this type of system. The water circulates from the collector to the tank automatically when the sun shines, and stops when it doesn’t. It might be limited to milder climates where freezing is not an issue. These systems are quite simple compared to solar PV with the associated power electronics and a heat pump water heater with all of its complexity. There just has to be a simpler way to heat water, especially in the south where I have to be careful to not scald myself when I try to rinse off with a garden house that has been laying in the sun.

David, Yes, the heating
David, Yes, the heating balance point temperature is a pretty rough “average” number that depends on a lot of things. I usually estimate by plotting the daily heating energy use as a function of the daily average outdoor temperature (this assumes constant indoor temperature). I then fit a straight line through the data. Then I estimate the heating balance point as the outdoor temperature at zero daily heating energy. The more scatter that you see in the data relative to the line indicates how many assumptions are being violated (changing temperature setpoints, varying internal loads, varying infiltration, etc.). Most of us don’t have this type of data, so we just settle for listening to heater operation and comparing that to outdoor temperature.

I have not used Manual J, but apparently from your description, it does not include internal loads. That seems like a significant gap so I can see how it would overestimate loads.

Solar thermal is obsolete
Solar thermal is obsolete economically. It’s better to have solar PV and heat pumps. Most climates, ASHP is the most affordable. Even ASHP for building heat with a indoor heat pump water heater is cheaper than heating water with a solar thermal system.

@Roy, as I’m sure you’re
@Roy, as I’m sure you’re aware, in addition to internal loads, the heating balance point for a given home depends on its thermal mass and local climate, and can also vary significantly from day to day with wind speed and trailing weather. I ask my clients to note the *highest* outdoor temperature at which they observe the thermostat calling for heat.

The balance point for my previous home, which was production-built, is around 45F based on observations over eight years. Early in the heating season, it’s not unusual to see the heat not kick on until it drops to 38F or 39F outside as there’s still a lot of residual heat in the ground and in the home’s mass. But I’ve also observed heat calls when it’s as high as 45F outside, typically later in the winter, and when it’s cloudy and breezy.

As an aside, Manual J dutifully tells us there’s a heat load if the outside temp is 1 degree below the design indoor temperature! But if we have an accurate assessment of the heating balance point as per the above procedure, another point can be established at or below the design outdoor temperature by cycle-timing the heating system. The line defined by those two points is the ‘true’ heat load, and is always significantly lower than what Manual J would have us believe. Having done this for different types of homes in several climate zones, I’ve learned how to ‘de-rate’ what Manual J comes up with. I can safely say that the tighter and more efficient the shell, the more MJ overstates the heat load, notwithstanding significant reductions in internal gains in recent years.

Perhaps are you looking for
Perhaps are you looking for an IAQ monitor for the air acceptability reporting. Two of my favorites are the Foobot and the uHoo, both of which I use in my home. And both of which I sell through my company, TruTechTools.com