3 Sources of Heat for High-Performance Homes
Earlier this week I wrote about the three main sources of heat in homes. At the end of the article I threw in a fourth one, solar energy, but then I dismissed it as something that people tried in the 1970s and ’80s and have since abandoned for home heating. And that’s still mostly true, but my Idahoan friend Skylar Swinford raised a good point on Twitter.
Skylar spends his days modeling Passive House projects and in that world, where you go above and beyond with the building enclosure, internal and solar gains are definitely more of a factor. Solar gains still have the problem of occurring during the day whereas peak heating loads normally happen in the dark of night, and they can cause problems with overheating if the designer isn’t careful. But internal gains are there for you through the night.
The diagram he posted above isn’t from a real house, though. Typically, Passive House projects still get more than half of their heat from the active heating source, which would be one of the three I discussed in the other article. Just to spell it out for the people looking for the list here, the three heating sources for high-performance homes are:
- Active heating
- Internal gains
- Solar gains
This discussion has reminded me of reading Amory Lovins discuss the high-performance home he built in Snowmass, Colorado way back in 1984. I don’t remember which book it was but somewhere he wrote about welcoming people — and their 120 watts* of heat — to his home. As you build a house that’s really good at not losing heat, those internal gains become more and more significant.
Thanks to Skylar for raising this issue on Twitter. For those of you who don’t know Skylar, he was quite a running back on his college football team. (And yes, he did get the first down on that play.)
* I don’t recall exactly what number he used. It may have been 150 W.
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We generally just count on
We generally just count on around 340 btus – or 100 watts of gain per person per hour. That’s not a bad general average since the generally accepted number is from 200 (at rest) to around 450 (fully active) btus per person per hour. We have homes in Asia in Siberian zones where the nearest electric power is more than 50 miles away and in the depth of winter when all the firewood is gone five persons sleeping in a 12×12 room can maintain an internal temperature of 20 Celsius (68F) through a Siberian winter night with NO other source of heat. They know by sleeping in the same room they survive – snoring be-damned, I guess. Working on a couple of polar region projects now – same rules apply. Stud framed has a tough time getting past the whole thermal bridging conversation – we are dealing with an entirely new set of challenges – and opportunities – that traditional stud frame homes could simply never experience.
Solar hydronic can also be
Solar hydronic can also be very useful if ground source, closed loop heat pumps are in use. During the colder days when there is not enough heat absorbed by the solar hydronic system, that 70-80 degree fluid can be circulated through a plate heat exchanger to transfer heat from the solar system fluid to the return water for the ground source heat pump. This not only provides the heat pump with more heat energy, but allows the heat pump to utilize the hot water generator to enhance heating domestic water whenever there is no demand for space heating. If nothing else, the added heat from the solar system reduces the need for auxiliary electric resistance heating.
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.
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.
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.
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.
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 was talking to a friend of
I was talking to a friend of mine who built a very high-performance home. I asked him about this heating balance point. He said it was around 50 F, which to me is not much better than current new construction. I asked him why it wasn’t lower when he had such a tight and well-insulated envelope. He replied that the internal loads were also much lower due to high-efficiency lighting, appliances, etc. I hadn’t thought about that before.
One of the other interesting issues with high performance homes is that if you reduce the heating load enough, you cannot economically justify higher-efficiency heating equipment, especially ground-coupled heat pumps or active solar systems. Some even suggest that electric-resistance heating is a cost-effective solution in these homes. If that is the case, then high-efficiency lighting and appliances might be working against you during the heating season. Of course, the opposite is true during the cooling season.
Roy, just to be clear, the
Roy, just to be clear, the “balance point” you’re talking about isn’t the temperature at which the heating load and heat pump capacity are the same, right? That’s typically in the 30s Fahrenheit. From the context, it seems you’re talking about the base temperature, which is the temperature dividing line between needing or not needing heating. Correct?
Yep, that is correct. I wasn
Yep, that is correct. I wasn’t even thinking about a heat pump during this comment. Just “active heating”.
@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.
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.
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.
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.
Not a heat source, but heat
Not a heat source, but heat loss avoidance is also a key to minimizing heating loads. As two individuals who do not smoke or often open doors and windows, we lessen our home’s heat loss. The quest for ventilation I sometimes observe can be tailored to individual circumstances. Is there a air acceptability gauge or meter available to provide more accurate air acceptability reporting?
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
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