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Passive solar design before you build

On a new build, your very first stop is passive solar design. Strutting forward and designing a bad old stickhouse, and then bolting on solar as an afterthought, is wasteful.

Passive solar design means engineering the building so it does not need active heating (except on rare occasion. The occasion is rare enough that you can get away with cheapie resistive electric heat).

One way to make passive solar work is to angle the windows and overhangs so the buildings reject sun by summer, but drink up sun by winter... And capture the prevalent temperature (solarization by winter, shade by summer) in the building's rather considerable thermal mass. This does not happen by default obviously, you have to swerve out of your way to design the building for that. Design time is the time to do this.

Another method is to make a building partially earth-sheltered, so that by summer, you have an easier time resisting solar heat. Several feet of earth is great insulation and you can't beat the price. Obviously you have to compromise that with having a nice well-windowed home.

Another design aspect is heat pumps, though with passive solar design they are 99% of the time in A/C mode. And ideally you want to interchange with groundwater as your ultimate heat sink. See, normally, AC/heat pumps are pushing heat "uphill": it's 90 degrees out, and you want your house at 70, so you're dumping the heat in the 90 degree outside air - "pumping uphill". If your ultimate heat sink is 55 degree ground water, you are "pumping downhill". It's still pumping, but it's much more efficient.

Think about what it means for heat-pump (A/C is a type of heat pump) efficiency when the pl, since it is cooler than you want your house to be, and therefore in A/C mode, your heat pump is "pushing heat downhill. Which makes it more efficient.

The overall point is, don't design for inefficiency just because it is conventional, and then try to bolt on efficiency as an afterthought. Really explore the state of the art.

Another option is a double roof. It doesn't need to be a contiguous roof; it just needs to put the primary roof in shade. Solar panels might do that, but firemen are usally kitchy about covering too much of your roof with solar panels.

Use the grid as a buffer... or not?

How your electricity is billed is a big factor. If you are on net metering, meaning every KWH the solar generates unwinds a KWH off your bill, then you are using the grid as a storage device - you can run a 9pm dryer load using power you generated at noon.

However if you are paying peak/off-peak charges, or are credited a lower rate for reverse-flow power than forward, then you want solar to operate when you are at energy consumption peaks - in other words when the A/C is working the hardest. The good news is that A/C requirement is due to solar load, which is exactly what makes solar panels work, so they work in unison.

The bad news is, in reality, A/C need considerably lags solar load; 3 hours after sunrise my house is chilly, 3 hours before sunset it's unbearable, even though they have exactly the same solarization as far as the panels are concerned. I suspect that is the house's normal thermal mass at work. That is why one strategy is to greatly increase that thermal mass and insulation, to greatly slow that rate of swing.

Assuming that you are (a) on the regular electric utility grid and (b) are on the regular natural gas delivery system (i.e., don't require propane deliveries), as a general rule, natural gas heating will be most cost-effective in most parts of the US. If you are not on the regular (utility) natural gas system then electricity has some advantages. Alternatively, if you are not on the regular electricity grid then solar panels have huge advantages (i.e., better than just relying on diesel or natural gas generators).

But assuming this is a normal (utility gas & electric) setup, I would separate the solar panel issue from the heating issue. Since you are in Texas, air conditioning is probably more critical much of the year than heating. Plus, solar panels only work during the day and the biggest heating need (and some times of the year the only heating need) is at night. So solar panels without battery storage (e.g., Tesla Powerwall) would help with overall power costs but not much with your heating per se.

Solar Panels without Battery Storage

Solar panels without battery storage are typically used to generate power that is converted from DC to AC (inverted) and fed into the local grid. Effectively, on the best days you actually generate more than you consume (though air conditioning can have a huge impact on this) and the electric company pays you for providing power when they need it the most. Depending on local electricity tariffs, you may do really well with this based on "peak demand" or it may be less helpful based primarily on usage (kWh).

Solar Panels with Battery Storage

If you add battery storage (a significant additional upfront cost) then you gain a few advantages:

• Power Shifting - If you generate more than you need during the day, you store up extra energy to use at night. If you don't get much from the power company for generated power then this can save you more (by reducing total usage) than you would gain by selling the excess to the utility.


• Battery Backup - Instead of just a small battery backup for your computer, you get a battery backup for your whole house - lights, computer, TV, heater (if you use natural gas, you need some electricity for the controls & blower - which the battery backup can supply quite easily - but I wouldn't put straight electric heating on a battery backup system), etc.

Electric Heat

There are a number of different types of electric heat. Resistance heating is basically big coils - like a toaster for your house. Resistance heating is very simple (and therefore relatively inexpensive) but is very inefficient. Actually, it is efficient in converting electricity to heat, but that is economically inefficient compared to other fuels, unless you actually produce all your power via solar panels (and therefore don't pay the utility for electricity), but typical residential solar panel installations only provide a fraction of total electricity used.

A heat pump - essentially an air conditioner that can run in reverse to produce heat instead of taking away heat - is a great alternative. It can be more cost effective than resistance heating, providing you don't get really cold. When it gets really cold, a heat pump will use "emergency heat", which is typically resistance heating, because a normal heat pump simply can't do much if the outside air is too cold. Fortunately, in Texas that works well - e.g., Dallas has average lows in the winter in the 30s, so emergency heat wouldn't need to be used very often.

If for some reason a heat pump is not an option (including "just don't like 'em" which I think is much more of a problem in the colder places where they don't work as well), then natural gas furnace (using the same air handler as the air conditioning) is the way to go.

Do the Math

A lot will depend on:

• Cost of Solar Panels


• Cost of Battery Storage


• Local cost of electricity


• Local cost of natural gas


• Utility Payment (if any) for your solar panel generated electricity


• Expected total & peak electricity usage

My gut feeling is that with the gradually dropping cost of both solar panels and battery storage, a full solar + battery system and a heat pump for heating & cooling would likely make sense, particularly as part of new construction in a warm & sunny place like Texas. But your mileage may vary.

If you design the solar thermal correctly, then given the annual solar insolation ( higher than my location in EU by about a factor of 2) then with good insulation you can avoid heating at all.

But you will need to maximize the passive solar gain - allowing the sun's energy to enter the building in winter for example.

Use underfloor heating driven from the solar thermal collectors and its associated hot water storage tank - this is low temperature compared to "normal" radiators.

Start with efficiency

Harper's overall point about starting with an efficient envelope is dead-on; managing solar gains is going to be a huge part of it down in Texas, and you'll also need to be careful with how you apply insulation and air barriers to avoid trapping water vapor or moisture in wall or roof assemblies. The ultimate test of this is a blower door test that actually measures the amount of air that can leak in and out of your house; this test, along with infrared thermography (which is good at pinpointing leaks), are the bread and butter of home performance analysis, alongside some intuition and understanding of how heat, air, and moisture/water work.

Vented attics and ducts do not mix!

One classical mistake in Southern homebuilding is the use of a vented attic to house HVAC equipment and ductwork. This not only shortens the life of the equipment due to the high ambient temperatures present in attics in the South, but wastes valuable power and capacity on overcoming heat gain through the ducts during cooling seasons, often to the tune of 10-25%.

As a result, if you are using a ducted system, you are far better off putting the ducts inside the building envelope fully, one way or another. "Hot roof" configurations can work well in Texas due to the lack of ice dam concerns there, but basically require a roofing system that can withstand high temperatures. There are also configurations, such as scissor trusses, that form a mechanical cockloft space between the finish ceiling and the ceiling envelope plane aka where the attic insulation lives, if you do not mind the extra structural complexity that entails.

Beware the Ductopus, Strangler of Air Conditioners!

Atop this, Southern residential construction compounds the error of putting ducts in the attic with the aid of a few bags of flex duct and a harried installer or three, resulting in the organic, many-tentacled monstrosity known as a "ductopus" spawning in one's attic. The unstretched flex duct creates excess friction losses, which reduces the efficiency of the blower at delivering conditioned air to indoor spaces, thus forcing further upsizing of the system to overcome this condition.

The antidote to this is proper duct design and installation -- applying ACCA Manual D rigorously, installing the ducts properly, and avoiding the indiscriminate use of flex will give you a fighting chance of being able to use Manuals J and S to right-size your HVAC system for your house, instead of taking the "bigger is better" approach to the problem, because bigger is not better when it comes to HVAC, especially if you are building a high-performance home. Of course, you will want to test your ducts when installed to ensure that the static pressure across the blower is acceptably low; this is a good measure of the overall workmanship of the HVAC techs, as well.

Play to your strengths

Texas is a mostly-cooling climate, by and large. As a result, since you will want some sort of air conditioning anyway, the logical choice for turning electricity into heat here is a heat pump. Furthermore, the use of electric resistance heat is a gross waste of power when your electricity is coming from solar panels; as a result, you will want a heat-pump setup that avoids heat-strip "emergency heat".

This leads us squarely to mini-split heat pumps, of both the ducted and ductless varieties. Instead of leaving the blower on full-bore and using resistance heat to both fuel the defrost process and make up the missing heat, mini-splits stop their indoor blower during defrost, thus allowing them to defrost without burning power in resistance heat strips or blowing cold air everywhere.

Mini-splits also have the advantage of being able to use either multiple ductless heads or low-profile, in-ceiling ducted heads that make going without a full-sized attic or basement to house HVAC ductwork an easier design proposition. There are also full-sized air-handlers available for today's mini-splits if you want to use one to drive a fully ducted system. You do need to be careful with duct design and installation with this approach, though, as their blowers can't provide quite as much static pressure as the ones on more typical split-system air-handlers.

Why not gas?

The issue with gas in a climate like yours is that for you, gas furnaces come in one size: too big! For a climate with light heating loads like yours, even the smallest gas furnaces are going to be vastly oversized compared to your actual needs, especially in a high-performance building envelope as heating loads fall off faster than cooling loads when you add insulation and remove air leakage paths.

Given the comfort and efficiency drawbacks of an oversized furnace (even if it can modulate), this makes mini-split heat more attractive, especially with today's mini-splits that can handle just about anything Texas will throw at them for outdoor temperatures. However, I would not completely discount gas heat: modern modulating-condensing gas hydronic boilers are highly efficient, and can also feed an indirect hot water heater to provide domestic hot water as well as feeding an air-handler coil, radiators, or floor heat for space heating. This allows the boiler to operate in an efficient range year-round while providing efficiently easily comparable to high-end gas appliances for water heating and space heat.

If you have enough solar in place to manage cooling during the summer, you probably have enough solar to manage heating during the winter. The question is if you can get more money from selling the solar to the grid, and using that money to buy gas for heating. The answer is usually yes, as long as you can sell to the grid at a decent price.

Probably better for the environment, too. Most electricity comes from fossil fuels, and that's less efficient than burning them for heat. By pushing solar to the grid, you can reduce the fossil fuels needed for power.

My preferred way is small split ac, reversible, COP of 1 to 3. Service can be expensive.