Row of hot water panels on a roof.

Incoming water temp can determine size of collectors, storage

As you get ready to design a solar thermal system for your home, there’s one factor you may not be considering that could make a bucket load of difference: What’s the beginning temperature of the water you’ll be heating?
In the United States’ northern climes, it’s likely 50°. In warmer, more southern climates, you’re likely beginning with 70° water.
And common sense tells us, the colder the water you start with, the more solar uumph you’ll need.
I’m asked all the time how to design a residential system. And, as I worked on the attached chart, I realized it pretty much tells you the basics of what you need to know – the size of the storage tank and the number of collectors.
This attached chart will give you an idea of what to expect. The chart allows you to determine if, for instance, you’ll be heating an 80-gallon tank in a system sized for up to three people, or a 119-gallon tank sized to serve three to five people. I chose these sizes because they are averages for family-sized systems. These numbers can be easily customized.
So, to read the chart: Say you’ve decided on an 80-gallon tank and you live in a colder area, then you’d chose line 1. And, reading across, it will tell you the number of solar collectors you’d need based on the sizes of the collectors you’d like.
Continuing this example, you decide that your home in Michigan has a large roof area and you’d like the largest collector — the 4’x10’ collector. Based on this chart you’d need two of those large collectors — or 80 square feet of collector real estate — to heat that 80-gallon storage tank.
As someone who lives in a northern climate, I shake my head sometimes knowing that we colder people all need a little extra uumph. But I know that we snow people – I live in northern Utah — will enjoy our hot water all the more.
Just as a side note, the input water temperature also has a bearing on how steep should you tip your collectors, believe it or not. If you live in 70 degree, you’ll living in a 30 to 40° latitude, so you’d use a tilt or angle of 30 to 40° for optimal performance. Likewise, those in the colder climes with colder water probably live in a 40 to50° latitude, therefore using a 40 to 50° tilt or angle.
To download an Excel file of the chart, click here.

Sizing home system based on temp of incoming water, with helpful chart

Sizing home system based on temp of incoming water, with helpful chart

In recent years, I published a chart that helps size a solar hot water system for your home, but after hearing from some people I realized I needed to make it more simple.

So, to back up and provide a bit more information:

This chart shows what system is right for you, using actual sizing logic instead of guessing. (Believe me, I see that more than you’d think.) Instead of a rule of thumb, it considers a southern or northern location in the U.S. to determine what the incoming water temperature might be. The amount of energy you’ll need is based on raising the water temperature from where it starts as it comes into your home. With a little math, we can easily determine how much solar will be needed to fill up the tank, based on the incoming temperature.

The chart lists four choices of collectors with differing measurements. For example, if the load requires two 10-foot collectors and your roof height is only 7-feet tall, you still have several choices to make them fit nicely on your south-facing roof. (Unless you live in Brazil, then please face them north.)

Chart for sizing solar based on temp of incoming water. Click on chart image for full-sized PDF.

Drainback with tankless is less costly, more efficient

Drainback with tankless is less costly, more efficient

Today, as part of our ongoing series of basic solar designs, we look at a drainback system using an efficient solar storage tank and a tankless water heater as backup.

This design is fairly uncomplicated and is, perhaps, the least costly system for residential applications. For all its simplicity, it delivers freeze and overheat protection with water. It also features low heat loss.

The tank is the solar storage and the drainback tank combined. The hot water storage tank I’m using in the drawing is made of expanded polystyrene (EPS), which has an insulation factor of about R5 per inch. Dimensions of the tank are 60 inches in height and 30 inches outside diameter, with 4-inch walls. Usable Btu storage is 81 gallons,  which when filled with solar energy up to 160 degrees equals about 67,000 Btus of energy.

The system has very few components — basically the storage tank, pump, controller and collectors. The tank contains a 50-foot 1″ HX coil. Potable water is drawn through the coil, picking up heat from the contents of the tank. The backup water heating is located after the coil exits the  tank. A tankless gas or electric water heater works well as a backup, or this could be connected to any existing water heater and used as a preheat design.

This system is an excellent choice for the HTP Hydra Smart tankless water heater. That is the tankless water heater I’ve used in the design drawing. For more information on this advanced, modulating tankless, click here.

Like other drainback designs, all plumbing must be sloped from the panels for complete drainage.

Here’s more general information on how tankless water heaters work.


Flat plate vs. evacuated tube: Which is the winner?

Flat plate vs. evacuated tube: Which is the winner?

By Tom Scheel

Red vs. Blue. Tastes Great vs. Less Filling. Kirk vs. Picard. Epic battles for which there really are no right answers. Enter into that pantheon the great debate between flat plate vs. evacuated tube solar thermal collectors. The answer, much to the chagrin of the true believers on either side is: it depends. Let’s first briefly look at the two types of technology.

 Flat plate solar thermal collector:

The flat plate collector has been around forever (at least since the 1890s). An insulated box, with glass on one side and copper tubing running through absorber plates is the basic design for flat plate collectors.

Evacuated tube solar thermal collector:

Instead of evacuated tube, think thermos bottle. Each tube is actually its own mini-collector. A clear thermos bottle allows sunlight in, an absorber plate absorbs more heat, and the vacuum holds the heat in. Most designs have a heat pipe – a small diameter, sealed copper pipe inside the thermos bottle. Inside the heat pipe water, antifreeze, alcohol or some other medium, under a mild vacuum, flashes to steam around 80F. So the liquid heats up, turns to steam, rises in the heat pipe to the heat exchanger (usually a larger section of the heat pipe dry fitted to the water-or-glycol carrying header), gives up its heat, turns to liquid, flows to the bottom and repeats the cycle.

Comparison of generic evacuated tube and flat panel collectors

Evacuated tube* Flat plate*
Dimensions 9’X7’X6” 4’X10’ X4”
Weight 250 pounds 150 pounds
Effective absorber area ~30 square feet ~38 square feet
Piping Top only Top and bottom
Max weight per heaviest piece 105 150
BTU output Depends Depends
*off-the-shelf, readily available models. The point is not a particular brand/model, but roughly comparable panels between the two styles.

Each type has a role to play, depending on where the project is, and what the heating needs are. If you are installing a solar water heater in Phoenix, the flat plate solution is the obvious choice. If you are installing a space heating system in Burlington,VT, evacuated tubes are the obvious choice. Almost anything in between requires some analysis.

To start the analysis, let’s look at price, performance and ease-of-installation.


The advantage goes to flat plate collectors, but not by as much as is typically assumed. For example, general prices for the two panels above are: flat plate: about $1,100; evacuated tube about $1,400. How and if discounts are available from list prices is a whole other subject in the game of solar.


Regardless of price, how much energy can we get from a panel? Here we will use data from the SRCC OG-100 tables. Data for any rated panel can be found here: Because panel performance varies greatly, you should check the specific performance of the panel you are considering.
Performance comparisonThe table (click to enlarge) is based on three different sunlight conditions: Full sun, partly cloudy and cloudy. At those three conditions four different temperatures were tested (the temperature is the difference between the outdoor temperature and the water you are heating at the inlet to the collector). Pool 1 is an estimate of pool heating requirements in a warm climate; Pool 2 is an estimate of pool heating requirements in a cool climate; the same applies to DHW 1 and DHW 2. Black plastic pool panels are recommended for seasonal pool heating.

Somewhere in row C the performance per panel begins to favor evacuated tube collectors. Note that this compares a ~55 square foot evacuated tube panel with a ~40 square foot flat panel. If you are minimizing the solar footprint on the roof, flat plates probably win (but you may not get the performance you want in the winter). This graph presents the same information in a slightly different way.

As you can see from the chart, somewhere between 70 and 80 degrees Fahrenheit delta T between your incoming water at the solar panel and the ambient air, the evacuated tube is more efficient. An example would be heating 120°F water when it is 40°F outside. The 80°F differential is right at the efficiency crossover. This is a typical case for overnight storage in a solar hot water heater. You want to heat the water to 150°F or higher so you can use that heat energy for night-time and early morning hot water demand.

However, if your ambient temperature is 50°F or 60°F then the flat plate is more efficient almost the whole time. The ambient temperature we are considering is the average from 9am to 3pm (the solar window).

Now let’s consider space heating – as a stand-in for winter, we will assume partly cloudy conditions (~800 watts/square meter)

Partly cloudy efficiencyNow the crossover point is about 60°F. If the outdoor temperature during the solar window is 40°F then heating water over 100°F benefits (at least slightly) from evacuated tube collectors.


If you are a one man crew, the evacuated tubes are the hands down winner. Other than that, the flat plate wins. Flat plates are big, heavy and bulky. Full roof harness safety gear for a pitched roof and some sort of mechanical lift are required to get them on the roof safely. A 3 to 4 man crew can muscle them up by hand, but even with a full crew this is a challenging step in the process.

Both styles require some frame to attach to, and typically require a tilt-angle frame to lift them away from parallel to the roof. The panel or panel header/frame is set in place once the tilt-angle frames are built and installed. It will take a few people to get the flat panel positioned, whereas one person can maneuver the manifold and frame of an evacuated tube collector. While the flat panel is ready for piping, the evacuated tube panel still needs all its tubes put in place (the correct order is complete piping, and then put in tubes). It takes roughly 60 seconds per tube. If you are putting up a single 30-tube evacuated tube collector that is an additional half hour of time. If you are putting up a 10 collector array, that is an additional 5 hours.

Miscellaneous considerations

* The evacuated tube collector is more efficient at chasing a dwindling resource. So the gross BTUs the extra efficiency gets you are not as large – but they may come at the critical time (typically winter conditions).

* Windy conditions affect flat panels much more than they do evacuated tubes (due to the surface area of the un-insulated glass). If your project site is typically windy during the day, you might want to give evacuated tube collectors extra consideration.

* Evacuated tube collectors only require flow through the top (header). This can be very helpful when installing drainback designs (the most efficient freeze-proof design).

* Flat plate panels melt the snow due to their high losses through the un-insulated glass. Evacuated tube collectors are so well insulated they do not melt the snow. If the south facing roof at your location does not routinely melt the snow during a typical winter, flat plates are probably a better choice. The evacuated tubes seem to work well under light-to-medium frost. And they will collect energy when half covered with snow (about half as much as when fully exposed).


The chart at left (click to enlarge) highlights the advantages of each type of collector. As the issue can be sliced many ways – cost per square foot, cost per BTU, performance per square foot of collector, of absorber, of net aperture, etc., the debate can be never ending.

Tom Scheel is the owner of Radiance Heatingand Plumbing in Flagstaff, AZ. His website is This article was originally published in Contractor magazine. Originally posted on on March 12, 2012.