Filling the pressure: Sizing an expansion tank correctly

Filling the pressure: Sizing an expansion tank correctly

You’ll need a few basic details about your system.

Here are some good questions about the correct sizing for an expansion tank from a conscientious installer — which I know includes all of us.

Question 1: Fill pressure vs. expansion tank size? (Or in other words, what should the loop pressure be and how much pressure should be in the expansion tank?)
Question 2: Expansion tank size vs. system fluid volume/ number of collectors? (Or, how to size the expansion tank to your solar system, including collectors, length of pipe, etc.)
Question 3: Fill pressure vs. PRVs? (How to size the pressure relief valve.)
Question 4: Fill pressure vs. head? (Does head or distance to the panel affect fill pressure?)

If you can visualize the concept of a correctly installed pressure glycol system, the answers will become more evident.

The goal is to not blow off when the power fails on a super-hot day. To do this, the system needs to be designed along the lines of a drain-back system. Sometimes it’s called a steam-back system. Sizing an expansion tank for solarThis is the only sure way to design this to make it work correctly.

Let me explain, and your other questions will start to become clear once you understand how it works.

All plumbing coming out of the collector array MUST drop down several feet to form a thermal lock or trap for the steam in the collectors. The liquid/steam must not migrate at all. Water expands 1,130 times when it flashes. If in any way it can migrate and flow out of the collectors, it’s not possible to have a large enough expansion tank.

With this understanding, you can now start making the calculations you need. For example, a single 4’x10’ collector, holds 1.3 gallons of water. So, if you have a string of eight 4’x10’ collectors, the total is 10.4 gallons. This should have 1″ pipe (copper handbook). Never oversize the plumbing or you lose control.

As the array heats and starts to flash, it will start to push the fluid back and DOWN the plumbing. Once it settles, the collectors now contain 10.4 gallons of steam volume, and about 10.4 gallons have been pushed down the plumbing into the expansion tank. Calculate expansion for the liquid in the plumbing (maybe 0 to 200°), and you now know what size the expansion tank needs to be. Or, I have an Excel spreadsheet I created years ago that will help you with the calculations. (Go to the “Sizing Tanks, Panels and WH” tab.)

Most systems have between 20 to 40 psi. Remember, every 100 feet of water has 44.4 psi. Add this to whatever you start with. Oh, and make sure you size the PRV with a good margin above set pressure.

The expansion tank should have a cold unloaded pressure set before you start to fill that is close to the psi your system will be set at. You cannot change or check the pressure on an expansion tank after it has been loaded. The reading will be wrong.

You might get some other suggestions for setting the pressure. I know Velux and Heliodyne have some interesting guidelines. Then too, I’m not a pro on pressure glycol systems. We do drainback every chance we can. But, European collectors generally can’t do drainback when they have a serpentine riser inside.

Good luck — or, do it correctly and you won’t need any luck!

Incoming water temp can determine size of collectors, storage

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

Earlier, 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.