Solar Water Tank Info

A solar tank is a critical part of your solar hot water system. The type and size of solar tank you need is often a poorly understood part of a system, mainly because there are so many variables to it.

Many systems installed by companies often attach a small box of tricks to the side of your existing domestic hot water tank. The justification for this is partly that often you don’t have room for a bigger tank in many mechanical rooms and it also saves you buying another larger tank. While this is true, it also means that your solar system will store very little heat during the good sunny days.

This article will also describe various types and sizes of solar tanks that can be used for solar water heating. It will describe pressurized and un-pressurised solar tanks including some places you buy them from.

Most solar water heating systems installed in people’s homes either re-use the existing water heating tank or maybe add a second tank of 60 - 80 gallons in size. We also described in this article, why the tank size you use is VERY important to the solar hot water system. Although the solar systems still work on smaller tanks, you are also missing out on much better performance with a larger tank.

What are solar hot water tanks?

The tank is simply like a battery for electricity, except it stores heat in the form of hot water. Normally a tank is used to store the heat in hot water.

The solar tanks are mainly split into two categories; Pressurized and Un-pressurised.

There is another store people can use for solar heat. It is possible to connect the solar collectors direct to an in-floor heating supply. In this system the concrete becomes the storage tank. This can work in some scenarios where temperature control is not as important such as in a garage or a shop.

Pressurized Solar Tanks

These are like the "normal" tanks you will have in your mechanical room. They are heavy and designed to hold pressurized water for your house. The "pressure" is normally supplied by the pressure of your homes incoming main cold-water supply from the city or well.

They hold the water coming into you house from the city water which will be under pressure from the main city supply. An empty 80 gallon tank can easily weigh 200 lbs.

You can use a "standard" tank such as a Bradford White tank with no internal coils and then install your own external side arm heat exchanger onto it. This will work OK for a small solar water heating system, i.e. one or two 30 tube collectors. With this "normal tank" you spend less money on the actual tank but will spend a bit more in time and money adapting the side arm heat exchanger to the ‘solar’ tank.

However, you can buy tanks specifically designed for solar hot water systems that have internal heat exchanger coils built into them specifically designed for solar hot water systems. The lower coil is attached to the solar system then the upper coil can be used either (a) for connecting to a backup boiler (i.e. put extra heat into the tank) OR (b) or connecting to heating such as a pool or in floor heat (i.e. to draw heat out of the tank)

This diagram shows the inside of our SolarStor Water Tanks with two heat exchangers. These are good solar tanks but cost considerably more than a standard tank without a heat exchanger.

Buderus Solar Water Heating tank

Our double wall heat exchanger tanks come in two sizes, an 80 gallon and a 119 gallon. We also have single coil solar storage tanks available in 50 gallon size.

Weight – Our 119 gallon solar tanks but these will weigh +300 lbs so even moving them into the house is hard work. When they are full of water, they are very heavy and you need to make sure your floor is strong enough to hold this weight.

The reason for the difference in price between the "expensive" solar tanks and "cheaper" solar water tanks is often not obvious to most people. 80 gallons is 80 gallons, but the quality of the internal parts of the tank can vary enormously.

Some of the things you need to check:

  • The diameter and length of the heat exchanger in the tank, i.e. the square foot of metal. More square feet = better heat transfer from the solar collectors to your tank. Our heat exchangers are 7.8 ft2.
  • More expensive tanks will have bigger (thicker, longer and better quality) heat exchangers = they should last longer. Our heat exchangers are 1.5” in OD diameter.
  • More metal = better heat transfer to the tank. The material, if the heat exchanger needs to be stainless steel or similar. If it’s plain steel tank then it will likely rust and fail prematurely. Look for a heat exchanger that is coated to prevent rusting.

Adding multiple collectors on a single tank?

Yes, you can add as many as you want to a single tank and the tank will heat up quickly but consider this.....

Let’s say you buy a normal gas fired 40 gallon water heater from Home Depot. Say the gas burner in this is 40,000 BTUs. Say this will heat the 40 gallons of cold water to 130 Deg F in 30 minutes.

If instead of this, say you buy the same sized tank but this one has a 160,000 BTU gas burner ( 4 x the size). All this means is the water will be heated 4 x as quick. You still only have 40 gallons of hot water.

With a gas burner this is OK, when the sun isn’t shining the gas will heat more water. With a solar collector this can’t happen, you cannot heat the water with solar when the sun isn’t shining! 40 gallons of hot solar water will not last long when the sun goes down. If you are trying to heat a home or garage with it then the heat in the tank will be gone very quickly.

If you put 3, 4 or 5 solar collectors on a 40, or even an 80 gallon tank, you will have the same problem this. It will heat the tank quickly, often by midday the tank will be at nearly boiling point and you then have to install systems to "dump" the heat. Unless you have a pool to use the solar heat, oversizing collectors on a tank is not a great strategy.

Adding more tanks for more storage

One easy way to gain more storage is to add more pressurized tanks. If you have 3 x 80 gallon tanks you can plumb them together so the water flows through them all together giving you an effective 240 gallons. When adding tanks together to from more storage, it is important to make sure the flow rates are balanced. If you get the "balancing" wrong, then all the flow from the solar collectors goes the path of least resistance and only one tank heats up which is a waste of money and stored energy.

Multiple pressurized solar tanks

Multiple pressurized solar tanks

Certified Tanks

Remember to install a pressurized tank in Canada and the USA it has to have CSA/UL approval which not only limits the availability of suitable tanks but adds to the cost of the tanks. As you go to larger sizes above 120 gallons, the costs skyrocket as the certification cost versus the demand requires manufactures to recoup their cost by raising prices. There are pressurised "solar tanks" with heat exchangers for sale everywhere but a lot of them are not CSA approved and it is technically illegal to install them. In the case of a pressurised tank, the CSA approval is important because if it blows up and there is no approval, not only might someone get hurt, but your house insurance might not pay out!

Un-Pressurized Solar Tanks

Un-pressurised solar tanks - All an un-pressurised solar tank consists of is a huge tank which holds water. The solar heats the water but this water does not get replaced or used in your home or building. It simply gets hot and holds heat.

You then move the heat energy to your home or building by running it through a heat exchanger or copper coil immersed in the large solar tank or and external heat exchanger such as a plate heat exchanger outside the tank.

Un-Pressurized tanks come in almost any size you want but the standard range from 100 gallons up to 2500 gallons. These tanks are very well insulated and provide a cost per gallon below what you could hope to buy with the pressurized tanks. These are excellent tanks suitable for residential and commercial applications but still cost a considerable amount of money, but cost less than multiple pressurized tanks.

You can see several coils hanging in the tank. These are sized so you can add multiple banks of collectors to put heat into the tank and also so that you flow you’re in floor heating or the domestic hot water line through the tank where they are pre-heated.

Trendsetter - large un pressurized solar tanks

Trendsetter - large un pressurized solar tanks

There are other manufactures of such un-pressurised tanks such as Cocoon Water Tanks.

These tanks range in size from 300 gallons up to 10,000 gallons in size. Even the 10,000 gallon tanks are designed to fit through a normal doorway then are built on-site in the room. This way you can get tanks of 10,000 gallons into an existing building. They are super well insulated with wall thickness up to 8”. The advantage of the Cocoon Tank is that it is made mostly of foam and as such is affordable to purchases and to ship.

The DIY Large unpressurised solar storage tanks

Professionally Manufactured un-pressurized tanks will work very well, they are very well designed, have a warranty and a track record with certifications and engineering. However, they do cost a lot of money.

If you don’t mind getting your hands dirty, and are feeling adventurous then the DIY tank approach will save you lot money. Almost any large vessel can be modified to build a large un-pressurized storage tank.

You can also use almost any other tank you can find.

  • Scrap yards - find old milk tank trucks, old oil filed tanks etc. We found a 1000 gallon insulated stainless steel milk tank at a Calgary scrap yard for under $1000.
  • Old oil field tanks - modern oil tanks need to be double walled to prevent spillage should one wall get a leak. Hence there will be single walled tanks for sale. They will be messy and need cleaning but could work.
  • Call into your local plumbing supplier / wholesaler. Ask it they have any gas or electric tanks that have been returned for a warranty reason. Very often the tank is still OK but the electric element might have failed. All you are after is the tank.
  • Try Kijiji or EBay for old tanks.

There are some limitations to using a plastic tank.

  • You will need to use some ingenuity to build and modify the tank to your use.
  • You will need to insulate it yourself or come up with some novel insulation methods as described below. Read this story on heating homes with solar collectors.
  • The plastic tanks are not usually designed to take very hot water. A custom solar tank will be capable of storing hot water 80+ deg C (170 Deg F), but a cheap plastic tank is likely to start to get "floppy" about 55 Deg C (130 deg F). However remember if you use a very large tank then it is unlikely to overheat.

Larger solar tanks mean your system works more efficiently

A simplistic explanation of what this means in practice is that with a very large solar tank when the tank starts at 50 degrees F in the morning. The solar pump stations are generally set to turn on the pump when the collectors are 10 degrees hotter than the tank. As soon as the sun comes up and the collectors heat up to about 60 degrees F they will be heating the tank.

The temperature differential in this case is only 10 degrees so they are working very efficiently.

However, as the sun goes down in the afternoon and evening, the tank temperature is still fairly low but holding a lot of energy (large volume of water), the collectors can still heat the tank even as the collectors start to cool as they are still much hotter than the tank right down to after sunset. In this way we are maximizing the amount of heat energy being stored throughout a day.

If you only have a small tank, as the tank heats up quickly the collectors can only heat it when they are hotter than the tank. In summer a 40 or 60 gallon tank can be very hot 60 to 80 deg C (140 Deg F or 176 Deg F) by midday. This means all afternoon the solar collectors can do nothing as they slowly start cooling down through the late afternoon and evening but the tank will still be very hot so no solar heating of the tank can occur.

Remember in September, the collectors can still be well over 100 Deg C at 5 pm. A 60-80 gallon tank in a normal home will have reached 80 + Deg C by midday. This means all afternoon the tank will be at its maximum temperature (set for safety) and not only are you wasting solar heat (no more room to store it) but you will spend money running a pump to dump the heat to a heat dump unless you have a pool to dump to.

Temperature is not energy – Most customers often assume a hotter system means more energy. This is not true. In fact in solar water heating theory, the colder the system operates at, relative to the outside temperature, the more efficient it will be. Put in another way, a collector is most efficient when the fluid it is heating is the same temperature as the outside temperature. This happens when the collector first starts up as the heating fluid in the collector is the same temperature as the outside temperature. As the heating fluid gets hotter than the outside temperature the power of the solar collectors decline.

All solar collectors (evacuated tubes, flat plate collectors and un-glazed pool heaters are more efficient when heating cold water than they are heating hot water. However vacuum tubes have the best efficiency curve for high temperature differential heating. When you buy a solar collector, you want it to be heating water for as many hours as possible. There is no point buying it if it is only going to heat water for a couple of hours in the middle of the day. A larger tank will ensure this is possible!

Overheating of solar systems – Expansion Tank

Whenever a solar storage tank reaches the maximum temperature it can hold (set by you) the solar pump turns off to protect the tank. At this point in a closed loop solar system the fluid in the collector header will boil and turn to vapour and you get a expansion of the fluid in the entire system as a result of the steam gas. This is why in a closed loop solar system you will need a properly sized expansion tanks to control this over pressuring. If you don’t then something in the system will burst.

Sizing the expansion tank is very important as is the type of tank you purchase. Most expansion tanks such as Amtrol are not meant for the higher operating temperatures in a solar thermal system. All our pre-packaged solar heating system come with the appropriately sized solar expansion tank made by Italian solar manufacture Zilmet. These tanks have a high temperature EPDM rubber bladder able to withstand higher temperatures.

As the solar system increases in size, controlling this heat becomes more important and only a properly engineered solar water heating system should be installed.

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