How to compare solar water heating systems when purchasing

If you have talked to a number of different suppliers about solar water heating, you may be a little confused by the conflicting information provided. So what is better?

Flat plate or glass tube?
Direct systems or indirect/glycol?
Thermosyphon or pump circulated?
Stainless steel or aluminium collector frames?
Copper or plastic pipe?
Simple controller or liquid crystal display?
Elevated or flush on the roof?
New cylinder or existing cylinder?
Mains, medium or low pressure?

The answers to these questions will depend on your own individual situation. Each of these choices needs to be made with your own circumstance in mind. We will try to give you a little guidance based on our experience.

Flat plate or glass tube? (Performance)

There is no performance difference between flat plate collectors and vacuum tube collectors. To compare the two technologies, you need to compare the aperture of the systems you are being recommended. The aperture is the size of the part that collects heat from the sun. For a flat plate, the system aperture is the width, times the height of the heat plate – do not include the frame as this collects no heat. For a vacuum tube system with no reflector, the system aperture is the diameter of the inside tube, times the length of the tube, times the number of tubes. There are two tube diameters available – 58mm (47mm id) and 47mm (37mm id). So for a 20 tube system with 1.8m long by 58mm diameter tubes, the system aperture is:1.8m x 47mm x 20tubes = 1.69 square metres. For a vacuum tube system with a reflector, the system aperture is the width times the height of the reflector. At Mirak we manufacture our own aluminium framed collector with vacuum tubes and a reflector. The reflector material we use has an exceptional lifetime, and is self cleaning.

Flat plate or glass tube? (Frost Protection)

The vacuum tubes have a definite advantage when it comes to the frost protection of direct systems. Flat plate collectors are single glazed to allow the greatest amount of sunlight onto the collector plate possible. When the outside air temperature falls below zero degrees, the flat plate has to be kept from freezing to prevent damage. The single piece of glass is a poor insulator and heat from the plate is easily transferred away. In a pump circulated system, warm water from the storage cylinder is circulated to prevent the panel from freezing. For a thermosyphon system, there is an expansion tube for the water to prevent damage. A vacuum tube system prevents freezing in a direct system in the same way as a flat plate system. The vacuum tube system has only a fully insulated manifold to keep warm, and will therefore use a lot less energy in this situation. There is no difference in frost protection of a flat panel or a vacuum tube collector if the system is glycol filled. In Mirak’s opinion, all flat plate collectors in the South Island of New Zealand, and some parts of the North Island should be glycol filled.

Direct or glycol system?

A direct system is one which circulates water from the cylinder through the collectors as the heat transfer fluid. A glycol (indirect or closed loop) system uses glycol as the heat transfer fluid. The glycol system is not as efficient as the direct system. This is because the glycol itself cannot transfer the heat as efficiently as water, and there is a heat exchanger required in the storage cylinder which reduces efficiency. Glycol systems become more efficient as the climate becomes colder. There is a balance between summer and winter efficiency which needs to be considered. At Mirak, we recommend direct systems anywhere on the coast of the South Island, and glycol systems depending on how far inland you live. We may also recommend a glycol system if there is poor water quality in your area. Glycol systems are more expensive than direct systems.

Thermosyphon or pump circulated?

Thermosyphon systems use the change in temperature of the water to circulate between the collector and the cylinder. As the water heats, it becomes less dense and rises. For this system to work, the cylinder needs to be above the panel. Pump circulated systems are just what they say; a pump is used to circulated water between the panel and the cylinder.

Thermo siphon advantages:
Simple system – therefore more reliable and generally cheaper. (No pump)
You will have solar heated water even when there is no power. (Some pump circulated systems can do this also)
Lower system cost

Pump circulated advantages:
The cylinder location is not set by the collector position.
The cylinder can be vertical instead of horizontal, allowing the water inside to sit in layers in the cylinder.
The pipe run from the cylinder to the point of use is generally shorter, meaning you will get your hot water sooner and waste less energy heating up pipes.
There is generally more feedback on how your system is working.
Easier to install.
Can be retrofitted onto your existing cylinder.
Generally lower building consent charges.
Less weight on your roof, so no extra bracing required.
Generally there is easier access to your cylinder.
Lower cylinder losses as the cylinder can be inside the building envelope.

At Mirak we sell only pump circulated systems. We believe these suit New Zealand homes and the New Zealand building code far better than thermosyphon systems. Almost all of the thermosyphon systems for sale in New Zealand are Australian manufactured. These systems suit Australian conditions very well.

Stainless steel or aluminium collectors?

Most people believe stainless framed collectors are a good material choice for New Zealand. This is not true. The majority of New Zealand homes have light weight roofing material (Coloursteel or corrugated iron). Stainless steel is not compatible with some of these materials, and can void the warrantee on your roofing material if installed over it. The grade of stainless most of these systems are made from is not allowed to be installed in some environments in New Zealand as it will corrode. Some of this information is included in the building code, and some of it is provided by roofing material manufacturers. Most councils we have spoken to at Mirak are not aware of these requirements. Mirak collectors are manufactured from marine grade aluminium. They are compatible with most types of roofing, and can be installed in any environment in New Zealand.

Copper or plastic pipe?

This is up to the system manufacturers to specify. To be able to use plastic pipe, the system must not exceed the temperature and pressure limits of the plastic. The best plastic pipe can take a very small pressure at 95 degrees C maximum temperature. Most systems can easily exceed this. Some systems have temperature relief valves which prevent the collector from overheating. Most systems that use these valves dump heated water onto the roof or into a drain. This is a waste of water, and in most cases is prohibited by the building code. Standards are being changed to prevent this type of installation. Mirak specifies copper pipe in every installation and does not dump water when the system has no load.

Simple controller or liquid crystal display?

Your supplier may have offered a display unit to show how your system is operating. At Mirak we believe this is an essential part of the system for two reasons. The first reason is that with a display you will get a much better idea about how your system is working and how your hot water is used – it gives you more control. The second reason is if you have a problem with your system, you can call us, tell us what the display is showing, and we can get a very good idea over the phone about what is happening.

Elevated or flush on the roof?

This decision comes down to your individual situation as well as personal choice. There is a mixture of roof pitches and orientations that work well for solar. You need to look closely at how your roof is set up, and how your plumbing system works. Different installers have differing recommendations here, and there is no right or wrong answer. One possibility if you have a flat roof is to increase the collector area rather than elevating the system. Other factors that require consideration when elevating a collector include: increased wind loading on your roof structure, how the system looks, and what other sources of water heating you have e.g. a wet back fire.

New cylinder or existing cylinder?

This is relevant if you are fitting the system into your existing home. Pump circulated solar can be fitted to your existing cylinder. It is up to the individual system installers to assess the condition of your existing cylinder and recommend a replacement if required. Reasons for replacing your existing cylinder include:

Age – cylinders have an expected life between 10 and 25 years depending on the cylinder type and the quality of the water it has heated. Installing solar on a cylinder that is too old could lead to a leaking cylinder.
Efficiency – Cylinders suitable for the New Zealand climate will be labeled ‘A Grade New Zealand’. Cylinders that do not meet the NZ A grade standard will loose a lot more heat overnight.
Size – A well set up solar water heating system needs to store at least enough hot water for one days use. This means when there is a rainy day, and no heat from the sun, you will not run out of hot water. If your existing cylinder is too small it should be replaced.

Mains, medium or low pressure?

The operating pressure of your hot water cylinder should not affect how your solar water heater operates – but it will affect how much hot water you use – and this affects the size of your system. The higher the pressure of your hot water cylinder, the more water disappears down the plug hole of your shower. e.g. A low pressure shower with a flow rate around 6 litres per minute will use about 35 litres of hot water for a 10 minute shower. That same shower at mains pressure, with a flow rate around 20 litres per minute will use about 110 litres of hot water. A solar water heater for the mains pressure system will need to be 3 times the size of the low pressure system to have the same effect in reducing your water heating cost. However, mains pressure can still be used with some careful selection of shower fittings.

For more information please call Mirak on 0508 4 Mirak. Your call will be answered by either Martin or Sonia Perry. We are here to help. The consumer website also has a very good guide on purchasing solar water heating, as does the recently released smarter homes website.