Hot Water Tank Options

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Do I need to replace my tank when installing solar thermal? No

For those of you thinking about installing solar thermal (solar hot water) and unfamiliar with the options, I'd thought it would be useful to summarise the different options available and the common approaches people take to integrating solar hot water with their domestic hot water (DHW) tank.

Solar thermal is all about heating water in your solar panels (collectors), and somehow transferring the heat energy collected into the water in your hot water tank. We'll cover solar panels elsewhere and for now we'll focus on your DHW tank. (You can also use solar hot water without a water tank, e.g., of you have a combi boiler system in your house, but this not at all common and we'll ignore for now)

The two main approaches to tanks I believe are as follows:

[1] The least effort / cost approach, whereby you try and use the hot water tank you've already got. Whilst perhaps not technically ideal in many cases, you can end up with quite a saving in both effort and cost and yet still attain most of the benefits afforded by the more expensive option of replacing your DHW tank.

With this approach you already know the size of your tank and all you need to do is decide on the size of panels, something that is generally determined by the tank size, and you also need to decide on your favoured method of transfer the heat from the panels to the tank.

[2] Work out how much hot water the household needs, then work back from this to compute both your required panel and tank sizes, and then purchase a solar (twin coil) tank of the appropriate size to meet those theoretical requirements.

Approach 2 is that most often taken by the commercial solar thermal installer, which, not surprisingly, tends to be the most expensive option. Approach 2 is also taken by the likes of myself who didn't already have an existing DHW tank due to the fact that the previous owner of the house allowed Phoenix Gas to rip the old hot water tank out.

Approach 1 tends to be taken more by DIYers on a budget.

Nothing much wrong with either approach - It really depends on the householder's objectives, capabilities and/or means.

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If you are retaining your existing tank all you really need to think about with regards the tank is how you want the heat to be transfer ed to the water in the tank.

Most existing tanks will have one 'boiler coil' already. This is used to transfer heat from an oil or solid fuel boiler, but won't have a spare coil to use for bringing in the heat from the solar panels.

The main options are:

[1] Direct flow. Here the water in your DHW tank, which ends up coming out of your hot taps and shower, is exactly the same water as that heated in your panels.

[Plumbing schematic for direct flow to be inserted here.......]

[2] Indirect Flow. Here the water/fluid heated in your panel circulates from the panel to some sort of heat exchanger either connected to or inside your DHW tank. The hot water loses some of its heat by conduction through the metal walls of the heat exchanger, thereby heating the water in your DHW tank , before the water/fluid is returned to the panels for reheating by the sun. Water/fluid circulated from panel to tank and back is not allowed to mix with the water in the tank.

[Plumbing schematic for indirect flow to be inserted here.......]

Advantages of Direct Flow tank:
Simple & inexpensive

Disadvantages of Direct Flow tank include:
a)Not suitable in hard water areas and the calcium/salts will precipitate in the hot water and eventually clog up your dhw tank and solar pipework.
b) Because normal water circulates in the panels you have to do something about freezing.
c) If the system is not 'tuned' correctly you can end up with buckets of lukewarm water and not necessarily the small quantities of hotter water (generally households prefer smaller qtys of hot to larger qtys of lukewarm)