Solar Hot Water

In Winter, our Apricus evacuated tube solar hot water heater works more like a solar warm water heater. Annoying design glitch leads to petitions and complaints by Carlyle Gardens residents in Stage A.

Jeff came around Carlyle Gardens collecting details of solar hot water complaints in August 2011. Ray had a note to Stage A residents asking if they had issues with the solar hot water, to follow up with the Resident's Committee.

Solar Hot Water in Winter

My only complaint is in Winter, our Apricus evacuated tube solar hot water heater works more like a solar warm water heater. According to the Apricus evacuated tube solar hot water heater manual, this may be working as designed.

Some home owners make the mistake of thinking that once they install a solar water heater they can turn their element or gas booster off whenever there is sunlight. This is incorrect for two reasons. Firstly it is a requirement to heat the water to 600C on a regular basis to kill Legionella bacteria. Secondly, solar radiation is only half or one third as strong in the winter months compared to summer, and therefore is not able to provide the same amount of hot water as in the summer. Your solar system is designed to meet virtually 100% of summer hot water needs and will provide about 50% in the winter, averaging about 60-80% annually, depending on your location.

My view is that there is a design flaw in the installation of virtually all solar hot water heaters. Essentially they produce a surplus of hot water in summer (except when cloudy), but a shortage of hot water in winter. I believe this is because installers treat them as if they were solar power panels for electricity. Solar power needs to maximise total output (because you can sell the surplus back to the grid). However you can neither sell nor store surplus hot water. Therefore the solar hot water panels should be installed so as to maximise the number of days in which you get sufficient hot water. This means reducing solar input in summer, and increasing it in winter. The panels should be at a higher angle (probably add 23.5 degrees to your latitude). So for Townsville, an angle of around 43 degrees.

Australian Standard AS/NZS 3500.5 clauses:

3.4.1 Storage Temperature

Hot water shall be stored at a minimum of 600C so as to inhibit the growth of legionella bacteria.

3.4.2 Sanitary fixtures delivery temperature

All new hot water installations shall, at the outlet of all sanitary fixtures, used primarily for personal hygiene purposes, deliver hot water not exceeding 500C.

Since it costs money to buy a copy of the Australian Standard, I am not going to attempt to interpret how you avoid legionella buildup in dead end pipework, taps and shower fitting that will always be below 500C. I am also not going to try to work out whether storage at 600C is adequately covered by boosting tank temperature to this level once a week, as recommended by several Australian government departments promoting energy conservation.

I am also not going to speculate as to whether full tank temperature hot water should be supplied to non-sanitary fittings, such as dish washers (if they accept a hot water feed), and especially laundry tap fittings. However I do not believe homes in Carlyle Gardens built by Finlay have full tank temperature water in the laundry.

Is Legionella a Problem?

Legionella based disease is associated with aspirating water particles that contain legionella. It is something you breath, not something you pick up from contaminated water. Lots of water has legionella in it, and that is not a problem. It is part of a normal environment. Large scale outbreaks have always been associated with droplets from air conditioning water towers and similar. However there is a theoretical risk associated with showers and spas. Worse, the risk is greater for older folks, and those with compromised immune systems.

A long paper about Controlling Legionella in a large environment. However the basic solution for a hot water service is to ensure water is heated above 600 C from time to time.

I am not convinced there is any authenticated case of legionella based disease being contracted from shower water. There are cases where legionella disease has been diagnosed (which typically takes several weeks to confirm from a spit test), and subsequent testing of a hot water system has shown legionella present. However random testing of any water source may show this.

Electricity Required to Heat Hot Water

The energy needed to raise the temperature of any substance is a physical property called specific heat. For water, this is 4.2 Joules (J) per gram per degree Celsius.

You have a 250 litre Apricus hot water tank, which is 250 kilogram. So it takes 1050 kJ to raise it one degree. If the water starts at 400C (mine was 430C in the laundry), to raise it another 200C to 600C takes (4.2 by 250 by 20) kJ, which is 21,000kJ or 21MJ.

You convert MegaJoules (MJ) to kilowatt hours (kWh) by dividing by 3.6, so 21MJ needs 5.55kWh. Since the Apricus solar hot water system has a 3.6 kW booster element (15 Amp at 240 Volts) this means running the electric booster for one and half to two hours ensures the water temperature reaches at least 600C.

Cost of Electricity for Boosting Solar Hot Water

I am supplied with electricity by Ergon (the default supplier). As at August 2011 when I wrote this, the Ergon electricity tariff 33 (including GST) was 13.673 cents per kWh. Your air conditioning and hot water should be connected to Tariff 33.

So boosting your solar hot water tank will cost you around 50 cents per hour.

Your minimum payment per month on Tariff 33 is $6.09, whether you use electricity or not. So you can use about 44 kWh per month without additional cost. This equates to running the hot water booster about 12 hours a month, or three hours a week.

Since Tariff 33 is a controlled time supply, you will need to ensure that the hours at which you switch on your hot water booster are not times at which the Tariff 33 electricity supply is actually switched off. Weekends are generally good. Plus, to have the best solar hot water temperatures already in the tank, you want an afternoon, when you have not drawn much hot water, except say for a morning shower. For these reasons, I favour Sunday afternoon to give the solar hot water a boost.

Quotes from Erma Bombeck My theory on housework is, if the item doesn't multiply, smell, catch on fire or block the refrigerator door, let it be. No one cares. Why should you?