Advantage in turning off a solar hot water system mid week for a weekend house?

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Advantage in turning off a solar hot water system mid week for a weekend house?

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Started 11 years ago by Andrew Rogers
Latest reply from Inspector

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Andrew Rogers
Member

Hi,

I have a hot water system in a house which is currently it's not used Monday to Friday. The existing system is solar with a separate tank located above the panels only 2 metres or so away.

My question is if I were to connect a timer to turn the system off Monday to Friday, will more energy be used in bringing the water up to temperature having been switched off for the previous 5 days than it would to simply maintain the temperature in the tank with no water usage for the same period?

Has anyone considered this previously or tested by metering for power consumption?

Regards,

Drew

Posted Wednesday 27 Oct 2010 @ 12:50:16 pm from IP #
Russell Moore
Member

I assume you mean turning off the booster for those days? My thoughts, and they are only that, would be to turn off the booster from Sun-Thu, and have it turn on Fri-Sat. That way you would have hot water on arrival Saturday morning, either from solar and/or boosting, and assuming the booster comes on at night, you would have hot water on Sunday.

To test this just take a reading of your meter from Sun-Sun with the booster on, then the same with the booster off Sun-Thu. Of cause the problem with that would be how much was the solar contributing in those times....?

Posted Wednesday 27 Oct 2010 @ 10:57:33 pm from IP #
thatmosis
Member

Question, why do you employ a booster unit at all? We have a solar hot water unit that has no booster and it is adequate for our needs. Of course we are only 2 people with a 130ltr tank on our solar but there is enough for a morning shower each, the washing up and a shower each at night. We clothes wash in cold water to save our hot and that is all.
An adequate Solar Hot water system shouldnt really need a booster unless of course you are in an area that doesnt receive at least 5 hours of sun a day.

Posted Thursday 28 Oct 2010 @ 12:25:53 am from IP #
Buzzman
Member

drew

The simple answer is yes, it will use less power. Cloudy days mid-week would see the booster cut in unecessarily - unecessary in the sense that there is no one in the house to make use of the hot water that the energy has been used to heat.

If the su keeps it hot, no cost to the home owner, no problem.

Only problem is if the booster is Off and Thursday and Friday are overcast. With no sun heat, and only overnight Off-Peak boost, that means no hot water before Sunday if you notice on Saturday morning that there is none.

So, either arrive Friday night - so you can turn the booster on; befirend a neighbour who you can phone and check the local weather, and get them to turn the booster on if required; or as you've suggested, install a timer so that it will only come on, on Friday night, if required by the thermostat.

Check out Hager's website for DIN rail model timers that go in the fusebox alongside the Hot water Main switch/circuit breaker. You'd need a 7-day timer that would enable only one off-on during that period.

The EH010 is the simplest but not sure if it would do that. It's a day-by-day timer I think. My Mum has one on her HWS but it is designed to turn it off at night so the heat pump doesn't waker her up.

Posted Thursday 28 Oct 2010 @ 12:59:13 am from IP #
Benny
Member

Drew - in terms of energy required, think of it this way...

energy is required for 2 reasons, 1. to heat the water from incoming cold to required hot output, 2 to cover losses due to imperfect insulation. Energy 1 is always the same to get your water hot. Energy 2 is dependent on how hot the water is in the tank. The hotter it is the more losses. So you are better off leaving the booster off mid week and reducing the heat loss.

As others have said as well, I'm surprised you even need the booster esp in summer. Esp if you have good insulation it should be hot after 5 days of no use. Then again if it is hot turning on the booster won't matter as the thermostat will stop it turning on. Don't just turn on the booster automatically though, if its sunny and you have morning showers, the booster will come on before the sun has a chance to re-heat the water. Depends if you are connected to off peak though.

Posted Thursday 28 Oct 2010 @ 1:38:58 am from IP #
Buzzman
Member

More to the point, here's a formula that enables you to calcualte how much it would be costing you to leave it on.

http://www.leaningpinesoftware.com/hot_water_heater_tank_insul.shtml

Posted Thursday 28 Oct 2010 @ 4:32:47 am from IP #
sun2steam
Member

Thanks for the calculation.
If you add that the solar heater will produce enough heat to cover the losses most of the time. And if there is no water being used during mid week, the temperature will stay above the booster thermostat level and there will be no boosting.

I guess we have already wasted more energy writing this thread than the electrical booster will ever need. As I had posted already in another thread, we paid just about $3 (offpeak1) for the whole winter quarter when the booster had not been turned off (Central Coast, 2 people, 30 tube collector 420L tank). And this is our main home, not a weekender.

Result: just leave the booster always on. Legally it must be on anyway!

Posted Thursday 28 Oct 2010 @ 4:50:00 am from IP #
Sunshine
Member

sun2steam,
I don't find any evidence that it is illegal to leave the electric boost off on a SHWS.
The Australian government website for hot water systems can be found below:
http://www.yourhome.gov.au/technical/fs65.html#solar
An extract from the Booster Systems section says "Boosters can be manually operated or automatically controlled by a thermostat that cuts in when tank temperatures fall below desired levels. If boosters are not appropriately designed and operated they can defeat the purpose of having a solar water heater by reducing the solar contribution."
Sounds like they recommend to leave the booster switched OFF.
I have only boosted once in two years now for a total of two hours, so don't boost unless you need to. Why heat the water with electricity in the evening, when the next morning the sun will do the job for you for free? It defeats the purpose of having a solar collector. The website says the same thing.

Posted Thursday 28 Oct 2010 @ 12:55:07 pm from IP #
dymonite69
Member

I believe that there is a Standard that says that storage HWS systems can't lose more than 2 kWh per 24 hour period. That might amount to about $73 wasted electricity or 730 kWh.

The tank must reach 60 degrees at least once during the day to kill Legionella. According to research this does not happen in a substantial number of systems. But according to the Vic DH website thankfully no cases of Legionellosis from shower heads ever reported.

Posted Thursday 28 Oct 2010 @ 1:09:42 pm from IP #
Diver
Member

Dymo; Is this a new revision?
"The tank must reach 60 degrees at least once during the day to kill Legionella".

Your advice conflicts with the following which I am unaware of being either under review or updated.

Legionella: The Australian Standard AS3498-2009 related to plumbing products requires that: "For a storage water heater, heat pump heater or solar water heater, at least 90% of the stored water is heated to 60oC for at least one single period of not less than 32min each 7 day period."

The recent posts regarding hot water recirculators have also caused me to consider the consequence of 'dead water' in pipes, although Legionella, if present, needs both a nutrient supply and favourable temperature, however, dead water would have already been heated to 60 degrees C., killing any Legionella present.

Posted Thursday 28 Oct 2010 @ 4:17:43 pm from IP #
sun2steam
Member

http://www.health.sa.gov.au/pehs/PDF-files/ph-factsheet-legionella-pneumo.pdf
page1: "Hot water systems
Hot water systems have the potential to harbour Legionella pneumophila in places where there may be stagnant or warm water (25-50°C). Examples include shower nozzles, hot water taps, hot water storage vessels and hoses or filters attached to shower roses or tap outlets.
Hot water tanks need to be set to store water at 60°C or more to reduce the risk of Legionella multiplication. If you are unsure of how to set the temperature of your hot water tank, seek advice from the manufacturer."

Posted Thursday 28 Oct 2010 @ 11:14:03 pm from IP #
Diver
Member

Legionella:
Below 20 degrees C. Dormant.
20-25 degrees C. Essentially dormant but some growth still possible.
25-30 degrees C. Very slow growth.
30-37 degrees C. Increase in growth rate.
37-43 degrees C. Optimum temp for regrowth.
45 degrees C. Maximum temp for regrowth.
46 degrees C. Will die after approx 1 week.
55 degrees C. Dies in 1 hour.
63 degrees C. Dies in a few minutes.

The text below is found in the following link.

Multiplication is encouraged if nutrients are available, temperature is appropriate, and the
water system is shutdown, used only occasionally (say, less often than weekly) or the pipework
contains “dead-legs”. A dead-leg is a length of pipe that is capped at the end or provided with
an end fitting through which water flows only rarely when the fitting is opened.

http://enhealth.nphp.gov.au/council/pubs/pdf/legionel.pdf

Posted Friday 29 Oct 2010 @ 12:45:59 am from IP #
Andrew Rogers
Member

As a new member to ATA the very detailed replies are a nice welcome so thank you.

We've only had the house for about a year, and the solar system has been in place for the last 20 years or so I expect (maybe less). The existing configuration is simply what was there when we arrived.

We're currently seeing around $20 to $80 on the quarterly electricity bills for off peak power and I'm quite certain that only the hot water is connected. So there's obviously a bit of boosting in use at the moment.

I'll take some meter readings over the next month or so both with the booster turned on and off and report with the results.

Buzzman - thanks for the link to the timers, I'll definitely look into these. A timer the turns the system off Sunday evening and then back on Friday evening sounds like it may work well.

Posted Saturday 30 Oct 2010 @ 10:20:56 am from IP #
Inspector
Member

My SHWU has a manual boost switch in the kitchen (it's 28y/o). One of the recent summers we used a measly $2-something for off-peak tariff (there must've been a rather overcast week that summer). Most of the time the switch is off, and we only turn it on before bed when it's been overcast all day (having already had a bath using the hot water from the previous day's free heating). I've seen alot of switchboards where the off-peak switch (SHWU installed) has been switched off, and my in-laws do the same at their holiday house - why waste electricity/money heating water when it's not needed? Obviously having the switch inside the house is more convenient than the switchboard outside, but it would usually be an easy job for a sparky to put a switch (and neon light to remind you from a distance that it's turned on) in a convenient location inside the house.

Posted Saturday 30 Oct 2010 @ 11:03:00 am from IP #

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