Solar Hot Water - Electric boosting in the mornings? « Alternative Technology Assn Forums

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Solar Hot Water - Electric boosting in the mornings?

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Started 7 years ago by LMcLaggan
Latest reply from sunbeam

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LMcLaggan
Member

This has been bothering me for a little while now.

I bought a house last year and it has solar electric hot water i believe an edwards or something. I have no technical information on the unit unfortunately and it is a few years old (around 5 years). I have been having showers in the evening just because i am paranoid about the system using the electric booster and getting a massive electricity bill (and emitting greenhouse gas emissions). I know the hot water storage tank is insulated; but what if me and my partner have a shower in the mornings (8L/min showerheads at 6min x 2 people = 96L) and the water temp has dropped overnight? If it is stored at 60 degrees in an insulated will it drop that much?

But no one really talks about changing habits to shower in the evenings to reduce the risk of the booster turning on so I wonder if I am worrying in vain...

Posted Wednesday 6 Feb 2013 @ 5:30:22 am from IP #
Lance
Key Master

Having showers in the evening, before the boost period starts, will make it more likely that the system boosts as you are removing some of the hot water and making the tank colder. Morning showers, after the boost period (I am assuming it is off-peak boosting on an overnight tariff) means that you draw off hot water in the morning, cooling the tank, which gives the solar something to do. If the system boosts at night and you use no hot water in the morning, the water is already hot and the solar input is going to be limited.

Posted Wednesday 6 Feb 2013 @ 5:35:48 am from IP #
bundy
Member

Best solution would be to have a timer on your booster so it only boosted if you needed it. Set the Booster to come on say around 4pm and switch it off by 6pm ( By then the tank is pretty much as hot as it will get for the day and if it needs more it will be ready for you) If it has been a wet cloudy day or two and you need to boost it will do its thing if not you wont use power.

If you have this setup it really doesnt matter if you shower in the morning or evening, just regulate your boosting to only occur at the end of the day after it's had a chance for full solar gain. Only thing is make sure you dont boost AFTER you have used the hot water otherwise you will boost overnight and next day still have a hot tank and nothing for the sun to do as Lance mentions above

Posted Wednesday 6 Feb 2013 @ 7:32:03 am from IP #
Sunshine
Member

Even better is to leave the boost off, and only boost when required. Problem solved.

Posted Wednesday 6 Feb 2013 @ 7:34:19 am from IP #
Morbo
Member

Am in Melbourne, too, switched the booster off 3 months ago and maybe get another 2 months. No point having it on, since the water is at 70° today.

Posted Wednesday 6 Feb 2013 @ 10:07:21 am from IP #
SgtThursday
Member

You might be surprised at how good the tank insulation actually is, there should be quite a bit of heat left in the mornings, particularly over summer. My household has mostly morning showers and we don't have any problems.

My preferred time for boosting is around dusk, we're on a flat tariff for imports so there's no advantage to us boosting overnight. Boosting at dusk (provided it finishes before you have any showers) seems to be the most energy efficient for the reason outlined by Lance.

Posted Thursday 7 Feb 2013 @ 9:49:10 am from IP #
hamish05
Member

Morbo said:
Am in Melbourne, too, switched the booster off 3 months ago and maybe get another 2 months. No point having it on, since the water is at 70° today.

+1 Morbo....Same here.

During the colder months my boost is determined by the upper tank temp and a time switch set to supply power between 4:00 and 6:00 hrs, this ensures I only utilize my cheapest TOU (O/peak) electricity.

If the upper tank temperature falls below 55deg, the tank will heat between 4:00 and 6:00 hrs. OR until the tank reaches 60 deg. The temps are adjustable, but after a fair amount of fiddle those temps work well for me.

The most power my SHW consumes is 7.2kWh/day, but on most occasions much less is used to bring my 315L (Apricus branded) tank up to 60 deg.

So first thing in the morning, we always have hot water between 55-60 deg . After morning showers etc, the sun can do its bit during the day, and if at 04:00 hrs the next morning the temp has dropped below 55deg it will electrically boost.

Some stats for the last 2 years, 2 adults (usually 2 showers each / day) dish washer on Hot water, no laundry usage, some guests and grandchildren occasionally.

Feb10 to Dec31-2011, 111 Boosts (800kWh) Total cost (inc GST) $80.31

Jan01 to Dec31-2012, 135 Boosts (837kWh) Total cost (inc GST) $101.16

I commend the Apricus branded tank. It is made to suit solar operation with separate specifically positioned ports for both temp detection and water connections and that makes so much sense.
Interestingly, the upper tank temp is measured half way UP the tank....

I originally had an Aquamax tank which failed prematurely and I was fortunate enough to have it replaced with the new style Apricus model.

Posted Thursday 7 Feb 2013 @ 10:52:04 am from IP #
TonyT
Member

For 2 people, the annual heat-energy used for hot water heating in our bathroom plus laundry plus kitchen totals 2,000kWh/yr.

About 2/3 of this is the heat-loss of 3kWh/day through the 50mm insulation of the new 317 Litre tank.

The Siddons heat pump delivers an annual average of 2.7kWh heat from one kWh of electricity so our annnual electricity for heating domestic hot water is
2000 / 2.7 is 740kWh/year.
The Coefficient of Performance COP of the Heat Pump is shown as the bottom red line on the second Chart at
http://www.ata.org.au/forums/topic/6704#post-38041

Using the off-peak 14cent/kWh tarrif (available for 10hrs each night) the annual cost for this electricity is $104.

This is less than the annual cost for the electricity that is used in the booster element of many solar thermal hot water systems.

Additional water heated by our Siddons is used through hydronic wall-mounted radiators to heat 3 small rooms (total floor area 28sqm) at a low cost of 5.6cent/kWh of heat.

Using a CO2 heat pump (e.g. the newly available Sanden), the cost of heat would drop to 3.4cent/kWh heat as shown in the top Chart at
http://www.ata.org.au/forums/topic/6704#post-38041

Using a source of heat-input that is warmer than the (winter, night-time outside-air) and / or a heat-output that is cooler than wall-mounted radiators (e.g. hydronic under floor heating) would reduce the cost of heating to below 3.4cent/kWh of heat.

Posted Thursday 7 Feb 2013 @ 10:15:10 pm from IP #
Bushwalker
Member

I can back Tony's figures above from personal experience.

If the concern is runaway electric booster on a solar HWS, then the simple answer is to pull the fuse and see if you can exist without boost, perhaps modifying your hot water usage to suit the changed availability.

From my investigation prior to installing a Siddons, I concluded that anywhere south of Sydney was at risk of high boost costs that would alter the economics of solar hot water.

Posted Thursday 7 Feb 2013 @ 10:30:20 pm from IP #
Bushwalker
Member

hamish05 said:
Some stats for the last 2 years, 2 adults (usually 2 showers each / day) dish washer on Hot water, no laundry usage, some guests and grandchildren occasionally.

Feb10 to Dec31-2011, 111 Boosts (800kWh) Total cost (inc GST) $80.31

Jan01 to Dec31-2012, 135 Boosts (837kWh) Total cost (inc GST) $101.16

This is almost equal to our yearly costs for our Siddons heatpump hws under similar load.

Posted Thursday 7 Feb 2013 @ 10:33:02 pm from IP #
LMcLaggan
Member

So looks like i am climbing on the roof tonight

Posted Thursday 7 Feb 2013 @ 10:47:05 pm from IP #
dbindoff
Member

My solar hot water boost is 'mixed in' with the air source heat pump heater so I can only estimate my usage. My winter boost is on a simple programmable 7 day, 1 minute timer, installed in the switch board. I operate it over around 100 days in Winter, I estimate yearly boost around 200 kwh. (2 persons usually)

I program it to boost at dusk normally.

Posted Thursday 7 Feb 2013 @ 11:05:34 pm from IP #
Sunshine
Member

LMcLaggan,
Why climb onto the roof?
The fuse/circuitbreaker is in the switchbox.

Posted Thursday 7 Feb 2013 @ 11:54:47 pm from IP #
LMcLaggan
Member

Sunshine .... for the solar electric hot water unit on the roof? I would need to adjust the boosting setting on the unit ... wouldnt i? Unless i just pull the fuse out?

Posted Friday 8 Feb 2013 @ 12:00:22 am from IP #
Bushwalker
Member

If the unit uses a circulation pump, pulling the fuse might not be the best idea.

Posted Friday 8 Feb 2013 @ 12:28:59 am from IP #
sun2steam
Member

If the heater is on a special night tariff the circulation pump is powered from another power circuit.

Posted Friday 8 Feb 2013 @ 12:38:30 am from IP #
LMcLaggan
Member

Mk so really i need more information about the system

Posted Friday 8 Feb 2013 @ 1:31:41 am from IP #
SgtThursday
Member

Don't climb up and start twiddling, there are usually no controls that a user can set on the HWS itself.

There should be a separate circuit breaker in the fuse box which is connected to the HWS and nothing else.

You can get an electrician to install a timer switch in the fuse box which lets you control when the booster comes on.

Posted Friday 8 Feb 2013 @ 2:52:55 am from IP #
Sunshine
Member

LMcLaggan,
The electrics are in the switchbox. There are no boosting adjustments to be made on the system on the roof.
Either you manually operate the circuit breaker in the switchbox, or have a timer installed in the switchbox for the electric boost circuit, or alternatively get a sparky to install a switch in the house to save the walk to the switchbox.

Posted Friday 8 Feb 2013 @ 7:38:17 am from IP #
Morbo
Member

booster defies purpose of SHW because in winter it would rather loose heat, when it pumps water through the panels? In winter ours is at 25-30 deg, if booster would boost it to 55 deg, it would do nothing. Wouldn't an instantaneous booster be better? 25 deg in winter is still better than 10 or so, so even then the SHW does something

Posted Friday 8 Feb 2013 @ 10:25:40 am from IP #
sun2steam
Member

There are three issues with instantaneous electrical boosting:
- it is a really high electrical load, potentially higher than available in some buildings
- it does not address the legionella health risk
- it is more expensive than a standard heating element sitting in the tank

If you have instantaneous electrical boosting, it should be just in front of the user to avoid unnecessary heat losses in the pipes.

Posted Friday 8 Feb 2013 @ 11:49:39 am from IP #
Morbo
Member

I think a normal gas heated tank is only 80L, solar hot water starts at 175 or so. So to make matters worse you now have to keep a way bigger tank at 55+ degrees. Defies the purpose of SHW, as the whole thing needs more energy than not having one?
Reg legionella, our inst. gas booster whips it up to 55° or more. And at present its at 70°, no need to boost.
How much does an electric instantaneous booster draw, seen them in some high rises, you would not want to wire 10kW to each flat? Maybe 3 phases to every house would be nice, like in other countries.

Posted Sunday 17 Feb 2013 @ 9:23:06 am from IP #
Aussie Wide Solar
Member

@ Bushwalker: "anywhere south of Sydney was at risk of high boost costs that would alter the economics of solar hot water"
I have to comment on that statement. If properly sized and installed, a quality hot water system will will rarely require boosting, even in Tasmania. Yes, boosting will be needed more often there than in the Top End, but not so much that it becomes untenable economically as might be taken from your statement. Solar hot water systems using the STC process have a higher rebate value in more southerly/colder climes for just this reason so that Customers may still get similar performance and efficiency by spending that extra rebate on slightly larger collectors/etc.

To All:
It is not generally recommended to switch off your solar boost. Still water has a natural tendency to breed bacteria, in particular legionella, and for safety stored water is must regularly hit temperatures over 60 degrees to inhibit growth. To this end, the auxiliary booster is best left permanently on. A solar water heater system under AS 3498 must ensure that "at least 45% of the water contained within the storage tank is heated to a minimum of 60C daily".
In practice, the water is rarely stable or settled enough to allow for bacteria to become a big enough problem before the next solar heating cycle, I will admit. It is even rarer that a properly installed solar system does not achieve 60 degree heat on a daily basis, but a string of cold, overcast weather, combined with heavy draws off the hot water system may create a problem, which, rare as it may be, every manufacturer and installer is required to recommend against.
If you are to to turn off the auxiliary heater against all recommendations, SgtThursday had it right, one of the safest methods is to flick the switch in your fusebox, thereby cutting off any current at source, and ensuring that the wires are no longer live. Just remember to turn it back on again!

@Morbo:
The water is only pumped through your collectors when the controller thinks that they are hot enough to heat the tank water. A correctly installed controller/pump will cycle only when the water in the collector is hotter than the water in the tank. In winter the boost will only heat your tank water, The controller should not cycle then.

"keep the way bigger tank at 55 degrees" - as per above, only 45% of your solar tank needs to reach 60 degrees, which is why many tanks have elements and sensors in the middle, at that 45% point. The element does not heat the entire tank of 100 or 400 litres, just the top half. Keeping in mind that solar collectors should still contribute heat every day and that the tank is well insulated, the auxiliary heater should never have to heat the tank from cold but only top up on existing heat. If you are regularly using your element consider the condition and size of your collectors.

Posted Tuesday 5 Mar 2013 @ 1:54:56 am from IP #
sunbeam
Member

I have a 10kw PV system that was meant to be wired to heat the water tank. I noticed from my last bill that I'm still using 4.7kwh overnight on a dedicated circuit. This is exactly the same amount as prior to the PV installation. Given there are only 2 of us living here now ,I think the tank should be easily heated from our daytime production. Is there something I can do to stop the overnight heating of the tank? The installer said that the heater would only use one of the elements if water cooled down a little and wouldn;'t use the same amount of electricity. Alternatively can i tell from the fuse box that he did indeed wire the tank up to accept the pv production?

Posted Sunday 7 Jan 2018 @ 11:59:09 pm from IP #
Benny
Member

When you say dedicated circuit - do you also mean via a dedicated meter ? We don't have these in WA but I read that many over east have off peak/water heater supply via a separate meter. In this case I presume the meter disables the supply to the water heater until off peak time ? The PV system will be wired in to the "normal" house supply meter. If your water heater can't access the "normal" supply during the day then it can't receive power from the PV system. I think the water heater needs to be connected to the regular supply to get power from PV.

I winder also if its related to your hot water use ? Do you have showers in the evening or morning ? If evening then the water will cool down and be reheated overnight - before the PV has a chance to supply power to it. Could you turn the water system off for a day or so - use up all the hot water then turn it on at say 11am. You should see (from meter display) if the heater is operating - LED flashing 1'ce per sec.

Posted Monday 8 Jan 2018 @ 3:30:06 am from IP #
sunbeam
Member

I'm with Powershop in VIc, This is what I see on my weekly update:
Dedicated Circuit
4.7kWh
CZ303847:E4 per day
Time switch controlled hot water services, space heaters and other dedicated loads.
I don't have a space heater or other dedicated loads AFAIK. I moved in a few months ago and had the panels installed.
There is a HW Main Switch in the meter box and another switch labelled Main Switch/Storage Heat/Aux Element.
We both shower in the morning.. I don't know which of the two HW related switches to turn off unfortunately . The HW Main switch is a black box with large flick switch. The other one looks more like a circuit breaker with 2 flick switches. It has 20A written on it,
I think I will talk with Powershop about my contract now that the PV is up and running and see if they can organise to change it remotely.

Posted Monday 8 Jan 2018 @ 6:52:24 am from IP #
Ben1
Member

When there is a dedicated off-peak meter for in-slab electric heating or off-peak hot water, this meter will usually not register PV input as the PV does not pass through this meter. Sunbeam, if you can be confident that you are consistently overproducing by at least 3.6 kW throughout most days, it would probably be advisable to move your hot water system onto the regular meter (and tariffs). You then control it with a timer (eg Clipsal TC32V24) to ensure that it only runs during the bright daylight hours (say 10-4pm). Alternatively, you might consider using a solar load diverter to make sure that you don't inadvertently import grid electricity at shoulder tariff to heat water.

Posted Monday 8 Jan 2018 @ 6:58:47 am from IP #
sunbeam
Member

Ben!, We have plenty of surplus production so I will speak with retailer and do as you suggest. I expect once they are notified of our PV input, they will put us on a set 24/7 tarriff. I remember EnergyAustralia doing that, when I installed PV at previous address years ago. I will look into the Clipsal timer. Is that something I need an electrician to install?

Posted Monday 8 Jan 2018 @ 8:55:04 am from IP #
Ben1
Member

Yes, that's a hardwired timer so will require an electrician. They make a quiet, but continuous clicking noise as the gears move, so I would recommend putting it in your meter box (easiest for the sparky, too). You will hardly need to adjust it once installed and configured.

Posted Monday 8 Jan 2018 @ 9:52:31 am from IP #
sunbeam
Member

Excellent. Thanks for the advice.

Posted Monday 8 Jan 2018 @ 10:01:19 am from IP #

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