Solar Hot Water - How to use Efficiently?

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tdgeek

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#198262 2-Jul-2016 12:41

Partner to my heat pump thread

The house we bought has solar hot water. Cool. Hot seasons , turn off the power, see how that goes, great.

Now, reading the manual, and the excessive option LCD meter thingy, it seems more complex than I thought.

Apricus. 300l tank. Not insulated. There is a lower and upper element.

I was told that if the water is too hot, to bleed it off, and if going away, may need a bleed off to run some hot water into the kitchen sink. A concern

Not sure if I can just give it a whirl and turn off one or both elements to see how well the water remains hot and usable. In case that causes issues.

I gather the heated panel water (Collector) is pumped to the cylinder?

Now, this time of year no good to see what times of the year it could last on solar only, but ideally, I'd like to safely experiment that in the summer.

Is it typical that the cylinder is readable by the power company to allow off peak boost?

Again a solar HW noob, I recall a couple of you are quite knowledgeable on it. I gather it may not be the ideal solar option these days, but its here.

House is 2010. Showers are usually morning

nolanz

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#1584620 2-Jul-2016 13:05

When we built our last house in 2000, we had a Solar Edwards system installed. This is a system which uses thermosyphon to circulate the water from the solar panels through a heat exchanger in the hot water cylinder. The system would continue to heat the water when the solar energy was available. I originally (without knowing enough) felt that to eliminate a pump would save electricity and avoid maintenance of the pump itself.

One of the issues I never considered was that if we were not using the hot water, the temperature would continue to build up until the temperature got to 95 degrees in the cylinder. At 95 degrees a valve automatically opened and dumped a huge amount of the hot water. We only discovered this when one summers day, after not being in the house using water, we arrived back and found that the water was not hot enough for a shower.

The solution was to have to climb onto the roof and cover all or part of the panels if we were going to be away for an extended period.

Another problem was that the system used a header tank which had to be manually maintained, and topped up with a food grade glycol mix as it slowly evaporated. The solar was backed up by a woodburner with a coil which supplemented the heating in winter when there was less solar power.

I understand that the more modern systems using a pump will only transfer the heat from the solar panels when it is required, thus saving the need to worry about overheating the water in the cylinder.

tdgeek

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#1584621 2-Jul-2016 13:12

nolanz:

When we built our last house in 2000, we had a Solar Edwards system installed. This is a system which uses thermosyphon to circulate the water from the solar panels through a heat exchanger in the hot water cylinder. The system would continue to heat the water when the solar energy was available. I originally (without knowing enough) felt that to eliminate a pump would save electricity and avoid maintenance of the pump itself.

One of the issues I never considered was that if we were not using the hot water, the temperature would continue to build up until the temperature got to 95 degrees in the cylinder. At 95 degrees a valve automatically opened and dumped a huge amount of the hot water. We only discovered this when one summers day, after not being in the house using water, we arrived back and found that the water was not hot enough for a shower.

The solution was to have to climb onto the roof and cover all or part of the panels if we were going to be away for an extended period.

Another problem was that the system used a header tank which had to be manually maintained, and topped up with a food grade glycol mix as it slowly evaporated. The solar was backed up by a woodburner with a coil which supplemented the heating in winter when there was less solar power.

I understand that the more modern systems using a pump will only transfer the heat from the solar panels when it is required, thus saving the need to worry about overheating the water in the cylinder.

Interesting stuff. What bothered me was I expected an overflow if the water got too hot. Dont think there is one. But I am going to ask for a visit, to explain the ins and outs of it.

Not sure why the cylinder isn't insulated though. Its in an insulated cupboard, maybe thats why

1eStar

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#1584642 2-Jul-2016 14:41

You may find you can operate with just the top element switched on. That means you always will have hot water. But you might run out of hot if there been no sun for a while. If you simply leave both elements on during summer there is a good chance it will boil.

Aredwood

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#1584838 2-Jul-2016 20:26

I have solar hot water on my own house which I installed myself. 50 solar tubes(30 tube+20 tube panels) and 300L Stainless steel Hot water cylinder. And supplementry heating from my waste oil fired radiator central heating system. (Im a plumber / gasfitter)

In summer definitely turn off both elements. And if you run out of hot water, turn on just the top element to reheat.

And for winter - leave the top element on all the time. And put a timer on the bottom element, set to only run the bottom element at night. And switch off before the morning showers. Although experiment with leaving the bottom element off during winter - if you don't use much hot water, you will still be fine. Yet get good savings as the solar will be able to take the chill off the water, even in the middle of winter. Which reduces the amount of heat needed from the top element.

Even during winter, on a sunny day like today and yesterday, my system still produces a reasonable amount of hot water. And due to how well the tubes work, on overcast days they can still heat the cylinder to approx 25-30 deg. Your savings depend alot on how you manage the system. Do you want it to be automatic and never run out of hot water? Or do you want to manually manage it to get the highest savings. But sometimes run out of hot water? Also depends on how you are billed for power - flat rate? Night rate? Wholesale via Flick Electric? Peak / Offpeak ect.

[Edited to add]

There will also be at least 1 TPR valve on the system, assuming mains pressure. (Temperature and pressure relief valve) This is the valve that dumps all the hot water if the system overheats.

Also do you have any other heat sources connected to your cylinder? As I configured mine to send heat into the central heating system if my cylinder ever gets too hot. And from there into the spa pool. Don't want the radiators going in the middle of summer. No more cylinder overheating but can only heat the spa from 15deg to about 22 deg. But is still free spa heating.

tdgeek

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#1584958 3-Jul-2016 08:28

Aredwood:

I have solar hot water on my own house which I installed myself. 50 solar tubes(30 tube+20 tube panels) and 300L Stainless steel Hot water cylinder. And supplementry heating from my waste oil fired radiator central heating system. (Im a plumber / gasfitter)

In summer definitely turn off both elements. And if you run out of hot water, turn on just the top element to reheat.

And for winter - leave the top element on all the time. And put a timer on the bottom element, set to only run the bottom element at night. And switch off before the morning showers. Although experiment with leaving the bottom element off during winter - if you don't use much hot water, you will still be fine. Yet get good savings as the solar will be able to take the chill off the water, even in the middle of winter. Which reduces the amount of heat needed from the top element.

Even during winter, on a sunny day like today and yesterday, my system still produces a reasonable amount of hot water. And due to how well the tubes work, on overcast days they can still heat the cylinder to approx 25-30 deg. Your savings depend alot on how you manage the system. Do you want it to be automatic and never run out of hot water? Or do you want to manually manage it to get the highest savings. But sometimes run out of hot water? Also depends on how you are billed for power - flat rate? Night rate? Wholesale via Flick Electric? Peak / Offpeak ect.

[Edited to add]

There will also be at least 1 TPR valve on the system, assuming mains pressure. (Temperature and pressure relief valve) This is the valve that dumps all the hot water if the system overheats.

Also do you have any other heat sources connected to your cylinder? As I configured mine to send heat into the central heating system if my cylinder ever gets too hot. And from there into the spa pool. Don't want the radiators going in the middle of summer. No more cylinder overheating but can only heat the spa from 15deg to about 22 deg. But is still free spa heating.

Thanks Andrew for your reply.

There are two power cords, so I guess thats top and bottom.

How does the water circulate? Power? Is that connected at the collector end? I was worried about inadvertingly turning that off

Im keen to manage it manually for cost savings.

Ill need to check the power bill, and call them to see what options there are, if any, to have the cylinder on a night rate.

Where does the water get dumped? Reason I ask is the the previous owner said to bleed it off with a gadget connected under the kitchen sick on a timer if we go away on holiday,

which seemed to indicate there is no overflow. Ill re read the brothers and manual.

I don't know if it is mains pressure. Its 5yo, but the shower is fine but not as good as our last house which had a 180l mains pressure cylinder. That could be the shower head though. The cylinder is upstairs, next to our bedroom, and adjacent to our ensuite, which also backs onto the public bathroom/shower

If I can be sure that the two power feeds are "just" the two elements, I can have a play. the meter has Collector temp, upper temp, lower temp, and I guess capability for fancy settings. Ill take a pic later and post.

Edit: No other heat sources connected

Cheers again

Aredwood

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#1585253 3-Jul-2016 19:58

The pump is most likely powered directly from the controller. Try switching off the power to the elements during a sunny day then watch the controller. The panel temp should increase to approx 10deg above lower cylinder temp. Pump should switch on and run until panel temp drops to about 5deg above lower cylinder temp. This will confirm that pump is still working despite power to elements being off.

The TPR valve will be on the cylinder. It sounds like the device underneath the sink is simply a solenoid valve connected to a timer. The solar controller might have an output that switches on if the cylinder gets too hot. This will be far better to use for controlling that solenoid valve. As it would only dump the min amount of hot water. While the TPR valve on the cylinder will dump almost the whole cylinder. (Has happened to me).

tdgeek

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#1632017 17-Sep-2016 07:29

@Aredwood

Help!

I turned the bottom element off the other day. The bottom cooled, the top was over 60, cool. But the top has dropped to below 60, so I assumed maybe it only heats at night. But it continued to drop and is 33 this morning. Turned the bottom element on, and its slowly heating the bottom, so hopefully crisis averted. The solar control panel gives no error codes, and shows the after heater is on, which is bottom element.

It seems the top element has failed? Checked the circuit board and all are on. Or could it be that the solar control panel has got confused? Id have thought not. Bit concerning as I checked Apricus and they said it should be fine to turn the bottom off, and the manual once reading in detail also shows this. I should call a plumber, bit i'll wait for your reply. In ChCh

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Aredwood

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#1635658 19-Sep-2016 01:43

@tdgeek It is probably just the overheat cutout tripped. They activate if the cylinder temp exceeds 80deg. Built into the thermostat tiny button marked reset.

Usual precautions for working around mains power apply to access the thermostat. You can get solar versions that self reset. The solar might have simply gotten the cylinder hot enough to trip it. (Used to happen alot on my system until I got my heat dump working properly) But could also be a failed thermostat - failed permanently on, element keeps heating until cutout trips. Try resetting it and allowing it to reheat. If it trips within a day without enough solar input to cause it - replace thermostat - call a sparkie.

Also what brand cylinder do you have? As cylinders that use curved bolt in elements will sometimes overheat if the element is bolted in upside down.

Hammerer

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#1635664 19-Sep-2016 07:24

nolanz:

I originally (without knowing enough) felt that to eliminate a pump would save electricity and avoid maintenance of the pump itself.

There is no power saving by not having a pump and they generally run for more than 20 years without any problem. The additional efficiency gained through the better heat transfer in a forced cycle over a thermosiphon is greater than the cost of running the pump.