I am considering installing a 150Lt Runonsun solar water heater http://www.runonsun.com.au/ on a roof in Sydney with all day sun. Their system runs at mains pressure with tank on roof and vacuum tubes. We have existing point of use electric heaters so the tank will not need a booster. Runonsun advise that 'overheating can lead to excessive venting'. They also sent us the following:
"Here is a Email from a customer who had overheating problems.
We have an inlet directly under the P/T valve for hot climates and one into a water tunnel for cloudy climates.
The customer origionally conected to the water tunnel and has since changed to the other.
Andrew in regard to my last contact re excessive hot water venting I have changed the inlet to directly under the temp relief valve plus 100 mm pipe protrusion into the tank & now when it vents it only lets quite a small amount go and stops as the cold water reaches it quite quickly A vast improvement. Also 50 mm ag soakage pipe /hose with a 20 mm strip cut out clips nicely onto the tube / tubes to reduce the efficiency. All in all the unit is working great."
Any experience/advice on venting would be most appreciated.
overheating (venting) of evacuated tube system
(40 posts) (18 voices)-
Posted Tuesday 17 Nov 2009 @ 1:18:13 am from IP #
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Sunshine
MemberBoyd,
I am not familiar with your system, but with the Hills evacuated tube system, the manifold on the roof has a capacity of only 1.7 litres, so on hot days, when the tank reaches 95 degrees and pump shuts down, there would only be a cup or two of water being vented.
Posted Tuesday 17 Nov 2009 @ 3:31:44 am from IP # -
Sunshine
MemberDymonite69,
If the storage tank on the ground reches 95 degrees the circulating pump stops and any excess steam that is generated from the 1.7 litres in the manifold is vented. The manifold should still have water in it..only excess pressure due to expansion of water is vented.Posted Tuesday 17 Nov 2009 @ 4:37:37 am from IP # -
Sunshine
MemberIt wouldn't be much, only excess pressure is vented. I wouldn't call a manifold a steam generator, it's a heat transfer device.
It may well generate steam, but as it is under pressure, it may still be below boiling point, eventhough manifold temperature may be around 150 degrees (certainly higher than 100 degrees)Posted Tuesday 17 Nov 2009 @ 5:45:34 am from IP # -
JohnSamin
MemberHi Boyd,
Is your solar heating loop via a heat exchanger into your tank? Closed loop? If so and you install an expansion tank then the expanding water won't vent. I'd hate to see what would happen if the overpressure vent fails... these should be a safety feature, not system pressure regulation design.
If the solar loop is closed circuit then the expansion tank will be small, maybe 5 ltrs or so. So it's also not very expensive, my 5ltr expansion tank was $35.
I've got a direct feed of 60 ev tubes to 300 ltr tank (winter heating), my expansion tank is 24 ltrs - for 12 ltr working volume. To keep it cooler I have covered both racks with 70% shadecloth and temp is still 65 degrees.
Maybe this wont be a problem when ejector jet A/C goes mainstream!
Samo
Posted Tuesday 17 Nov 2009 @ 10:34:19 am from IP # -
Sunshine
Memberdymonite69,
I don't know, have a look on Hills website or contact Hills if you really need to know. If it works, who really wants to know at what pressure does the valve pop off?
All I know is that it reliably supplies hot water in my household with no electric boost.
What more can you ask for?Posted Tuesday 17 Nov 2009 @ 2:22:34 pm from IP # -
dymonite69
MemberI would also like my solar heater not to be over-stressed and last as long as possible.
I can't imagine that it would be very good for the manifold cycling under pressure changes. If your system is optimised for winter, it will receive 2-4 times excess heating energy for the size of the tank by summertime.
Most installers consider overheating as a issue.
Apricus have directly addressed this by selling heat exchanging radiator to dump heat if required:
http://www.apricus.com/html/solar_heat_dissipator.htm
Alternatively you could derate the system by removing tubes or covering them up during summer.
Posted Tuesday 17 Nov 2009 @ 10:04:27 pm from IP # -
Lance
Key MasterOne solution to overheating solar water heater: http://www.solarblind.com.au
Just be aware that with evac tube systems, if you use the solar blind you should make sure that the blind completely covers each tube, not just halfway down or whatever, as uneven heating of the tubes may cause failures...
Posted Wednesday 18 Nov 2009 @ 12:14:04 am from IP # -
franks
MemberOwners connected to tank water supply, should take summer over solar output, (Venting excess steam) the water waisted could be a issue.
If you can tilt the angle of your solar collector your lattitude +15' would improve winter solar output and reduce summer output.Posted Wednesday 18 Nov 2009 @ 12:27:45 am from IP # -
Sunshine
Memberdymonite69,
The system is not overstressed, it is designed to shut down to stop it from over heating.
Marine grade stainless steel is a quality steel that's been around a long time in water heating applications.
I can't see any dramas with it.Franks,
A cup or two of water is not a big issue even on those on tank water, as manifold only contains 1.7 litres of water.Posted Wednesday 18 Nov 2009 @ 1:28:44 am from IP # -
Sunshine
MemberI would say the amount of water vented would be very minimal, only a cup or two, as if there is 1.7 litres in the manifold, the whole 1.7 litres of water wouldn't need to be purged, just a small amount of water due to expansion in volume when 1.7 litres of water is heated.
If anything is purged from mine it would flow into the water tank anyway, but a cup or two would probably evaporate on a hot day anyway before it gets to the tank!
The system on the Hills is programmed to keep pumping until the tank temperature reaches 95 degrees, so it would only reach this temperature only on the odd summer's day, or if the household goes on summer's vacation for a week or so.
We don't go away in summer on vacation, so we are always using hot water daily, so it may never reach shutdown. I should really check it on the weekend in this heat wave and see if it actually does shutdown.
I have been told that flat plate collectors have more of an issue with this as they have a much larger volume of water in the collectors, so they end up purging more water than this. A friend of mine has one, and reckons they were losing over 100 litres of water a day, which is definately too much.Posted Wednesday 18 Nov 2009 @ 2:37:14 am from IP # -
dymonite69
MemberSunshine,
I don't quite follow what you are saying. You say you have enough heating capacity to heat your entire tank to 60 degrees during winter. In summer your tank reaches 95 degrees and the systems shuts off. That still leaves excess solar heating energy to boil off at least another 75 litres.
So if your hot water tank has reached capacity only one of the following to occur.
a) The system boils off the remaining water in the collector and it is left dry; the collector surfaces continue to rise to over 120 degrees in the sun
b) New cold water continually is added and the system continue to boil off that water i.e. 75 litres is lost over the course of the day
c) The water left in the manifold is overpressurized to prevent the water from boilingThis places an abnormal stress on the system due to a daily cyclic cooling/heating or overpressure/normal pressure. This is not the same situation as a tank placed under a fairly constant heat load.
Posted Wednesday 18 Nov 2009 @ 4:52:41 am from IP # -
Sunshine
Memberdymonite69,
Not sure what you mean by "That still leaves excess solar heating energy to boil off at least another 75 litres"
I don't see 75 litres per day in summer disappearing down the drain.
a) I have never opened the manifold on the roof, when the system shuts down to see if the 1.7 litres is still there or not. Doesn't really interest me, nor do I care. A stainless steel manifold is not going to be ruined by sitting in the sun with no water in it!! It's not made of plastic.
b) The controller stops pumping at 95 degrees, so tank temperature doesn't go above 95 degrees, so no boiling in tank occurs- that's straight forward.
c) The manifold has a pressure relief valve so no excess pressure is placed on manifold, unlike the Apricus system.
System works wonderfully well for me, should have bought one years ago.
What system do you use?Posted Wednesday 18 Nov 2009 @ 5:17:14 am from IP # -
Ammo
MemberI just purchased a 40 tube Apricus system with 315 ss tank. I am in Melb.
The tubes are mounted at 52 degress on stands to minimize summer generation and venting. Even with this, each day i loose on average 9 litres. I have put a bucket under it to measure.I have contacted my installer who spoke to Apricus who confirmed the problem.
My installer recommended an expansion tank as well, but said i would still loose water each day, i would loose cold water instead of hot.Apparently the exapansion tanks are required for installs in WA but not Vic.
My family of 3 uses 90% of our hw in the morning. The ss tanks only have a temp sensor at the bottom which stops the pump when it hits ~ 70c.
Appreciate any advice.Posted Wednesday 18 Nov 2009 @ 8:54:06 am from IP # -
JohnSamin
MemberAmmo its Sammo...
With those figures it sounds like you have a direct heating system on your 315 ltr tank, expansion of 315 ltrs from 0 to 100deg is around 12ltrs (4.8% ball park). You're heating from say 40 to 80? So 9 sounds feasible, hard to say without knowing exact temperatures and system volumes.
Water gets hot, use some, cold water in, water volume decreases because tank is cooler, water gets hot, it expands, vent some.... use it, cool it, heat it, vent it.. cycle.
Expansion tank wont stop thermal run away ie. steam venting due to boiling, but properly tank, should stop expansion venting...
Hydronic systems rely on this and are designed to operate from around freezing to around 80 deg (maintaining 3bar / 300kpa). So for 12ltr expansion I'd be shooting for 24ltr expansion tank with internal air bladder - abt $120.
Read up on expansion, calculator available at this site too...
http://www.watts.com/pro/divisions/watersafety_flowcontrol/learnabout/learnabout_thermexpansion.asp
samo
Posted Wednesday 18 Nov 2009 @ 9:45:51 am from IP # -
Ammo,
What is actually the problem?
The water you lose is not lost by excessive heating of the collectors. It is just the normal water loss in hot water tanks under mains pressure that heat up and cool down. As there must be a non return valve installed in the inlet to the water tank to prevent back-flow into the cold water pipes, the additional volume caused by the heating of the water has to go somewhere. And it is just being released through the safety valve.With a solar system this is just the same as with any electric hot water tank. There is water release thing happening, just at a different time as most electric hot water tanks will be heated by the cheaper offpeak1 tariff during night time.
Is it really worth the effort to install an additional bladder expansion tank to save a part of the 9 litres of water a day? How about just using this water to water your garden or a few flower pots? Or just run it into your rainwater tank. You can run the water uphill with a hose as it is released under pressure. Just make sure for safety reasons that the hose can not block up.
Posted Wednesday 18 Nov 2009 @ 11:31:20 am from IP # -
Ammo,
What is actually the problem?
The water you loose is not lost by excessive heating of the collectors. It is just the normal water loss in hot water tanks under mains pressure that heat up and cool down. As there must be a non return valve installed in the inlet to the water tank to prevent back-flow into the cold water pipes, the additional volume caused by the heating of the water has to go somewhere. And it is just being released through the safety valve.With a solar system this is just the same as with any electric hot water tank. There is water release thing happening, just at a different time as most electric hot water tanks will be heated by the cheaper offpeak1 tariff during night time.
Is it really worth the effort to install an additional bladder expansion tank to save a part of the 9 litres of water a day? How about just using this water to water your garden or a few flower pots? Or just run it into your rainwater tank. You can run the water uphill with a hose as it is released under pressure. Just make sure for safety reasons that the hose can not block up.
Posted Wednesday 18 Nov 2009 @ 11:32:26 am from IP # -
Ammo
MemberHi Sun2stream and Sammo,
Thanks for the links on expansion vessels and explanation.
My post was just to share as a consumer I was not made aware of the daily water loss of the system when we purchased it. I thought the water loss from our old gas HWS was due to a leaky old pressure relief value.
Posted Thursday 19 Nov 2009 @ 1:24:28 am from IP # -
Skysolarman
MemberTo all re overheating of direct systems.
A heat pipe system will expel more than the header volume per day , SUNSHINEs comments relate to his installation where his collector faces west , i think he would fine if his was due north his results would astound him
Expansion vessels are only really usable for indirect SWH systems as they are a closed circuit and not susceptible to pressure change. Direct systems are inherently unstable meaning always experience system pressure fluctuations..ie..they waste a lot of water in the summer months.Your EV's bladder needs to be able withstand temperatures of 110 deg + if not when the tank is in overheat so will the bladder , it will then rupture.
If your cylinder is stainless steel get the installer to set up the solar controller to 85 degrees max tank storage temp and not 70.it will keep your system running longer and will help to avoid stagnation periods in the collector
If your tank whether it be stainless or enamel gets to its max storage temp but there is high levels or irradiation around your collector will build up pressure as it gets hotter , that pressure is invariably exerted down into the tank from your solar piping circuit taking the heat with it causing your tank eventually to overheat , when the TPR valve on the tank detects 95 degrees it will open up and exhaust that excess temperature , unfortunately it will only cut off once cold water from the tank bottom has worked its way up to the PTR probe to cool it off to shut it down..meanwhile you have lost 90% of your stored hot water...suggestion...when you purchase a system set it up for your peak weather periods..that is to say buy enough tubes or whatever for your usage versus tanks size for these peak times and take what you get in the winter time.
The worse thing you can do is put your collector into overheat or stagnation conditions often , the result is it stresses all the weld joints etc and limits the lifetime of the collector..bare this in mind when you are thinking about warranty issues.
Another option for overheat problems apart from covering a % of the collector is to ask your installer to install a Cold water Expansion valve(CWEV) at the cold inlet to your tank , this valve is preset at a lower expansion rate(750kpa i think from what i can remember) the CWEV is generally set 200-300kpa expansion point less than the PTR , which means when your tank gets hot and the pressure is up to 750kpa it will expel cold water from the CWEV , when the valve opens and water is expelled it also replaces that expelled water in the bottom of the tank with colder water therefore the sensor will tell the sensor at the collector that it needs more heat reducing stagnation time of the collector and the chance of the PTR to go off and totally dump the hot water contents of your tank.
Air vent valves(AVV) are a total waste of time , each time the header goes in stagnation it turns the water to steam just like a jug at home , when the temperature cools a certain amount of air is created in the header through the recondensing process of steam to water, the next time when steam occurs the AVV opens up to exhaust the air but the steam through being higher in pressure is admitted as well causing a problem . The additional problem is when you turn on the household hot tap you lower the systems working pressure which in facts lowers the boiling point as well encouraging the collectors water content to constantly create steam , this is not good , it means your controller will go into lock out and not transfer any of this heat...if your tank contents is dumped late morning for example and the collector is in lockout your reheat can only be done by auxilliary , another problem it may cause is because the steam is so hot or super heated it will extract any impurities from the water supplies just like a still and line the inside of the copper piping within your collector.
SUGGESTED WARNING: Don't oversize your SWH systems , i strongly recommend that you take caution in sizing your system for summer conditions only unless you have more than adequate pre cautions in place to protect from overheat..
So what options are there..
1)Cover the collector
2)Use Indirect SWH only
3)Use a heat sink system..ie bypass to external radiator or buffer tank , spa pools
4)Larger size tank compared to usage
5)Size SWH to summer months
6)Install CWEV
7)Install expansion vessel no less than 19 liter vessel and bladder temp resistent to 110+(always massively oversize)These are my comments only , i expect you won't agree with it all but if there is some stuff that is usefull then great good luck
8)Maybe install oversized systems on a west face , would be safe from dumping but efficency would drop...worth a thought thoughPosted Tuesday 19 Jan 2010 @ 10:37:57 pm from IP # -
Gandini
MemberSkysolarman,
Re your comment about pressure "exerted down into the tank from your solar piping circuit taking the heat with it" - I don't think that pressure can “take heat with it".
If the pump is not operating due to stagnation conditions, the temperature of water in the tank could increase only by conduction from hotter water in the pipes from the manifold. Several posters have indicated the pipes become very hot which means the water in the pipes is very hot.
Posted Wednesday 10 Feb 2010 @ 11:13:18 am from IP # -
Skysolarman
MemberGandini
Close your lips together and try and blow...pressure is created in your mouth..as you try and increase pressure in your mouth you get to a point where your lips can't hold it the pressure any longer and out the air comes...same principle as per my explanation...As the temperature rises in the collector so does the pressure..applies moreso when the collector or manifold is at stagnation temp(steam)...temperature is conducted through water and through the pipework as you suggested...pressure on the steam side(collector side) is at a higher pressure than that of the water in the pipework connected to the tank..as the steam boils the remaining water in the pipework the pressure of steam gets closer to the tank
Posted Wednesday 24 Feb 2010 @ 11:56:27 pm from IP # -
Traveller
MemberStill not sure about the stagnation problem. I will be on holidays for 1 month in the middle of this summer. I am concerned this could lead to problems with my Apricus system as it won't be used. As the pressure builds the overflow valve on the tank will dump pressure, but there won't be cold water to the tubes. Or is the design of the Apricus system set to handle the overheating? As an aside, I have had this system for 8 months and it's working great.
Posted Sunday 12 Sep 2010 @ 8:04:07 am from IP # -
gully
MemberWhy can't you just throw on an expansion tank in just before the cold water inlet to your SHW tank? There is NO reason for the expansion tank to be connected to the hot water side at all, and the expansion tank is the only REAL solution for the real issue which is thermal expansion.
CWEV just wastes cold water rather than hot water. Suppose you could direct this to a rainwater tank though.
This issue is not just for solar systems guys... it will happen on any hot water system.
Posted Monday 13 Sep 2010 @ 1:38:02 am from IP # -
thatmosis
MemberWe have a RUNONSUN solar hot water heater and we had a slight problem with the overheating and noise through the roof but I enlarged the vent and also fitted a hinged screen with shade cloth on it that I can lower it over the tubes on excessively hot days. This only occurs once in a while and its easy to rig up so that it is able to be operated from the ground. Ours is a gravity fed system.
Posted Monday 13 Sep 2010 @ 5:04:23 am from IP # -
peter69_56
Memberthatmosis said:
We have a RUNONSUN solar hot water heater and we had a slight problem with the overheating and noise through the roofDid you tilt the system at all, what is the tilt and your latitude? How many tubes, size of tank and number of people????
Posted Monday 17 Oct 2011 @ 3:25:26 pm from IP # -
reelmick
MemberI dont know why people waffle on when they dont know about these systems ,i fitted one before summer and its working great ,no moving parts, free hot water ,no overheatventing yet (lake macquarie) running through existing hws and hotwater part of bill is nearly zero .people will only confuse Boyd when they dont know the systems,they are the best type i think and very inexpensive.got mine from FxLarkins ,have a look at there website page 5 Mick
Posted Saturday 12 Nov 2011 @ 10:13:23 am from IP #
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