[Sudanpan]

We have 3 solar panels on our roof for hot water heating.


The system has been working for nearly 2 years - we are very pleased with the results and our oil costs have dropped considerably. 


However - last year after some really hot weather - we noticed that the system pressure had dropped. This was about 9 months after the installation, we checked everything thoroughly for leaks and couldn't find any. We topped up the system it only took a small amount of fluid before the pressure was back up. Everything stayed fine until just this last week or so (1 year later) after another period of the temp in the panels being really high and we have lost pressure again 


The pressure in our expansion tank is about 2 bar.


Should we be having to top up the fluid in the system every year 


Any suggestions? We have tried the technical 'helpline' at Worcester Bosch - but they insist that you have to take one of their courses before they will give you any support 

[mab]

This is a guess (only had a home made solar water heater which never got that hot, and my knowledge of real systems is only theoretical).

If the panel gets very hot it will boil the water/fluid, causing the pressure to go up; as it's a sealed system there is a relief valve that opens if the pressure gets too high and vents the steam/fluid, then reseals once the pressure is below to limit. my guess is that this is what has happened.

The question then, is why the panel temp is going so high - if the system is running and pumping fluid through the panel it shouldn't get that hot.

Do you know what your system controller does when the water in the tank is already hot? sometimes they have a 'heat dump' - a radiator in the loft for e.g. - and divert the excess heat into that.

marcus

[Sudanpan]

With the double coil pressurised water cylinder that is plumbed in with the system there seem to be a million and one safety features:
1. when the water in the tank gets to a particular temp (60 degrees) then the solar panel loop is blocked from running through it so that the water doesn't get super heated.
2. The solar panel controller switches off the pump when the temp in the panels go over a certain temp thereby stopping the fluid circulation


I've seen the temp in the panels get to 159 degrees




To our thinking - and o level physics - the situation then is created whereby temp goes up, volume stays the same therefore pressure increases - just as you say Marcus. However we don't seem to have a heat 'dump facility to stop the over-pressurisation situation each time    The safety shut down protocol seems to exacerbate the increasing temp.


The stupid thing is we are in Cornwall - not exactly known for its long, unrelenting hot summers...... so how do people's systems cope in places where the sun stays out for oooh lets say more than 6 hours!! 


Ver ver irritating 

[Small Farmer]

If such a system were installed in a commercial building someone would be sent on a course to understand it.  With domestic installers you're lucky if they leave you the instructions.


MAB is right.  There will be a pressure relief valve in the system: we've had the same issue with our Vaillant system (never, ever buying Vaillant again).  A bigger storage tank (swimming pool?) or some other heat dump is the only answer I could find. 


Our son who is notorious for liking long hot baths has migrated to showers for the summer.  Mind you last year we found that he'd also left the immersion heater on for three weeks in the summer when he wasn't in the house.  Not happy.

[mab]

Hm well 60°C is the max safe temp for domestic hot water; (although, if I were installing, Id let the tank go up to ~90-95° and fit a thermostatic mixer valve to the outlet of the tank to limit the temp at the taps to 60°).

But without wishing to cast aspersions on the installer of your system, what did they think would happen to the heat if you stop circulation to the panel?

I can't help thinking it's bad for the panels letting them get that hot - aside from making the system emergency vent (which it should never do if it's designed & working properly), if you use some of your water and the pump turns on it's going to send cool water into a VERY hot manifold causing sudden cooling and thermal stress.

I'd look into getting a diverter valve and 'dump' radiator fitted.

m

[Small Farmer]

I looked into this and was told it was controversial in the industry but I can't see why it should be.  Some of the controllers including ours have provision for a second cylinder to be heated after the first one has reached its programmed temperature.  It should be possible to use that feature.

[Sudanpan]

We put the system in ourselves - when I say 'we' of course I mean the Royal 'we'    OH is a dab hand at all the plumbing and he is an electrician by trade so he does know what he is doing with controllers 


The installation instructions were followed assiduously - diagram and everything and there is no mention anywhere about a heat dump facility. The controller instructions go on and on about how everything is safeguarded to stop the circulation once the temp gets above a certain level.....


There is a safety release system (this was an additional option) that we decided to have connected to the expansion tank - this is located in the cylinder cupboard, and there is an automatic air vent up at the highest point in the system. The loss of the system liquid seems to be from the air vent and the safety release system.


It does seem a bit cuckoo to stop the fluid circulation when the temp goes up. We have got a thermostatic mixer valve so will try jacking up the limit on the cylinder - it has its own pressure relief safety system so should be able to deal with all eventualities 

[chrismahon]

Sounds like your expansion vessel isn't pressurised. You may be reading 2.0 bar but is that the pressure of the water in the system or the air in the diaphram? To read the pressure in the diaphram the system pressure must be zero. What's happening is the expanding water is going out of the safety over-pressure valve. You top the system up again- the water gets hot and expands and blows out again. Seen this dozens of times with sealed central heating systems. Although they are supposed to on the annual service, no-one checks the vessel pressure.
 
I don't know what the normal operating pressure of your system is, but it is usually less than half of the safety blow off valve setting. Check the vent pipe outside. PM me if you need help -I have just retired as a Plumber. Only available 2nd to 5th September as I'm move coops to France tomorrow and moving us and the chickens on the 6th. Signing off for at least a month then.

[mab]

Umm.. OK so it's all according to the plans.

Chrismahon raises a very good point about the pressure in your expansion vessel; I hadn't registered that detail (my home made system was open-vented).

I suppose the system could (when set up correctly) allow the water to vapourise in the panel and the expansion vessel should accommodate the extra volume. Can't say I like that way of doing it for the aforementioned reasons, but...

You might want to get Chris's advice as he's the plumber, but (if I understand these things correctly), you set up as follows (no sun conditions):

release the pressure on the water side of the expansion vessel (so gauge reads zero) then check the air pressure on the expansion vessel (using the car tyre type valve on it) and set it, as Chris said, to < half the relief valve pressure (the ones I've seen default to 3 bar).

Then refill the water side to a bit less than the air side pressure.

Well, that's what I'd try  .

m

[Sudanpan]

Thanks for the replies -


At the moment the pressure gauge on the water side reads zero when the system is cold. When the sun is out and the system is up and running the pressure on that gauge goes up to 2.5 or so. Before the minimum pressure that the system used to show when cold was about 0.5 or so. This remained like this from last year, all through the winter and spring and then up to very recently. Presumably if the loss was due to a leak in the actual system then it would lose it gradually and steadily.






We took the expansion vessel pressure in the morning - when the system pressure (according to the gauge) was zero.


Given how the expansion vessel works if it wasn't pressurised then the vessel would be full of fluid yes? This is not the case. The vessel is a 25l one - the system has 3 solar panels.


A query - if you have to check the expansion vessel when not connected to the system how is that done without draining the system? There are no cut-off valves between the vessel and the system, and presumably if an engineer comes to check the system on an annual inspection this would generally happen during the day when the system would probably be running and therefore under pressure?


Chrismahon - thanks for the offer - we might very well take you up in it    Good luck with the moves....

[mab]

A query - if you have to check the expansion vessel when not connected to the system how is that done without draining the system?

You don't need to drain the system, or disconnect - but the fluid pressure should be zero - That way when you have pressure on the air side you know the vessel is full of air.
If the fluid pressure isn't zero you need to open the fluid side, which will release some fluid to release the pressure (sounds like you don't need to though).

We took the expansion vessel pressure in the morning - when the system pressure (according to the gauge) was zero.

You mean you took the expansion vessel [air] pressure? what was it?

Given how the expansion vessel works if it wasn't pressurised then the vessel would be full of fluid yes? This is not the case.

If there was no air pressure then yes, it will likely be full of fluid, which will cause the system pressure to shoot up as soon as it warms up. When you say 'this is not the case' do you mean it IS pressureised, or that it ISN'T full of fluid?

At the moment the pressure gauge on the water side reads zero when the system is cold. When the sun is out and the system is up and running the pressure on that gauge goes up to 2.5 or so

Assuming that's without the panel reaching silly temperatures, then that to me sounds like the pressure vessel is full of fluid.
What you want is (when cool) for the expansion vessel to be full of air.

With the system cold, lets say you pressurise it with air to 0.5bar (and the fluid pressure is less than 0.5bar). When the fluid warms up (and expands) the fluid pressure will rise; when the fluid pressure = 0.5 bar, it will start flowing into the expansion vessel, compressing the air and the pressure will then only rise a tiny amount (as the air is compressible, and only a small %age of the volume will be used by the expanding fluid).


If the fluid boils in the panel then the expansion vessel will have to accommodate the displaced fluid which will cause the pressure to rise more, but hopefully not to vent pressure.

[Sudanpan]

Mab - when we took the pressure of the expansion vessel (as in air pressure) when the fluid system was registering zero the vessel showed an air pressure of 2 bar.
Today the sun was out -    - and the fluid side of the system was showing 3 bar when the solar panel pressure was showing 75 degrees. BUT the expansion vessel was still empty of liquid and the corrugated pipe leading to the vessel was only warm for the first 6 inches from the system side.
From recollection I have NEVER known the expansion vessel to be even remotely warm - and if the system is reaching the safety release pressure you would expect the vessel to be full of very hot fluid thus exacerbating the pressure increase because all the extra available volume has been absorbed so the pressure just goes up. Perhaps we have a faulty expansion vessel?

[mab]

I'm not sure the vessel would get hot - the fluid expands relatively slowly so it would cool on the way in and once in it's not part of the circuit.

Perhaps we have a faulty expansion vessel?

If there is a difference of pressure between the air side and the fluid side then it's working fine.


2 bar? Is that the recommended pressure in the setup instructions? seems a bit high to me.


OK, If your relief valve opens at 3 bar (I'm assuming), then:-

With the empty (of fluid) pressure at 2 bar, then the pressure vessel will be 1/3 full of fluid at 3 bar (compressing the air into 2/3 of the volume raises the air pressure to 3 bar).  which gives you 25/3 litres of expansion room (= 8.3 litres) before the relief valve opens.

if you reduce the air pressure to 0.5bar, then the vessel will be 5/6ths full of fluid at 3 bar, which will give you ~20 litres of expansion room.
 
My guess is that you need to lower the air pressure (or raise the relief valve vent pressure - but I'd say 3 bar is high enough).

[Sudanpan]

We lowered the pressure in the expansion vessel and then topped up the fluid system side pressure


It has been rock solid since 


Thanks for the input   

[chrismahon]

Glad you got to the bottom of it Sudanpan. Glad I've finished plumbing as well!


So there was nowhere for the expanding fluid to go except out of the safety valve, which sounds as though it could be venting early, not at the 3.5 bar it should. The 2.0 bar in that setup sounds high though, they are normally 0.8 bar (12psi).