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Author Topic: What makes a "Solar" cylinder special?  (Read 2139 times)
Axel
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« on: April 01, 2016, 06:18:18 PM »

What makes a "Solar" cylinder special?

Is it because of the secondary coil, the size of the coil, greater temperature & pressure ratings for the coil?   

or is it some renewable energy quota thing?

i spoke to technical at Santon today and asked him what was the reason why someone should not use a normal indirect cylinder primary coil as part of a solar primary circuit.

he said that the coil is stainless steel and the 3/4" thread is cut into the end of the steel coil pipe so there is no soldering issues. But he then got annoyed when I explained that I wanted to use an old one with solar and said i was breaking the law building regs etc etc.  i tried to explain I was going to possibly do this in another country but he hung up on me!

Please can someone explain the technical reason why it would be dangerous to put the (potentially very hot steam high pressure etc) solar circuit into the coil. (Glycol issues?)

Thanks
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biff
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« Reply #1 on: April 01, 2016, 06:49:15 PM »

Hello Axle,
           And welcome to the forum. I just googled your question and got reams of info, all about the different types of cylinders,,and copper ones and S/S ones.
  And of course the difference between a solar cylinder and an ordinary one..I will finish reading and get back to you.
                                                                                                       Biff
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Axel
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« Reply #2 on: April 01, 2016, 09:21:48 PM »

Thanks.

It's a stainless steel unvented cylinder.  The coil is welded onto the cylinder, the ends of the coil are threaded - there's no solder in it!

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TheFairway
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« Reply #3 on: April 01, 2016, 10:41:25 PM »

Unvented cylinders are subject to building control and some authorities will not allow them to be fitted in some scenarios. I had great difficulty finding someone who could install an unvented cylinder (you need the relevant unvented cylinder qualifications and certificates to install and/or work on them) - almost unheard of in my area.

At a guess, its the unvented bit causing Santon helpdesk the problem rather than solar/direct/indirect.
« Last Edit: April 01, 2016, 10:43:45 PM by TheFairway » Logged

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Axel
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« Reply #4 on: April 01, 2016, 11:51:02 PM »

I am G3 qualified, although that doesn't help if I'm misusing a cylinder, but I have some idea of what I'm doing / not doing.  I do have a heat exchanger & spare pump can use to keep the solar circuit separate from the coil in  the cylinder although the heat will still get through it, but not the pressure or the glycol.
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dhaslam
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« Reply #5 on: April 02, 2016, 12:59:54 AM »

Solar coils need to have a big surface area because  often heat needs to be transferred at  at just a few degrees  difference.  The same applies to coils for heat pumps.    Boiler coils are often very short  but much of the time there is difference of thirty degrees or more.
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RIT
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« Reply #6 on: April 02, 2016, 01:43:02 AM »

This is a nice overview

    http://www.viridiansolar.co.uk/Solar_Energy_Guide_3_8_1.htm

As you will see one difference is that the solar input is at the bottom of the tank. As dhaslam has already noted there maybe only a few degrees difference between the input and the tank so convection is used to cause the heat to rise and collect at the top of the tank, while also allowing a standard boiler to be used.
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« Reply #7 on: April 02, 2016, 08:07:42 AM »

That's a really useful set of articles for the layman. Wish I knew about it when I started down this road, before I discovered the Navitron Forum...

Mind you, I wasn't too sure about the reference to "compression fittings with a high temperature solder"?

SB.
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Axel
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« Reply #8 on: April 02, 2016, 08:41:03 PM »

Many thanks for your replies.

I think the coil is low enough in this one.

I was more concerned about potential safety issues & equipment failure.



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dhaslam
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« Reply #9 on: April 03, 2016, 12:48:00 PM »

Solar panels can reach very high temperatures if the circulating pump isn't working for any reason.  Typically during a daytime power cut  the temperature builds up in the panels to over 100C.  The pipework near the panels  should  be  copper and  at least the size of  the  panel connections in order to allow the steam to expand.  The controller typically stops circulation and may not  restore circulation when power is restored but the  normal system with compression fittings and copper pipe can cope.   What this means that the  coil in the cylinder  may not have to deal with  temperatures over 100C at all.   Solar thermal works best with low water content  in the pipes so if there is more than a few metres  distance from panel  to  cylinder the pipe should be  10mm, that is unless there are multiple large panels.   It isn't always  emphasized that there should be at least a metre of larger pipe  connecting to the panels.     
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dimengineer
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« Reply #10 on: April 04, 2016, 10:07:16 AM »

Solar coils need to have a big surface area because  often heat needs to be transferred at  at just a few degrees  difference.  The same applies to coils for heat pumps.    Boiler coils are often very short  but much of the time there is difference of thirty degrees or more.

Agreed.

Although a boiler might be transferring 18kW at a 30 - 40C delta T, a solar coil will maybe be transferring 5kW at 6 - 12C delta T. So all round, it may not make much difference.

AFAIK, a solar cylinder has 2 coils, with the solar coil below the boiler coil, and is bigger. "Normally" a std cylinder might be 150 litres, but a solar cylinder might be 250 litres, with the bottom 100 litres below the boiler coil with the solar coil in it.
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DonL
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« Reply #11 on: April 04, 2016, 10:09:23 PM »

Surely the simple answer is "nothing". It is perfectly acceptable and proven to retrofit a coil for use with solar panels into a domestic hot water cylinder. The surface area or position may be less than ideal and the amount of heat you can store limited by this and the size of the cylinder but it is perfectly practical.

To get MCS certification is (or was?) a different matter but that won't worry you.

Don
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