evacuated solar hot water tubes (thermal grease, when to change?)

rgormley · Dec 15, 2023

i installed a 15 tube evacuated solar hot water tube heater
been working great 13 years now

my question is where the copper tips go into the manifold with the thermal transfer grease, how often does this thermal grease need to be cleaned off and replaced?.

i have not pulled a tube out and looked so i don`t know what condition the grease is in?

any ideas?

Cheers,
Richard

WindWizard · Dec 15, 2023

l have a similar setup and the answer to your question is to not change the thermal grease because it is still good. However, I have found that the evacuated tubes do go bad. I have had to replace around 15 or so that have gone bad on my setup. I think it is due to the harsh winters in Maine that kill them.

A few clues as to when your tubes have reached their end of life.

1- Water in the bottom of the evacuated tube.
2- When the sun is out then the tubes should be cold. If you put your hand on the evacuated tube and it is warm, then it is defective and needs to be replaced.

rgormley · Dec 16, 2023

does anyone have some photos of what a good/bad tube looks like?

WindWizard · Dec 16, 2023

On the upper part of the tube the glass in normally darkened. On the ones that are going bad, the dark material is starting to lighten a bit.

The sure way is to measure the temperature of the lower part of the tube. If the tube is warm when the sun shines, then it is a defective tube.

If you have noticed that your indirect water tank is not getting as hot as it used to, then you have some bad tubes.

If you look at this picture of the back of the tubes, the tube that is 12th position from the left, the top part it lighter. The tube itself looks darker. This tube has gone bad and was replaced.

JohnSolar2009 · Jan 30, 2024

WindWizard said:
l have a similar setup and the answer to your question is to not change the thermal grease because it is still good. However, I have found that the evacuated tubes do go bad. I have had to replace around 15 or so that have gone bad on my setup. I think it is due to the harsh winters in Maine that kill them.

A few clues as to when your tubes have reached their end of life.

1- Water in the bottom of the evacuated tube.
2- When the sun is out then the tubes should be cold. If you put your hand on the evacuated tube and it is warm, then it is defective and needs to be replaced.

Here's a picture of my roof showing a bad tube. One of the 60 evacuated tubes is different. It's partially filled with water.

Screen Shot 2024-01-30 at 5.06.09 PM.png

JohnSolar2009 · Jan 30, 2024

I hope I don't have an issue with my manifold. I find it hard to believe that this came from condensation so perhaps it's cracked. Will have to wait and see to get up on the roof. Not fun.

WindWizard · Jan 30, 2024

JohnSolar2009 said:
Here's a picture of my roof showing a bad tube. One of the 60 evacuated tubes is different. It's partially filled with water.

That tube is defective and needs to be replaced.

The water that is inside the glass evacuated tube comes from the copper tubing developing a leak. The evacuated tubes are seperate from the manifold.

The way the evacuated tubes are constructed is that inside the glass tube is a copper tube. The tube has copper fins attached. The sun warms up the copper fins and they heat up the copper tube. The copper tube has a small amount of water in it. The water boils and then the steam rises and heats a copper bulb at the top of the evacuated tube. The copper bulb is sandwiched in between a copper manifold. The manifold gets very hot and water is circulated in the copper manifold. Some of the water may be a mixture of water and antifreeze.

JohnSolar2009 · Feb 2, 2024

WindWizard said:
That tube is defective and needs to be replaced.

The water that is inside the glass evacuated tube comes from the copper tubing developing a leak. The evacuated tubes are seperate from the manifold.

The way the evacuated tubes are constructed is that inside the glass tube is a copper tube. The tube has copper fins attached. The sun warms up the copper fins and they heat up the copper tube. The copper tube has a small amount of water in it. The water boils and then the steam rises and heats a copper bulb at the top of the evacuated tube. The copper bulb is sandwiched in between a copper manifold. The manifold gets very hot and water is circulated in the copper manifold. Some of the water may be a mixture of water and antifreeze.

Update on my tube. It was cracked along the top near the manifold. The outer glass was cracked which allowed rain water to enter, then during a cold night the stored rainwater froze and broke the inner glass. This happen over the course of a few days. I had other issues leading up to this, but won't get into it here.

PS. I believe the interior copper tube with the bulb has a mixture of water and perhaps acetone or another liquid to lower the boiling point from 212 F. With a lower boiling point the vapor/condense action is increased at lower sun intensitys.

evacuated solar hot water tubes (thermal grease, when to change?)

rgormley

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WindWizard

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rgormley

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WindWizard

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JohnSolar2009

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