Installing the hot water evacuated tubes

Today we installed the solar hot water evacuated tubes. The hot water service is already installed, so I have just been waiting till my plumber can come and do the tubes and commission the unit.

The tubes are 1.8m long, and there are 30 of them.

One end goes into a manifold where the water from the tank runs through, and the other end just goes into a plastic mounting boot.

Each tube is like a thermos flask – its a double walled glass cylinder, and between the two glass walls is a near complete vacuum. Inside the cylinder runs a copper tube that has glycol inside it. The tube heats up from the sun, transfers the heat to the copper and thus the glycol. The glycol gets hottest at the top of the copper tube (the bit that goes into the manifold). Water flows through the manifold, and the very hot copper tip transfers the heat to the passing water.

The water circulates from the hot water tank (which in my case is up inside the roof), through the manifold, and back into the tank. The circulation operates by thermosyphon – there is no electric pump required. Cold water is drawn from the tank from an outlet that is low on the tank, the manifold  inlet is higher than this tank outlet, the manifold is on a slight incline, and the water returns to the tank on an inlet that is above the height of the top of the manifold.

When installing the tubes into the manifold, heat transfer paste is smeared onto the copper ends so that there is good thermal conductivity between the copper of the tube and the copper of the manifold.

Note: keep the tubes covered when they are sitting in the box ready for installation, else the copper ends will get too hot (from the sun striking the glass) to handle and apply the paste.

The manifold has rubber gaskets to ‘seal’ around the glass tube. These gaskets proved to be a real pain in the arse – we could not slide the tube into the manifold without pushing the gaskets off their seating. So, we ditched the gaskets – they were proving to be of limited value anyway. Probably really only used to stop a bit of moisture/rain  from entering the manifold, but I don’t really think that would matter much anyway.

Once all the tubes were installed, and the manifold pipes penetrating the roof were insulated and lagged with tape (to protect the insulation from birds and UV), we could fill the tank and test wait for sunshine to heat things up.

Basically the thermosyphon would work or it wouldn’t. If it didn’t, then the backup was to install a small circulating pump. We may install a pump regardless, because of frost. On a cold night we dont want the manifold water freezing, and bursting pipes, so keeping it circulating every half hour for a few minutes would solve that.