The Walipini Project (aka greenhouse shenanigans)

Only reason I ask is because some years ago a mate had something similar in his greenhouse.
The boiler was in a shed and the pipes came through the end of the greenhouse I believe he said the system was filled with water.

 

Do you have any horizontal ties on the roof, rafter to rafter forming a triangle.

The glass weight will tend to try and spread the walls, if you get my meaning, the downward weight becomes an outward force due to the shape.

Your extra uprights will help, but triangulation gives the stiffness to the structure,

 

Before electric pumps, fans, heaters etc one way of heating a greenhouse was have a boiler at the low point of the pipe work (in some cases in a pit in a shed outside). Then as the water was heated it rose and the circuit was completed by the cold return running into the bottom of the boiler.
When growing up we had a coke boiler in the kitchen, it was immediately below the hot water tank and worked on the same principle of hot water rising and cold water sinking.

 

That's how our old boiler used to work @NigelJ .
Gravity.

 

Stayed in a farmhouse in early 2000's that had a Rayburn in the kitchen that used the same approach for hot water, definitely showing its age though.

 

The problem is without any ties across its not a triangle
Just some ties would stiffen the whole thing up and make the glass weight a downward rather than a spreading force.
Just give yourself enough head room, if you can make it 6ft 6ins it would be good.

 

I was just thinking about those poor old walls, but if you have the rafters tied together that is really all I was getting at.

 

My 12x12 greenhouse has a couple of stainless steel rods going across, between a couple of brackets.

 

All I can say is WOW!!! I'm totally envious.

 

Turned out really well
Right, so next it's the coldframe.