28mm pipe

[Stochengberge]

If you start increasing flow rates, pipe size & resistance don't necessarily go hand in hand as with a 28mm pipe there is more internal surface area, therefore more stiction...
However, if all you want to do is move the water a Grundfos Alpha2 15-50 operating with a 2 meter head (I'm guessing at the height of your tank connections), will move 2.8 cubic meters of water per hour (or 0.7litres per second). At tick over, it only draws 5W and <43dB(A) so is also quite.
A lot of this depends on how big your boiler is and how much water (heat) you need to shift.

As a guide, a 15mm copper pipe can cope with up to 6Kw, 22mm - 14kW and 28mm - 28Kw.

From what you have described, there are those that would say it would thermosyphon, and those that say it won't. A very slight rise may be enough. How about using the pump on an injector T? Tee into the 28mm flow with a 15mm Tee and put a 28mm tee in a bit further up the line, but using it as an elbow - imagine the flow coming up into the Tee and turning left. On the right hand side of the Tee, have a 28x15 reducer (or a reducing end tee if you can get it) which connects to the 15mm Tee you cut in earlier via a small pump. BUT, instead of the 15mm pipe finishing in the fitting, you poke it through so it comes out the other side of the 28mm tee. That way if you have a power cut, there is a chance it will still do something. With a pump in the way, it won't do jack...

[clivejo]

On the right hand side of the Tee, have a 28x15 reducer (or a reducing end tee if you can get it) which connects to the 15mm Tee you cut in earlier via a small pump. BUT, instead of the 15mm pipe finishing in the fitting, you poke it through so it comes out the other side of the 28mm tee. That way if you have a power cut, there is a chance it will still do something. With a pump in the way, it won't do jack...

Interesting technique, have you seen this in action?  Does the flow of water under pressure from the 15mm pipe induce a higher flow rate in the 28mm pipe?

[Stochengberge]

That's the idea.
Not used it myself, but I have heard from several others that it works. There is no pump in the way to inhibit thermosyphoning, but you get the benefit of a gentle push from a pump. It sort of bump starts the thermo...