Using UFH manifold drain valve to drain Radiators

Hi all.

I have the boiler and radiators upstairs, underfloor heating downstairs.

I plan to use Adey MC3 to clean the system and then MC1 to maintain it. After I use the MC3 cleaner, I need to flush the whole system. The problem is I don’t have a drain valve anywhere, apart from the UFH manifold return drain valve.

Is it possible to drain the entire system (excluding the pipes in the floor), using the drain valve on the manifold?

I am concerned the pipe on the left (flow directly from boiler above) is lower then the return drain valve. Will the entire system drain if this pipe is lower then the valve? If not, I am thinking of using the self cutting drain cock to attach to where the yellow circle is.





Finally, once I drain the system, I plan to flush the UFH pipes as if I was re-filling them to clear out any old MC3 chemical. I will then refill the pipes, and bleed any air as per usual.



Also I have radiators which are lower then some of the flow and return pipe work where the boiler is. Will this stop the radiators from draining?

Any advice welcome.

Thanks

Here is another dilemma..

The lowest radiator upstairs (on the left of the picture) is lower then the flow/return pipe work which makes its way to UFH manifold (directly below the cupboard but on the ground floor).

If I try to drain the system from the manifold, only the pipe work will drain and the water will remain in the rads.

I have draw RED lines to show the FLOW, return runs next to it pretty much...