Cannot Achieve 12 Degree Drop

It doesn't matter which end the LS valve and TRV are placed. You just adjust the LS valve to give the required drop. Just remember to take the head of all TRVs while balancing.

The water is not backfeeding. You have found out, the hard way, that adjusting one LS valve can have an effect on the others.

Start with all LS valves open 1/3rd of a turn. Adjust one rad at a time and make very small adjustments. Wait 5 minutes for the change to have effect.

 

Have a look here for Tom Plum's excellent guide to balancing radiators (think it's the third or fourth post down ( a video) http://community.screwfix.com/threads/radiators-bottom-of-the-rad-not-getting-hot.153255/

 

To answer you specific Qs, Dean - during balancing, the 'control' valve on your rads - whether 'manual' or TRV - should be FULLY open, and the lockshields at a 1/3rd-turn open starting point. With TRVs, best to actually unscrew the head and leave aside until you're done.

Don't touch the control valve until the balancing has been done. All the rads should be at roughly the same temp and heat up roughly just as quick and have roughly the same temp drop across them as eachother. Once the balancing has been completed, you then shut down the ones you don't want as hot (say, bedrooms) using the control valve, hopefully TRV types.

Your boiler probably won't need a by-pass rad as most have built-in by-passes should it be needed. In any case, you will have the rads in your 'room stat' room (sitting room? Hall?) without TRVs on them, so they will always allow 'by-pass'.

And then do as Mr Plum says

 

The purpose of balancing is to get the correct flow rate through all rads. You do this by having the same temperature drop across each rad. A 2kW rad with a 10C drop will have twice the flow of a 1kW rad with the same drop.

There will be water flowing through the return pipe, but it will be approx 10C cooler than when it entered the rad.

It doesn't matter which end the LS valve is, you still adjust the LS valve, as what you are doing is changing the flow through the rad. In fact, slowing the flow is often necessary to achieve the correct drop.

You need the flow to be about 75C to achieve the correct balance. If the flow temperature is about 30C you will never achieve a 12C drop.

What is a 'wild' rad??

 

Radiator balancing is one of those things that you can get hung up on, spending hours with rototherms strapped to the pipes and faffing about to get the exact temp drop is mostly a waste of time.
It is also dependant on the rads being correctly sized for the rooms, if they are undersized you are on a hiding to nothing.

Most of us that spent years learning to size rads according to the "U" values of the construction, type of room number of outside walls, yady-yady-ya and getting boiler / rad temperature differentials spot on, if we are honest when we look at a system do it by rule of thumb. Are all the rads evenly heating? Those that are not get the lock shield opened a bit, those that are robbing the rest get cut down, usually takes about 20 to 30 minutes

AS a matter of interest why do you feel the system wants re-balancing? My advise would be don't get too hung up on it, if they are all getting hot it ain't broke so don't fix it

If you can post what the problem you have is it might turn out to be something else and with luck we can help to fix it.

Best wishes,
teabreak

 

Think ya self lucky OP you aint got an ASHP feeding ya Rads!

Now that is a Balancing Act!