asalcedo wrote:
I need to attach a 35 mm to 22 mm endfeed reducer to a 35mm end of a
35mm x35mm x22 tee.
The problem is that the tee is already fully soldered and installed.
The end that I need to remove has a 35mm to 15mm reducer. I need to
remove this reducer and solder the 35 to 22 reducer.
By applying heat and using pliers I can remove the existing reducer.

Three questions:

1. Will it be ok to clean the inside of the tee with sand paper to
prepare it for soldering or will it be very difficult to get it good
and clean so that I can insert and solder well the new fitting?

If it were me, have your new reducer cleaned, fluxed and ready to go,
you _may_ get away with being able to whip it in while the heat is still
being applied to the joint so the old solder never cools.

If the joint remains in situ I don't think you stand much chance of
cleaning it out once the solder goes hard again.

2. I know that I risk damaging the other joints in the tee. What can I
do to avoid it?

Luck.
:¬)
You won't be able to apply enough heat to the joint to just melt the bit
you want. All other joints will go lose too so if you manage to get the
old one out and the new one in then make sure you re feed all the other
ends with new solder.

3. What is the best way to tackle this job?

Try it and be prepared to remake the whole joint with all new bits from
scratch. Far better to do it right once than spend a day attempting to
re-make the old joint.

You'll also need a good heat source to keep the heat local.

IMVHO
Pete