New rad question about draining the system

[Tally101]

I am having a rad changed this week for a larger rad, I have a system boiler and a tank in the airing cupboard. My question is when the plumber drains the rads will I lose all of the hot water in the hot water tank or will it only drain the heat side of the system.
Many Thanks

 

[terryplumb]

Domestic hot water is not drained ,only the heating system is.