Big washes are a bastard to stir, Ive found dissolving the sugar and other dry ingredients in smaller containers of hot water first is a good idea.
Once all the sugar is dissolved pour into the big fermented then top up with hot or cold water to achieve roughly 28C Then throw in the yeast.
A ferment that size should make enough heat to keep its self warm for the first few days if you wrap it up snuggly. After that it could need some help to stay warm. Its said that one constant temp throughout the ferment is more important for neutral washes than the actual temp.
HERE is a link to an old post I made when I was a newbie, It was winter and my TPWs had been slow, This experiment kept the temps reasonably stable though they did move around a bit, I didn't need added heat till the afternoon of day four it seems.
Anyway its worth a look might help someone, might not.