When I was processing wool in utility sinks in my garage, at my last house, this is what I did to keep the water hot during the washing process:
Wrap the sinks in insulation, and when they have the hot water in them, cover them with a sheet of the stiff foam insulation.
I started out with my sinks at 180 F, and with this insulation, kept them above 165 for at least half an hour.
(Getting the sinks to 180 F -- 1/2 fill with hottest water heater water, 1/2 fill with boiling water)
(posted by me on spindlers this day)
The picture is fleece from Sitka, a Cotswold Ram, spread out on my skirting table.