Keep the pot warm after the leaves come out
A fabric cover slows heat loss, but it does not stop the tea leaves from steeping. Separate those two jobs.
Warmth is useful after the brewing is done
The first cup from a pot tastes balanced, while the last seems harsh. It is tempting to blame the tea cosy because the pot has stayed hot. First ask whether the leaves were left in the water. A cosy slows heat loss; it does not separate leaf from liquor. If you want several cups at roughly the same strength, remove the infuser or strain the finished tea into a serving pot before covering it. This simple separation lets you judge the cosy on its actual job. You are trying to keep a drink comfortably warm, not extend the brewing period until the pot happens to be empty.
Check the fit with an empty pot
Some covers sit over the whole pot and must be lifted before pouring. Others leave openings for the handle and spout. Neither style should interfere with your grip, obscure a loose lid or drag into the stream of tea. Try the cover on the empty, cool pot first. Check that you can remove it without pulling the lid up with it. A wet or badly fitting cover is awkward to handle, and a heavy knitted flap should not hang near a heat source. A cosy is not a substitute for a heat-resistant glove and is not intended for use on a hob or over a candle warmer. Keep fabric away from flames and hot heating elements.
Decide whether the extra warmth helps
For a small pot emptied quickly, the difference may be unimportant. For a longer breakfast, a cover can reduce the need to reheat a serving pot. Try the same finished tea with and without the cosy, tasting at similar intervals and allowing each sip to cool enough to assess it. Do not expect a precise holding temperature from a fabric cover. The pot material, room and amount of tea all influence cooling. Wash the cover according to its material's instructions and let it dry completely before storing it. If it collects drips around the spout, adjust your pouring routine or choose another fit. The best cosy is a quiet convenience: easy to remove, easy to clean and useful for the way you share a pot.
