Due to the poor quality of our tap water where I live, I also use
distilled water or sterile water for irrigation as the base.
I was told to keep the distilled water in the fridge after opening it
up and only use it for a few days as bacteria can still grow in
standing distilled water.  I simply use the distilled water because it
is very inexpensive.  Sterile water for irrigation is a bit expensive
if you want to buy it yourself.  My ENT gave me a pretty good supply of
the sterile stuff following nasal surgery to use.

I heat the water up to about 104 deg F in the microwave,  add the
salt/buffer packet (Nielmed), and mix it up completely.
By the time I make it to the sink in the bathroom I recheck the
temperature of the water and it usually is right under 100 degF which
provides no temperature related discomfort.

I do not generally use the water pik, but the bottle in the Nielmed
sinus irrigation kit for most routine rinsing out of the allergens and
thick sinus drainage.  The key I think is to not do any physical trauma
to the sensitive tissue as that will obviously increase the swelling
problems.

My ENT wanted to make sure I understood the importance of keeping the
pik and bottle clean, dry, and as sterile as possible between uses.  So
make sure you are not breeding any mold or other creeping cruds that
you cannot see in your bottle or water pik.