I have 2x split systems that are terrible at heating my apartment.
I've been looking into getting a Rinnai unflued gas heater, which are around $1,100 for ~6kw of heat output and about another $300 to install.
I've also experimented just running my gas cooktop burners for 20 mins which did a pretty good job of heating up the place. I understand CO can be produced so you cannot use this method of heating bedrooms and the like, but for large spaces it should be OK. The apartment is hard to heat because it is a large space and high ceilings (open plan design with loft style bedroom, no doors) so I figured this could be OK.
I have never experienced headaches or anything and this is something I only do for say 30 mins on very cold days.
I've also read some stuff on how a radiant heat effect can be added by boiling a large pot of water as well.
I don't mind dropping the cash on a Rinnai but apart from adding a 'timer on/off' function is it really any better?
Mate has a Rinnai gas heater, the quickest way to heat a room in lest than 3min. Natural gas is great, but for me I use a 10 column oil heater which takes bout 10-15min to warm up a room depending on the size and how you seal the gaps, I turn it up to full for about 30min and then switch half of it off and turns it down to about 60%, works great and saves me money that way.
I also hate using the air conditioner as a heater, I get inconsistency and heating takes a while.