We have an old (15-18 year old) Hayward pool heater that has stopped working and we need to buy a new heater. What factors should I consider when buying a heater? My husband would like to replace it as soon as possible (my daughter's birthday party is in 3 weeks) and has ruled out solar because he thinks it will be very expensive and take a long time to install. We are very inexperienced with pool equipment (but thanks to this forum, I'm good at water chemistry) and would appreciate any advice. Thanks.