Here is a copy of the users manual for these pool heaters. There is a lot that could need to be adjusted or fixed if it is just not coming on at all. Try the troubleshooting guide.
What type of heater is this, millivolt or electronic, is there a standing pilot if so will pilot stay lit, if pilot does not light you may have an obstruction in it which needs to be cleaned. as well as checking gas orifices for blockage
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Here is a copy of the users manual for these pool heaters. There is a lot that could need to be adjusted or fixed if it is just not coming on at all. Try the troubleshooting guide.
http://www.backyardcitypools.com/images/Hayward/H100-Manual.pdf
What type of heater is this, millivolt or electronic, is there a standing pilot if so will pilot stay lit, if pilot does not light you may have an obstruction in it which needs to be cleaned.
as well as checking gas orifices for blockage