I'm mistaken. It's about 4 hours of sunlight, from 1-5PM.

I'm in Minnesota.

How hot is the exhaust from the gas fireplace vent and how far will the shrub be from this vent? I'd be most concerned about that. I don't have Lemon Lights but I have several others of the series in about that amount of sun and they bloom well. As long as the bed is well-drained, even moisture is good, but in general rhodies and azaleas don't do well with wet soil.

I would place it a good 2 1/2 feet to the left of the vent and a foot and a half forward from the wall it is on. Not sure how hot it gets, but I use the fireplace sparingly; never in summer; mostly on cold mornings for about an hour to warm the house a bit.

I would be worried about heating and then refreezing the plant

IMO 30 inches and 18 inches away from a hot air vent is not far enough away for the plant to experience it's winter without extreme temperature fluctuations.

I have Lemon Lights and Candy Lights in a bed that sees about five hours of sun a day and they are both doing well. The Lemon has been the slower growing of the two but it was far smaller initially.

I also have several Mandarin Lights in full blazing sun in a very dry area and they are doing even better. Go figure :)

I'd add width to the bed so you can move the plant out from the vent; as cearbhaill commented that is too close and the freeze thaw of the plant's tissues won't be healthy. I am not sure but I imagine that the acidity in the condensate may be an issue for the plant as well since condensate from our gas furnace has killed grass in that area even though it's fine with our acid soil. Also, as the plant grows it will get closer to the vent, so more potential damage.