As with everything, there are pros and cons. In the case of natural gas heaters, the biggest one is the cost. Initially, adding a gas spa heater is going to start at around $1000 and go up from there. This doesn’t include burying a gas line from the meter to the hot tub, or hooking it up, so that’s an additional cost that can get very pricey, because running natural gas lines is something that must be done by professionals who are certified to work with gas lines.
Also it’s important to keep in mind that natural gas is flammable, and has the potential for dangerous accidents. Chances are low, but it’s still an issue to keep in mind. Add to this issue that a gas heater can’t be placed inside the spa cabinet because it needs fresh air and exhaust in order to work, so it can’t be visually hidden the way electric heaters can.
Energy Costs
Let’s break this down a bit more. Assuming that your hot tub has the same rate of heat loss no matter what heater type you’re using, a gas heater can only be approximately 80% efficient, where electric ones are (in most cases) closer to 100% efficient.
However, when you’re looking at efficiency, there are a number of factors to consider, like the size of your spa, the outdoor air temperature (and wind speeds), and the quality of the hot tub’s insulation and cover. The best way to compare energy costs is to compare your natural gas furnace costs to electric air conditioning costs. The more efficient the system, the lower the cost.
Availability
At the time this article was written, you can still get a natural gas heater for your hot tub, but they’re mostly only available for custom, in-ground spas, and are much harder to find in a portable spa. The bigger trick might be finding a licensed contractor to run gas line from the meter to your heater (which is an additional cost that can often supersede the cost of the heater itself). However, there are several states discussing legislation that would essentially ban natural gas heaters in spas, so if you’re determined to invest in a natural gas heater, keep in mind that it may eventually have to be replaced for environmental law reasons.