This could be a direct profit by using a coin or token operated time switch for example heating the bay of a golf driving range with a 50p per 30 minutes timer.
Depending on the size of your heater and your electricity tariff, it might be costing you 42p – 84p per hour to run the heater, so charging a small amount per x minutes is going to, at worst, cover your costs and make a small profit.
In addition to any direct profit as above, there is always an indirect profit generated simply by extending the length of time an area remains comfortable for use by your customers.
If you are able to keep areas occupied that would usually be vacated when the temperature drops, those areas are generating revenue and making a profit.
With an average running cost per heater 20p per hour to run, even an extra hour that your customer stays on site is going to prove profitable.