All gas appliances are adjusted for a gas + oxygen mixture as 'sea' level. As you go up in altitude, there is less air and the combustion is less efficient.
At around 1500m (5000ft) we estimate that the HOTTAP will be around 10% less efficient. The higher up you go, HOTTAP will work but it'll just take longer for your water to heat up, or it won't get as hot. You can mitigate this by just recirculating water in a tub if you need to get it to reach a higher temperature.