We've had people use it for this exact situation. It works, but there are a few things to be mindful about.
The hottest increase the incoming water is by 30-35C (86-95F) above the incoming water. This is on the highest gas / lowest water flow setting.
Say the water coming in is 10C (50F) from the tap - the HOTTAP will get it to 40-45C (104-113F), but the water will be set to the lowest setting (i.e approx 4LPM (1GPM)). So it might take a while before the bath is fully filled up
Recirculating the Water
Another idea is to circulate the water, but using a pump. Start by filling your bath up with water, then use a pump to recirculate water inside the bath (simply put the suction hose into the bath) and whatever is coming out the Hottap goes back into the bath.
You might find it's quicker to fill up the bath from the tap, then just recirculate water, and over time it'll heat up nicely. Using this method, the longer you keep recirculating water you can heat it up quite hot - as the water inside the bath will slowly get warmer over time.
However using the pump, we only recommend a max cycle time of 20min at a time. We've seen our pumps working for more than 20min at a time without trouble but again something to keep an eye on so the pump doesn't overheat.
Protecting from Wind
Assuming the Hottap will be running for a while, and if it's in an area where it's prone to winds, there is a chance that the flame could blow out. If the flame goes out the Hottap won't automatically shut the water flow, so you'll have to keep an eye on it to make sure you're actually getting warm water coming out.
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.
HOTTAP uses two 'D-Cell' batteries, these power the unit itself as well as the LED screen and generally last for 3-6 months, depending on use.
The pump connects to a 12V supply (we give you a 5m cigarette socket cable) and draws approximately 3-4 amps depending on your settings.
The HOTTAP is designed with freshwater use in mind, using saltwater or brackish water can affect the internals and leave them susceptible to rust and corrosion.
It won't cause total damage immediately but saltwater, for example, will certainly reduce the lifetime of the unit. If you need to use salt water we'd recommend rinsing HOTTAP with clean water after use to minimise the risk of long term damage to the internals.
You can definitely use a gravity system at home to run the HOTTAP however please be mindful that you will require a specific set of conditions in order to meet adequate operation of the system.
You'll need 10PSI minimum to open up the pressure switch on the inside of the unit and you'll need 6LPM flow rate as well.
Usually we find that with gravity systems (this is assuming a tank 10-15m above the shower on the side of a hill) has enough pressure to fire up the HOTTAP on the lowest flow setting, but then once you try to adjust the dial to anything more than 1/4 water flow, the unit will cut out.
This is because there isn't enough water flow and pressure to keep that pressure switch inside the unit opened up, so it closes up and the system shuts down.
If the tank you're talking about is anything less than 10m above the shower, then you'll run into issues. If you're not running through the HOTTAP than gravity fed shower is OK, it's just when you want to connect to the HOTTAP you'll need to make sure you have enough pressure/water flow.
Reason for that is the size of the burners are designed for a certain water flow/pressure. If the water flow comes in at a much lower flow rate, then the water will heat up very very quickly way beyond the safety cut-off of 50C (basically it'll be useless as you can't have a shower).
From past experiences we have found in many cases it is easier to obtain a pump to enjoy the system to its full potential.
Protecting from the rain:
Your HOTTAP unit has been designed to be used outdoors however to ensure the unit remains in pristine working condition we strongly urge you to make all possible attempts to shield the unit from the elements. The unit is splash-proof however prolonged exposure to rain may result in the degradation of the internal components. The easiest way to protect your unit is to pack it away after use or keep it undercover to shield it from the weather.
Protecting from freezing temperatures and frost:
One last note regarding very cold temperatures - if you are in an area at a time when the temperatures approach or drop below zero you must drain the unit of water to prevent the water from freezing inside the unit and causing damage. Simply detach both water quick fittings and let the water drain from the unit. This will ensure that even if the temperature drops below zero your unit will remain safe and ready for next use.
The HOTTAP has a max temperature output of 50 degrees Celsius (122F). The temperature rise is dependent on the settings you have the HOTTAP on.
Low water & high gas = 35C increase (95F)
High water & high gas = 20C increase (68F)
For example, if you had the HOTTAP set at Low water and high gas and you were drawing water from a lake that was 10C (50F), the output would be 45C (113F).
In general, the answer is No! The HOTTAP must be used outdoors and in a well-ventilated area.
This applies to houses, sheds, shacks, camper trailers, caravans, houseboats, bus and van conversions, etc. You might want to check on the specific compliance requirements of your vehicle but we always recommend to use the HOTTAP outside.
If you need to get hot water inside, to an existing sink or something like that, this is still perfectly possible (and quick common amongst our customers) by setting up the HOTTAP outside the vehicle and quick connecting the hoses to your internal water lines.
The HOTTAP must be vertical when in use. This is because the flame (from the burners) heats the heat exchanger at its best possible efficiency when the unit is directly upright. The more the unit is tilted, the less efficient the heat exchange is – causing reduced water heating capabilities.
For peace of mind, the HOTTAP has a tilt sensor which will automatically cut gas to the unit if the HOTTAP gets tipped over.