From the FAQ on the trial page, it seems like it will simply not run if there is nowhere to dump the excess heat:
How much of my hot water will it deliver?
A useful base load. If the heata unit ran all day it would deliver about 4.8kWh of hot water which is about the amount of hot water an average household uses. In practice the utilisation level will be lower and sometimes the water will be at temperature so the heata unit won't be able to run. We think it could deliver up to 80% of an average household's hot water, with good compute utilisation. This trial will help us learn more about how this will work in practice and enable us to give you a useful amount of free hot water at the same time.
How much of my hot water will it deliver?
A useful base load. If the heata unit ran all day it would deliver about 4.8kWh of hot water which is about the amount of hot water an average household uses. In practice the utilisation level will be lower and sometimes the water will be at temperature so the heata unit won't be able to run. We think it could deliver up to 80% of an average household's hot water, with good compute utilisation. This trial will help us learn more about how this will work in practice and enable us to give you a useful amount of free hot water at the same time.