That sounds right Paul. Given that used that way it has a lot less water to heat it ought to need a lot less power to get it up to speed. It still seems odd to me that it runs fans during start up though, they'd be using unnecessary power by blowing cold air till the water was heated. I wonder if the fan/fans (I assume just one) could be switched off during warm up, or at least till the starting sequence is completed. That would reduce the power demand at the crucial time. I fit a switch and give it a go. It won't harm the heater.
It'd also be worth starting it on hot water only and once that's running properly ( a while after start up) switch it to heating and hot water.
Surely the fan should be thermostatically controlled, only coming on when the water is up to temp?
Alan I am glad to report that you are correct, starting the Hydronic on hot water first and then switching to heating/hot water uses much less power.
Starting on heating/hot water uses over 17 amps for 3.5-4 minutes gradually decreasing to 2.8-3 amps after 12 minutes although still cycling up to over 17 amps every minute or so.
Starting on hot water first it cycles between 4 and 14 amps for 2 minutes the 4.2 amps for a further 8 minutes, then switch to heating/hot water. It uses a steady 2.8-3 amps there after, this is on the low fan speed.
So a noticeable difference and I will always start on hot water first from now on so I,ve learnt something from this thread.