Argentina!
Hi Mafulynch,
I get your point about not wanting to preheat water for washing - I suspect others are thinking of the British climate and not realising you're in South America. High altitude, I'm guessing: cold but sunny?
No, I don't think a small heated tank will restrict flow. I meant that there's not a problem having a small tank when the electric heating is off. Ignore my radiator comment - it was badly worded.
I don't know what your boiler manufacturer means by 'renewable compatible' - you'd have to ask them. Is it a combi boiler? If so, they may mean it will accept preheated water on the secondary side - which you aren't interested in.
SJB06085 is spot on with guessing my concern. IF you have a boiler that is a modulating condensing boiler it may be monitoring the return temperature and trying to adjust the pump speed and burner to maintain a set temperature difference between the flow and return. It would take some thinking to work out the possible effects of putting a 6kW gain into the boiler flow or return under varying conditions and heat requirements and how the boiler may respond. I would imagine the boiler would work to some degree, but it may cycle an awful lot and never actually get a chance to go into condensing mode. It might even detect the situation as a fault.
Some ideas of the problems you might face: assuming you get a 20°C rise off your electric, you could try to setup the flow to account for full preheat conditions (run the emitters at, say, 60/20 and then preheat back to 40°C, so the boiler runs 60/40) but the radiator output will be reduced. Or you could run the boiler 60/40°C and then superheat the flow to 80°. This will give hotter radiators and if you run the radiators 80/40, it's not bad.
Problems might arise with both experiments when the electricity goes off as the boiler is now running on a 40° drop it isn't designed for and the radiators may be too cool to be of much use. Unless you can make the tank designed to restrict the system flow and use some kind of modulating control to bypass the tank and so increase system flow as the output reduces. It would seem very complicated though and you'd risk having two systems trying to compensate for one another.
Rather than running in series, have you considered running through a low loss header if an accumulator is not an option? LLH are not without their effects on boiler efficiency, but would at least work.