Underfloor heating pipes through winter

Hello I found your details by a google search, wondered if you could give me the benefit of your experience, I have an unusual question...... I am building a house and have installed the underfloor heating pipes and screed but not yet got a boiler. For various reasons there will be no building over winter and I am concerned that the pipes will freeze. As the house itself has yet to be insulated, I cannot heat the house to keep the floor warm. And I cannot drain the system (already pressure tested so still full of water) as there is no drain valve or low point. So I was wondering if I could plug in a small electric hot water tank, say 50 litres, and just leave it on and pump the water continuously if the temp drops below freezing for any more than a few days. Do you think that would be enough heat, with the continual flow, just to keep the water pipes above freezing? Been racking my brains on this one to find a solution, would be very grateful if you had any thoughts or advice. Many thanks Stephen