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solar hot water for heating house - doing the maths

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I got second-hand a number of hydronic radiators, and now want to get the risers and water storage tank to get these working.

As anything like that is so expensive here in New Zealand, I am looking at getting a one-off shipment from China, but want to do the maths on what I need for size.

I have a sort of insulated, very drafty 100 square metre / 1000 square feet house.

In winter our average minimum is 2C and and record minimum is -4C as we are near the coast.  I am aiming at 16C.

We are going to make an enclosed porch running the sun-facing length of the house that we hope will pretty much solar heat it on sunny days, and help stop the drafts.

Going by this figure of 65W per square metre - I would be looking at 13kW per hour though the night on fine days, but if we expect bad weather, we can light the fire in the dinning/lounge kitchen area and drop this to 3-5kW per hour in the bedrooms.  

So planning on 16 hours x 13Kw that is 208kw for overnight on fine days, or two winter days with the fire going.  We are setting up in-home childcare, so we need to heat most rooms for most of the day.   Where do I go from there in terms of Wattage of risers and size of tanks, as there are other factors such as

* how effective the radiators will be as the water cools  (what is the minimum temperature you would want your water to drop to?)
* How many days of cold water weather is it worth storing up (that is, how quickly will the tanks lose their heat naturally?)


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