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solar hot water for heating house - doing the maths

 
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Hi,

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.

http://www.theheatpumpshop.com/pdfs/heating_sizing_chart_guide.pdf

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?)

Annie


 
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