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I have a cottage that is off grid. I want to plumb radiators on it that wont use a pump, or minimise usage of a pump if at all possible. What is the best way to do this?

Hot water tank is raised to roomceiling level, expansion and water tank are in very small attic, boiler is at floor level, and there is three loops, two with one rad a piece, one with five.

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You may need to be prepared to rearrange your setup to make it work optimally. Is your arrangement based on something you read up on? –  Jack Dec 28 '13 at 2:28
    
No, its laid out as I bought it, but the plumberripped it out and is putting in a modern system, which is dependant on an electrical pump. –  Tom Carty Dec 28 '13 at 16:46
    
If that fails, I want to try the old system. –  Tom Carty Dec 28 '13 at 16:46
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1 Answer

Back in the days in the beginning of the tall cast iron radiators, they were set in the loop so that the hot water would rise and by the time it went through the radiators in its zone, and by the time it went through the radiators and cooled it was on the downward leg of the run back to the boiler. This would work best in a two story home where it would go through a few on the first floor, a few more on the second floor, so it would be cool enough to make its way back down to the boiler in the basement, repeating the cycle. It is called a gravity system. there is a small article near the bottom of the page here

12-28-13 edit taken directly from the article

" After a few fires and explosions, Perkins modified his system by installing a temperature-limiting device, operating the system at a lower pressure and temperature. The Perkins’ system was very popular in England until the 1850s but then died out. The idea of high-temperature hot water systems languished until they were revived in the 1920s."

The article goes on to state how the system grew in popularity

I would presume that old tried and true technology would work now. I think that with a little research, since these systems really need to be designed by an engineer to get the flow right, a slightly more efficient system could be had.

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You'll need to use LARGE pipe and be very careful to slope it correctly - thermosiphon is an extremely wimpy way to move water, compared to even a 5 or 10 watt pump. –  Ecnerwal Dec 28 '13 at 2:35
    
Yes it was a very wimpy way to do it but it worked. The idea at first was to use large pipes but later smaller diameter, thick wall pipes were used, as mentioned in the article I linked to in my answer. The modern equivalent would probably be close to copper pipe with a wrap of quality closed cell foam insulation. –  Jack Dec 28 '13 at 2:54
    
I know they work - I've lived with them. However, the "small diameter pipe" system mentioned was operated at temperatures and pressures no sane person puts in their house (or has for 100 years), if you read the article - 200PSI and 400F - massive steam explosions. In a typical gravity system fed by a 15-30PSI hot water boiler, large diameter (2 or 2-1/2 inch) makes a very significant impact on effective circulation, and 1 inch pipe chokes it. Steam is, in some ways, easier without a pump, but still sensitive to slope. –  Ecnerwal Dec 28 '13 at 3:48
    
It may be ineffective to move water but it work well for steam. You would have to calculate you energy use for a less efficient steam boiler versus a more efficient pumped hot water. –  mikes Dec 28 '13 at 14:42
    
IMO, the money saved in piping and efficiency using a conventional pumped system would easily pay for the photovoltaics and batteries needed to run the pump. Obviously this increases complexity, but the pump itself is the only thing that might be considered slightly unreliable. –  bcworkz Dec 28 '13 at 21:59
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