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Author Topic: Quick question - surface area of DHW coils in thermal stores?  (Read 2126 times)
julian
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« on: September 29, 2012, 11:31:46 AM »

Im having trouble finding the details - im sure theyre somwhere close, but i just cant locate them!

Can anyone tell me the surface area of the mains pressure DHW coils in a thermal store?

To be sure, i mean the coil that gives on-demand mains pressure water from a vented store.

Thank you,
Julian
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dan_c
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« Reply #1 on: January 16, 2013, 08:53:30 AM »

I've seen them as small as 3m2 in a 300litre store, as big as 5m2 in 500l stores, and then 7-18m2 in really big stores from 700-3000litres.

Does that help?
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julian
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« Reply #2 on: January 16, 2013, 09:03:15 AM »

Yes, thank you.

I dont have a proper tank at the moment, and keep swapping between the idea of making one or just buying one.

I do have a mini-store, called a "gledhill assist" unit, but, so far as i can calculate, the coil in that is only about 2m2 : (

(it was a unit intended to add solar pre-heating to a suitable combi boiler)
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biff
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« Reply #3 on: January 16, 2013, 09:25:56 AM »

Hi Julian,
       The DHW cylinder which i removed was sold to me as a "quick recovery" with twin coils both 8mtrs long,It was only 4ft high x 20inches wide. The first cylinder which arrived to replace it was a poor effort of a replacement.I measured the coils with nylon string and a vacumm cleaner,I recall one being just less than 4mtrs and the other 3.5mtrs,So back it went.There are massive coils in our present cylinder,550 x 1800mm.Good long coils are really necessary to heat the water quickly especially if you are connected to a solid fuel boiler.
                                                                      Biff
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julian
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« Reply #4 on: January 16, 2013, 09:46:53 AM »

What im thinking of doing is -

Using the 'thermal assist' tank as a 2nd tank, in series with another.

Ill plumb it so as -

The solar heats the small tank, then swaps to the larger.
The small tank is heated 'direct' from the boiler, if it ever gets too cold, so as to maintain DHW, but not heat a large store with the boiler.


I know direct heating and solar really dont mix, but i figure that the temp of the small tank should never really drop when the solar is running, as the water coming into it will be up to temp from the large tank anyhow.
If it starts to waiver in normal running, the solar controller will swap the flow to keep it up to temp (*see possible flaw below)
Then *only* if the large tank has gone cold so its now taking heat from the small tank also, will the boiler fire.

This will, hopefully, mean that i can have a large 'large' tank without the same 'lukewarm' issues that ive read about from solar and excessive sized tanks.  Even if its only warm, hopefully, this will be enough to preheat the water enough to maintain the temp in the small tank for some time before it drops low enough for the boiler to work.


*This plan will not work, however, if the small tank has to be maintained so hot as to provide hot water when the large tank is out - i mean with its 2m2 coil, i assume it will provide DHW, but only if the body of the tank is hot enough.  And if the temp that the boiler comes on is almost the same as tank switch over on the solar controller, it wont work so well.
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