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Hot Water Tank Coil

 
Domestic Hot Water & Cylinders
Last Post by Judith 9 months ago
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 JohnnyB
(@johnnyb)
Estimable Member Member
Joined: 2 years ago
Posts: 114
Topic starter 21/01/2026 12:33 am   [#2764]

My DHW tank has been fitted and the immersion heater isn't screwed in yet.  Without it in place I can partially see inside the tank and it looks like the coil is half 'dumped' in the bottom third or less of the tank.  When I look at pictures online coils seems to be shown as a solid pipe formed in a coil that goes a reasonable distance up the tank. The coil in the one I have looks like a flexible metal pipe, similar to this picture but it's doesn't look like it is wrapped in a coil around the tank but crosses over itself a lot in the bottom third of the tank. (It is hard to see exactly what it does but it isn't a single or double coil around the outer edge of the tank)

image

 

I'm wondering if this is normal, or if not normal is it likely to be efficient for heating the hot water.  I can't see anything like this online and I'm wondering if the coil is too compacted together so that a larger coil will be pointless.  I asked for the coil to be as large as possible and they said the biggest they could fit is a 4 mtr coil instead of the usual 3 mtr. The label still says 3mtrs but the plumber has insisted it is a 4mtr coil. It's a 300ltr tank, tall but still around 600mm diameter.

 

What is the best way to fit a coil in a hot water tank, does it need coiling towards the outer edge of the tank and going a good distance up the tank or is heating the bottom stil effective?



   
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 Judith
(@judith)
Prominent Member Member
Joined: 3 years ago
Posts: 554
21/01/2026 9:19 am  

I hadn’t realised they were loose coils until I saw your post and picture. It really is only due to area and possibly the more random the better for the warm water rising(!). As much as can be fitted into the bottom really, then all of the tank is hot. The take-off is from the top anyway.


2kW + Growatt & 4kW +Sunnyboy PV on south-facing roof Solar thermal. 9.5kWh Givenergy battery with AC3. MVHR. Vaillant 7kW ASHP (very pleased with SCOP >4) open system operating on WC


   
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