A 200 L tank at 60°C in a 20°C room, wrapped in 15 cm of wool-like insulation, loses heat at about 28 W and cools by about 2.9 K a day; the same tank bare loses about 1 kW and cools by 4.6 K every hour. Wrap the tank in at least 15-20 cm of dry insulation, put the most on top, insulate the pipe connections too, and keep the insulation dry, because loss falls with thickness but never to zero.
Part of Hot Water Tanks
Super-insulation is a thick, continuous layer of dry insulation around a hot store, thick enough that the water cools over days rather than hours.
Derive The Loss, Do Not Guess It
The parent page says a well-insulated 200 L tank loses 1-2°C per hour and a poorly insulated one 5-10°C per hour. Check the first figure.
Inputs, assumed: tank 200 L as a cylinder 1.2 m tall (radius 0.230 m, from π r² × 1.2 = 0.2 m³); tank at 60°C, room 20°C, so ΔT = 40 K; conductivities from the corpus’s insulation table: wool 0.041 W/m·K, straw 0.096 W/m·K (Natural Insulation Materials); heat capacity of the water 200 × 4.18 = 836 kJ/K.
For a layer of thickness t around a cylinder: side loss per kelvin = 2π k L ÷ ln(r₂ ÷ r₁), with r₂ = r₁ + t; the two ends are approximated as flat layers, k × area ÷ t.
| Insulation | Thickness | Loss per kelvin | Watts at 40 K | Fall per day |
|---|---|---|---|---|
| Wool, k = 0.041 | 5 cm | 1.85 W/K | 74 W | 7.6 K |
| Wool, k = 0.041 | 15 cm | 0.71 W/K | 28 W | 2.9 K |
| Wool, k = 0.041 | 30 cm | 0.42 W/K | 17 W | 1.7 K |
| Straw, k = 0.096 | 15 cm | 1.66 W/K | 66 W | 6.9 K |
| Straw, k = 0.096 | 30 cm | 0.98 W/K | 39 W | 4.0 K |
Worked line, 15 cm of wool: side = 2π × 0.041 × 1.2 ÷ ln(0.380 ÷ 0.230) = 0.309 ÷ 0.502 = 0.61 W/K; both ends = 2 × 0.041 × π × 0.230² ÷ 0.15 = 0.09 W/K; total 0.71 W/K. At 40 K that is 28 W = 2.4 MJ a day, which cools 836 kJ/K by 2.4 MJ ÷ 0.836 MJ/K = 2.9 K a day.
So a 15 cm wool jacket cools the tank about 0.12 K an hour, one eighth to one sixteenth of the parent page’s 1-2°C per hour. Its figure corresponds to a loss of 1 K/h × 836 kJ/K ÷ 3,600 s = 232 W up to 464 W, which would be a lightly insulated tank, not a 15-20 cm jacket. For comparison, a bare tank: surface 2.07 m² at an assumed 10-13 W/m²·K (bare steel in still to breezy air) loses 830-1,080 W, which cools it 3.6-4.6 K an hour, close to the parent page’s “5-10°C per hour” for a poorly insulated tank in wind. The top of that range, 1,076 W, against 28 W for the 15 cm wool jacket, is a ratio of 38.
Doubling the thickness from 15 to 30 cm only reduces the loss by 41% (0.71 to 0.42), because the cylinder’s outer surface grows as the layer thickens. Past a point, thicker insulation is a poor investment; closing leaks and thermal bridges is better.
Where The Heat Really Leaks
- Top: the hottest water sits at the top, so the loss per square metre is highest there; the parent page asks for the most insulation on top.
- Pipes and fittings: an uninsulated pipe stub leading out of the tank can lose as much as a square metre of wall. Insulate the first metre of every connection.
- Moisture: wet insulation conducts many times more than dry. Keep a vapour-proof layer on the outside and the whole jacket dry.
- Foam against a hot wall: rigid foam can soften when held at a hot surface; check the foam’s temperature limit and keep a mineral or air layer next to a tank hotter than about 70°C.
What You Need
- Dry insulation at least 15-20 cm thick (Insulation Materials Ranked)
- A wooden or sheet enclosure to hold it in place
- Waterproof outer layer
- A thermometer on the tank
Method: Wrap The Tank
Step 1: Check the tank for leaks before wrapping it.
Step 2: Build the enclosure with a gap on all sides equal to the insulation thickness.
Step 3: Fill or wrap, tight against the tank without gaps; tuck into every corner.
Step 4: Insulate pipe connections for the first metre, and close the top last.
Step 5: Measure: note the temperature drop over 24 hours with no draw. Compare with the table.
Common Mistakes
- Gaps at the seams. A haybox in this corpus loses 3-5 times what the ideal value predicts when it has leaks (Thermal Insulation Basics).
- Compressed insulation. It loses its still air.
- A damp wrapper.
Safety
A well-insulated hot tank stays scalding for days
Super-insulation keeps water at 60°C for days, so a leak, a valve or a drain at that temperature scalds severely in a few seconds, and the outside of the jacket may hide a leak. Keep combustible insulation (straw, paper, dry leaves) clear of any part of the tank or pipe hotter than the insulation’s tolerance, and keep an open vent or relief on the tank. Temper the delivered water.
What’s Next
- Learn next: Stratification & Draw-Off — getting hot water out first
- Combine with: Heat Loss Pathways — where heat escapes