**Heat loss in pipe insulation.**

On a cold day, I was on the (non heated) attic of my house where the central
heating pipes are running.

When I touched the insulation around the pipes, I noticed they were feeling
warm.

This means despite the pipe insulation, warmth is escaping from the central
heating pipes.

Via some measurements I try to find out how much energy the central heating
pipes are approximately losing.

I focus on the following two aspects.

- Heat loss via the insulation around the pipes

- Heat loss via the pipe brackets, through which the pipes are mounted to the
wall.

**The measuring set up**

The components used for the measurements:

- A copper tube, 28 cm long and 22 mm diameter, the bottom side of the tube is
sealed.

- A thermometer

- Two resistors of 12 Ω (series connected) which I
use as heating element.

Set up for measurement 1.The resistors are put into the
tube> |

Set up for measurement 2.The tube is insulated with a piece of pipe insulation with 9 mm wall thickness. |
Set up for measurement 3.The tube is insulated with 50 mm thick glass wool. After applying the glass wool, and winding tape around it, the glass wool was flattened to a thickness of 20 mm. |
Set up for measurement 4.A pipe bracket is attached to the tube. The pipe bracket is mounted on a brick The tube insulation is the same glass wool insulation as in measurement 3. |

**The set up for measurement 5** looks the same as the one for measurement
4, so also with glass wool insulation around the tube.

Only in measurement 5 an insulating cloth is applied between the pipe bracket
and the tube.

The cloth can't be seen, while it is covered by the glass wool.

But on the picture below, you can see what I mean.

I used a piece of a cleaning cloth with 1.5 mm thickness.

**The measurement.**

Before starting the measurement, the temperature of the tube must be equal to
ambient temperature.

The water used for filling the tube must also be of ambient temperature.

The begin temperature is noted, and the resistors connected to the power supply.

The tube filled with water now will slowly warm up.

On regular times the water temperature is noted.

One measurement can take some hours.

**The measurement results**

Table 1.

In the table below, the temperature increase of the tube with regard to ambient
temperature is given.

The temperature is measured with ± 0.5 ºC accuracy.

If no value is noted, no measurement is done on that time.

Time (minutes) |
Measurement 1 Without insulation |
Measurement 2 With 9 mm pipe insulation |
Measurement 3 20 mm glass wool insulation |
Measurement 4 20 mm glass wool insulation + non insulated pipe bracket |
Measurement 5 20 mm glass wool insulation + insulated pipe bracket |

0 | 0 | 0 | 0 | 0 | 0 |

5 | 4 | 2.5 | |||

10 | 7.5 | 6 | 5 | 6 | |

15 | 10 | 8.5 | 8 | ||

20 | 12.5 | 11.5 | 10 | 11.5 | |

25 | 14 | 13 | |||

30 | 16 | 15 | |||

35 | 17.5 | 17.5 | 18.5 | ||

40 | 19 | 20.5 | |||

45 | 21 | 23 | |||

50 | 22 | 24 | |||

55 | 22 | 27.5 | |||

60 | 29 | 25 | 27.5 | ||

65 | 26.5 | ||||

70 | 24 | 27.5 | 30 | ||

75 | 34 | ||||

80 | 25 | 29.5 | 32.5 | ||

85 | 30 | ||||

90 | 31 | 38.5 | 31.5 | 35 | |

95 | 26 | ||||

100 | 32 | 37 | |||

105 | 42 | ||||

110 | 27 | 35 | 39 | ||

120 | 34 | 45 | 40.5 | ||

130 | 28 | 37 | 42 | ||

135 | 35 | 48 | |||

140 | 38 | ||||

150 | 28.5 | 36 | 50 | 38.5 | |

160 | 36.5 | 39 | 45 | ||

165 | 52 | ||||

170 | 39.5 | ||||

180 | 53 | 40 |

De values from the table shown in a graph.

Graph 1:

Warming up of the tube.

Line 1 = without insulation around the tube.

Line 2 = with 9 mm pipe insulation around the tube

Line 3 = with 20 mm glass wool insulation

Graph 2:

Warming up of the tube.

Line 3 = with 20 mm glass wool insulation

Line 4 = with 20 mm glass wool insulation and non insulated pipe bracket

Line 5 = with 20 mm glass wool insulation and insulated pipe bracket

**Formulas for calculating the heat loss**

The power of the "heating element" is (10 volt)²
/ 24 Ω = 4.166 Watt, this power I call: P(in).

One part of this power provides the warming up of the tube with water, by which the
temperature of the tube increases, this part of the power I call: P(heat).

An other part of the power will get lost by heat loss, this part I call: P(loss).

At the beginning of the measurement (t0 = time zero) the heat loss will be zero,
because the tube has ambient temperature, all added power is then used for
warming up the tube.

What I want to know is the heat loss in Watt per ºC.

For the heat loss applies:

Heat loss (in W/ ºC) = P(loss.) /
T1 (formula 1)

T1 = measured temperature of the tube with regard to ambient temperature at
point of time t1.

P(in) = P(heat) + P(loss).

or:

P(loss) = P(in) - P(heat) (formula 2)

P(heat) = P(in) . dT1 / dT0
(formula 3)

Where:

dT1 = speed of the warming up of the tube (in ºC / minute)
at point of time t1.

dT0 = speed of the warming up of the tube (in ºC / minute) at point of time t0.

Combining formula 2 and 3 gives::

P(loss) = P(in) - P(in) . dT1/dT0

P(loss) = P(in) . (1-dT1/dT0)
(formula 4)

Combining formula 1 and 4 gives:

Heat loss = P(in) . (1-dT1 / dT0) / T1 (formula 5)

**Results of the calculations.**

With formula 5 the heat loss is calculated for the measured set up's.

Measurement | P(in) Watt |
dT0 ºC / W |
dT1 ºC / W |
t1 minutes |
T1 ºC |
lossW / ºC |

1 | 4.166 | 0.75 | 0.0375 | 130 | 28 | 0.1413 |

2 | 4.166 | 0.5666 | 0.0666 | 135 | 35 | 0.1050 |

3 | 4.166 | 0.6 | 0.1 | 165 | 52 | 0.0668 |

4 | 4.166 | 0.5 | 0.05 | 160 | 39 | 0.0961 |

5 | 4.166 | 0.6 | 0.1 | 145 | 43.5 | 0.0798 |

Table 2.

Calculating the heat loss for the 5 measurements.

The tube length is 28 cm, so the given losses are also per 28 cm tube.

For measurement 1, 2 and 3 we can calculate the loss per meter tube, by
dividing the results of table 2 by 0.28m.

This gives the following results:

**Measurement 1: loss is 0.5046 W / ºC.m (non
insulated tube)
Measurement 2: loss is 0.3750 W / ºC.m (pipe
insulation
9 mm)
Measurement 3: loss is 0.2386 W / ºC.m
(glass wool insulation 20 mm)**

The difference in loss between measurement 4 and 3 is the loss caused by **
one non insulated pipe bracket**,

this is 0.0961 - 0.0668 = **0.0293 W / ****ºC**

The difference in loss between measurement 5 and 3 is the loss caused by **one
insulated pipe bracket**,

this is 0.0798 - 0.0668 = **0.013 W / ºC**

**A practical example**

The central heating pipes on the attic of my house have a length of 16 meter,
so 16 meter flow pipe, and 16 meter return pipe.

The flow pipe is 80 ºC

The return pipe is 50 ºC

The temperature on the attic is 10 ºC

The difference with ambient temperature is: 70
ºC for the flow pipe, and 40 ºC for the return pipe.

By multiplying the heat loss in W / ºC.m by the pipe length and temperature
difference, we get the energy loss of that pipe.

The following table shows how much energy we lose with different kinds of
insulation.

Kind of insulation | length flow pipe (m) |
temp. difference flow pipe (ºC) |
energy loss in flow pipe (W) |
length return pipe (m) |
temp. difference return pipe (ºC) |
energy lossin return pipe (W) |
total energy loss(W) |

No insulation | 16 | 70 | 565 |
16 | 40 | 323 |
888 |

9 mm pipe insulation | 16 | 70 | 420 |
16 | 40 | 240 |
660 |

20 mm glass wool | 16 | 70 | 267 |
16 | 40 | 153 |
420 |

Table 3:

Example of energy loss at 16 meter pipe length.

We see, the loss can be reduced by hundreds of watt's with a good insulation.

Both flow and return pipe are on my attic fixed by 10 pipe brackets.

The following table gives the energy losses in the pipe brackets.

Kind of pipe bracket | number of brackets | temp. difference flow pipe (ºC) |
energy loss per bracket in flow pipe (W) |
energy lossin 10 brackets in flow pipe (W) |
temp. difference return pipe (ºC) |
energy loss per bracket in return pipe (W) |
energy lossin 10 brackets in return pipe (W) |
Total energy lossflow + return (W) |

Non insulated | 10 | 70 | 2,05 | 20,5 |
40 | 1,17 | 11,7 |
32,2 |

Insulated with 1.5 mm thick cloth | 10 | 70 | 0,91 | 9,1 |
40 | 0,52 | 5,2 |
14,3 |

Table 4:

Example of energy loss in ten pipe brackets.

Insulating the pipe brackets also gives a small saving.

Let's say we can reduce the heat loss by 250 watt through a better insulation.

If the central heater is heating for 500 hours per year, this leads to a saving
of 125 kWh.

At an energy content of 8.8 kWh per m³ gas, and a
heater efficiency of 85 % , the saving will be 16.7 m³
gas per year.

Below some pictures of the insulation I applied in connection with the
measurement results.

I choose to keep the original pipe insulation intact, and apply an extra layer
of glass wool around it.

I removed the glass wool (with permission) from a house which was being
demolished.

It was free, and it also reduces waste, these are also kinds of savings.

Insulating cloth between pipe and pipe bracket.

As we see, the pipe insulation wasn't fully applied on all places.

On other places, the pipe insulation was applied over the pipe brackets, but
because of the extra thickness of the brackets this led to a chink in the
insulation.

By applying the extra glass wool insulation, these points are now also warmly
packed.