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The thermal oil Feroline U in Boiler is not gaining the desired temp of 240 Degrees. We have physically measured the temp using digital thermometer at a distance of 20 cm where the boiler temp sensor is installed. 40 degrees are lost in 20cm insulated area. The boiler is well insulated with Rockwool & oil quality is good (recently tested).

What could be the reasons? Any solutions please? enter image description here

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  • $\begingroup$ When physically measuring the temperature, is the temperature of 240 deg supposed to be the temperature of the metal tube underneath the insulation or the temperature on the atmospheric (outside) side of the insulation? Essentially, are you measuring the temperature correctly? $\endgroup$ – Fred Aug 12 '18 at 17:19
  • $\begingroup$ @Fred 240 deg is temp shown at A (on HMI) it is the temp of sensor that is installed inside the boiler, we measured the temp at point B correctly, exactly at the metal tube inside the insulation. Yes we measured the temp correctly, making sure we are not measuring the temp of insulation. $\endgroup$ – Jahangir Aug 12 '18 at 17:57
  • $\begingroup$ "Yes we measured the temp correctly" Sorry, I don't believe you. A digital thermometer tells you the temperature of the thermometer probe, not the temperature you are trying to measure! Unless you drilled a blind hole into the tube at point B and sealed the thermometer sensor inside it, you are measuring some unknown combination of the tube temperature and the surrounding air temperature. you must have made a hole in the insulation to get the thermometer probe close to the tube inside, and that hole will let the outside air get to the thermometer probe! $\endgroup$ – alephzero Aug 12 '18 at 20:11
  • $\begingroup$ @alephzero The digital thermometer probe is very thin in size & is quite very sensitive & accurate. We were getting different temps,relatively higher,while measuring somewhere inside insulation say 210 degrees, but when the probe was hit against the tube it read 200 degrees. You are right, the method we are using is an approximation, but difference of 40 degrees between point A & B not understandable. $\endgroup$ – Jahangir Aug 12 '18 at 21:05
  • $\begingroup$ So what happens when you measure the temperature with both methods at the same location? i.e. take the distance out of the equation. $\endgroup$ – hazzey Aug 15 '18 at 14:21
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There are a couple things that could be going on, but most likely it has to do with how and where the temperature is measured rather than some sort of unknown heat loss. People are being critical in the comments, because the two measurements (based on the info given) should be nearly identical.

Seeing your replies in the comments, I am assuming both your temperatures were measured with the same measuring device, placed between the insulation and the metal pipe/vessel, and given time to reach equilibrium.

I assume A is your vessel and B is your outlet.

  1. Is B in fact the Outlet and not the Inlet? It is sometimes easy to mistake the flow through a plumbing system. Things are also not necessarily labeled correctly. It is very possible B is the Inlet to the heater and this temperature difference is the heat that is being delivered to users as planned. This is the most likely scenario with the info I have.
  2. Have you checked temperature B in different locations about the diameter? Maybe the flow is all to one side or there is rust/scale near the measuring location.

Have you checked temperature A in different locations on the vessel?

  1. How close is point A to the heating element? (not sure of the energy input type, but the location/proximity of the electric heating element or heat exchanger will make a big difference. This is also a likely scenario.)

  2. How close is A to the inlet? (this is where the cool oil returns in a closed loop system) (this is more likely to make A colder than B, but something to think about as well)


Other things to consider is that the steel thickness at B may be thicker or the insulation may be thinner. This results in your measurement being closer to ambient temperature than the the measurement at A. When we place a thermometer below insulation we are making the assumption that the insulation is much much more resistant to heat flow than the metal. Most the time this assumption is true, but it is something to consider when looking for accuracy.

If none of those items help you track down the issue please include info like flow-rate, energy source, energy users, etc.

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  • $\begingroup$ Thanks for your time & detailed answer. Both temp are not measured by the same device, temp A is the boiler thermocouple & Temp B is the one we measured physically. A is boiler thermocouple location & B is 20 cm away from boiler thermocouple. Yes we have measured temp B on equal distances like 30cm on the whole pipe (B), the temp almost same between points. A is 1.5-2m away from inlet. Flowrate 0.5 cubic meter, Thermal oil Feroline is energy source, energy users are heatexchangers & coolers. $\endgroup$ – Jahangir Aug 14 '18 at 8:33
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If the equipment sensor is in a well, which it should be, remove it and put it in boiling water. Now put the equipment sensor in cooler water and check temperature. This will confirm span and accuracy, assuming all readings check, for the equipment sensor and indicator system. While performing the above take readings with your portable thermometer also. Record and compare the readings. Draw valid conclusions based on the data.

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