Heat detectors- old and new-

[kurnal]

This one is just out of interest.
When I was a lad,  heat detectors used bi metal strips and different metals whose expansion rates in concert and in opposition were harnessed to give a fixed temperature and rate of rise alarm.

I remember the moreton instructors of the day, in between dozing and doodling,  telling us that after a couple of years service they were  liable to go way out of calibration and become become hugely inaccurate.

Does the newer technology now use solid state technology and is it any more stable? What is the basis of the newer technology- does it use a variation of electrical resistance with temperature?  Are heat detectors likely to be any more or less reliable than smoke detectors in terms of staying in calibration?

[Ricardo]

I would love to know the answer to this one as well Kurnal, I have just had the pleasure of auditing a large hotel, who have only heat detctors in both the bedrooms and the god forbid  the escape routes, I would have thought in our new world of FRA, and protecting all relevant persons, that this type of detector would now be deemed unsuitable to give a relevant person early warning in the room of fire origin, as would that not be the objective of the FRA?

I have been reliably informed from higherarchy that there is little that can go wrong with heat detectors, although I am not convinced with that answer, I would never come out with such a statement, not knowing all my facts on such a product.

The hotel was advised to install stand alone battery detectors in the meantime until they can get the escape route detectors changed. But as for the bedroom detectors, it seems they can stay, I am very uncomfortable with this.

I believe BS 5839-1:2002 indicates a method of testing heat detectors is with a heat source, but not a naked flame, can someone advise the prefered method of testing such detectors.

[Pete M]

The more recent heat detectors do use a thermocouple device (two actually, one exposed, the other in a controlled 'chamber') configured in what is known as a Wheatstone Bridge circuit.  The electronics continuously monitor the balance between the exposed and controlled element and enter an alarm condition when a preset parameter is exceeded.

Sealed bi-metallic strip type devices are still found in industrial and hazardous area applications due to their inherent mechanical robustness but then again these are supposed to be subject to much more demanding testing and maintenance regimes than would be the case in a commercial or domestic setting.

As for heat detectors being used to provide early warning of potential means of escape problems, the factor affecting tenability is paramount.  In all but a very small minority of situations, the problem lies with smoke affecting the means of escape - heat detectors would be of no use in this respect and smoke detectors must be used.  For the same reason, fusible link dampers alone are no longer permitted to be used to protect means of escape.

Heat detection in the room of origin should be OK - this will still provide warning before the means of escape (common corridor) is adversely affected and will also avoid false alarms due to smoking and water vapour from the en-suite shower etc.

[Wiz]

The electronic component most commonly used in modern automatic heat detectors for sensing temperature is known as a thermistor. The resistance it exhibits to an electrical current varies with temperature. Since the various complete electronic circuitries, designed to provide the appropriate response required for the class of heat detection response, use no moving parts it is generally considered to be highly reliable. Furthermore since heat is a simple variable to measure, the automatic detector doesn't have the same liklihood of other influences affecting it, as would, for example, an automatic smoke detector monitoring particles in the air that can be 'confused' by dust or other particles.

[Wiz]

.......I believe BS 5839-1:2002 indicates a method of testing heat detectors is with a heat source, but not a naked flame, can someone advise the prefered method of testing such detectors.

Obviously a naked flame could generate so much heat it could damage the detector housing. Even the products of combustion from a naked flame are likely to discolour/contaminate the detector housing.
Traditionally, fire alarm engineers have carefully used a hot-air gun (paint-stripping etc type) to test heat detectors although there are now a number of speifically designed products on the market for testing heat detectors with less chance of damaging/contaminating the detector.
All these tests are a basic test of operation but not of calibration (or more properly, in respect of heat detectors, response to fairly specific criteria) but this would be equally true of testing a smoke detector with a simple aerosol type tester.

[Wiz]

Heat detection in the room of origin should be OK - this will still provide warning before the means of escape (common corridor) is adversely affected and will also avoid false alarms due to smoking and water vapour from the en-suite shower etc.

All electronic equipment can be affected by excessive water vapour unless it is very well sealed against water ingress in some way. Pete M's advice is basically spot on, unless someone misreads it and believes detectors could be installed in or very close to a shower room and where excessive water vapour is likely to be encountered!

[Ricardo]

Heat detection in the room of origin should be OK - this will still provide warning before the means of escape (common corridor) is adversely affected and will also avoid false alarms due to smoking and water vapour from the en-suite shower etc.

Thanks for the above comments, and also from Wiz, but I still feel uncomfortable with having a heat detector in the hotel bedroom, as it surely wont give a quick enough response for the protection of the relevant person in that bedroom, BS 5829-1:2002, as far as I can tell, isnt taking into account all relevant persons in such a scenario, unless you are elderly or infirm then it seems to say the use of a smoke detector would be more apt.
It is more concerned with protecting the escape routes themselves, as opposed to the person in the room of fire origin?

I fully understand the reasons behind for decades now, having heat detection in bedrooms, and no doubt stats will tell us people have not been dying in hotel bedrooms of any quality since the introduction of the FPA71.
But can a  RP's FRA really justify that they are providing adeqaute protection of all relevant persons with the use of heat detection in bedrooms?