So, I've finally figured it out.... (apologies for the ultra-long post, but there's no point in doing a half-explanation in this case...!)
Ok, so the pressure switch (vacuum switch in my case) is not used to detect a blockage, but instead it's actually used to detect the build-up of ash.
The initial error-code on my stove's display showing '
ALAr deP FAil' should have been a warning for me to get the flue and exhaust cleaned. But in my case, it was actually a false-alarm, as it was the damaged pressure switch itself that had an intermittent fault.
How the system works....: The build-up of ash is detected by very clever use of
Bernoulli's principle. See the attached picture (taken from from tec-science.com website) showing a demo of the
Venturi-Tube in action. Ash build-up in the exhaust-pipe will cause a reduction in the effective internal diameter of the pipe. In this region, airflow will speed up due to the restriction. In turn, this will cause a drop in pressure at this point (yes.. a
DROP in pressure... read up on Bernoulli...!) And, if you happen to have a vacuum-switch connection-port at this exact point, then the low pressure will pull on the diaphragm, open the switch contacts, and the stove will correctly go into safe-mode.
To verify this, I did a little experiment. I simulated the build-up of ash in my exhaust-pipe using a custom made insert, which simply consisted of corrugated cardboard and play-dough modelling clay. (Yes... I remembered to remove these flammable materials before re-installing my stove...!) I paid attention to getting a good seal between my bespoke insert and the leading-edge of the exhaust pipe, and also a good seal around the small connection-port protrusion for the flexible switch-pipe. I wanted all the airflow to be directed through the centre of the insert (not bypassing it by going around the outside). Next, I directly wired the exhaust fan to mains power. This forced the fan to run at max RPM, which would be the case during the '
Foyer Cleaning' operation. (This cleaning action happens regularly during normal use of the stove.) Sure enough, when the fan came up close to max RPM, the pressure switch contact opened, and correspondingly, soon after turning off the exhaust fan the contacts closed again. I repeated this operation several times for confirmation. After removing my homemade insert, normal operation was again resumed. This design is ingenious, as you get lots of pre-warning ever before a complete blockage occurs. The only downside is that there are certain conditions where '
fail-safe' operation could be missed.
It was interesting to inspect the design of the exhaust pipe in the Perla. Tracing along from the fan end to the flue end, it transitioned from a rectangular 4536.5mm squared cross-sectional area shape, down to a circular 4185.5mm squared area shape. At the welded junction there was an abrupt change in shape making it more likely for ash to be deposited in the round pipe section where the port connection pipe attaches.
Just to point out a few more details:
(1) In the pressure switch datasheet, pay attention to the mounting instructions. In my case, for reliable operation it was best for me to mount the switch in a secure vertical position, and not at some odd angle.
(2) Some stoves will have their pressure-switch connected to the exhaust with two pipes/hoses rather than just one. Two pipes just means the
differential system is used.
(3) Some systems use NO rather than NC switch contacts, so it's worth labelling all wires
before disconnecting them.
(4) Ensure that your replacement pressure-switch has the same pressure trip-point rating, max-pressure rating, and the same electrical ratings as the one it's replacing. (Regarding electrical specs, be aware that some are rated for high mains voltage, while others are only rated for e.g. 30Vac).
For further reading it's worth checking out the following blog pages:
https://wowman.altervista.org/how-to-solve-pellet-stove-pressure-switch-alarm-depr-sic-fail/
and
https://wowman.altervista.org/how-to-test-pellet-stove-pressure-switch/
. . . . . . however, in this blog, I would recommend
skipping the part advising you to blow compressed air into the pressure-switch ports to clean them out. The delicate internal diaphragm will be damaged, and this is how I ended up in this predicament in the first place...!
Anyway, I hope this helps out anybody having pressure/vacuum switch issues. The world needs more well maintained wood-burning appliances rather than oil/gas/coal ones which are far from carbon-neutral.
Paul.
