weird behavior of the fan on my Bosch Home Professional tumble dryer, i'm curious to learn about it

[wholelottared]

hello everyone!! i've been using my Bosch Home Professional WTX87EH0TR tumble dryer for almost a year now, and I kept noticing new thing that I didn't notice before.

it always acts differently about this, and it's not something that it constantly does, which makes me wonder what's the logic and explanation behind it.

it's about the fan that's at the bottom right of the device.

don't know if it makes a difference but i always use this program: cottons with "low heat" setting.

here's a list of the different behaviors it does:

1. fan never turns on during the program.

2. fan turns on only right before the end of the program, on the last 11 minutes when AutoClean section starts.

3. fan turns on sometime during the program, and lasts throughout the whole program. it doesn't run continuosly though, but more like fan works for 2 minutes, stops for 2 minutes, then repeats.

4. fan runs through the ENTIRE program, never stops, and continues even AFTER the program is finished. like the program is complete, everything ended but the fan is still running. it stops after you open the door or turn off the machine. this is my favorite one, because it dries at a really low temperature this way, and the laundry comes out not even warm but still fully dry. wish i was able to choose this behavior manually somehow.

5. during the program fan is constantly running without stopping, but the condenser/compressor keeps turning off and on repeatedly. this one is the most weirdest one, because it only happened once, but i wasn't even sure if it was a glitch or not. i thought it was weird glitch, so i cancelled the program, took off the plug, waited a little bit, restarted the program, but then it did the SAME thing. so maybe it wasn't a glitch and maybe it was sensing something weird with the electricity or something. the compressor was running for like 15 minutes then the compressor was stopping even though drum continued to spin, and then compressor was starting again after some time, then repeating. as a result, the drying time was extremely long (like 2.5-3 hours, which normally would've ended in like 1 hour). everything was dry at the end, but it was the weirdest behavior ever that i couldn't understand why. it never did it again. the fan was working even after the program ended on this one too.

so, what could be the logic behind these behaviors? another thing i noticed is that the fan is pumping air inside the device, it's not pumping air outside. i realized this by putting a towel to the grid at the bottom, and it was holding the towel still, instead of blowing it away lol. is this the expected design or is the fan spinning on the wrong way or something?

sorry the post was a bit long, but i know that there are many professionals on this site so i would be soooo happy if someone can explain these all to me, and the logic behind them. my guess is that they are either glithces, or they are something that machine decides according to the electricity or the humidity of the room or something? i don't know, want to learn the details.

thank you so much!!

 

[gizmo]

This is a heat pump dryer. The airflow in the dryer is a closed loop - the air that dries the clothes is recirculated through the heat pump unit and the revolving drum. It is never blown out of the dryer.

 

There is a fan at the back of the cabinet which exists only for cooling the working components inside the dryer - removal of excessive waste heat. It only comes on when needed - the computer that controls the whole dryer switches that small fan on and off as needed.

 

Its operation will vary depending on the cycle, the temperature of the room, the size of the load, the starting temperature of the load, the moisture content of the load, and other variables.

 

It is probably operating correctly.

 

[me]

Don't own one, but I'm pretty sure that fan at the front is an electric fan to cool the compressor by blowing air from the room over it and will only turn on if the compressor gets over a set temperature. If the compressor's temperature exceeds the recommended maximum when the fan is already running, then I'd expect the compressor to shut down briefly, it might also do that if the temperature of the air in the drum gets too high.

 

[me]

Here's a tear down of a heat pump dryer if you are interested how they work. It's actually a Samsung one, but it will be broadly similar.

 

[brisnat81]

Generally that second fan is to remove heat from the refrigerant system to help with keeping the temperature correct within the air stream and to help with condensing the water.

My experience is only with the Mieles, but depending on the model there is either a condenser, or a series of hot refrigerant pipes. The fan is computer controlled to help remove heat from the system, whilst still allowing the cold side to be cold.

On the Mieles it’s generally only will run after the first 20-30 minutes and then it cycles on and off. During the cooldown phase it runs almost continuously.

 

[henene4]

BSH dryers (Bosch, Siemens, Neff, Constructa) of that timeframe don't have that secondary heat exchanger.
Very few current dryers actually have - efficiency measure.

However, as heat pumps got smaller and smaller, the pure heat transfer with that bit of airflow is enough to manage thermals under operating conditions.

That fan is temperature guided - but should behave the same on the same cycle.
However, many factors can alternate behaviour - room temp, dryer temp, load size etc.

 

[wholelottared]

thank you for all the replies & to henene4

thanks for all the replies!!

i have one last question, can you please check the number 5 on my post? what do you think was the reason for the condenser/heat pump stopping and then continuing repeatedly on that cycle?

 

[logixx]

Too hot

That happened to me one time when I was cycles back to back. The dryer was still hot from the first cycle, so the heat pump shut off twice during the seconds cycle. All the while the cooling fas was continuously running, venting pretty hot air.

Makes me wonder how some dryers get away without a cooling fan. I suppose it's the inverter heat pump in the likes of LG and the Haier Ultra Reverse.

In bens Appliance and Junk's video, the LG compressor even has sound insulation around it.