Loud AC on modern trains

[OneTrackMinded]

I’ve noticed on some Aventras (345s and 701s), when the AC is on, every few mins or so there’s two loud thumps and then the sound of whistling sound followed by an electrical noise, lasting for at least 3 mins, sometimes longer, in the centre of the carriage. It’s a very odd sound which retracts from the otherwise quiet cabin.

Is this something that other Aventras do, and by extension is it present on other modern fleets? Just one of those things that’s just a characteristic of modern AC systems?

 

[EuxtonNeutral]

Sounds a bit like the vcb opening through a neutral section…? But that wouldn’t be every few minutes.

 

[Skie]

It could be the compressor cycling off as the thermostat is reporting the temperature target has been met. The fans will still run but the systems can make some odd noises whilst temperatures and pressures normalise. Then it'll get warm enough to trigger a cooling demand again, and the compressor will kick back in. Repeat.

Could also be some sort of exterior air vent opening and closing to introduce fresh air into the system, but usually those don't make much noise.

 

[OneTrackMinded]

It could be the compressor cycling off as the thermostat is reporting the temperature target has been met. The fans will still run but the systems can make some odd noises whilst temperatures and pressures normalise. Then it'll get warm enough to trigger a cooling demand again, and the compressor will kick back in. Repeat.

Could be that, although I think it’s more likely to be the AC compressor turning on as it’s paired with the sounds of the fans outside the carriage ramping up.

== Doublepost prevention - post automatically merged: 31 May 2025 ==

Same thing appears to be present on Desiro Cities, albeit without the hissing at the start.

 

[Trackman]

It could be the compressor cycling off as the thermostat is reporting the temperature target has been met. The fans will still run but the systems can make some odd noises whilst temperatures and pressures normalise. Then it'll get warm enough to trigger a cooling demand again, and the compressor will kick back in. Repeat.

Could also be some sort of exterior air vent opening and closing to introduce fresh air into the system, but usually those don't make much noise.

I was thinking the compressor too. Fans still run as you say.

 

[Skie]

Could be that, although I think it’s more likely to be the AC compressor turning on as it’s paired with the sounds of the fans outside the carriage ramping up.

== Doublepost prevention - post automatically merged: 31 May 2025 ==

Same thing appears to be present on Desiro Cities, albeit without the hissing at the start.

Yeah. Different systems make different sounds. Got a portable AC system at home that hisses very loudly after the compressor turns off and then makes plenty of gurgling noises and manages to make noises that do sound like a VCB opening/closing now that I think about it (albeit less...harsh).

 

[43096]

Sounds a bit like the vcb opening through a neutral section…? But that wouldn’t be every few minutes.

But the OP mentioned 701s, which have no VCB as they are DC only.

 

[vicbury]

In modern offices, the trendy ones without ceilings with all the conduits exposed, it's amazing how many different noises air conditioning systems make. Hissing, clapping, banging, all quite common

 

[physics34]

The new Welsh units have extraordinarily loud AC. Disruptive to a quiet journey.

 

[OneTrackMinded]

The new Welsh units have extraordinarily loud AC. Disruptive to a quiet journey.

Which ones? The FLIRTs or CAF units?

 

[physics34]

CAF. Awful on the long run to Crewe when the loco hauled isnt avaliable

 

[tfw756rider]

The new Welsh units have extraordinarily loud AC. Disruptive to a quiet journey.

CAF. Awful on the long run to Crewe when the loco hauled isnt avaliable

Are you referring to this noise on the 197s? If so, do you know for definite that it's the Air Conditioning (AC), and if so, how? Transport for Wales are trying to find out internally for me exactly what it is, as some people in the 197 thread reckon it's something to do with the air compressor / braking systems, rather than the AC.

(recorded from the outside)

It should also be pointed out that the Northern 195s and the West Midlands Railway 196s make the same noise.

 

[physics34]

Are you referring to this noise on the 197s? If so, do you know for definite that it's the Air Conditioning (AC), and if so, how? Transport for Wales are trying to find out internally for me exactly what it is, as some people in the 197 thread reckon it's something to do with the air compressor / braking systems, rather than the AC.

(recorded from the outside)

It should also be pointed out that the Northern 195s and the West Midlands Railway 196s make the same noise.

Yes that's it. It could be the compressor yes but being external it shouldn't be that loud. It sounds like air con units on the roof/ceiling. I could be wrong though.

 

[Class777]

Are you referring to this noise on the 197s? If so, do you know for definite that it's the Air Conditioning (AC), and if so, how? Transport for Wales are trying to find out internally for me exactly what it is, as some people in the 197 thread reckon it's something to do with the air compressor / braking systems, rather than the AC.

(recorded from the outside)

It should also be pointed out that the Northern 195s and the West Midlands Railway 196s make the same noise.

I strongly believe it is AC. I asked a driver the other day whilst one was at Lime Street, apparently it's just the air filtering through the system new air as well as checking cabin temperatures to ensure optimal comfort? This may not be true but based off what I have heard (and also felt when it was hot it did do it more) it does seem true.

 

[tfw756rider]

I strongly believe it is AC. I asked a driver the other day whilst one was at Lime Street, apparently it's just the air filtering through the system new air as well as checking cabin temperatures to ensure optimal comfort? This may not be true but based off what I have heard (and also felt when it was hot it did do it more) it does seem true.

Thanks And of course, air conditioning systems have compressors!

 

[Class777]

Thanks And of course, air conditioning systems have compressors!

Yep, hope it helped!

 

[OneTrackMinded]

(and also felt when it was hot it did do it more)

Indeed, this is the same behaviour as the (albeit a very different noise) present on the Aventras and Desiro Cities.