Is TfL losing the battle against heat on the Victoria line?
113 comments
·May 25, 2025azernik
brookst
Yep. That 30% is a bad use of statistics.
28 degrees Celsius is not 30% warmer than 21 degrees Celsius. This same stat rendered in Fahrenheit would say 70 degrees -> 82 degrees, or 17%. In kelvin it would be 294 -> 301, or 2.3%
Or we could invent a new measure indexed to Celsius but offset by 20 degrees, and declare a 1 -> 8 change, a whopping 700%.
Y_Y
Feynman was complaining about this error appearing in textbooks back in the sixties[0].
The trouble (of course) is that Celsius properly is not a proper unit, but a "scale", or a "unit of difference" (equal to kelvin), or even torsor[1].
The trouble with the kelvin here is that if you see the 7 kelvin increase as a proportion of the 295K starting temperature the you only get a 2% increase. Nobody is going to buy your newspaper if you're putting up weak numbers like that.
[0] https://mathematicalcrap.com/2024/03/05/the-feynman-story/ [1] https://math.ucr.edu/home/baez/torsors.html
buran77
To make matters worse, not all ranges and percentages on that scale are equal, whether they're the same in absolute or relative terms. Humans have a narrow relevant "operational" temperature band. Even 20 degrees between 10-30C feel like nothing compared to the 5 degrees between 37-42C.
VBprogrammer
Not to mention that wet bulb temperature, measuring the effect of humidity, is actually the most important measure in those temperature ranges.
hnlmorg
You’re right in principle but that’s probably the worst example you could have given. So bad an example that I think it could easily be argued to disprove your point.
azernik
Then just don't use percentages, and rely on people realizing that a 7 degree difference is big!
OJFord
It would definitely be crazy in Fahrenheit, but in centigrade I think it makes some sort of intuitive (if not scientific) sense. (Together with the sea-level assumption we always make in casual temperature discussion anyway.)
azernik
It makes just as much intuitive sense in Fahrenheit as it does in centigrade.
detourdog
Reading this quote made me finally realize why the name centigrade exists. It’s a gradient scale of 100.
tim333
Maybe because I was brought up with centigrade it makes more sense to me. The centigrade number is how far you are from water freezing. If it goes up 100% then you are twice as far away. I'm not aware that doubling the fahrenheit number has a similar easy to understand meaning?
perching_aix
Why? The slope of the Fahrenheit scale is different to the Celsius and Kelvin scales, but the slope of the latter two does match.
strken
In both it makes a sort of intuitive sense. 7% of the way from freezing to boiling is a meaningful way to visualise temperature; 7% of the way from ice melting in a bath of salt to slightly above Mrs Fahrenheit's armpit temperature is also meaningful, although perhaps a little idiosyncratic.
Edit: this comment was deeply stupid for obvious reasons and I regret trying to interact with other people when I should be asleep.
movpasd
The issue is a percentage of a Celsius value is not that. For example, an increase from 1°C to 2°C is a "100% increase", but is only 1 percentage point from freezing to boiling.
zeristor
Yikes. I posted this, and I missed that, something I realised soon into my first year physics degree lab. I learnt more than just dipping calculators in liquid nitrogen for fun.
I apologise.
nelgaard
And they manage to make it even more crazy by also comparing it to average external temperatures.
== The Victoria Line average temperature in August last year was 60% higher in temperature than the average external temperature that month, measured at 19.5 degrees. ==
Certainly for January it must have been hundreds of percent higher.
And what would the numbers be for e.g., the Moscow metro in winter months where the average outside temperature is negative?
AStonesThrow
So how many decibels louder is it now?
moomin
I don’t know if I’m worried about it. While the measurement makes little scientific sense it makes intuitive sense, and, importantly, the intuitive implications are the scientific implications.
It’s a huge increase, if not for the reasons they describe.
StopDisinfo910
Is it? I think it puts the Victoria rise in perspective to the other lines quite effectively.
Everyone knows where the zero is in Celsius using countries anyway and days in the negative are so rare in the UK you can discount them (plus they are none inside the tube).
ch33zer
What if trains brought their heat with them instead of leaving it in the tunnels? I'm thinking some kind of large seal like thing in front of and behind trains that keeps the train surrounded by the same air. Stations serve as a spot to dump hot air and blow in cooler air. I guess the problem is it would take a lot of energy to push all that air around. Also the trains would be hotter.
Y_Y
> Historically, the Underground infrastructure offered a respite from warm weather, indicated in Austin Cooper’s ‘It is cooler below’ poster, issued in 1924 by the Underground group to promote a more comfortable experience of travel during warm weather.
A century of burrowing commuter-worms unfortunately managed to bake all the beautiful wet clay that kept the tunnels tolerable when the sun was shining about.
It seems straightforward to me that it would be enough to rehydrate the ground. Just need (approximately),
400km of track * 25m average depth * 3m tunnel width * 20% moisture content of wet clay
= 6 billion litres of water
avianlyric
I don’t think the hydration of the clay is the important element here. Rather I suspect it’s simple just the sheer mass of clay, wet or otherwise, that’s involved.
There’s a reason why ground source heat pumps work so well. It’s because the ground is such a fantastically huge heat sink/source that in most scenarios we consider it capable of sinking or sourcing a practically unlimited amount of heat.
Unfortunately one of the scenarios where this breaks down, is when you stick a bunch of tunnels in the ground, then pump a crap ton of energy into those tunnels years round, and expect the ground to sink all the heat away. Turns out, if you do that, the ground itself starts heating up, and given that clay is a reasonable good insulator, it’s like wrapping all those tunnels in wool jumpers.
I would point out as well that all these tunnels are “deep level” tunnels running at an average depth of 24 meters and getting as low as 67 meters. The heat of the sun on the ground surface will have approximately zero impact on the tunnel temperatures. 24 meters of clay is a lot of insulation to work with.
heisenzombie
<man walks into sauna room> Ooh, it's a bit hot in here! I better throw some water on these rocks to cool them down.
Joking aside, I actually don't know how dry it is in the underground, and therefore whether adding water for evaporative cooling would work. I would have assumed it was quite humid, but maybe not?
ajb
It seems implausible to me that the clay is dehydrated. The Victoria line was only built in the 60s and has a waterproof lining. (It's also built with asbestos cement, unfortunately, which is no doubt a problem when they need to cut it for whatever reason)
lloydatkinson
Have you thought of suggesting this to TfL? There has to be something here.
thyristan
Unfortunately, hydrating clay is extremely hard to do. Clay is what you use as a water-tight material in dams, artificial lakes, waste dumps and stuff like that, because water doesn't really pass through it.
rjh29
Yes, people whose day job is to explore all possible options have surely missed this extremely obvious idea.
chiph
Looking at the temperature chart and the significant drop in 2020 during the pandemic, the source is certainly the trains and people themselves. (fewer trains moving, less heat added back then). At this point I expect the infrastructure is heat soaked and will need a prolonged period of cooling to bring temps down. i.e. don't expect instant results.
Moving more air through the tunnels, adding A/C systems - both have a problem of needing room up on the surface for blowers and compressors, something that is hard to do in modern London. Tough problem.
euroderf
Big fans to pull surface air down into the tubes when the tubes are warmer than surface ambient ? Cool the tunnels, warm the surface.
avianlyric
Where there’s space, this has already been done. Unfortunately for many lines there simply isn’t space.
Just about every abandoned station, elevator shaft, and maintenance tunnel on the network is already fitted out with huge fans where possible.
TfL also runs a semi-continuous works project that looks a custom and novel one-off cooling solutions that can be retrofitted into whatever space is left. Including complicated hydronic systems that pump around huge quantities of water where the infrastructure allows for it.
netsharc
I wonder if they can carry hundreds of opened barrels of ice on open-bed trains through the tunnels at night, go slowly and let them melt to water (but kept in the buckets, because you don't want to flood the tunnels)...
Andys
Ice blocks were trialed in the past. https://en.wikipedia.org/wiki/London_Underground_cooling#cit...
thyristan
They could flood the tunnels with an appropriate amount of liquefied air.
meindnoch
This. Brake friction pumps heat into the ground at a higher rate than it could dissipate away.
eternauta3k
Aren't they using regenerative braking?
avianlyric
The trains in London can be up to 50 years old at this point. Where the technology was available during the building of the trains, you can generally expect regenerative breaking. But it’s far from universally available unfortunately.
Zigurd
Some parts of the London underground use passive energy recovery by locating stations nearer to the surface than most of the tunnel between them. Trains start by rolling downhill and when they approach a station, uphill.
zeristor
Believe regenerative breaking is used to supplement Oxford Circus’ electricity supply
Aachen
Isn't heat free energy in a place like London? I know very little about metro systems so please correct me if this is insane: wouldn't the people living above the tubes be happy to get a heat exchanger (passive) or heat pump (active, but takes more of the heat) that prewarms their hot water supply? People still take warm showers and boil tea and rice/pasta in summer, and in winter the purpose should be obvious. If the water comes in at 30 instead of 10 degrees C, you need to add only a few degrees for showers and floor heating
jodrellblank
A problem is the clay surrounding the tunnels insulates them - it traps heat because heat flows through it very slowly. So you drill down and put a heat exchanger pipe down there, you pump heat from 3cm of clay around the pipe and now no heat flows through the clay to your pipe even though there’s a lot of heat still down there.
Your pipe becomes a tiny worm of cold pipe in a big lump of hot clay and you’ve done very little to cool the underground or warm your water. That is, if heat moved easily through the stuff then the problem of heat buildup would be easy to solve but in that case heat wouldn't build up so there wouldn't be a problem; and vice-versa.
aaron695
[dead]
hnlmorg
The problem isn’t so much finding uses for the heat, it’s getting the heat out of the tunnels to begin with.
These are some of the deepest tunnels going under some of the most built up parts of the UK.
BJones12
I wonder if an extremely tall subterranean windcatcher [0] with its bottom at the top of a tunnel could passively cool the tunnel.
avianlyric
These basically always exist around the network. But that simply isn’t even close to moving enough energy to make a difference.
tobylane
There is at least one on this line (north of Kings Cross) and one on the Northern line (north of Moorgate). It's for district heating or electricity generation.
gruez
>wouldn't the people living above the tubes be happy to get a heat exchanger (passive) or heat pump (active, but takes more of the heat) that prewarms their hot water supply
Ground source heat pumps are expensive to build, even more so in a dense area like London. So even if everything you said is true, I suspect the juice isn't worth the squeeze.
cranky908canuck
I think you are correct.
In this case, though, the excess heat is a major burden, so there is room to negotiate with a district thermal provider that pays that provider to absorb and redistribute the heat, as long as it's less than the cost to pump it out to the environment.
I'm not saying it's easy (it will likely be a bespoke solution). Given the organizations, I expect the difficulty to be as much business (setting the prices) and political (defending the prices set) as technical.
joshlk
What do you do in the summer when the homes don’t want the heat?
crote
That doesn't have to be a problem in practice.
The entire issue is that the earth surrounding the tubes is acting as a giant buffer. Enough heat has been dumped into it over the years that it has permanently warmed up. Draw heat from it during the winter to warm up homes, and it'll be able to absorb more heat from the tunnel air during the summer.
TheOtherHobbes
And because it's permanently warmed up, the long term consequence is the line becomes a health hazard and has to be closed for increasingly long periods.
When wet bulb > body temp people start getting heat stroke, which leads to fainting and potentially death - a bad look for a public transport system.
The likely remedy is to install gigantic refrigeration units in the ventilation shafts and pump in cold air. This will be hugely expensive to build and run.
But the alternative is a tube line that can't be used. So there may not be much choice.
Aachen
It won't be zero so spreading it across enough people might already solve it. If that still leads to insufficient demand during the hottest weeks, idk, it's energy, surely there's something useful you can do? Store it for next week, pre-heat water for the nearest steam engine (e.g. gas power plants are steam engines running on methane, so if they have to heat the water by fewer degrees.. The problem will be finding a steam engine close to the heat source), supply it to an industrial process that needs temperatures above ambient (egg breeding for vaccine production? Idk), create electricity from the temperature differential between this system and the Thames water using the Peltier effect
I've surely got a too naïve view of economics but if the goal were to not waste resources then there will be things you can do before dumping it into the hot summer air
jairuhme
People still take hot showers and use hot water
metalman
Your are correct in principal, though implimenting your idea, now, is essentialy impossible as installing the plumbing after the fact might cost more than just starting over with a whole new line, and would in fact make things much worse durring the many years it takes to find out if the added systems even work. Given that there is only clay under London, it is by far better to start over and build a whole new line, and/or go all in on a mega high tech ,high pressure refrigeration systems for the human occupancy areas, and hope that there are no break downs in the "hot zones" orrible mess
philjohn
Yep ... know people who commute on the Victoria and it doesn't sound fun. I've had to get it when my usual commute on the Northern line was impossible (only once, thankfully) and it was horrendous.
And the Northern line is no picnic either.
philshem
In case you also couldn’t guess the “f” in TfL, it’s Transport for London.
moomin
You can blame Blair, I think, for the fashion of putting “for” into the names of administrative organisations.
devnullbrain
In case you missed the 4 places it's made explicit in the graphs, or 'London' in the URL.
zeristor
I had an idea, perhaps a weird fantasy.
Of a special tube train with blocks of ice. You’d need to have various pits dug in, and pumps to drain the water. Yes water and power electronics is “fraught”.
I just like the idea of trains trundling along, blocks of ice being carted out and gradually melting.
Another idea is to move mechs-bots via Underground in a post-apocalyptic scenario, but that’s not so relevant here.
laurencerowe
This was actually tried: https://www.theguardian.com/uk/2007/jun/05/transport.world
LeoPanthera
This would also increase the humidity to swamp-like levels.
zeristor
Not if it was kept separate a metal box with fins could absorb the heat without leaking any humidity.
Besides at that temperature, more water can be absorbed in the air, so not just latent heat of melting, but heat absorbed in evaporation too.
Of course that would have to be wafted out, and not pumped were it just water.
cherryteastain
> tunnel ventilation installations, chiller systems pumping chilled air into mid-tunnel shafts and regenerative braking to reduce heat generated by trains breaking
The hoops TfL jumps through just to not extend AC to the rolling stock in more lines are baffling. At least we finally got some AC in the new Piccadilly rolling stock.
pitaj
AC will only make the problem worse in the long term. Picadilly got AC because it has above-ground sections.
cherryteastain
You can redesign the signalling systems etc to work at even 40C, plenty of countries do it. You can't redesign humans to feel comfortable inside a stuffy carriage at 35C.
crote
Sure, but that means the stations will also have 40C air. Can the humans handle that? And it's going to be 42C the next year, 44C the year after, and so on...
raattgift
What do you do if some incident halts full trains (possibly depowering them but for things like emergency lighting) near the midpoints of longer sections of 40 degC deep tunnels?
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gilbertjolly
This is a generational buildup of heat, being tackled seasonally.
TfL must run its cooling operations in the winter as well as the summer.
It’s about net energy difference over the whole year.
Finding cold air in the winter will also be substantially cheaper
MichaelZuo
I dont quite understand what’s stopping them from just buying hundreds of chillers, putting them on the surface close to each station, and running chilled water loops down. Other than cost of course.
mnw21cam
They're running trains. Trains use a lot of electricity, and they turn almost all of it into heat. You'd have to have as much chilling capacity as the current electricity demand of the entire tube line, which is quite a lot.
However, if the buildings above were to sink ground source heat pump loops into the warmed ground to heat the buildings in winter, this would basically be what you just suggested, and would be a win-win situation.
MichaelZuo
Huh? Modern modular air cooled chillers go up to 800 tons each and can remove multiple MW of heat load continuously pretty much 24/7.
500 of them could remove 1.4 GW of heat.
Of course there are many ways to improve efficiency, but even assuming the worst case it’s still technically feasible to remove many times more heat than the line generates.
Havoc
Is there a reason why they can't drill a couple bowling ball sized holes at strategic intervals and put some high speed extraction fans in?
Stations entrances are open to outside so if you create enough negative pressure the hottest parts in the tunnels it'll pull in air. Do that long enough and presumably ambient & clay cools?
Presumably engineers dismissed this already, but why?
CorrectHorseBat
They have added ventilation to the tunnels, it's mentioned in the article. It's more than just a couple bowling sized holes, but apparently still not enough
thyristan
Unfortunately, hydrating clay is extremely hard to do. Clay is what you use as a water-tight material in dams, artificial lakes, waste dumps and stuff like that, because water doesn't really pass through it.
CorrectHorseBat
I think you replied to the wrong comment?
isaacremuant
Losing the battle or not even trying to fight? It's
"The average temperatures on the Victoria line have risen by almost seven degrees since 2013 – nearly a *30%* increase.
Conversely, the increase in the average annual temperatures across all Underground lines from 2013 to 2024 was merely *seven percent*, placing Victoria’s temperature rise vastly above that."
Using percentages to talk about changes in non-Kelvin temperatures is crazy.