A look inside an old Honeywell thermostat.
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- čas přidán 31. 01. 2016
- When I moved into this house it had an old traditional thermostat controlling an oil boiler. One of the first things I did was replace it with a wireless digital thermostat/timer, as it gives more control, is easy to install (no wiring) and allows more efficient use of fuel through more precise programming and control.
When I removed the old thermostat I thought it looked interesting enough to make a video about. - Věda a technologie
We had the same model when I was growing up but it had remote control. The remote function worked by my Dad bellowing " HEATING " and I would go turn it up or down depending on what the last setting was. I still have the nightmares....
Your dad is a cruel but clever man
Ha Ha... Much loved and missed. A truly kind man was he but your comment made me laugh, it's perfect....
;)
Yep, between my sisters & I as kids my dad had multiple multi-purpose remotes and my mum even had a remote that went round the local shops for odds & sods, notably including cigarettes for her and the occasional cigar for my dad! Them were the days eh!
Clive, you found a keeper. The 'bellows' (in mechanical engineering terms) is an _aneroid capsule_ and you are correct, they are quite accurate. The temp change expansion/contraction is the basis of energy input for the Atmos clock. No spring, battery or mains input, the only power input is air temp. I recommend everyone look them up and admire the elegance.
+dieselscience I've often wondered how changes in barometric pressure effect the temperature setting on that type thermostat.
I took apart a thermostat once, and found a mercury switch inside.
Which I re-purposed to sound the air horn on my bike if moved off its side stand.
Stop showing off fella!
The resistor is used to reduce the hysteresis of the switch. When on, it reduces the switch off point by local heating.
+BersekViking it is only on when the unit powers up the boiler right? not running the resistor all time time?
+BersekViking That's what I thought too. A 'wee bit' of positive feedback to avoid chatter. Very clever.
jusb1066 Yes, only when switched on.
Called an anticipator resistor.
Couldn't believe it when I saw the picture of this, we had one of these on the wall in the back room of the house I grew up in. Not seen another since the mid 90's
Hi Clive thanks so much for explaining the workings of the stat. the penny dropped for me when you pointed out the excelerator resistor as was trying to understand my old T40 honeywell which also has a bellow type inside .. Great video!!
Honeywell refer to that type of 'stat as a "vapor filled dual diaphragm". The vapour in the one I last took apart was from a small charge of a volatile liquid hydrocarbon, possibly chlorinated. I've always assumed the reason they're used is to give a faster response as the thermal inertia is a lot less than that of an equivalent bimetal element.
The little resistor isn't "to generate airflow". It's switched with the heating load and has the effect of reducing the switching differential from around 2C to something more comfortable. But does need a neutral connection which means 3+E cable.
Note for faded stickers etc:
UV torch as well as an IR torch (or some DIY contraption to the same effect).
UV and IR response can be remarkably different, even for faded ink.
I like these vintage teardowns. Really cool stuff.
I recall taking a similar-looking vintage unit apart and found that inside it had a coiled metal strip (sort of like a clock spring) with a mercury tilt switch on the end. The thermal expansion of the metal would change the length of the coil and change the angle of the end of the spring. the knob for setting the temperature just rotated the coil so that its tipping point would be different.
I still have an original 1958 honeywell controlling the heat in my house! Works perfectly.
Really enjoyed that one, thanks Clive, I really do need to get out more.
We use to have a oil furnace too. The massive tank was in our basement and had been there since the house was built in the 50's. Needless to say the tank had to be cut up and taken out piece by piece. Something around the size of a full size propane tank. The last years of it's life before we updated the hvac around 2000 it had to be manually started due to broken part's. I believe it as well had a Honeywell or GE thermostat. The furnace we got in 2000 was so unrepairable we had to replace that. Funny how vintage appliances last way longer...
Thanks of all the videos BigClive! Your in my top 10 youtube favorites and defiantly my favorite electrical/electronics/chinese eBay channel. Keep it up.
As someone else has likely noted, the classic Honeywell (at least in the USA) is circular. That is a later model. I think that lookalike ones have been made more recently than the classics, looking superficially alike but I think with different guts.
Honeywell was taken over by Allied Signal, which then assumed the Honeywell name. This is rather like a horror movie in which a beautiful woman has a much bigger monster slip inside her as if she were a costume, somehow fitting inside, and assume her identity... very creepy!
My Honewell thermostat is much the same as yours, but has a built in thermometer. It was installed with a new CH system in 1979. The thermostat is still working, but I always reckoned that the dead band was a bit too large.
My Mum's house has a very much earlier Honewell thermostat, about 1964 and that still works perfectly
Bellows are more sensitive, and more accurate for heating/cooling, do you remember the days when car cooling system had bellows before waxstats ?
bellows are used in a lot of older industrial heating/cooling systems, we had a lot of danfoss controls where i worked, bellows also allow for a pressure pipe to a bulb for remote sensing, or as a pressure switch on fridge plants.
I never got around to trying it, but the bellows could pop if overheated (hint) :D
had one of these in our house when we were kids in 94. it was really shabby but still worked fine. Within in minutes of a new one being installed i had stripped it.
Clive needs a Twitter account, over the past year or two from watching, the amount of subscribers are flowing thick and fast now! Brilliant video like usual, we all get to learn a few new tricks from this.
Ha! We have that one, been going solidly for past 35-40 years. Reminds me of my dad, wish I could show him the video.
I wonder if our modern thermostats will be functioning in 40 years time? I know places where these old Honeywells are still in use.
+Choppington Otter Probably not. But the extra functionality and convenience means they will probably pay for themselves several times over per unit.
+bigclivedotcom Do you really save all that much? I mean, if the house cools down, it will require more enegery to warm back up again.. There are inertia in the house, but once it cools it will require even more energy to warm. Unless you live in a unisolated house. :) So in my mind it's best not to have big temp differens over time. Maybe just 1-2°C at night for comfort. I guess the way you warm up the house matter alittle, maybe. But interesting video as always. :) Thx
+Choppington Otter Yep those old ones just last forever.
+kruppin I put in a Honeywell CM707 which is a digital thermostat and timeclock. It made a big difference, noticeable fuel saving and keeps the place at a far more even temperature. It profiles how long it takes your heating system to respond to avoid overshooting etc.
NanoCottage Yeah, ofc it helps for things like that. But I mean about what clive said in the video, that he lowers the temp in the house when he is not there and then take it higher for when he comes back. Which in my mind will cost more than keeping temp constant, like you (and me) are. :)
We had one of these in the family house which was a new build back in 1975, the childhood memories I can still picture it on the hall wall :)
The diaphragm is cool, wasn't expecting that amount of expansion / travel from it at all.
Hi, thanks for this video! I have the same thermostats in my house in Finland, and one just stopped regulating, is behaving now as on/off switch. I have a electric ceiling heating. House was built in 1980. I guess that the bellow is damaged and is not expanding anymore with the increased temperature. I have ordered the new modern programable thermostat, but I must say these ones seems to be pretty accurate.
The trouble with bimetallic strips is that they take a long time to heat up or cool down, so they don't react quickly to temperature changes. An alternative design of thermostat senses temperature changes more quickly using a pair of metal discs with a gas-filled bellows in between. The discs have a large surface area so they react quickly to heat. When the room warms up, the gas in the bellows expands and forces the discs apart. The inner disc pushes against a microswitch in the middle of the thermostat turning the electric circuit (and the heating) off. As the room cools, the gas in the bellows contracts and the metal discs are forced back together. The inner disc moves away from the microswitch, switching on the electric circuit and turning the heating on again. That version of Honeywell thermostat was available from the early 70s through around the mid 90s. That one seems to be the 70s-80s vintage. Anyway, from another electronics designer, thanks for the videos, keep up the great videos.
Big Clive,
Look closely at the front of the thermostat. To turn it UP you turn is ANTI clockwise. This operation is still used on the Honeywell thermostats made nowadays.
I realize this video is 18months old, but something I didn't see mentioned is most thermostats for furnaces (at least in North America) DON'T operate on line voltage, they operate on 24VAC. There ARE line voltage thermostats, but they are usually for baseboard or in-floor heating, and the wiring used is standard 12gauge, vs. the 20gauge used for 24VAC systems. Most wireless thermostats (like the Nest) operate on 24VAC systems only...I had the toughest time trying to find a wireless thermostat that would operate on line voltage.
Seem to remember the power station where I worked having quite a considerable number of these little units knocking about! We overhauled quite a bit of 11KV, 3.3KVand 415V switchgear. Wish they were that simple to dismantle and set up!
Its hard to express how much I need a sticker or plaque that says "Lets Take it to Bits" in my office at the lab. My philosophy on life to a T.
+Lettuce Or maybe a T shirt that says "hold on, I might need to use violence"
Utterly fascinating. Have you thought about doing a series of videos where you explain what some of the basic components are for those of us who don't know the difference between a capacitor and a resistor?
We have a very similar unit in our kitchen, with individual switchnig for heating and hot water. I give it an annual (ish) service (blow the dust & spiders out of it and put a tiny bit of appropriate lubricants on moving mechanical parts) It is one of the more modern 'bits of the house'. I will probably replace it if it ever gives up but as long as it keeps working there is no need to change it, We live in a mining village in the Ayrshire coalfields & still using coal fire for all heat & hot water so an electro-mechanical clickety switchey oney offey thing is pretty modern for over here ;-) Enjoying Your videos, Keep up the good work Clive :-)
I found this video really interesting. We still use one of these with a 30yr old glow worm boiler. I wonder at the minute if it is faulty as there is the click at the lower end to turn it off but it appears to have lost it’s higher region click and although it is set to 25 it definitely isn’t 25 in the house. I had hoped to do a straight swap replacement as like yours there would be a rectangular gap were I to put in a newer square Honeywell. Alas I don’t seem to be coming across one and may have to wait until later in the year when we replace the boiler.
Interesting video nonetheless
We had one of these in our living room until about 5 years ago when I replaced it with a more updated mechanical thermostat, never thought to take the vintage one apart to be honest. I remember when I was a kid seeing a one with what I can only assume was a mercury switch inside or something? I just remember a glass bulb of some sort with mercury in so I guess that was it (memory is hazy.) I haven't come across one since, but would be really interested if you could do a video on one. Loving the videos you've uploaded, really learning a lot, thank you :)
My House in Australia had that exact thermostat, to control the ducted AC. The ac unit was installed circa 1983. so that can give a very rough time frame. And yes it was mains voltage wired in to it at 240V
It may be old but it's robust, reliable, and doesn't phone home to an offsite server telling God-knows-who when the house is not occupied.
I have a Honeywell wifi T-stat and it has 2 'features' that piss me off. The Wifi portion is cloud w/account only (not mentioned anywhere on the box ) that forces all usage data and wifi functions to be funneled through an internet connection and an offsite server. The other stoopid bit is it's locked at 1 degree temperature differential. This will cause your HVAC to cycle a lot more often and wear out contactors / compressors and short-cycle older furnaces. I apologize for the mini-rant. I think folks should be aware that the latest and greatest is not always the best for you.
If I had that I'd block the wifi access through my network until it could be replaced by something with a better attitude...
Mines still in use! Works perfectly. Always has done.
I still use my old one from 1987! It works great!
A black-light might help you read the label a little better. There is something so elegant about old electrical assemblies like this.
#bigclivedotcom
I have been watching your videos for the last 3 days and I can safely say that I have learned more, than in a year of fiddling with (destroying) electrical/electronic devices.
Thank you so much.
WOW you like a cold house. My flat is at 24c 24x7x365. That is in a country that swings from 40c in Summer down to -18c in winter. Double wall cavities etc help. 10c does not 'keep out the chill'. 10c is the chill.
Oh! That clicky switch... in the age of touch screens, One truly appreciates a nice, beefy click... *rewinds again*
The house I grew up in had one of these and it was pretty old then. I'm an 80's kid so I'd think it'll be 70's tech.
The electrical resistor is there to establish a model for the heating system and makes this controller into a feedforward (as opposed to a feedback) controller. It is usually referred to as an "anticipatory" circuit. Cheers
My parent's have one with the bi-metallic strip inside it controlling the boiler. Still works after all these years!
The problem with a barometric thermostat is they are barometric and will change the range of operation with the weather. They were an early attempt at removing mercury from out of the hands of the common peons who apparently don't know that eating mercury switches or thermometers is not the proper and accepted usage for them. They then introduced Hall effect and optical switches but since most people have never heard of them it demonstrates how well they failed. The most reliable thermostats are still the mercury capsule switched or magnetic tripped micro-switch when they are used with the correct hysteresis unit/adjustment.
The problem of the thermal mass effect is offset by the higher heat dissipation rate of maintaining the higher temp 24/7. Less so if the building is well insulated as the cool down rate will be extended. If the building is well insulated and leak free the savings from setbacks will be much less and the recovery time is shorter.
Thanks to China not really giving a toss you can still buy new mercury switches on eBay.
And thank Pasta for that. They were declared dangerous therefore they must be dangerous, just like lead solder.
Perhaps the best thing about chingeneering is that many specialty items like thermostats are now more reasonably priced even from big name manufacturers. And then there is the addition of stores like Dollar Tree and Poundland that wouldn't be around without eastern goods. Yes, I know, there were five and dime stores and discount houses before but most of them had closed or severely down sized before the interwebs opened an easily accessible channel to the worlds market place and information. And we need more stuff to take to bits.
My school uses this brand of thermostat. But they look older than this one, as they have that curvy 60's look. Oh and we're still using boilers and radiators
We use that resistor for lowering the room temperature when we leave the house. So right next to outside door is the switch and turning it to not home position energy goes to every thermostat resistor and room temp goes 5 degrees lower approximately.
These thermostats are actually a ambient temperature controlled PWM controllers. The pulse period is in maginitude of few minutes. The resistor and the pressure capsule form an oscillator and the difference between setpoint (preload on pressure capsule) and ambient temperature sets the on/off time ratio (= pulse width). This way the heat transfer delay from radiators to thermostat is compensated. Without the resistor the radiators will get really hot until the thermostat switches them off.
The typical bimetallic models had an adjustable anticipator resistance so you could control the overshoot. My electronic one has a swing setting to do the same thing.
I thought I'd hit the wrong channel & John Ward had suddenly got a Glaswegian accent when I first stated watching this vintage electrical stuff :)
despite it's issues and how it'll take longer to respond, less functionality ect, I can't help but love that honeywell thermostat for that brilliantly simple design.
We moved into our current house in 1988, the Glow-worm boiler and Honeywell thermostat, fitted sometime between 1976-1988, before we bought the house, are still in operation. The Danfoss Randall programmer has only been replaced once in nearly 32 years of living here.🙂
we had that exact model in our house when we moved in!
I came here expecting mercury, dangit! Clive, you let me down!
+Alex Barlow Ebay has mercury tilt switches in bulk from China if that's a help?
... I guess the hazmat shipping wouldn't be applicable to that quantity...
+Alex Barlow Mercury shouldn't be shipped on planes at all as even a drop can cause serious damage to aluminium structures by defeating aluminium's self repair characteristic. (instant formation of oxide)
But mere things like that don't bother China.
I've seen Mercury switches installed new as late as 2003 - not in the HVACR industry so could be they still install em heh.
In Canada 24 Vac, and the heater was referred to as an anticipator
nice demonstration
We've only just changed one of these thermostats for a modern wireless one. Very interesting!
Any videos?
@@gokayguran4506 no lol
I've learned something new. Intriguing.
here in the US old fashioned Mercury switches in thermostats were used in air conditioner units in older houses before 2003 and I was born in 1992
I have one of these old style barometric thermostats and had to repair it after the armature had been snapped off. I was getting very hot due to the micro switch jamming on. I'd love a digit replacement.
Still have one of these on the wall downstairs controlling the central heating. Lost its accuracy when the face fell off and was stuck back on with no reference so it also works as an on and off switch.
Still got one of those on a wall now....'old faithful'!
Bought a digital one Years ago but never found a reason to replace it....
Interesting. I would have expected a mercury switch in a thermostat of that style and age, but then again, in the US, the majority of residential thermostats run at 24v, not mains voltage. I have an old Honeywell Chronotherm in my house that is 20+ years old and still working
Im guessing 1970's for that particular Thermostat. We had one in my house when I bought it in 1983.
I remember these.
I was going to ask how old they are but thought I'd look through the comments first.
Thanks!
I remember putting these in when I was apprentice back in the early / mid 70's Clive
I pulled apart a very similar looking one, but with a slider for temperature, and it used a mercury bulb with terminals at each end. When the mercury inside expanded it made the contact. Adjusting the slider would tilt the bulb so that it would change the temperature at which the contact was made.
They look vintage but it seems they still make these. The one I had was made in 2004!
Clive when I was younger, I remember reading about phreaking and all those lovely coloured 'boxes'. Did you ever come across any? Would be nice to see!
+Smmmile:) Let's just say that I had a scary moment when they clamped down and I was phreaking some calls to friends and a police car suddenly turned up outside the phone box. I was using a technique where I had a stack of coins on the top of the phone that I pretended to be putting in during the call, so they drove off after a while. scary though, and the technique I was using was blocked within days....
I was on a ship that was fitted with a similar Honeywell thermostat for the Aircon. The switch was a mercury tube type. You can imagine the problems when the ship rolled. Needless to say, all were replaced.
My old farmhouse (built in 1910) had one of those before we upgraded to a digital but not smart thermostat.
It is neat, simple and doesn't need batteries. I like it.
We had one of these at our old house that controlled a fan in the wall that moved air between rooms.
So much better than the modern ones that are so damn complicated to set and use, also the new ones use batteries. Many elderly people have huge gas bills because of modern thermostats that are too complex. These older types are the best.
Resister is to provide hysteresis. It stops the thermostat constantly cycling between on and off when it is near its set temperature by heating some part of it. (pretty much all mechanical thermostats have one). The modern replacements may be more versatile but they are vastly more expensive and I doubt they will last anywhere near as long. KISS applies here and I like the old ones. They just work. It's similar to how old non condensing system boilers just work and keep on going for decades and the modern condensing combi's break. down. constantly. then need complete replacement after 10 years because "we can't get the parts sir".
+pklongutoobe Actually it's the other way round - it's to reduce hysteresis.
Hey Clive, you might be interested in having a look at these fancy Fast Charge power supplies for compatiable mobile devices, which can alternate between 5v and 9v.
The round diaphragms used to be filled with freon gas up to early eighties when it was change to butane. The heat anticipatory resistor cycled the themostat on and off as the room temperature increased, significantly improving the control temperature and reducing overshoot.
There are THREE different voltages used on thermostats in the US. Which one you use is dictated by the controller on the furnace. Aside from main voltage and 24 volts, there is a low voltage version, which uses the low voltage provided by the thermocouple which is heated by the gas pilot light. These are fairly rare, but can be noticed by the fact that there is NO 24 volt transformer mounted on or near the furnace.
The resistor is used as a heater to make the thermostat turn off a bit sooner, thus narrowing the temperature gap between heat on and heat off.
I'm fitting a wireless thermostat tomorrow,they are all the rage now.If I get a job that requires a new roomstat I normally by the 240v model,works on 12v,24v & mains voltage
I live these old things, I have an old pic of one. What modle is it?
I still have the same units on my wall that control the underfloor heating. Mine are labeled Nettle as the make.
The resistor is a heat anticipator. It purpose is to heat the bellows a little while the heat is on to stop heat glide. If it wasn't there the heater would overshoot the setpoint. Most mechanical thermostats had adjustable anticipators that were set to the control system amperage so the heat level it produces would be the same for all installations.
+Ken Wolfe What is heat glide?
I am from Australia. We do not have things like thermostats, it is just air conditioning hard wired on 100%.
+Godfrey Poon, in a fossil.fueled heater, the interior heat exchanger where the burners send hot gas and flame, the metal gets hot. When the thermostat shuts off the burner because the room temperature has raised to the setpoint of the thermostat. The system can't just shut off completely.. The heat exchanger is still hot and is holding a lot of heat. The blower needs to fun to cool off the system and also push the residual heat into the conditioned space. This extra heat that is pushed into the room is called glide. The heat anticipator or in this case the resistor heats up slightly to get the thermostat to shut the burners off a little bit before the setpoint is reached. The heat that needs to be removed from the system will then finish raising the temperature of the conditioned space using the blower, to the setpoint of the thermostat. The new digital thermostats compensate for heat glide by learning the extra heat rise and deciding when to shut the burners off ahead of time.
+Ken Wolfe Ah that explains it! The instructions for a satchwell bimetal thermostat said that you didn't have to connect the neutral if you were using local electric heaters (the thing was rated for 20 amps resistive!) The lack of heat glide as you call it would be the reason. Thanks!
It'd be interesting to hook it up and see how accurate it is after all these years :-)
Thank you for the video! I want to control an old thermostat by a microcontroller. In fact, I just have to demonstrate that I can control the set temperature of thermostat by my microcontroller (microcontroller-relay-thermostat). I wonder the wiring of the relay and thermostat. Should I wire the relay in point 1 and 3 (in the video)?
If it's an identical thermostat switching between 1 and 3 that sounds right. Keep in mind that it may be switching mains voltage.
Hi... I just moved into a flat, and it has this type of thermostat and I want to change it, can you help me pls
My old house had a Mercury Thermostat. A little glass vile of mercury on a big spring so as the spring heated and cooled it moved the mercury as a switch.
Very reliable. A lot of products suffered from the banning of mercury switches.
I have one that looks similar thats is actually still the control panel for our heating floor but we can't seem to get it to work? Or turn on
they typically have a refrigerant charge in those bellows because of the equil expansion rate of the gas.
Thanks for the Video.
I have the same one in my house out side the kitchen door and we have to turn it ON/OFF Manually all the time. Its pain full to get up middle of the night.
Winter is just hitting here in UK. Im thinking to change it with some modern Thermostats, Can i Change it with Nest ? Im very poor with wiring and i think Honeywell takes just two wire straight in Red and Black. Do you know if Nest Thermostat would be easy to replace it with old wiring ?
You'd have to know the voltage (possibly mains voltage) and you'd need to use a thermostat designed to operate with just two wires. Another option is to use a combined wireless timer/thermostat and fit a receiver in the heating unit itself.
There seems to be some misunderstanding about the purpose of tne resistor, the heat from the resistor makes the stat cycle around the setting of the control knob and the ambient temperature. This gives a much better control and reduces the hysteresis due to the long lag when heating a building. When the stat is used to control electrical loads of some 2KW or so, there is sufficient heat generated inthe internal wiring etc and the connection to the resistor becomes unnecessary
This is how an accelerator heater works.
Imagine the thermostat is set at 20°C. Typically it will have about 2°C hysteresis, meaning that it turns on at 20°C and off at 22°C.
If the room is at 18°C it is below temperature and the thermostat calls for heat. It turns on the room radiators, and also the accelerator heater. Soon the temperature in the thermostat is above the room temperature and the thermostat reaches 22°C when the room is at about 19°C, at which point the radiators and the accelerator heater are turned off. The early turn-off is important because there is hot water in the radiators when they are turned off. This hot water raises the room temperature from 19°C to the comfort temperature of 20°C.
Of course, as soon as the thermostat got to 22 °C and turned off, the thermostat started cooling down to the room temperature which was 19°C when the heating turned off and is creeping up to 20°C. If we've got things right, as soon as the room hits 20°C, so does the thermostat, and it turns on the hot water supply to the radiators so they are getting hot just as the room's temperature is falling below 20°C.
The effect of the accelerator heater is to turn the radiators off before the room gets to the set point temperature, and turn them on again before the room temperature drops below the set point. It's called an accelerator because it accelerates the thermostat, making it operate, on or off, before the room temperature would normally do so.
The accelerator heater does not reduce hysteresis. It uses the thermostat's innate hysteresis to reduce hunting, the variations above and below the set point that would happen without its presence and results in a much more uniform room temperature.
Many modern control systems use 2-wire low-hysteresis thermostats with hysteresis around 0.2°C and no accelerator heater. They may seem better but they are not ideal. A typical wet system is slow to respond. It is best controlled with a slowly-responding thermostat system.
Having said all this, modern wet systems have thermostatic radiator valves throughout and it is these that do the main controlling with a single zone stat in the coldest place to turn the whole system off when the coldest place is up to temperature.
Cool how that works
I still have one of these for my Central Heating :D :D
omg, I have this exact model in my own home!!!
I used to have that exact thermostat :)
Lol I have this exact thermostat in my house, along with a Potterton boiler.... bought many years ago for several thousand pounds.
my mum worked for this company for her entire working life up to retirement. as a kid I would play with the thermostats...I really don't know why...I guess it's what kids do !!!
What - no mercury tilt switch? This thermostat may have been a later design, when concern about mercury in the environment began to be a concern. Some old Honeywell thermostats of the 1960s-1970s had a mercury switch in a glass vial that would tilt when a bimetal coil unwound with temperature changes. The mercury blob provided hysteresis (non-adjustable), and the sealed nature, in combination with switching low voltage and current, gave them essentially infinite service life.
I have the same one, but the under floor heating is not working. Perhaps replacing the thermostat may fix it, any idea of one to replace this unit with?
I still have that exact thermostat in my house. xD
That's an early 1970's stat, i would think 71 or 72, I have seen them in both metric and imperial. In fact I saw an imperial one of these yesterday still working well.
Honeywell isn't really vintage. They are very much still in business and among many other products still makes state of the art thermostats for the home. In the US we don't use mains connected thermostats, only low voltage .
Removed one of these just yesterday!