Room thermostat hysteresis

I think an electronic 3 wire might suit you better. Unlike mechanical ones you can set the hysteresis in software and they will generally be a lot tighter.

E.g., this looks a nice replacement for a mechanical and isn't 'smart' and suffering from 'creeping featurism'. Claims ± 0.6°C

This kind of thermostat often has 'anti-hysteresis' built in, in the form of a resistor of 220K or so across the switched AC physically near/below the thermal switch. This may have gone high or o/c with age.

Indeed. The resistor needs to be between switched live and neutral so that when the heating has been switched on by the stat, the resistor gets slightly warm and warms the temperature sensor which has the effect of reducing the hysteresis.

The OP should check: a) that there *is* a neutral wire, and b) that live and switched live are connected to the correct terminals and are not crossed.

If they were crossed, the resistor would be heated permanently - not just when the heating is on - and that would defeat its purpose.

No help, just a comment. My dad had one of a similar age which used a Mercury tilt switch operated by a bimetallic spiral. The temperature setting knob just rotated the assembly as this was easier than altering the Earth's COG.

If yours is the same, you should only approach it with full Hazmat and breathing apparatus, and definitely not release the Mercury and flick it around on the bench like we used to in physics class.

Thanks for the suggestions. There is a neutral wire and what looks like a 470k resistor as an anti-hysteresis measure, and wired up the right way around, but it's not mounted very close to the bimetallic strip, so I suspect it's never been all that effective.

I'm inclined to get one of the ESRTD3 "digital" room stats that was suggested by The Natural Philosopher. They are only £22-99 if bought direct rather than via Amazon, so not a large investment to try one out.

I'll report back if this works out.

Ah, I have not come across that type. Fortunately my existing thermostat just has a bi-metallic strip.

However I do just about remember playing around with mercury in class - but I have a vague feeling that the element itself isn't as dangerous as many compounds of mercury. If it was then we all escaped lightly.

You could also have a programmable stat - so the chance to have a number of different preset temperatures throughout the day to better match what you are likely to be doing etc.

Please. Id like to know if they work OK

Clive Page snipped-for-privacy@page2.eu wrote in news: snipped-for-privacy@mid.individual.net:

In case you've not bought yet I'd second John's suggestion of a programmable stat as a slot in replacement for your existing one which will offer better control & economy.

Whilst you can pay a lot for these, Salus models offer decent functionality at a reasonable price eg this one for 40quid:

I had thought of programmable control and they may suit some, but our activities vary so much from one day to the next that I can't see any way of taking advantage of programmability except by continually re-programming, which slightly defeats the idea.

On 09/12/2025 20:07, fred wrote: offer better control & economy.

I've had the radio version one of these for several years. It works as a thermostat very well, although I don't require a timing function as I just carry it from the living room to the bedroom and back again and set the temperature I require accordingly. It's very reliable and the batteries last a long time.

However, it's ergonomically not the best design I've ever come across. If you pick it up it's almost impossible to avoid touching one of the six buttons on the front. You'll suddenly realise there's been a change to the display, but you're not certain exactly what you've done, and therefore how to get it back to what it was showing and the particular function you had set. And that stand at the back soon comes loose (mine's held on with blue tack!), and if you're not careful when you put it down you'll find it's front-heavy and falls forward, touching one of those damn buttons again.

Clive Page snipped-for-privacy@page2.eu wrote in news: snipped-for-privacy@mid.individual.net:

I totally get that, it's not for everyone.

I use a base program and then use overrides to cater for variations in my schedule. I have an older property with thick walls and high thermal mass so I need to have a base heat on for most of the cooler months.

eg. I have a warm base temperature set during the day with a cooler one programmed to kick in overnight. If I'm going to be in all day then I can simply adjust the demand temperature up from base. For extended absences I'll use the holiday setting which will control at a lower, frost avoidance, setting.

You will know best what will suit you.

Jeff Layman snipped-for-privacy@invalid.invalid wrote in news:10ha7oc$pese$ snipped-for-privacy@dont-email.me:

Yup, they're cheap for a reason!

Honeywell wins hands down on useability but I hate that you can't override the mimimum cycling rate (3 per hour on the last one I had, 6 default) which I have found takes its toll on motorised valves (their own). Their zebrastrip connected lcds tend to fail far too frequently too. The Salus controls well and to within a degree on it's simple on/off mode but it's hardly elegant.

The spring coil with the mercury switch capsule, has an adjustment. What we would call "hysteresis", the inventor calls it an anticipator.

If the heat anticipator is malfunctioning, your furnace will short cycle and the room temperature will not reach the temperature displayed on the thermostat."

The thermostats actually have manuals (if you can find them). This is the traditional design, approved by Rube Goldberg hisself.

But at least in principle, that "nub" you are supposed to fit a pencil tip into, to make adjustments, that allows more or less current to flow through the heater and affect the response.

Until I read this stuff just now, I thought there was an adjustment for hysteresis, but it involved something "pressing" on the spring. But according to that, it's some sort of heat-related thing achieved by a current flow.

Red (power) White (call-for-heat) Yellow (call-for-cooling) Green (fan) Blue/Black (common)

If they want to heat something, the power is available on Red and Black to do it. But the tiny transformer in the heating device, is as far as I know, impedance protected, and likely has a limited output current for fiddling the thermostat that way.

In the video here, a guy with well-worn fingers explains what the anticipator is doing.

The thermostat in both cases, is triggering at 69 and 71, and those values are related to the temperature setting knob. A two degree difference.

Via "anticipation", the cycle can be extended or shortened, according to that rotary adjustment of "current".

The thermostat does have hysteresis (2 degrees) but that aspect is fixed, and the anticipator further adjusts the cycle rate via shortening the cycle. (The temperature in the room is lower than the temperature inside the thermostat housing, when it trips off.)

# Checking anticipator draw, adjusting anticipator knob

The meter the guy is using, is here. He appears to be using AC amps on the dial adjustment.

A digital thermostat does not need a Rube Goldberg implementation.

But a digital thermostat could require a battery, and as well might have a different number of wires running to it. You want to read up on it, before buying it (download the PDF manual for the stat you like the look of). I got mine home, opened the instructions, and realized my cable from upstairs to the heating, didn't have enough wires.

Paul

I believe it's not good to breathe the vapours around it.

Some mercury is sequestered in the environment, and it is bad to stir it up. Once stirred up off the bottom of a water course, it gets back into the food chain.

There are some kinds of fish you should not eat every day, because of the mercury level.

"Swordfish has an average mercury load measuring 0.995 ppm and highest loads measuring 3.22 ppm"

"chunk light (skipjack) tuna having much lower amounts (around 0.12 ppm)"

"Cod has low levels of mercury, generally averaging around 0.111 ppm" [bottom dweller] [not available, never recovered]

"If a child eats 1 serving of brook trout in a week, no other fish should be eaten that week." {More to do with the child than the trout] That is likely to be a reason that swordfish was not available when you wanted some. Rather than scarcity being the only reason.

Yes. And you may already have a timer on the boiler etc.

I thought cod were predators, not bottom feeders. And the mercury is/was from what they fed on.

"Species of fish that are high on the food chain, such as shark, swordfish, king mackerel, bluefin tuna, albacore tuna, and tilefish contain higher concentrations of mercury than others. Because mercury and methylmercury are fat soluble, they primarily accumulate in the viscera, although they are also found throughout the muscle tissue. Mercury presence in fish muscles can be studied using non-lethal muscle biopsies. Mercury present in prey fish accumulates in the predator that consumes them. Since fish are less efficient at depurating than accumulating methylmercury, methylmercury concentrations in the fish tissue increase over time. Thus species that are high on the food chain amass body burdens of mercury that can be ten times higher than the species they consume."

Trouble with mercury is that unlike say radioactivity, its a cumulative poison...

We care not very good at excreting it and neither are the fish.

That suggests, that the essential neutral has not been provided/connected. Does it only have two wires, plus earth connected to it, live and switch wire? Idea of the neutral, was to power a small resistor, which when it switched on, provided some heat to the mechanical part, to reduce the on/off hysteresis. Perhaps, that resistor has failed?

Even if it hasn't the hysteresis is still quite large.