Are there any thermostats with a good range of user adjustable hysteresis?

[DIYNotIan]

Hi

Sorry if the title is a bit vague. I'm struggling to think of the best terms to describe or explain this, and as such I'm not finding out much when googling either....

What I want to know is.... If a room thermostat is set to say 16c, presumably it will heat to above and below this. I'm sure I once read that someone had theirs set at 16c but programmed to heat up to 18c and then not come again until it dropped to 14c. But that was a long time ago so I can't remember for sure.

If setting a range / parameter / envelope is a thing, how is it usually achieved? At the thermostat? Or at the boiler or heating controls?

If anyone could set me in the right direction with the correct terms and some basic info, I can get back to google and ask more meaningful questions!

Many thanks

 

[Swwils]

This is called the hysteresis and is an old fashioned way of thinking about heating control.

 

[JonathanM]

The technical term for that behaviour in a thermostat is hysteresis. Thermostats always used to turn off a bit above their set point and come back on a bit below. Often it's called the switching differential, or just differential. But I've also seen it called swing, span or lag. With most thermostats you don't have any choice as to the amount of hysteresis, but a few do. With mine you can choose between having it +/-0.25C or +/-0.5C.

However, most modern thermostats work a bit differently using an algorithm (like TPI) to give more precise control, or they vary the flow temperature from the boiler instead, to maintain a steady room temperature (weather compensation or load compensation).

 

[ian1182]

All thermostats need some degree of hysteresis. If they exactly switched at the set value your heating would be switching on/off for a very small change in temperature. If your set value was 16C and the thermostat went off at say 16.1C and came back on at 15.9C that's not good. It's better if it cuts out at 16.5C but does not switch on again until 15.5C. Modern smart thermostats better understand room characteristics and switch the heating on/off to maintain a steady temp, but even here they have some hysteresis.

On a hot water tank thermostat the difference is often 5C. You can hear the clicks by rotating the dial.

 

[DIYNotIan]

Great, thanks for those quick replies. Now that I've got the correct term, I've changed the thread title and in the meantime will start to look into it myself. But if anyone knows of any that wold be a great help. As much as +/- 2c would be excellent, so long as it can be adjusted to other ranges as well.

Many thanks

 

[JonathanM]

What is it that you are wanting to do? Do you want a programmable thermostat where you can set different temperatures at different times of the day. Is it going to be hard wired or wireless.

Most people want close control of the room temperature, so that they don't notice the difference too much!

 

[DIYNotIan]

No, I don't need any other type of control or "programmability". Just the adjustable hysteresis.

Our current thermostat is wired. I'm not sure how much work would be involved in changing to wireless, so I'm not sure but wouldn't rule it out (depending on cost etc).

Most people want close control of the room temperature, so that they don't notice the difference too much!

Our needs are quite different to the norm, and also I tend to feel heat differently than the average person!

Many thanks for your reply.

 

[Swwils]

Your heat source is far too hot if you are getting such wild swings. You need to match the heat input to the heat loss.

Modern systems and controls are more focused to this (thermostats are designed for bygone era where the heat source could not modulate).

Accurate room temperatures are all that matters.

 

[DIYNotIan]

I'm not sure what you mean tbh. I'm not getting wild swings. The hysteresis is something I want, not what I have.

My system cannot modulate, it is an old oil boiler which is either on or off.

Personally, I find it hard to get to the right temperature. I'm usually either too cold or too hot. As such, I'm tending to be a manual hysteresis appendage to my thermostat! And that suits me fine and is how I like it. I'd just prefer it to be automated....

Many thanks

 

[JonathanM]

I actually looked into this for myself a few months ago. I only found one model these days on sale in the UK, with that level of adjustable hysteresis, and even that is a discontinued model. It's the Drayton MiStat (N) and it still seems to be available from Screwfix and City Plumbing. It is expensive, though, and some reviews suggest it sometimes doesn't last very long before breaking. Also, it seems to be wireless. The only other model I found was an Italian brand called Seitron, which you could get through EBay. I can find the model if you want. And @Johntheo5 had another Italian brand with adjustable differential.

I am only working from the manuals, though! It is possible that other thermostats might do be able to do this, but the info is not in the manual. I think that's unlikely, but I just thought I should say. The replacement for the MiStat is the Digistat range, and I couldn't find anywhere in the manual to adjust the differential. But it might be worth contacting Drayton to make sure.

The thing with hysteresis is that, because of overshoot, it always gives more hysteresis than you set it to. If you set it to +/- 1.0C, you might end up with +/- 1.5C or more in practice. Also, it depends where it is located. If you place it in the hall +/- 1.0C might mean the living room is effectively +/- 2.0C. EPH for example make a thermostat with a smaller adjustable hysteresis.

This is the parameter table for the MiStat. Parameter 9 is where you choose the method of temperature control and the amount of hysteresis. Full manual below.

https://www.draytoncontrols.co.uk/sites/default/files/UG. IG. MiStat N_0.pdf

 

[Friday round]

I remembered my old Honeywell DT90 have various settings that I think will work for what you need in the installers configuration menu. Look for bandings and offsets.

https://www.plumbworld.co.uk/documents/honeywell-dt90e-digital-room-thermostat-data-sheet.pdf

 

[JonathanM]

I remembered my old Honeywell DT90 have various settings that I think will work for what you need in the installers configuration menu. Look for bandings and offsets.

There are some useful settings on the DT90 but I don't think you can actually adjust the hysteresis. You can set the proportional band and the cycle rate and also the minimum on/off time.

 

[muggles]

Heatmiser Neostat has adjustable hysteresis. A better solution is to get a control which is intelligent enough to learn the rate of temperature change in a room and turn on and off to suit. Turning your flow temperature down might help too

 

[MJN]

A Shelly Plus H&T and paired Shelly 1 smart relay would provide exactly what you’ve asked for as the combination separates out the temperature detection and response, the latter achieved through a completely customised rules-base approach (e.g. on below 14C, off above 18C as per your example).

However, I would hesitate to recommend the setup for a couple of reasons:

1) It’s not so quite so ‘plug and play’ as a normal off-the-shelf single purpose thermostat and so requires a bit more setup, programming etc.

2) I really don’t see a positive outcome from the path you looking to take. A wide hysteresis cannot possibly lead to high comfort levels (or energy conservation, but you haven’t mentioned that as a motivator but it is also a consequence of hysteresis) and what you are seeking is the polar opposite of what thermostat designers are trying to achieve.

My suggestion would be to go for a decent TPI thermostat (eg one of Honeywell’s offerings) in order to benefit from the highly stable temperature control that their algorithms are capable of achieving even with an on/off binary heat source. If the stable room temperature still doesn’t feel comfortable for you - ie one minute you’re feeling too cold, the next too hot - then no amount of automation will be able to compensate for what is a physiological condition (and I’m not mocking you in saying that; it is a fairly common condition and arguably one that affects us all to a greater or lesser degree). Or is there additional context behind your question you haven’t mentioned (quite possibly unknowingly!) that might be relevant to perceived comfort levels.

 

[oilhead]

What you are experiencing is very normal. There are so many other factors that affect temperature control in a room. Your thermostat is probably working to an acceptable differential, but it can only sense temperature at its location. I have a digital thermostat, and I also have a digital clock that shows temperature and pressure but in another part of the room. Sometimes, both read the same temperature, but at other times they can differ by 1-5oC. I find the best way is to rely on your own senses, and in some instances i feel cooler than the temp, in others I feel hot, and then there is is usually two of us in the room who feel differently. And then, 19oC is sometimes warm and other times quite cold. The most consistent performing I've ever known was on a warm air system that was controlled by a return air sensor.