Room thermostat hysteresis

A follow up as requested. I bought the ESRTD3 thermostat from the ebay-site of esicontrols, where it was £22-99, significantly cheaper than using Amazon. I'm sure Jeff Bezos needs the money, but so do I. The thermostat was posted the same day and arrived two days later, delivered by Amazon, so maybe Jeff got a small cut anyway?

It came with a small instructions booklet and was easy enough to fit, but as expected the backplate had different screw hole positions to the old one so I had to drill the wall (screws and plastic plugs were included). It's set up to imitate a simple on-off thermostat, which is what I wanted, but there are DIP switches that can be used to enable "chronoproportional control" - which is cycling. Since our boiler has proportional output I didn't see any point in the cycling. Another option is a delayed start if the temperature is already close to that requested, but that didn't seem all that useful to us either.

Good points: it displays the current temperature (just whole numbers of degrees Celsius no fractions) on it's display all the time, using an internal battery to power the display for up to 8 hours when the power is off. Turning the knob shows a flashing display of the requested temperature. A flame symbol shows when the heat demand is actually on.

Not so good: the knob just has a scale of 1 to 5, not calibrated to temperatures at all, so you have to look at the screen when adjusting the temperature. One of the DIP switches is labelled "heat" but with no explanation of its function; I left it alone.

We've now had the new thermostat running for a couple of days and it seems to maintain the room temperature to the desired value with perhaps fewer excursions than before, but we've had fairly mild temperature here in the south over the period so it's not yet been put to a serious test. If I understand it right the spec is for backlash of less than 0.6 C. That may be true, it's hard to tell as the display has no decimal places. So: not much dearer than the simplest bi-metallic thermostat and probably a bit better.

Yup. My home rolled ones I had to actually introduce some hysteresis to..but I shoukldnt have worried., By the time the underfloor heating has ramped up it overshoots a whole degree and a half, there is so much thermal mass.

I am quite surprised that anybody still uses room thermostats - I chucked mine out more than 20 years ago and have had TRVs ever since.

Yes, but ordinary TRVs still leave the boiler cycling on its own thermostat when all the room demands are satisfied - rather than shutting it down - which is contrary to Part L of the Building Regs.

I am quite surprised that people still have radiators to put them on . I chucked mine out about 20 years ago

If you've found a brand that actually works I'd like to know. I've had several of these of different brands over the years, and they work for a year or two then somehow jam and fail to operate thereafter. I have used pliers to free up the tiny shaft that moves the valve so it works again for a few months - and then they fail again.

Having a thermostat in such close proximity to the heat source also seems to me to be a poor design choice, so I'm at preset sceptical as to whether they would be effective even if the mechanisms were reliable. But your mileage obviously varies...

The room I am in right now is kept warm by a radiator with a TRV which has been in place for about 25 years - its a Westherm "Four"

To be fair however, I have subsequently been advised to avoid them like the plague.

Theres some anecdotal evidence to suggest that splashing out a load of money on valves is a waste of money and that you may as well make do with the cheap ones from Screwfix, which is what I do now.

Theres no "clarity of expectation" on their service life, but if I have a rummage through my junk box in the garage I would be surprised if I could find more than 4 or 5 dead ones, which is not a terrible return after (I think) 26 years considering that I have 10 radiators.

I have had a few occasions where they have seized up a bit but towards the end of summer I usually flush the system and put in new corrosion inhibitor and while I am at it I check all the TRVs and bleed valves, armed with a pair of pliers and a can of WD40 - I would say that the outlet valves at the other end of the rads are just as likely to seize up as the TRVs

I appreciate your concerns about the proximity to the heat source, but my theory is that there will be a constant movement of cool air which is being drawn in to the base of the radiator - so it might balance things out a bit. Whatever the reasons are, the fact is that as the room warms up, the valve slowly closes until it reaches some sort of equilibrium and the rad remains warm to the touch until the system is turned off.

I am not sure that the boiler cycling is much of a problem, and some people are of the opinion that constant stopping and starting of the pump causes more wear than just leaving it on – its on the lowest setting anyway and allegedly draws less than 10 watts

In the meantime, my 250 year old cottage is exempt from the demands of building regs and remains free of hysteresis

I also found that the tiny shaft will jam, but it works again with the aid of a pair of pliers.

I did work out why. The setting is for the room temperature you require. In the summer you get extended periods when the ambient temperature is higher than that, so the valve shuts off hard and it sticks in that position. By the time I want the heating again and I free off the shaft, the ambient temperature is lower than my desired room temperature so it stays free and working.

What I do now is when I am about to turn off the central heating for the summer I turn it on one last time and turn each TRV to maximum and make sure that the radiator does get hot. Then I turn off the central heating option on the boiler, leaving just the hot water setting functional.

When it comes to the time when I want the central heating again, I turn the Central heating function back on and check that all the radiators get hot, and once they are hot I turn each TRV down to its desired setting (I have marked each one with a permanent marker which saves me wondering what each should be). They all work OK for the winter after that.