Accuracy of Nest TPI control?

[MJNewton]

Hi everyone,

My first post here, although I've been reading with fascination many of the threads in the archive.

For many years I've been using a Honeywell CM927 thermostat that has managed rock-solid control of room/house temperatures. You can see the stability in the graph below - noting in particular the grey line representing the temperature in the room with the stat an the red line showing the temperature of the air being extracted from the house by an MVHR and hence representing something of a whole-house average. The heating isn't on overnight so it's the heating periods (high sections of the graphs) that's the area of interest. Even squinting at the graph reveals a very obvious change in stability on Sat 30th Jan and that point being when I swapped out the Honeywell for a Nest...

(Here's a link to the full size image - the forum-attached image is really small: http://www.newtonnet.co.uk/permanent/preandpostnest.png)

Both the Nest-read temperatures and MVHR-read temperatures show a fair bit of oscillation which leads me to believe the TPI control of the Nest is nowhere near as effective as the Honeywell. Or am I being too impatient in reaching this conclusion and should give the Nest longer to learn the responsiveness of the house and heating system?

[MJNewton]

Just to follow up on this for the sake of the archives (although the lack of responses suggests it's only me that's had an issue!) I've found that the Nest's control of the temperature has improved somewhat now that's it's been installed for a week or so. Perhaps it did just have to be given a bit of time to learn the response rate and in the meantime was using some fairly liberal settings.

[mikey8156]

I am not sure if the TPI control is self learning.

[MJNewton]

I am not sure if the TPI control is self learning.

Are you sure? The plots definitely show clear improvements in stability over time so I'm wondering what else it could be?

http://www.newtonnet.co.uk/permanent...t-longview.png

[kevinsmart]

I don’t think Nest employs TPI in the same sense as the CM927, switching on/off in 10 minute intervals.

But it’s True Radiant algorithm does learn how long your room takes to heat up or cool down and from memory, when I had one, eventually uses a very narrow maintenance band, something like 0.6C.

I also switched from a CM927 to Nest when they came out and was initially disappointed by its temperature control.

[MJNewton]

I don’t think Nest employs TPI in the same sense as the CM927, switching on/off in 10 minute intervals.

Can you expand on that? How else could TPI work if it's not pulse-width control? Nest describes itself as a Class IV thermostat and hence 'An electronic room thermostat that controls both thermostat cycle rate and in-cycle on/off ratio of the heater proportional to room temperature.' I don't know if the Nest employs a 10 minute cycle (unlike the CM927 you can't see/adjust it) but it must surely work in the same way regardless of cycle length.

[kevinsmart]

There’s an explanation here:

https://support.google.com/googlenes...-radiant-works

Whereas with Honeywell TPI where there is a regular on/off cycle when within 1.5C, Nest learns when it should turn off early to meet the set-point and later when to turn back on.

In practice in my house at the time it would be off for over an hour which didn’t suit some of the colder rooms in the house. Whereas the Honeywell thermostat kept pulsing to balance the heat loss.

[MJNewton]

when I had one

I forgot to ask: what do you have now? Evohome? I actually thought this was an Evohome forum when I first arrived as it seems to attract the lion's share of discussion.

[MJNewton]

Whereas with Honeywell TPI where there is a regular on/off cycle when within 1.5C, Nest learns when it should turn off early to meet the set-point and later when to turn back on.

I think you might be mixing up two aspects within True Radiant? Google uses the term to cover not just the TPI element but also what others call Optimum Start.

Whereas the Honeywell thermostat kept pulsing to balance the heat loss.

That's exactly what my Nest does - sometimes pulsing for only a minute which is annoying, not least given 20s of that is just the zone valve opening and boiler preparing to fire.

[kevinsmart]

I forgot to ask: what do you have now? Evohome? I actually thought this was an Evohome forum when I first arrived as it seems to attract the lion's share of discussion.

Yes, Evohome