TPI thermostat and very old boiler

[JonathanM]

I still have a 32 year old FS Baxi 24KW, so obviously non-condensing, and doesn't modulate at all.

I've tried my thermostat on TPI for the first time, to try minimise the overshoot on the standard 0.5C hysteresis setting. Minimum 6 cycles per hour on this thermostat, only other choice is 9CPH. In successive ten minute cycles the boiler ran for 3 minutes, 1 minute and 1 minute. The return temperature never got above 30C.

Bearing in mind previous suggestions not to run these boilers at low temperatures, I'm assuming this can't be good for my boiler? Is there anything to try to get it to run longer. Usually a minimum nine minutes is needed to get the return above 45C.

Pity it doesn't have 3CPH. Basically, TPI too accurate, and hysteresis not accurate enough.

EDITED to clarify: It was the return temperature which never got above 30C. I had originally written flow temperature. Changed above to avoid confusion.

 

[JonathanM]

Forgot to say, it started 0.5C below set point. By the time the second cycle started it had gone up 0.2C. By the end of the third cycle it was bang on set point.

 

[JonathanM]

Sorry to bump my own post. But I'd made a crucial mistake, now edited to explain. Didn't want to confuse people.

 

[Johntheo5]

When in non TPI mode in a roomstat with a hysteresis of 0.5C, is the SP the switch off point of the stat or the switch on with a switch off of 20.5C, or is the switch on 19.5C with switch off at 20C?.

 

[JonathanM]

When in non TPI mode in a roomstat with a hysteresis of 0.5C, is the SP the switch off point of the stat or the switch on with a switch off of 20.5C, or is the switch on 19.5C with switch off at 20C?.

They vary. Mine is called +/- 0.25, so when set at 20C, switch on would be 19.75 and switch off 20.25C. I think most are 19.5C on and 20C off, though.

 

[Johntheo5]

Ah yes, that's where the proportional part comes in so?.... I installed a standard one in my daughters house a few days ago and left the adjustable hysteresis, (0.3C to 2.0C) at its default setting of 0.3C but didn't have time to monitor it but she said she thought it was switching on at her SP of 21C but hadn't monitored the off temperature, the display is in 0.1C increments.
They call it the adjustable differential in the leaflet so possible switch off at 21.3C? (SP+0.3).

 

[JonathanM]

Ah yes, that's where the proportional part comes in so?.... I installed a standard one in my daughters house a few days ago and left the adjustable hysteresis, (0.3C to 2.0C) at its default setting of 0.3C but didn't have time to monitor it but she said she thought it was switching on at her SP of 21C but hadn't monitored the off temperature, the display is in 0.1C increments.
They call it the adjustable differential in the leaflet so possible switch off at 21.3C? (SP+0.3).

Are you sure it's not on TPI? I've been looking again recently, and most thermostats now seem to be TPI only, or at least set to TPI by default.

I think the proportional part is to do with TPI. Usually, until it reaches say 1.5C below SP, it is outside the proportional band, and the thermostat will ask for heat non-stop. Then it switches to the algorithm, and will fire in short bursts, then cut off at SP.

I've read most of the manuals, if you want me to have a quick look.

 

[Johntheo5]

Pretty sure, its a FartiniCosmi CH but might be switching on at the SP-diff/2 and switching off at SP+diff/2, will get it monitored mre closely.

 

[JonathanM]

Pretty sure, its a FartiniCosmi CH but might be switching on at the SP-diff/2 and switching off at SP+diff/2, will get it monitored mre closely.

Have read the manual. I don't think it is TPI. As you thought, it seems to be a standard on/off thermostat, but with an adjustable switching differential. Actually might be perfect for me! The 0.5C differential on mine is too big, but 0.3C should work, as long as the sensor responds quickly enough. I think it would control the overshoot much better. It's not a brand I've seen here in the UK, though.

EDITED (for clarity)

 

[Johntheo5]

I don't think it is TPI, just works on/off with a differential. Actually might be perfect for me! The 0.5C differential on mine is too big, but 0.3C should work, as long as the sensor responds quickly enough. It's not a brand I've seen here in the UK, though.

Its the sensing part that I can't figure out, there is no slots anywhere in the housing, so presume the sensor is behind the display window?

 

[JonathanM]

Its the sensing part that I can't figure out, there is no slots anywhere in the housing, so presume the sensor is behind the display window?

I've never thought about that!

The speed of the sensing seems to be really important.

On further testing, I think I need a 0.2C differential, to give a decent burn for this boiler, but not to overshoot too much. Knowing what I know now, I would actually have bought a Drayton MiStat, which has an adjustable differential, in 0.1C increments, from zero to 5.0C

 

[Johntheo5]

Have just installed one in my own house (FantiniCosmi) so will report back.

 

[Johntheo5]

Watched it for a few cycles, SP 20C. (differential 3C). The boiler fires when the stat indicates 19.9C and cuts out about 1 minute after a indicated 20.1C, so just maybe a proportional band of 3C, ie +/- 1.5C?? the temp then rises to 20.5C, a overshoot of ~ 0.4C, not bad IMO as the ambient is 14C just now.

 

[JonathanM]

Watched it for a few cycles, SP 20C. (differential 3C). The boiler fires when the stat indicates 19.9C and cuts out about 1 minute after a indicated 20.1C, so just maybe a proportional band of 3C, ie +/- 1.5C?? the temp then rises to 20.5C, a overshoot of ~ 0.4C, not bad IMO as the ambient is 14C just now.

That's a lot better than mine.

Mine will hold pretty much rock steady when set to TPI, but I don't think the very short cycles and low temperature can be good for my old boiler.

But when I it change it to the switching differential of +/- 0.25, with an SP of 17C, it comes on at 16.8, goes off at 17.3 - so that's as expected - but then it continues to rise to about 18.5. It's not as bad as the old mechanical ones. But my tests show that if it turned off after a 0.2C rise, that would give a ten minute burn and a rise of exactly one degree, which would be a good compromise.

EDIT: mine is in the lounge, which heats up quickest. Temperature rise in rest of house is only about half as much.

 

[Johntheo5]

Mine definitely is operating as a proportional band but like you, because of the tighter control the other rooms are not getting up to temperature even though normally we have the two combined rooms set to 22C and all the others, 19C, with bedrooms at 17C/18C.
I have TRvs (fully open) where the room stat is/was so I will set one to ~ 18/19C which means the boiler run time will be longer and allow the other rooms to hopefully reach temperature. Of course my mechanical EPH stat did this anyway!!. but this new stat hopefully wll give less room temperature overrun.