TPI thermostat and very old boiler

[JonathanM]

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.

What if you keep the TRVs the same, but set the switching differential a bit higher?

 

[Johntheo5]

I will play around a bit once the the ambient falls a bit down hear in the sunny south, its presently 14.5C.

 

[Johntheo5]

Took a few more readings, it seems to be (just) a differential stat, when set to 20.5C, and a differential of 0.3C, it cuts in at a indicated 20.4C and cuts out at 20.8C with a max overrun of 0.2C to 21C, not bad I suppose, this is after the heating has been on for a few hours. (ambient at 11C/12C). This only takes 15 minutes with a mean (2 rads) temperature of 43C which is reached in 3 or 4 minutes, now, the plot thickness, it then takes 1 hour and 33 minutes for the room temperature to fall from 20.9C/21C to 20.4C and boiler cut in, I reckon that the 2 rads output while rising/falling from 30C/43C is ~ 650 watts. So why the huge discrepancies??, in the times?

 

[vulcancontinental]

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.

As the boiler is 32 years old I'm not sure the temperature of the return will affect the remaining life. At the last service did the engineer send his brushes down the exchanger and take out a dustpan full of rust flakes, if no rust, worry not.

The Baxi floor standers were/are great boilers and tough as old boots but expect a Land Rover to perform like a Land Rover; drive it like Ferrari performance expecting Ferrari performance and you will be disappointed.

It has always puzzled me why manufacturers divide thermostats into decimal places of a degree, the human body will not notice gradual changes in temperature of up to 4 degrees as long as the rate of reduction is slower than 1 degree per minute (in still air, if in a draft it's noticed immediately). Usually this comment is followed by something along the lines of 'you've not lived with my wife' but I find it's the case. Site the thermostat in a place unaffected by fires, cooking TV, occupancy, pets solar gain and start from there.

 

[JonathanM]

At the last service did the engineer send his brushes down the exchanger and take out a dustpan full of rust flakes, if no rust, worry not.

No, they've never done that. They usually just put the probe in the flue! I will see what they think next time. The heat exchanger was changed 11 years ago because it leaked though. They blamed corrosion. Free under service contract.

The Baxi floor standers were/are great boilers and tough as old boots but expect a Land Rover to perform like a Land Rover; drive it like Ferrari performance expecting Ferrari performance and you will be disappointed.

It is a great boiler, which I am wanting to treat respectfully. I am trying to drive it like a Land Rover. That was the whole point of my post. I was worried that the default setting, TPI, was making it act like a Ferrari. Luckily the thermostat I bought turned out, by chance, to have the option to revert to old fashioned swing/hysteresis. Not many do these days. I am happy now that it controls to 1.5C overshoot.

It has always puzzled me why manufacturers divide thermostats into decimal places of a degree, the human body will not notice gradual changes in temperature of up to 4 degrees as long as the rate of reduction is slower than 1 degree per minute

It is, however, very useful for working out what's going on when setting up.

 

[vulcancontinental]

It's either a Baxi FS balanced flue or open flue or the one that followed that, the Boston. I'm guessing balanced flue if they're taking a sample of flue products. Four wing bolts remove the burner and four more remove the top plate, brush straight down. Easy to remove the burner and wash any lint. You're not getting a service.

 

[JonathanM]

It's either a Baxi FS balanced flue or open flue or the one that followed that, the Boston. I'm guessing balanced flue if they're taking a sample of flue products. Four wing bolts remove the burner and four more remove the top plate, brush straight down. Easy to remove the burner and wash any lint. You're not getting a service.

Thank you. Yes, it is the balanced flue version, FS 801 RS. They used to check the burner to monitor a minuscule crack, but not the past two years. When I've asked why they don't do a more comprehensive service, they say, with a boiler of this age, it's best to leave it alone. It's very hard for me to know what's best!

 

[Johntheo5]

It has always puzzled me why manufacturers divide thermostats into decimal places of a degree, the human body will not notice gradual changes in temperature of up to 4 degrees as long as the rate of reduction is slower than 1 degree per minute.

My comfort zone is 21/22C , if it fell to even 19C I would certainly notice it and would be uncomfortable at 18C, I would think that the reason for the 0.1C resolution is to help prevent overshoot. I am only getting 0.2C overrun with a very basic non TPI roomstat set to 0.3C differential, my old mechanical EPH which appeared to have a differential of 1 C would result in a overshoot of ~ 2C.