Why should a Digistat sometimes only call for heat for 40 seconds?

[PaulUszak]

Hiya.

I have one of these:-

It's brand new. Can anyone explain why it should call for heat for only 40 secondsish every hour or so? I can't see any purpose for operating the boiler for so short a time. That just seems like it's ruining the boiler as it clearly can't heat anything in < 1 minute. It takes ~ 10 minutes to properly warm up the radiators.

General hysteresis also seems a bit too small for my liking, but I read that 'they' are all like that now though.

So, err..?

The boiler is an open vent system, not combi.

 

[DP]

Boiler fires, runs for several seconds, room reaches temperature and thermostat puts the boiler off.

As weather gets colder, boiler will stay on longer

 

[old duffer]

The clue is in the boiler plus badge. Maybe you need to do a little reading on the concept to give you an answer to your question

 

[PaulUszak]

Not really the detail I was hoping for

These devices work by measuring the gap between what the internal temperature is and what the user wants it to be, and modulates the boiler so that it only uses as much fuel as necessary to close the gap.

That's called hysteresis in control systems nomenclature. No boiler can change my room temperature by 0.5 degrees centigrade within 40 seconds. Does anyone have any more details why they think it can? Is it an attempt at a PID controller? And can this 'feature' be turned off?

Otherwise I think I'll be reverting to the old mechanical thermometer.

 

[old duffer]

Not really the detail I was hoping for



That's called hysteresis in control systems nomenclature. No boiler can change my room temperature by 0.5 degrees centigrade within 40 seconds. Does anyone have any more details why they think it can? Is it an attempt at a PID controller? And can this 'feature' be turned off?

Otherwise I think I'll be reverting to the old mechanical thermometer.

Good for you...the principle of Occams's razor is always correct

 

[Gasguru]

A lot of this tech is garbage...the problem is the boiler manufacturers, the pump manufacturers and the controls manufacturer all have different regs to comply with and when you stick it all together it's a shambles.
Add in government policy by Carrie and a bunch of clueless wonders and we get Part L building regs that require it.

 

[old duffer]

A lot of this tech is garbage...the problem is the boiler manufacturers, the pump manufacturers and the controls manufacturer all have different regs to comply with and when you stick it all together it's a shambles.
Add in government policy by Carrie and a bunch of clueless wonders and we get Part L building regs that require it.

The UK market is dominated by poor quality products (throw away design) because of the low standard of the market operators )gas safe register payment fee is all you need to claim you can fix a boiler.

These devices are easily diagnosed, they even have test points and the values are easily available for reference but.... the fitters don't know how to check with a multimeter and would not know an Ohm from a gnome

BY contrast, when you use quality Japanese or Korean builds, you get boilers that operate with ARM cortex IC's running linux. The irony is that these high end, superior products that have 20 year life cycles of perfect operational performance are actually the same or lower price than the big box crap.

The Koreans have as standard, built-in condensate neutraliser, air intake temperature sensors, water intake/output sensors, ability to switch between room thermostats and heating flow temperate at the flick of a switch.
Settings that automate the heating operation providing efficiencies that significantly reduce gas consumption (50% plus) for the same comfort levels. As standard features...

You are very much correct in your observations but there are superior alternatives readily available at lower prices so there is no excuse

 

[PaulUszak]

@old duffer

Okay, thanks. But you are talking about the boiler. Mine's a Glow Worm 30FF - 9kW (brilliant even after 26 years running). But I'm talking about the room control. Are you suggesting that my Digistat is incompatible with such an oldie? I can't see it myself. The thermal characteristics of the room are what influence the thermometer and it's call for heat. Even the best foreign built boiler running the latest Linux kernel can't change my living room's state within 40 seconds. I guess unless it's vastly over powered (50 kW+?)...

So, alternative question. Are there other examples of controllers only operating for 40 seconds?

 

[old duffer]

Back to my occams's razor...if its not working as you expected or not doing what you need it to do..then occams razor says the obvious answer is the one you have already reached...its incompatible with your system

 

[MJN]

Back to my occams's razor...if its not working as you expected or not doing what you need it to do..then occams razor says the obvious answer is the one you have already reached...its incompatible with your system

Is any system compatible with a controller that might make a call for heat as short as 40 seconds? What might be a good example of one that would suit such short bursts? It does sound a bit nuts to me and I can see why @PaulUszak is asking if it's normal or not. Might it be something that can be adjusted? I know such a feature (minimum on time) exists on my Honeywell stats (and is explicitly stated as to reduce unnecessary wear and/or damage from such short bursts).

 

[old duffer]

If you can't find what you want, make it yourself https://www.instructables.com/Raspberry-Pi-Home-Heating-Controller/

 

[old duffer]

Is any system compatible with a controller that might make a call for heat as short as 40 seconds? What might be a good example of one that would suit such short bursts? It does sound a bit nuts to me and I can see why @PaulUszak is asking if it's normal or not. Might it be something that can be adjusted? I know such a feature (minimum on time) exists on my Honeywell stats (and is explicitly stated as to reduce unnecessary wear and/or damage from such short bursts).

The logic of the very low cost and simple risc chips don't allow for "adjustment". They operate very simple instructions based upon a resistance value. So its getting readings that are outside of operational expectations or perhaps not getting any values, no idea other than to test...if you have access to the schematic. You could of course create a simple circuit with some switches to test the logic but then what would you do if it did not work!

Asking the tech support of the manufacturer would have been my first step

I've posted a link to a low cost DIY approach. Swap out the hardware for the new pi zero w 2 (£8)

 

[Harry Bloomfield]

Can anyone explain why it should call for heat for only 40 secondsish every hour or so? I can't see any purpose for operating the boiler for so short a time. That just seems like it's ruining the boiler as it clearly can't heat anything in < 1 minute. It takes ~ 10 minutes to properly warm up the radiators.

The simple fix, would be to have the thermostat's output calling for heat, fed to the boiler via a 40second plus delay timer circuit. Then if the call for heat has switched back off at the end of the delay timer - the boiler will not then start.

 

[MJN]

The logic of the very low cost and simple risc chips don't allow for "adjustment". They operate very simple instructions based upon a resistance value.

As I say, my Honeywell allows such an adjustment so I'm not sure where you're going with that one.

I've posted a link to a low cost DIY approach. Swap out the hardware for the new pi zero w 2 (£8)

The downside of that particular approach is that it is quite a crude controller and doesn't provide TPI control and so would be a step backwards in terms of stability of output. If @PaulUszak was looking for a replacement I'd recommend a Honeywell CM927.

 

[polesapart]

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