Room temperature - controller setting differential 4 degrees

[commelect]

Good day, folks,
I have an S-Plan system with a Baxi Solo boiler and Smiths Centroller 3000 programmer with room and hot tank senstats.
The system seems to be working OK, except that the boiler doesn't switch on when the room temperature drops below the Centroller setting until there is a temperature difference of about 3 to 4 degrees. I'm measuring room temperature close to the senstat and using several different thermometers.
In order to achieve 19-20 degrees in living rooms, the Centroller is set to 23 degrees. The Senstat has been replaced, with little change, and the clock settings work OK.
The programmed settings on the Centroller are retained correctly.
The circuit diagram on the Centroller installation instructions shows the Senstats connected to a "temperature control module", which has a physical connection to the clock, and presumably compares the Senstats with the programmed settings.
Or is there a problem external to the boiler/controller, such as a zone control valve malfunction?
I would very much appreciate advice from a competent person on the likely cause of the problem, as a replacement Centroller would be quite expensive.
With thanks - commelect.

 

[MrTherm]

The problem described is similar to having a room stat with no anticipator differential correction.

Does the room stat have three wires, one of which is a neutral or a just two (live in and out) ?

If three wire (230v) check the neutral for continuity and ohms reading through the resistor.

When working correctly this should bring the diff' to at least two degrees, one to sense down and one as latency.

If digital two wire, there is usually one degree down from actual to activate 'on' which usually pre-set.

 

[commelect]

Thank you for your prompt response.

Senstat is two-wire low voltage digital.

I concur the normal latency of about 1 degree, but the system now has a gap of 3 to 4 degrees before it switches on.

For example, this morning the CH didn't come on at the morning on time. The Centroller was set to 22 degrees and the room temperature at the Senstat was about 18 degrees. When I changed the morning temperature setting to 23 degrees the boiler fired immediately.
It's not exactly critical, as the max temp on the Centroller is 25 degreesC, but it ain't right!
What do you make of it?

Thanks again - commelect

 

[MrTherm]

O.k

You've proved the system responds when upping the room senstat, so the function appears correct at least, logic says the senstat is somehow not accurate in its temp response yet a replacement (of the correct type Type 67-040-20) has been fitted. The Temp/cont/module connections are 8 and 9 for the room senstat, 8 and 10 for the cylinder senstat.

First i would check these connections are o.k, perhaps disconnecting and reconnecting the room senstat wires. I would further experiment by temporarily extending the senstat to an alternative unfixed (wandering) position and observe any differences however small as a testing indicator and being further away from any affecting surfaces. In these alternate locations, activate the senstat with say a warm hairdryer while the system is off then again when the system is on and compare any differences in your thermometer readings to original position. Another check with the *cylinder* senstat temporarily disconnected at terminals 8 and 10 might also show different uninfluenced temperature results.

I would also definitely check all wiring connections at the temperature control module for integrity. Somehow, although i doubt it, there might be a voltage influenced problem, the senstat's low voltage can be affected by suspect (wire grade) wiring runs.

It isn't much to go on but it might give a slight clue and then take it from there.

 

[commelect]

Thanks, Mr Therm.
I was wondering what to do with my Sunday!

This may take a while.

I know that the Senstat terminals are secure, and the voltage across them is stable.

Do you think the Centroller 3000 may have a fault? The Senstat is wired directly to it.

The hot water stat is OK, and switches on and off within a couple of degrees.

Thanks for all the help and guidance. - commelect

 

[lauriehurman]

Hang on a minute ...

If the room stat is working correctly - that would have been my first guess but you have fitted a new one and the problem persists - then the problem must be in the zone valve.

As you know in a conventional S plan the boiler and pump are called by the switch in the zone valve.

Either the valve is jamming open or the switch is sticking on.

Try there first.

Laurie

P.S. (thinking about it) I've got this the wrong way round. It's sticking OFF not sticking ON.
I'd still check the zone valve is receiving a signal and what it's doing with it.
If you room stat is volts free then you don't have an old fashioned mains voltage system.

 

[The Baker]

With respect, and allowing for the fact that as I rarely work on "Y"/"S" Plan I seem to recollect a sparky bloke in Daventry saying its possible to use the micro switches in the 2 port/3 port heads for 24 volt & No Volt stuff.

But do realise I may have miss read his postings.

Tim

 

[jeff the gasman]

With respect, and allowing for the fact that as I rarely work on "Y"/"S" Plan I seem to recollect a sparky bloke in Daventry saying its possible to use the micro switches in the 2 port/3 port heads for 24 volt & No Volt stuff.

But do realise I may have miss read his postings.

Tim

You can use the two port valve for low voltage connections but not the three port valve

 

[commelect]

Reply from Commelect to Mr Therm: Tried relocating Senstat and checked wiring - all OK. Please see below.

Reply to lauriehurman: I had checked that the piping was hot on both sides of the CH motorized valve, but then ran checks with the motorized valve on manual open. The boiler immediately responded with a quicker heating cycle and the room temperature climbed quickly to the demanded setting.
When the valve was returned to Auto, and a lower temperature set, the boiler continued with the heating cycle and only very slowly responded to the reduced demand (more than an hour).

This seems to confirm your recommendation. Sticky motorised zone valve.
Many thanks