Myson boiler short-cycling / nest thermostat Y-plan system

Evening all,
Pointers appreciated.
Myson Apollo 50B boiler, serviced annually. Set to 'hi' which I think outpu ts water at 80 degrees. Nest 3rd Generation system. Gravity fed. Header tank, yellow cylinder in ai ring cupboard etc.
When we moved into the house we had an old Randall 103 programmer and no ro om thermostat which has since been replaced by the Nest system including th e installation of a cylinder stat and Honeywell 3 port mid-position valve.
Around about the same time the boiler has developed the annoying habit of s hort cycling (turning off and on at intervals of between 30 and 60 seconds whilst the system is still calling for heat) after about 30 minutes of cont inuous operation on either heating or hot water or both (i.e. when it's bee n running for a while). It never did this before installation of the Nest s ystem. I cannot say for certain whether the short-cycling started at the ex act time of the Nest installation.
In an effort to solve this problem I have:
Balanced the radiators Replaced the gas valve (service showed a slight leak) Replaced the pump overrun stat Replaced the 'hi' stat Replaced the 'lo' stat Replaced the thermal cut-off device
All with no effect.
With the heating turned up to 30 degrees to force it on and the boiler work ing in it's on/off short cycling mode I have measured voltage across Neutra l and pin 3 (heating call-for-heat) on the Heat Link and with the system ca lling for heat this is at 240v as expected. When the heating is turned down the system stops calling for heat and it drops to 120v and then 0v. This b ehaviour is also as expected, although I don't know the significance of the 120v reading, if indeed there is one.
Crucially, with the system calling for heat the voltage remains at 240v whe n the boiler is in the 'off' phase of the on/off short cycling. In layman's terms (if my logic is sound) the system is calling for heat but the boiler is not complying.
A plumber mate has suggested my circulation pump but I'm not convinced. The pump L and N are connected to the boiler, which in my mind means the boile r controls the pump. Boiler off = pump off, boiler firing = pump runnin g.
I'm looking for some next steps. Thank you kindly in advance.
Jon
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Jon Parker wrote:

Try turning up the pump to max speed (usually a three position switch) maybe the new three posn valve has increased flow resistance and the heat cannot get away from the boiler fast enough and the over heat stat is cutting in. BTW 80 c flow temp is ridiculously high so you are probably very close to the over heat stat switching point. Also the efficiency will be down as the boiler will rarely be in condensing mode.
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On Thursday, February 2, 2017 at 9:59:38 PM UTC, Bob Minchin wrote:

Pump already at maximum speed.

I'm now struggling to remember where I read that the 'hi' output temp was 80. I may have got that wrong.
Thanks for the suggestion.
JP
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On Thu, 2 Feb 2017 13:11:26 -0800 (PST), Jon Parker

Are there any ball or gate valves in the primary circuit that have been left partially shut?
--

Graham.

%Profound_observation%
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On Thursday, February 2, 2017 at 11:56:44 PM UTC, Graham. wrote:

Don't think so. There are valves either side of the circulation pump. I forget the name. I want to call them flange seals? Both of these are fully open as far as I can tell.
Cheers Jon
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On 02/02/2017 21:11, Jon Parker wrote:

It seems to me that there are two possibilities. Either the boiler is cycling on its own built-in stat which has too little hysteresis between on and off temperature, or the Nest thermostat is telling it to turn on and off.
If this problem started when the Nest and 3-port valve were installed, you should first make sure that the valve is wired correctly. Can you trace all the connections and reconcile them with a Y-Plan wiring diagram. In particular, the programmer (Nest in this case) must have a HW OFF output which makes the valve's grey wire live when hot water is not required. Similarly, the cylinder stat must have 3 contacts - with the HW Satisfied contact again connected to the valve's grey wire.
I don't know about the internal logic of the Nest, but many digital thermostats cycle their boiler output on and off when approaching the temperature set-point in order to avoid overshoot. There is usually something buried in an advanced menu somewhere which enables you to specify minimum on and off times in order to minimise short-cycling.
With a Y-Plan system, when the HW demand is switched off and/or satisfied, the boiler and pump are switched on and off by a micro-switch in the valve - which connects the white and orange wires together. So, in your case, the CH demand from the Nest will go to the valve's white wire. That being the case, if the Nest is causing the short-cycling, I would expect to hear the valve whirring away every time the boiler goes on and off, because the microswitch only operates when the valve moves to a different position.
When you talk about measuring voltages on Pin 3, is that in the box which connects the Nest to the system? If so, that box is switching mains (or not) to CH ON, and to one or other (but not both) of HW ON and HW OFF. It is never *generating* 120v. However, if you are measuring 120v, this is almost certainly being fed back by the valve when it is *not* receiving a CH demand. So, if that voltage is going from 240v to 120v in time with the boiler going on and off, it is pretty certainly the Nest which is causing the short cycling - in which case you need to see whether you can fine tune the way in which it operates.
Hope that helps!
--
Cheers,
Roger
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On Friday, February 3, 2017 at 12:53:04 AM UTC, Roger Mills wrote:

Indeed. I'm fairly happy that the testing I've done rules out the Nest. When it calls for heat the terminal 3 (which is the HEATING ON terminal) goes to 240v and an audible click is heard as the relay closes, the valve moves and the pump spins up.

Yes, I can confirm all of that. My father-in-law and I installed the system ourselves, he an accomplished electrician of 35 years experience. I have opened up the junction box today and rechecked everything and it is all where it should be.

I have been through every option, there is no such option.

It doesn't. During short-cycling when the boiler cuts out the valve stays put.

It's the Central Heating call for heat terminal.

I think it has. I'm convinced the Nest is not at fault.
I have the option of trying another circulation pump on a sale/return basis. I think this might have to be the next step.
Jon
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On 03/02/2017 08:10, Jon Parker wrote:

I'm still not sure which terminal you mean. Do you mean in a 10-way junction box often used for central heating systems? If so, that's probably being fed by the nest, and in turn by the 3-port valve. I can't see how that can be 120v when there *is* a demand for heat - so I'm still not sure that you can rule out the nest.
Can you get at the demand terminal on the boiler itself. If you can measure the voltage at that when the boiler cycles, it will throw a bit more light on the subject. If that stays at 240v throughout, the boiler is cycling on its own stat. If that goes off when the boiler does, the boiler is cycling on its external control - i.e. the Nest.
--
Cheers,
Roger
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On Friday, February 3, 2017 at 9:28:17 AM UTC, Roger Mills wrote:

No, I don't. On the Nest heat-link (which is the brain of the system and makes the electrical connections to all the other components via the junction box) Terminal 3 is the CH ON terminal.

Yes I can do that, I'll report back once it's done.
Jon
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On Friday, 3 February 2017 08:10:54 UTC, Jon Parker wrote:

Hot rads means you've got enough circulation. If you still suspect the pump, open it to see the impellor, it might be blocked. No point in a new one if it spins ok.
NT
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On Thursday, 2 February 2017 21:11:28 UTC, Jon Parker wrote:

puts water at 80 degrees.

airing cupboard etc.

room thermostat which has since been replaced by the Nest system including the installation of a cylinder stat and Honeywell 3 port mid-position valve .

short cycling (turning off and on at intervals of between 30 and 60 second s whilst the system is still calling for heat) after about 30 minutes of co ntinuous operation on either heating or hot water or both (i.e. when it's b een running for a while). It never did this before installation of the Nest system. I cannot say for certain whether the short-cycling started at the exact time of the Nest installation.

rking in it's on/off short cycling mode I have measured voltage across Neut ral and pin 3 (heating call-for-heat) on the Heat Link and with the system calling for heat this is at 240v as expected. When the heating is turned do wn the system stops calling for heat and it drops to 120v and then 0v. This behaviour is also as expected, although I don't know the significance of t he 120v reading, if indeed there is one.

hen the boiler is in the 'off' phase of the on/off short cycling. In layman 's terms (if my logic is sound) the system is calling for heat but the boil er is not complying.

he pump L and N are connected to the boiler, which in my mind means the boi ler controls the pump. Boiler off = pump off, boiler firing = pump runn ing.

It's the boiler's inbuilt stat that keeps stopping the flame. Likely cause is not enough primary cct water flow. But a misbehaving 3 way valve can als o cause it. My first question would be how hot are the radiators?
NT
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On Friday, February 3, 2017 at 3:41:28 AM UTC, snipped-for-privacy@gmail.com wrote:

Radiators get hot as usual. The 3-way valve is brand new and appears to be moving correctly when heating and hot water are turned on and off by the stat. When the boiler is doing it's short-cycling the valve doesn't move when the boiler shuts off.
This further lends weight (I think) to the boiler cutting out, rather than casting suspicion on the Nest.
Jon
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On 03/02/2017 08:04, Jon Parker wrote:

How hot? 80C is too warm I think, depending on where you measure.

I can't see how it could be the thermostat. Presumably it's just on/off, with no fancy weather/humidity sensors causing the boiler to modulate or cycle on/off?
--
Cheers, Rob

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On Friday, February 3, 2017 at 8:21:58 AM UTC, RJH wrote:

I haven't measured, just recalling a section of the boiler manual which gives maximum output as 82 degrees.

It does have a few fancy features but these are all off and I'm just using it as on/off.
Jon
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On Friday, 3 February 2017 08:04:23 UTC, Jon Parker wrote:

use is not enough primary cct water flow. But a misbehaving 3 way valve can also cause it. My first question would be how hot are the radiators?

e moving correctly when heating and hot water are turned on and off by the stat. When the boiler is doing it's short-cycling the valve doesn't move wh en the boiler shuts off.

n casting suspicion on the Nest.

We know it's the boiler from info in the OP. Hot rads eliminates the momo v alve as the possible culprit. You either don't have much hysteresis in the boiler stat, or more likely with the cold weather the boiler exchanger is j ust cooling rapidly, causing faster cycling. Old bimetal stat boilers didn' t have much hysteresis or time delay built in.
NT
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On 03/02/2017 09:35, snipped-for-privacy@gmail.com wrote:

It's most likely the boiler stat - but you can't be sure of that until you check whether the boiler demand stays on when the boiler goes off.
My Honeywell programmable stat (not in the Nest 'smart' stat category) keeps the boiler demand on continuously when the difference between target and actual temperatures is high, but then goes into 'proportional' mode as the target temperature is approached. This cycles the boiler - using on/off times which can be configured - *before* the target temperature is reached in order to prevent overshoot.
The Nest may do something similar - I don't know. It would be interesting to know how close the room temperature is to the target temperature when this cycling occurs.
--
Cheers,
Roger
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On Friday, February 3, 2017 at 11:25:35 AM UTC, Roger Mills wrote:

Sometimes it's as much as 1.5 - 2 degrees.
Also, in testing I have turned the stat up very high, like 25-27 to eliminate the possibility of any effect similar to the one you describe.
Jon
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On Friday, February 3, 2017 at 9:35:42 AM UTC, snipped-for-privacy@gmail.com wrote:

valve as the possible culprit. You either don't have much hysteresis in th e boiler stat, or more likely with the cold weather the boiler exchanger is just cooling rapidly, causing faster cycling. Old bimetal stat boilers did n't have much hysteresis or time delay built in.
Okay, but why did it not short-cycle prior to the installation of the Nest and 3 port valve then?
Jon
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On Friday, 3 February 2017 14:08:36 UTC, Jon Parker wrote:

mo valve as the possible culprit. You either don't have much hysteresis in the boiler stat, or more likely with the cold weather the boiler exchanger is just cooling rapidly, causing faster cycling. Old bimetal stat boilers d idn't have much hysteresis or time delay built in.

t and 3 port valve then?

I don't know. Possibles are colder weather, a 3 way that lets some water in to the HW exchanger when the CH is on, more crud in the pump or a drifting boiler stat. It's not something I'd lose sleep over.
NT
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I have revisited the radiators and found there does seem to be a small drop in performance, and the radiators at the end of the circuit are suffering particular poor performance.
Jon
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