Programmer still firing boiler despite set point achieved

[diyn24318]

Kit: three monthly old Honeywell Sundial RF2 pack 2 controlling a Vaillant 428 boiler for central heating and hot water stored in cylinder

I hope this is not too dumb a question! I would have assumed that, when a central heating ON program is current, if the set point of the wireless room stat is less than the current temperature (as shown on the wireless room stat), the boiler would be asked to (continue to) fire; when the set point is reached, the boiler should be shut off.

I believe the Honeywell (if not most/all other) programmer fires the boiler when the temperature = the set point, so the room maintains the set point (although that's fairly annoying as it can push the room temperature up more than we'd wish it - though I know this can be tweaked with the boiler cycle rate/hr setting on the Honeywell).

However, what is very odd to me is, when the temperature is GREATER than the set point, the boiler/CH is still firing and the rad's are getting warm!

Am I correct in thinking, given I am aware of the Honeywell boiler cycle rate setting and how this (should) work, and the ability to adjust some Vaillant timings, that the stat or programmer is faulty?

 

[D_Hailsham]

However, what is very odd to me is, when the temperature is GREATER than the set point, the boiler/CH is still firing and the rad's are getting warm!

1. Are you reading the set and actual temperatures from the Honeywell DT92E?

2. When the problem condition occurs, does the boiler run for long or short periods?

3. Is the HW timed so it can come on when the heating is on?

4. Do you have one or two motorized valves?

5. Has the output of your boiler been adjusted for the heat loss of your house?

6. There's no point fiddling around with the cycle rate as the stat automatically adjusts it; the setting is only a starting point.

 

[diyn24318]

However, what is very odd to me is, when the temperature is GREATER than the set point, the boiler/CH is still firing and the rad's are getting warm!

1. Are you reading the set and actual temperatures from the Honeywell DT92E?

Yep, OP edited to reflect.

2. When the problem condition occurs, does the boiler run for long or short periods?

I'll monitor and report back - gut feel would be Short.

3. Is the HW timed so it can come on when the heating is on?

Yes, should I make the run times mutually-exclusive?

4. Do you have one or two motorized valves?

Two

5. Has the output of your boiler been adjusted for the heat loss of your house?

Yes, in fact it's probably set a little under, though I recall it did the same before I started balancing/limiting output (below the default of 30).

6. There's no point fiddling around with the cycle rate as the stat automatically adjusts it; the setting is only a starting point.

OK, thanks - did not know that.

 

[D_Hailsham]

2. When the problem condition occurs, does the boiler run for long or short periods?

I'll monitor and report back - gut feel would be Short.

3. Is the HW timed so it can come on when the heating is on?

Yes, should I make the run times mutually-exclusive?

If the boiler is running for both CH and HW, the TPI control system built into the RF2 Pack2 is effectively disabled by the HW thermostat which keeps the boiler running continuously until the required HW temperature is met. This means the rads could get hotter than required. Making the times mutually exclusive would prevent this.

If you have a well insulated tank, which retains the heat overnight, it could be heated up after the CH has gone off.

 

[diyn24318]

3. Is the HW timed so it can come on when the heating is on?

Yes, should I make the run times mutually-exclusive?

If the boiler is running for both CH and HW, the TPI control system built into the RF2 Pack2 is effectively disabled by the HW thermostat which keeps the boiler running continuously until the required HW temperature is met. This means the rads could get hotter than required. Making the times mutually exclusive would prevent this.

Wow, I did not know that. I suppose, should I replace the boiler in the future, I'd want to get a separate HW and CH controllers on the boiler to circumvent this issue.

I will adjust HW times based on this info and report back - many thanks.

 

[D_Hailsham]

I suppose, should I replace the boiler in the future, I'd want to get a separate HW controller to circumvent this issue.

It's a bit more difficult than just having a separate HW controller. The boiler needs to have separate control connections for the HW and CH. In any case, mutually exclusive timings are necessary. otherwise the HW will always take priority over the CH. This is because HW always needs the boiler to run continuously at full output until the HW stat is satisfied.

Combi boilers do all this automatically as CH is turned off while a hot tap is turned on. But if you have a HW cylinder, you may have to purchase the boiler manufacturer's own controls to achieve the desired effect.

 

[diyn24318]

I suppose, should I replace the boiler in the future, I'd want to get a separate HW controller to circumvent this issue.

But if you have a HW cylinder, you may have to purchase the boiler manufacturer's own controls to achieve the desired effect.

Yes, I think that was what I meant by my comment, but I sh/could have included some more prose around it to be perfectly clear! Will update my post. In reality, I'm likely to have an unvented cylinder installed so I'd hope this issue would go away.

UPDATE: I have just witnessed the stat reading 21.5C (with set point at 21C) and the firing symbol coming on whilst HW was off. Any further suggestions or comments please? (I'm thinking the boiler firing cycle *always* happens for the minimum amount of time - Honeywell default 1 min - but this is daft if the temperature is greater than the set point, surely?)

 

[D_Hailsham]

UPDATE: I have just witnessed the stat reading 21.5C and the firing symbol coming on whilst HW was off. Any further suggestions or comments please? (I'm thinking the boiler firing cycle *always* happens for the minimum amount of time - Honeywell default 1 min - but this is daft if the temperature is greater than the set point, surely?)

You saw the 'flame' symbol appear on the stat; but did the boiler actually light?

The 'minimum on time' is not fixed; it can be overridden by the system's logic and reduced to zero. It's really saying: if the boiler needs to run, this is the shortest time allowed.

Do you have one or two motorized valves to control the HW and CH?

 

[diyn24318]

UPDATE: I have just witnessed the stat reading 21.5C and the firing symbol coming on whilst HW was off. Any further suggestions or comments please? (I'm thinking the boiler firing cycle *always* happens for the minimum amount of time - Honeywell default 1 min - but this is daft if the temperature is greater than the set point, surely?)

You saw the 'flame' symbol appear on the stat; but did the boiler actually light?

Pass If it occurs again, I'll go and eyeball the boiler.

The 'minimum on time' is not fixed; it can be overridden by the system's logic and reduced to zero. It's really saying: if the boiler needs to run, this is the shortest time allowed.

Ah OK, makes sense. And the flame symbol would appear regardless of whether the boiler fired too?

Do you have one or two motorized valves to control the HW and CH?

Two.

 

[D_Hailsham]

And the flame symbol would appear regardless of whether the boiler fired too?

That's a distinct possibility. The User Instructions say:

The flame symbol indicates that the boiler is on for heating. The flame will disappear when there is no heating demand.

I have a Honeywell CM927 (wireless programmable stat) and I frequently have the flame showing but the boiler stays off. Honeywell explain it thus:

The heat demand is send to the relay box as a value between 0-100%. The BDR91 will adjust the boiler on time to meet this requirement. That is why the heating will be off sometimes even if the symbol is displayed. (The relay box is the equivalent of your ST9420C programmer.)

This raises a question, which I have never thought of before: Does the "minimum on time" (MOT) mean: (a) the boiler will not turn on unless the demand is more than the MOT; or (b) if the demand is less than the MOT, the boiler will run for the MOT? I have always assumed option (b), but, now I think about it, option (a) makes more sense.

Do you have one or two motorized valves to control the HW and CH?

Two.

There is a possibility that the heating valve is sometimes sticking open. This can be easily checked.

Make sure heating is OFF
Move the lever at the end of the valve from Auto to Man
You should feel resistance
Release the lever
It should automatically return to the Auto end
Do this several times to make sure.
If OK, the valve is not sticking.
If the lever is loose in the slot and stays wherever you put it, the valve is sticking.

If it is a reasonably modern valve (post 1980's), you can remove the actuator (box on top) and get access to the valve spindle. This should turn easily with the fingers, but only 10-20 degrees. If it is stiff, try some silicon lubricant.

 

[diyn24318]

This raises a question, which I have never thought of before: Does the "minimum on time" (MOT) mean: (a) the boiler will not turn on unless the demand is more than the MOT; or (b) if the demand is less than the MOT, the boiler will run for the MOT? I have always assumed option (b), but, now I think about it, option (a) makes more sense.

Agree, however I would imagine it is (b), though I will see what mine does if I can monitor carefully enough.

There is a possibility that the heating valve is sometimes sticking open. This can be easily checked.
[checking instructions removed]
If the lever is loose in the slot and stays wherever you put it, the valve is sticking.

It behaves per your checklist, though there is some play nearest the Auto end. It looks older than the HW valve so I may look to check/replace that as and when I do the same with the bypass valve.

Will report back with boiler firing when I can monitor.

 

[diyn24318]

This raises a question, which I have never thought of before: Does the "minimum on time" (MOT) mean: (a) the boiler will not turn on unless the demand is more than the MOT; or (b) if the demand is less than the MOT, the boiler will run for the MOT? I have always assumed option (b), but, now I think about it, option (a) makes more sense.

Agree, however I would imagine it is (b), though I will see what mine does if I can monitor carefully enough.

Will report back with boiler firing when I can monitor.

Apologies to you, @D_Hailsham, for the lateness in coming back on this.

However, I can confirm that the boiler fires (when CH only in operation) whilst the temperature is higher than the set point, and I have witnessed it in two different circumstances:
1) boiler fires, flow temp quickly(/instantaneously?) exceeds maximum set temp, boiler goes into anti-cycle mode - this sequence takes (as a guess) around 10-15 seconds
2) boiler fires, flow temp increases but boiler turns off before temp reaches maximum - this sequence takes approx 1 minute (though there is 30 or so seconds between boiler request and it firing) - which implies (b)

One other thing to note - the fire request seems to occur every 5 minutes, not 10 as I'd expect the setting "boiler cycle rate per hour = 6" to imply.

The upshot is this is rather less than desirable as it's heating the rads despite the stat being a full degree higher than the set point. Anything else I can do? Is the stat/prog actually working correctly??

 

[D_Hailsham]

I can confirm that the boiler fires (when CH only in operation) whilst the temperature is higher than the set point, and I have witnessed it in two different circumstances:
1) boiler fires, flow temp quickly(/instantaneously?) exceeds maximum set temp, boiler goes into anti-cycle mode - this sequence takes (as a guess) around 10-15 seconds.

This suggests that the boiler is oversized. I see you have a Vaillant 428 boiler, which is a very large boiler for the typical house. Who selected it and on what basis?

2) boiler fires, flow temp increases but boiler turns off before temp reaches maximum - this sequence takes approx 1 minute (though there is 30 or so seconds between boiler request and it firing) - which implies (b)

This could be the room stat turning the boiler off

One other thing to note - the fire request seems to occur every 5 minutes, not 10 as I'd expect the setting "boiler cycle rate per hour = 6" to imply.

The five minute interval may not be caused by the room stat but by the boiler's own stat, which will turn the boiler off when the water reaches the required temperature if the boiler cannot modulate any lower or if the rate of temperature rise is too fast (case 1, above).

 

[diyn24318]

I can confirm that the boiler fires (when CH only in operation) whilst the temperature is higher than the set point, and I have witnessed it in two different circumstances:
1) boiler fires, flow temp quickly(/instantaneously?) exceeds maximum set temp, boiler goes into anti-cycle mode - this sequence takes (as a guess) around 10-15 seconds.

This suggests that the boiler is oversized. I see you have a Vaillant 428 boiler, which is a very large boiler for the typical house. Who selected it and on what basis?

It could, however I have previously reduced the output to 11. I think the actual problem here, after balancing my rads with around a 12C drop across most of them, is the insufficient difference between the flow and return temperatures - and therefore the boiler regulating. I have "rebalance rads with a greater drop" on my TODO list...
(P.S. Boiler was inherited from purchase last year!)

2) boiler fires, flow temp increases but boiler turns off before temp reaches maximum - this sequence takes approx 1 minute (though there is 30 or so seconds between boiler request and it firing) - which implies (b)

This could be the room stat turning the boiler off

I think this is the MOT in operation - were this scenario to occur all the time I'd suggest the "system" is working as designed, however bad/odd that design is - and it would put me off recommending the Honeywell to others on that basis.

One other thing to note - the fire request seems to occur every 5 minutes, not 10 as I'd expect the setting "boiler cycle rate per hour = 6" to imply.

The five minute interval may not be caused by the room stat but by the boiler's own stat, which will turn the boiler off when the water reaches the required temperature if the boiler cannot modulate any lower or if the rate of temperature rise is too fast (case 1, above).

Which would imply the boiler is turning itself back on after 5 mins off, otherwise it would be 10 mins and the stat turning it back on! Most strange either way.

 

[D_Hailsham]

This suggests that the boiler is oversized.

It could, however I have previously reduced the output to 11. I think the actual problem here, after balancing my rads with around a 12C drop across most of them, is the insufficient difference between the flow and return temperatures - and therefore the boiler regulating. I have "rebalance rads with a greater drop" on my TODO list...

The boiler is designed to have a 20C drop.

The 400 series boilers has caused problems with Micro-firing. A modified PCB is supposed to fix this.

This could be the room stat turning the boiler off

I think this is the MOT in operation - were this scenario to occur all the time I'd suggest the "system" is working as designed

MOT?

The five minute interval may not be caused by the room stat but by the boiler's own stat, which will turn the boiler off when the water reaches the required temperature if the boiler cannot modulate any lower or if the rate of temperature rise is too fast (case 1, above).

Which would imply the boiler is turning itself back on after 5 mins off, otherwise it would be 10 mins and the stat turning it back on! Most strange either way.

Yes, the boiler could be turning itself back on after five minutes. It's all to do with the anti-cycling time. The manual says that this is 20 mins but, as the link above explains, this is not a fixed time. It varies with required flow temperature.

Have you made a note of the Status Codes when the various events happen? They would tell tell you what is causing the boiler to go on and off.

The Fault (F) codes may also provide a clue.

My experience of Honeywell TPI thermostats (3 properties) is that they do what it says on the tin.