Boiler cycling after fitment of new thermostat

22 Dec 2005
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United Kingdom
We have a Glow Worm Ultracom 24hxi, installed approx 5 years ago.

I recently replaced the standard rotary thermostat in the hall with a centaurstat 7.

The thermostat does everything i expected it to, but seems much more sensitive than the old one.

When the house has got up to temperature the thermostat is very quick to cut in or out. The net effect of this is that whenever the thermostat calls for heat the boiler fires for 2-3 minutes, the thermostat trips and the boiler goes into bypass. During the 5 minutes that the bypass is running the thermostat will call again and the boiler will immediately fire once the water pump overrun time has been reached.

Typically, the boiler will cycle twice before the temperature of the house stabilises for a while.

Is there any way to reduce the sensitivity of the thermostat or adjust the settings of the boiler to prevent this from happening so that the boiler fires for long enough that there is a bit of temperature overshoot to prevent the cycling?
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I think the boiler will have a setting to stop cycling.
You may be able to set it to 10 or 20 minutes to stop this kind of thing.
Consult the manual.

d02 parameter.

Should default to 20 minutes though. Unless you are running your heating very hot at 80 degree in which case it is 2 minutes. Turn it down to 70 degrees and you should get 20 minutes between firing.
Manual says it should be set to 20 minutes. Boiler set temperature is 73 degrees.

How does this setting work? Does it set a minimum time between ignitions?
d.2 is a table that varies the time. However on Vaillant it does not directly relate to the actual decimal value of the parameter. I do believe that the glowworm is based on the Vaillant so you could see if it matches the table. There is a countdown timer that show the actual remaining minutes within the settings of the boiler.

So with it set to 20 you are getting between 2.5 and 1 minutes anticycle. Crank it up to 60 and see if it extends.

Also on the gloworms I have worked on they sit for 5 minutes before ramping up not sure of the hxi though so this could also be having an effect. Why are you running so high as well? I would look to reduce this somewhat which will have a two fold effect of being able to extend the anti cycle time.

I would recommend the myson mprt stat this does a very good job of keeping return temps below 55 and also has 48 set points per day. Any chance of taking your one back? Or you could get the honeywell 9x series which also has tpi but a bit more expensive.
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Setpoint is as it was set when we bought the house.

What would you advise we set it to?

I played with the anti cycle d02 setting and increased it from the default 20 to 25. This had the effect of increasing the countdown timer figure from 6 to 7. I don't know if this is minutes, but as the pump overrun timer is set to 5 minutes and the boiler seems to restart 6 minutes after going into overrun it would make sense.

However, the thermostat call seems to override any anti cycle setting.

I lowered the thermostat set temperature below the shown temperature by 1 degree to shut down the boiler. The anti cycle countdown timer duly started to count down. I then increased the thermostat set temperature above the shown temperature and within 10 seconds or so the countdown timer went from showing 6 minutes remaining to zero and the boiler relit.

That's not how i would expect it to work.

The boiler is controlled by a separate, non glow worm control panel and thermostat. Could this explain it?
Can anyone help me with this?

I can't get my head round why the thermostat seems to be able to overide the anti cycle timer.

Thanks in advance.
My Vaillant 418 contains the same logic as the Ultracom HXI. It behaves in exactly the same manner regarding the anti cycle timer, so it looks like this is normal behaviour unfortunately.

All I can suggest is to replace the thermostat (again) with one that has proportional control such as a Drayton (Invensys) Digistat.

Sorry I can't be of more help.

I should point out that I am not a professional, just an experienced DIYer.
are you sure you are in anti-cycle mode? What s code is being displayed? You may find that the boiler has not actually cycled. Strictly speaking a cycle is when return has come within the delta of the flow according to whatever the designer of the boiler thought was the time to switch the burner off. To explain, say a flow temp of 65 is set at the boiler when the return gets to 62 the boiler has run out of headroom ie heat input is greater than you are loosing, either via the heat emitters or the cyclinder coil. The boiler will then truly cycle and start the timer. Playing with the roomstat can be determined by the software as something completely different on some boilers depending on the logic on the pcb.

Please don't think that you can also mimic a true cycle by adjusting the flow temp of the boiler as this once again will complicate issues.


flow temp of 40 anti cycle set to 60 which is 38.5mins if you then waited 20 mins before turning the flow to 60 which has a timer of 17 mins you wouldn't expect the boiler to fire for 17 mins but it actually already knows it has waited 20mins so you are on on a negative integer of 3 hence boiler fires but the table says it shouldn't for 17mins. So who knows what will happen if you start messing with flow set points.

Ps I would bin the stat you have in favour of one with tpi such as the myson I suggested. You may have to set it in gas mode if you boiler sits on low gas for a long time before ramping up. You could try this for me by having a flow temp of say 60 wait for the return to get up to say 55 then remove call for heat from roomstat for say 2mins. Now call for heat again and see how long it takes the boiler to ramp up to high gas
Based on your description, i understand a bit more about what is happening.

When the house is heating from cold the boiler gets fully up to temperature and then the stat turns it off when its up to temp. That is a full cycle so when the stat calls for heat approx 10 minutes later the anti cycle timer has gone down to zero and the boiler relights. However, it doesn't take much heat to trip the stat so the return hasn't got up to temperature before the stat trips again and the boiler doesn't see it as a cycle and therefore ignores the anticycle timer when the stat calls again thereafter.

What does the Myson stat do differently?
Ps I would bin the stat you have in favour of one with tpi such as the myson I suggested. You may have to set it in gas mode if you boiler sits on low gas for a long time before ramping up. You could try this for me by having a flow temp of say 60 wait for the return to get up to say 55 then remove call for heat from roomstat for say 2mins. Now call for heat again and see how long it takes the boiler to ramp up to high gas

The Ultracom HXI will light at about 80% for a minute or so, then ramp up immediately to full gas. It will probably ramp up far too far and then go into anticycle, if the pump speed is too low :mad:

Lower, proportional control means that the heating will only come on a set number of times per hour - usually 3. It adjusts the length of each burn based on the room temperature. So you are always having 20 minute cycles, rather than on-off-on-off in short order.
The myson will stop overshoot and try to maintain an average temperature. Over a few days it will "learn" the characteristics of the building fabric and also take into account how much heat rise is needed and cycle the boiler accordingly. To explain

Say you have a set back nightime temp of 15c @ 6am you like to have the room where the stat is @ 21c. The stat will calculate that you need a 6c rise and will call for heat until around 20.6c it will then stop calling for heat and cycle the boiler on and off (obviously a radiator continues to give out heat after a stat stops calling for heat). It will then continue in this manner which will have the effect of keeping the return hopefully below 55c. I find this system works well and is good for comfort and also seems to save £'s. However it does depend on radiator sizing and also location of the stat. You may find that you need to balance the radiator where the stat is to take less flow to ensure other rooms get a chance to get up to temp.

It also has the advantage that you set the temp that you require at a specific time and it will call for heat to try and get the room to that temp by that time i.e. Say you get up @ 6am set the stat for 6am 21c the stat will call for heat if a 3 deg rise is required at 5am if there is only a 2 deg rise required it will call for heat at 5:20am

Does this it explain it well enough?
It does explain things thanks.

The Centaurstat 7 that i have claims to have TPI, but certainly doesn't limit the number of times it calls for heating over the course of an hour or do anything clever like start warming the house at different times depending on how cold the house is.

Our house is 90 years old, albeit with good roof insulation and full double glazing. The boiler fires and modulates perfectly and the flow and return temperatures are correct. Even when the stat turns off the boiler, the flow and return are a good 12-14 degrees away from each other so there is no issue with the boiler cycling from that respect and it would appear that the rads are sized correctly.

The only issue is that the old stat seems to be less sensitive so allow more temperature overshoot and then the temperature to fall further before calling for heat. Hence less frequent cycles. The more modern and more sensitive unit is the one that is actually causing the boiler to fire in a less efficient manner. Its a shame that there is no way to use the boiler settings to compensate.

I guess i'll have to live with it or buy a new programmable stat that can compensate for its greater sensitivity....
Overshoot is a bad thing you may be surprised that you are thinking you are using more gas but perhaps arn't. Overshoot is a bad thing as is letting the temp drop too far. Have had a quick google and it does say it is tpi but nothing in the manual about it. Check your gas usage over a period of days to see if you are using more gas. If the rooms are still comfortable and you are not over shooting you are using less gas in my book.

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