Vaillant is a German boiler manufacturer, which is well known in the UK. In May 2011, a Vaillant press release announced that it had achieved UK installation of one million EcoTEC boilers. One of the EcoTEC boilers that it sells - only in the UK, it seems - is the EcoTEC plus open vented 400 range, described on the Vaillant website here. We know that with our 400 series boilers there is a problem, whereby under certain conditions the boiler cycles, i.e. turns on and off rapidly and repeatedly. This shouldn't happen, but it does. Whether it happens with all such boilers or only some of them, we do not know, but reports on the web suggest that it is also seen by others.
The purpose of this blog is:
- to share information about the problem
- to seek contact from others who have experienced the problem
- to try to persuade Vaillant first that there is a problem, and second to do something about it.
The 400 series is supposed to be "Compatible with the full range of easy to install, advanced controls from Vaillant". The controls are listed by Vaillant here, and include weather compensation controls (VRC430f and VRC470). The problem is apparent in our hands when the boiler is used with the VRC430f, and we have not yet heard either way whether it is seen with the newer VRC470. In fact, we are told that it occurs more generally when modulating controls are used, including the VRT392 (which modulates boiler output based on room temperature, without taking into account the outdoor temperature).
Weather compensation
This isn't a blog about weather compensation, so we'll stick to a brief explanation. If you already know what it is, skip this bit.
Traditionally, and still in the vast majority of UK homes, heating controls are simply on/off. When on, water circulates through the radiators at a set, high temperature, say 70C (or through the underfloor heating system at a set, lower temperature). Control comes either (crudely) from switching the heating off - whether manually or according to a time programme or, better, under control of a thermostat somewhere in the house - or by using thermostatic radiator valves.
With weather compensation, "outside sensors measure temperature changes and adjust the boiler performance to maintain the desired inside temperature, allowing your boiler to operate with maximum efficiency", as Vaillant puts it. Thus when the heating is on, the water is not heated to a set, defined level. Instead the weather compensation function varies the water temperature as the outside temperature changes. As it gets colder outside, the water temperature is increased and vice versa. The amount by which the water temperature is varied is managed by a set of heat curves embedded in the controller. It is necessary to select the heat curve most suited to the construction and aspect of the property, which is something the installer should do. Once the correct heat curve has been selected, it should not be necessary to change it.
Then as much heat energy should be going into the property from the heating system as is leaving it through the walls, windows, floors and roofs, so there is thermal equilibrium and the house is kept at a constant temperature. This is not only more comfortable but is supposedly more efficient, since it means that the return water temperature can often be kept below 57C, and hence the flue gases are also at or below this temperature (their dew point), so they can condense. It is this that gives rise to the extra efficiency of a condensing boiler. It also means that the boiler turns on and off less, and instead enjoys a long, slow burn, which is all supposed to be better for it. Conversely, it is bad news if your boiler keeps switching on and off - especially if it does so every minute or two.
The problem visualised - short firing and micro firing
Of course, if the outdoor temperature is high enough, there may be no need for heat at all, so the boiler can switch off altogether. There is also a limit as to how low an energy output can be provided by a given boiler, i.e. how low it can modulate its output. According to their specifications, this lower limit varies from 5.0 to 6.3 kW for the boilers in the 400 series. Their maximum output is given (approximately) by the last two digits of the model number, so the 415 can provide up to 15.3 kW, and so on for the 418, 428 and 438.
You might imagine then that the boiler will be either on - at the temperature set by the weather compensation controls - or off, if it there is no demand for heat (or if the demand is less than the minimum that the boiler can provide). But, in the case of the 400 series (or at least our instances of it), you'd be wrong.
Here is what the behaviour of a 438 can look like over the course of 24h (pdf version available here):
This graph shows two distinct behaviours when the heating is on (note that the high peaks at around 70C are when the boiler is providing water for the hot water cylinder, and can be ignored for present purposes). In this blog, we refer to the two behaviours as "short firing" and "micro firing":
Short firing - instead of firing continuously, the controller asks for heat only for perhaps 10-20 min at a time, then reduces the target flow temperature to zero (in other words, asks the boiler to switch off) for some minutes. The cycle then repeats. Experiments appear to have revealed a way to abolish short firing, improving it to continuous firing - see the page entitled "What works and what doesn't".
Weather compensation
This isn't a blog about weather compensation, so we'll stick to a brief explanation. If you already know what it is, skip this bit.
Traditionally, and still in the vast majority of UK homes, heating controls are simply on/off. When on, water circulates through the radiators at a set, high temperature, say 70C (or through the underfloor heating system at a set, lower temperature). Control comes either (crudely) from switching the heating off - whether manually or according to a time programme or, better, under control of a thermostat somewhere in the house - or by using thermostatic radiator valves.
With weather compensation, "outside sensors measure temperature changes and adjust the boiler performance to maintain the desired inside temperature, allowing your boiler to operate with maximum efficiency", as Vaillant puts it. Thus when the heating is on, the water is not heated to a set, defined level. Instead the weather compensation function varies the water temperature as the outside temperature changes. As it gets colder outside, the water temperature is increased and vice versa. The amount by which the water temperature is varied is managed by a set of heat curves embedded in the controller. It is necessary to select the heat curve most suited to the construction and aspect of the property, which is something the installer should do. Once the correct heat curve has been selected, it should not be necessary to change it.
Then as much heat energy should be going into the property from the heating system as is leaving it through the walls, windows, floors and roofs, so there is thermal equilibrium and the house is kept at a constant temperature. This is not only more comfortable but is supposedly more efficient, since it means that the return water temperature can often be kept below 57C, and hence the flue gases are also at or below this temperature (their dew point), so they can condense. It is this that gives rise to the extra efficiency of a condensing boiler. It also means that the boiler turns on and off less, and instead enjoys a long, slow burn, which is all supposed to be better for it. Conversely, it is bad news if your boiler keeps switching on and off - especially if it does so every minute or two.
The problem visualised - short firing and micro firing
Of course, if the outdoor temperature is high enough, there may be no need for heat at all, so the boiler can switch off altogether. There is also a limit as to how low an energy output can be provided by a given boiler, i.e. how low it can modulate its output. According to their specifications, this lower limit varies from 5.0 to 6.3 kW for the boilers in the 400 series. Their maximum output is given (approximately) by the last two digits of the model number, so the 415 can provide up to 15.3 kW, and so on for the 418, 428 and 438.
You might imagine then that the boiler will be either on - at the temperature set by the weather compensation controls - or off, if it there is no demand for heat (or if the demand is less than the minimum that the boiler can provide). But, in the case of the 400 series (or at least our instances of it), you'd be wrong.
Here is what the behaviour of a 438 can look like over the course of 24h (pdf version available here):
Green: outside temperature (as measured by the wireless outdoor sensor)
Red: room temperature (in the room in which the VRC430f is situated)
Blue: flow temperature (i.e. temperature of the water leaving the boiler)
Purple: flow setpoint (i.e. the flow temperature that the VRC430f is asking for)
This graph shows two distinct behaviours when the heating is on (note that the high peaks at around 70C are when the boiler is providing water for the hot water cylinder, and can be ignored for present purposes). In this blog, we refer to the two behaviours as "short firing" and "micro firing":
Short firing - instead of firing continuously, the controller asks for heat only for perhaps 10-20 min at a time, then reduces the target flow temperature to zero (in other words, asks the boiler to switch off) for some minutes. The cycle then repeats. Experiments appear to have revealed a way to abolish short firing, improving it to continuous firing - see the page entitled "What works and what doesn't".
Micro firing - this is the real killer, and the crux of the problem. The boiler can spend hours on end in this mode. It ignites, the temperature rises rapidly (as it should), but instead of settling down at the desired flow temperature, the flame switches off again after a few seconds. The boiler stays off for a few tens of seconds, presumably waiting until it has cooled down a bit, then it tries again. The result is a boiler that cycles on and off every minute or two. Over and over again. This is bad news for all sorts of reasons.
The detail of micro firing is not apparent from the graph above, and indeed it is hard to record since the resolution of Vaillant's data logging software (vrDialog) is effectively about 1 minute, whereas micro firing events happen on a smaller timescale. Here is a graph created manually from video observation of the boiler display, when the target flow temperature was 40C:
It shows the change in boiler state with time. The relevant boiler states are:
1 - Fan running
2 - Water pump running
3 - Ignition sequence
4 - Burner ignited (i.e. this is the only period, of about 12s each time, when the boiler is actually lit, leading to a brief peak in flow temperature)
5 - Fan and pump running (i.e. after the flame has extinguished)
7 - Pump over run (i.e. keeping the pump running after the boiler flame has extinguished - this seems to happen for a variable period before it goes through the whole sequence again)
When does micro firing appear?
Our observations are that micro firing is apparent when outdoor temperatures are moderate, typically giving rise to a target flow temperature of about 42C or lower. It's a bit unpredictable and erratic, but under the right (or, rather, wrong) conditions a boiler can spend all day micro firing.
How should it look?
Only Vaillant could tell us how they've designed their system to operate. However, there seems to be an obvious way to design it to behave, which I can put no better than a commenter on this blog has already written:
I would 'assume' that on a cold winters morning, initially the boiler would be on continuously until the room temp approaches set temp. After the room/house has reached set temp, the boiler will cycle on/off for varying lengths of time depending on insulation, outside temp, movement (doors opening etc) inside etc. If the initial 'continuous' period above is 'cycling', then I would envisage the system is not working 'as designed' by Vaillant (assuming the rest of the plumbing/rads etc is upto specification).The assumed behaviour described is in fact exactly how a 438 is observed to behave when short firing has been removed (by changing Pump Delay Time - see "What works and what doesn't") and when it's too cold outside for micro firing to happen (pdf version of graph here):
This shows the same four parameters as above (but some with different colours, sorry), and includes also the boiler state and the "Quick Veto Temp", which in these circumstances indicates the programmed temperature (except at night).
Although there's a good deal of data shown, it's not too difficult to see what's going on. The heating/hot water day starts at 6am, with a peak of hot water demand. Then there is a single, continuous burn up until about 9.30am, by when the room temperature has crept slightly above the programmed temperature, so the controller stops calling for heat and the boiler goes off for half an hour or so. There's another peak of hot water, and then there are three more periods in the early afternoon when the controller stops calling for heat for a while because the house is hot enough. From about 3.30pm, there is a continuous burn all the way to the end of the heating day at 10pm.
On a slightly colder day, there is a continuous burn all day long, since the house is losing heat at a greater rate than the minimum that the boiler can provide, so the boiler stays on all the time at an appropriate level to keep the house at a more or less constant temperature:
(note that I have since slightly reduced the heating curve, to obtain a better match between programmed temperature and temperature achieved, so that the overheating at the end of the day is not seen as it is here)
These graphs look just as we imagine they should. The boiler fires continuously at all times, except when the house is warm enough that it will overheat unless the heating shuts off for a while. This demonstrates that the system can operate correctly and sensibly under certain conditions. There is a stark contrast between this and short firing (when the boiler shuts off repeatedly even when the house needs heat and the boiler wouldn't be supplying too much if it stayed ignited), and an even starker contrast with micro firing.
Why do we think that something can and should be done about it?
Micro firing is bad for the boiler, bad for fuel efficiency and it's obviously not providing heat as intended. The general consensus seems to be that Vaillant boilers other than those in the 400 range do not suffer from the same problem when used with weather compensation. It therefore seems to us - and others - that there is something wrong with the 400 series.
We suspect that it is a problem that could be solved by altering the software in the boiler and/or the controller. Vaillant has so far shown no inclination to do this. So far, they have not even acknowledged that there is a general problem.
Our aim is to find others who have the problem, to collect together information about everyone's experience of the problem in a systematic way, and to gather and report on people's collective wisdom about what if anything can be done about it. If you're from Vaillant and you're reading this, there's an open offer to engage with you about trying to sort this out properly. Vaillant has so far failed us.
Other resources
There has been considerable discussion of the cycling problem on a forum called www.diynot.com. Try a search there for "Vaillant cycling" (without the quotes).
