Vaillant VRC430 - what works and what doesn't?

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I have a Vaillant Ecotec 415 open-vent boiler with VRC430 installed as a roomstat. I have had a lot of difficulties with getting it to work at all and the situation is still not satisfactory. I believe that I have a very good understanding of the problem. I have been contributing to a number of threads about the VRC430 and, if you are interested, you can find my stuff by doing a "search by author" which is tucked away as a link at the bottom of the search page.

The problem I have found seems not to be well understood other than by some rules of thumb - "Vaillant boilers need very good flow", "In this country you need to set a high heating curve" , "There is a cycling problem". It seems unclear if there is a real issue that might bite people who have taken reasonable design decisions and got a competent installation or the merely the inevitable results of bad planning, installation and tuning.

I had a response from a happy user recently under the thread "Vaillant VRC430" which clearly shows that sometimes it can all work very well:

I'm interested to know in what way it doesn't work very well.

The flow is heated according to the desired flow temperature and circulated at that temperature with TRV's turning off rads as room temperature is reached. That's how my rads work so at some point the boiler is heating just the towel rails which I have plumbed in open circuit. The demand from two towel rails in October with weather compensation is tiny and the flow temperature can be as low as 31 C yet my Vaillant 24k system boiler manages fine just heating just enough to maintain comfortable temperatures.

I don't see how the load however small is relevant to how the system works with a weather compensated modulating boiler, and if it was such all systems would have problems when their TRV's started shutting down.

Rather than describe the problem again in the depths of a thread which was about a specific aspect of the VRC430's behaviour, I thought I would package up my reply in a sort of survey. I'm hoping some other users or installers will write in with the details of their systems and how well they work.


My system details:

Small 3 bed semi with cavity wall insulation and double glazing. Main pipes 22 mm copper, radiators on 8mm copper but large double in living room on 10mm copper. Main rad is 3kw at 70C and easily large enough - it has fully open valves and no TRVs. Aim was to heat only main room but I now also have kitchen and bathroom rads on medium TRV settings because it makes the system work better and I can afford and appreciate the extra warmth. I heat the main room to a frugal 18C, more if I have guests.

There is a Honeywell automatic bypass valve and this is finely tuned to get the best balance between a good flow through the boiler and through the rads. From temperature rise across boiler and other complex calculations I think I have a total of 11 litres/min through the boiler but only about 4 is sure to go through the main radiator. I use a Grundfos 16/60 pump for better flow.

The problems in technical detail:

The house cannot usually sink the minimum 5kw from the boiler which therefore has to cycle. The boiler modulates 5-15kw but starts at a steady 12kw and cannot modulate for approx 1 minute. This heats water by 16C so boiler cannot restart until return water is this temp below target. With normal settings it will not have cooled enough so retries fail. The proportion of flow through the bypass means that the heat input during failed retries remains in the "bypass circuit" and further delays success in restarting. May never restart properly if d2 is set low. With d2 set to maximum 60 minutes (resulting in actual delay in range 10 - 40 minutes at temps I use) then retries will normally work for temperatures of 45C and above so that is set as minimum in the VRC430.

The water going through the bypass arrives back at the boiler cooled only by mixing with a lower proportion from the radiators. This used to happen before the boiler was ready to modulate and would also cause retries to fail at a late stage after putting enough heat in to seriously compromise further attempts. I have fixed this by installing in the bypass circuit a radiator which acts as a sort of buffer. It delays the return of the heated water and mixes it so that the temperature rise is smoother and easier for the boiler modulation to deal with. The radiator could not be put anywhere where heat was required so it is fully insulated and is just a tank. This has been very effective.

With these modifications and a heating curve of 1.5 the system will maintain my target room temperature quite accurately. The temperature will only very occasionally fall to 17.5 and a little more often rise to 18.5. Response to changing requirements is not so good though. In particular, getting up to temperature in the morning is difficult. As the room temperature rises and often the outside temperature too, the VRC430 reduces the target temperature and this brings back the problems of boiler restarts described above. Increasing the heating curve doesn't really restore responsiveness, is less efficient and causes room temperatures to often overshoot the target which is less comfortable. I deal with the morning problem by setting a target room temperature of 21C for an initial period of 40 minutes followed by an off period of 1 hour. The initial overhot fill of the radiator reaches my target temperature reasonably accurately and maintains temperature until the water has cooled off enough to go into steady state mode for the rest of the day.

If I have honoured guests and wish to raise the temperature to a sociable level, I override the VRC430 by setting the minimum flow temperature to some figure which will give a high enough rate of heating and disable the heating curve by setting it very low.

I have lately noticed during one of these manual heatups that the VRC430 has started controlling the anti-cycling itself by stopping calling for heat when the boiler stops even with room target not reached. Actually it is rather more conservative than the boiler with d2=60 and this is a bit irritating because it waits longer than necessary. I'm rather worried that Vaillant may have set it up to be too stupid to get it right but just clever enough to stop me outwitting it. Just today I felt inspired to see if this VRC430 smartness might work in the no-fly zone below 45C. But no, it just keeps calling and leaves it to the boiler which is too optimistic so 45C remains the limit.

Problem impact

Had to modify the system at some expense
Ridiculous effort to measure, analyze and tune the system with no help from manufacturer
System is reasonably efficient but not responsive and needs a lot of hands-on coaxing to get particular results.

The good news

The overall efficiency is now very good and heating bills very low.
 
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Thanks for the info. Ignore the end of my post in the other thread.

When you say you have an open vent boiler, do you mean you have a separate pressure vessel?

I don't want to get into the history, but I had my first condensing boiler fitted about 18 years ago. Into an open vent system with a tank in the loft and rads dating from the fifties but with TRV's on all rads. Nothing to stop the boiler cycling except the max temp sensor that had a max of 60C. I had a lot of problems with the system many of which were cured by getting rid of the tank and installing a pressure vessel. I cured the cycling by getting a Danfoss BEM 4000 weather compensation system fitted. The whole system including the boiler were experimental so none of this was an issue for me, and the boiler designer and system designers were making changes according to my observations. I'm an architect so it was all down to professional development. At home I can do experimental stuff you can't try out on a client.

My minimum flow temperature is set to 25 C. I let the Vaillant weather compensation take care of the flow temperature. My heating curve is set to 1.7 but I haven't experimented much with that to be honest. I have a large 1905 4 bed house, two living rooms, huge kitchen diner and heated garage.
No double glazing, solid walls but insulated internally on three sides of the house. The fourth wall is cavity with about 6" insulation as I built an extension all along one side. I had the same pump as you set on the no 2 setting. This was on the old boiler pre Vaillant with it's own inbuilt pump. I would not expect you to need more than the no 1 setting on the pump you have.

Maybe I should add I have the VRC 430 set to modulating. I think it would help if Vaillant published a tech pdf on how their system works. I had a lot of info from Danfoss on how their weather compensation works so I had a good grounding in weather compensation to start with and came to Vaillant with expectations based on the Danfoss system. I would say the Vaillant system is a bit better than the 17 year old Danfoss system. As you would expect really.
 
OP Do you have a hot water cylinder connected to your system as I haven't seen you have mentioned one? Are there any zone valves on the system?

As starters I would shut your bypass fully, you have a large rad without a TRV to act as a bypass, no need in heating the return prematurely. Also using D.1 pump over-run, try it on 25 and reduce your D.2 to 40. This will ensure the temperature drop during anti cycling is 'seen' by the boiler & 430. If you use a short pump over-run the status code on the boiler will show S.8 when the pump stops and return a 0C desired flow temperature on the 430. Also with the long pump over run the temperature in the property should be more stable.

Agreed the high output of the ecoTEC 415 in the first minute can cause overshoot on the flow temperature when WC is used. It can be altered in software but officially thats a job for a very knowing Group Service senior engineer who has expert Vaillant knowledge :cool:
 
OP Do you have a hot water cylinder connected to your system as I haven't seen you have mentioned one? Are there any zone valves on the system?

As starters I would shut your bypass fully, you have a large rad without a TRV to act as a bypass, no need in heating the return prematurely. Also using D.1 pump over-run, try it on 25 and reduce your D.2 to 40. This will ensure the temperature drop during anti cycling is 'seen' by the boiler & 430. If you use a short pump over-run the status code on the boiler will show S.8 when the pump stops and return a 0C desired flow temperature on the 430. Also with the long pump over run the temperature in the property should be more stable.

Agreed the high output of the ecoTEC 415 in the first minute can cause overshoot on the flow temperature when WC is used. It can be altered in software but officially thats a job for a very knowing Group Service senior engineer who has expert Vaillant knowledge :cool:

Hot water cylinder via 3-way valve which is controlled by a VR35 widget.
Works acceptably. No issues I want to raise.

Closing the bypass does not work. The problem is that the main rad is on 10mm microbore. Only about 4 litres/minute will go through it. The boiler spec says minimum 10.6 litres/minute. Switches itself off at really low flows - will work at less than 10.6 but the temp rise return/flow is even higher than 16C and that makes the cool down problem much worse. I have done the experiments.

When I planned this, I installed a much larger radiator and changed the 8mm drops to 10mm. These are totally adequate for the radiator. Knowing what I do now, I would have put in 15mm despite the extra work. Then I could have done without the bypass and things would work much better.

I have d1 = 40 at present. The pump has to be run to cool the water in the boiler and main circuit. Otherwise the restart problem is made worse. Actually 40 minutes is not really long enough but I don't really like to run the pump all the time as I begrudge the 60 watts. I'm working on a circuit which will control the pump and run it intermittently until the system is cool. I have found by controlling the pump with a manual switch that a cycle of 1.5 pump then 3 minutes off gives the best results. Not only does it reduce pumping costs to 1/3 but actually cools the system faster. This is because the radiator water stratifies during the off phase.

I'm not really understanding your idea for d2. In the beginning it seemed that the VRC430 would always keep calling for heat and the boiler would handle anti-cycling. As I have explained, only the very longest period will do. I have seen the VRC430 taking over rather unhelpfully and spinning it out even longer - that might seem to make d2 irrelevant. Thing is that it doesn't seem to do that reliably. For example, I experimented at 43C minimum yesterday and it just kept calling. Boiler used 42 minutes anti-cycling which is not long enough.

I have very carefully explained this problem to Vaillant and they have measured my system. They haven't officially admitted the 1st minute problem. Are you absolutely certain that there is something fairly simple that could be done about it? As it stands they have simply left me in the lurch with a system that as far as they know hardly works at all. Actually I would find it incredible that they could fix it reasonably simply and have not bothered.

You mention the Ecotec 415 as having this problem. Do you know which of the Vaillant boilers are affected? One of the points if this post is to try
to find out what the scope of the problem is. Of course I would love for Vaillant to discover that it has enough impact for it to be worth their while to fix it.
 
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I guess you mean the VR65. You have got D.70 set to 0 haven't you? :eek: :eek: :eek:
 
MHOL";p="1775679 said:
You mention the Ecotec 415 as having this problem. Do you know which of the Vaillant boilers are affected? One of the points if this post is to try
to find out what the scope of the problem is. Of course I would love for Vaillant to discover that it has enough impact for it to be worth their while to fix it.

Yes I do, and it brings the appliance out of approval so can not officially/legally be done I'm afraid.
 
I guess you mean the VR65. You have got D.70 set to 0 haven't you? :eek: :eek: :eek:

Crossed in the post there. Good guess. Haven't changed d70 just checked it is 0. I'd be seriously miffed if there was a magic fix. I had hopes for d17 but experiments showed it to be unhelpful. Otherwise I only have d71 and d78 set in the "second diagnostic level".
 
I may be duplicating some stuff which has gone before, but the pump is wired to the boiler I take it. Also as you have rads 1.7 on the curve is quite low, have you had much success with it around 2 and range rating on D.0 to 9Kw ish.

It would be a lot easier to see the whole picture with VRDialog on the laptop and be on site.

Will be out now till after 8:30pm

Bunny
 
Interesting thread with some good points someones got time on there hands! Poor system pipe design I think ? Tryin to get software to overcome to many abnormalities 831. ???
 
I just had a thought that my gas consumption figures might be of interest.

The cost for heating and hot water 24/7 with a two degree set back at night for 6 hours is £510 p.a. which for a large 4 bed detached 1905 house as described in my post above I think says a lot for weather compensation.
 
Interesting thread with some good points someones got time on there hands! Poor system pipe design I think ? Tryin to get software to overcome to many abnormalities 831. ???

Getting my system to work as well as it now does has had to be a sort of hobby. Certainly no-one else was going to help me. It is my good luck that I'm retired and have the time.

On the piping. I'd guess that a lot of people replacing a boiler in an existing microbore installation would not suspect that this would be problem. Actually, I thought myself very clever to have been able to work out that I needed 10mm for the new main radiator and went to a lot of trouble to rip out the 8mm and replace it. I didn't see anywhere warnings about microbore and condensing boilers. Otherwise the pipework is fairly sensible 22mm stuff.

My other abnormalities are actually in accordance with the overall wish to use as little gas as possible. Heating only the main room and that to a low temperature. That is one of things that Vaillant have tried to say - what I'm trying to do is somehow abnormal and illegitimate.

If you look at the trends - better insulated smaller properties, less money, more expensive fuel, concern over climate change, then I'm thinking I'm doing something which ought to be the norm.

If Vaillant's kit doesn't work for installations like mine, they should say so up front. The fact that other manufacturers have similar issues shouldn't be relevant.
 
I may be duplicating some stuff which has gone before, but the pump is wired to the boiler I take it. Also as you have rads 1.7 on the curve is quite low, have you had much success with it around 2 and range rating on D.0 to 9Kw ish.

Bunny

The pump wiring is from the boiler at the moment. I'm now using 1.5 heating curve. This maintains my target room temperature well, avoids temperature overshoots and means that I'm often operating with efficient temperatures. I had to use higher numbers before my latest changes and they didn't completely overcome the problems or make it any more responsive. If my intermittent pumping scheme works as well as I think it will I might even edge the heating curve lower.

The reason why higher heating curve isn't a magic bullet for responsiveness is that the VRC430 moves the target temperature goalposts during heatup and then I get failed retries.

It is range rated at 10kw. I had that at 8 before the "great leap forward" but that seems to significantly impact heat up from cold.
 
I just had a thought that my gas consumption figures might be of interest.

The cost for heating and hot water 24/7 with a two degree set back at night for 6 hours is £510 p.a. which for a large 4 bed detached 1905 house as described in my post above I think says a lot for weather compensation.


My gas bills are around £200 p.a. and that includes daily use of a gas hob. I wonder how much it says about weather compensation as opposed to "modern condensing boiler" though. From the figures I have seen there is maybe 8% extra from weather compensation used well as you seem to be doing. Say £40 p.a. That would pay for the VRC430 in 3 years. If you made a lot of changes to pipes, rads etc to use low water temperatures, then the value is less clear.

If you add in costs associated with kit which doesn't work or is hard to tune, then you can see why there might be a lack of general enthusiasm.

If that is to be turned around then the kit needs to work and be easy to set up. So far there seems to be no public awareness of the sorts of issues I have found and it seems that industries try to get away with it unless they are held to account.
 

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