Reverse boiler flush? (Viessmann Vitodens 100)

[Phill Peck]

Your boiler has no idea if it is doing heating or hot water.

It is a heat only all it knows is that it has a switch live supply so it fires.
It then modulates its output passed on flow and return temperatures.

IF YOUR BOILER CIRCULATES WITHOUT CYCLING ON HW ONLY, THE PROBLEM IS NOT IN THE BOILER.

 

[MikeAustin]

Your boiler has no idea if it is doing heating or hot water.

It is a heat only all it knows is that it has a switch live supply so it fires.
It then modulates its output passed on flow and return temperatures.

IF YOUR BOILER CIRCULATES WITHOUT CYCLING ON HW ONLY, THE PROBLEM IS NOT IN THE BOILER.

Maybe you need to know the full background. It had the same problem 6 years ago. I asked the installer to change some pipework that I suspected. He did this, but said it needed a new PCB so replaced that. He also ran FX2 through it (no power flush). The system then seemed to be OK - not brilliant, but OK.
A few weeks ago, having these problems again, I asked for a boiler service and a check. When he flipped down the control panel, the boiler fired up when both CH and HW were switched off. It went up to 75C. There was no demand from CH or HW - he checked the contacts to confirm this. That was quite bizarre behaviour from the boiler! He phoned Viessmann support, but they were too busy and never phoned back. He did not service the boiler because of this problem, but suspected the PCB. The cost of replacement PCB was prohibitive and I considered a replacement boiler. All I am doing now is to try and make it limp through the winter with as little cost and effort as possible so I can get the whole system overhauled when it is warmer.

 

[Agile]

There is no mention of clearing air from the heat exchanger in either the installation instructions, service instructions or fault finding. Nor is there any mention of filling the HE. Are you familiar with the Viessmann 100c and clearing/filling its HE? Or does anyone else know?

Unfortunately that is quite correct.

But you have to realise that what you are doing is what is normally done by gas registered engineers who have been on the Viessmann boiler course at Telford to obtain their accredited installer status ( and a longer warranty period ).

But there is nothing very special about flushing it to get rid of the air. All you need to do is to find a way to connect a high flow rate of mains water into one end and take to other end outside or into a drain. But it does need a really good flow rate to ensure all the air is expelled.

Call me if you need further help.

But it would be very good if you could do this. It could well be the cause. But like all diagnostic work you have to eliminate all the possible problems before considering something else. It does not seem likely that this has been done adequately in this case.

Tony

 

[MikeAustin]

There is no mention of clearing air from the heat exchanger in either the installation instructions, service instructions or fault finding. Nor is there any mention of filling the HE. Are you familiar with the Viessmann 100c and clearing/filling its HE? Or does anyone else know?

Unfortunately that is quite correct.

But you have to realise that what you are doing is what is normally done by gas registered engineers who have been on the Viessmann boiler course at Telford to obtain their accredited installer status ( and a longer warranty period ).

But there is nothing very special about flushing it to get rid of the air. All you need to do is to find a way to connect a high flow rate of mains water into one end and take to other end outside or into a drain. But it does need a really good flow rate to ensure all the air is expelled.

Call me if you need further help.

But it would be very good if you could do this. It could well be the cause. But like all diagnostic work you have to eliminate all the possible problems before considering something else. It does not seem likely that this has been done adequately in this case.

Tony

Thanks again Tony. When it was installed eleven years ago, and when the installer reworked some pipework five years later, he did not do this. He put in the mains loop, which is what I have been using. Could it have been running with air in all this time?
I'm off to do another mains flush of the boiler - not done one for two days now! I have been on my roof fixing a leak. Seems my roof may be flowing faster than my CH system!

 

[Agile]

Don't fall off the roof.

You know what you mean but I don't.

If you think that it is the same thing then as an engineer I need figures!

So what is the flow rate?

What I am talking about is a high velocity of water to expel air. Not a slow flow to change the water.

Tony

 

[MikeAustin]

Don't fall off the roof.

You know what you mean but I don't.

If you think that it is the same thing then as an engineer I need figures!

So what is the flow rate?

What I am talking about is a high velocity of water to expel air. Not a slow flow to change the water.

Tony

The installer used the mains loop that he put in. I have used this as well. This is a simple flexible hose connection from the mains feed to the F&E tank into the vent pipe, with all the necessary valves in place.
With a reverse flush, measured a few moments ago, it was 8.6L/min. With a forward flush (cleaning out the return pipes) it was 12.0L/min. I would need to know the diameter of the pipes in the HE to calculate the velocity, but I guess flow rate is what you are after.
The reverse flush passes from just upstream of the pump to the boiler flow and out through the boiler return. The Magnaclean, just upstream of the boiler return, is shut for this. The main resistance here is the boiler HE and the drain cock (I guess).
The forward flush passes through the pump to any number of radiators (I did them individually and collectively) through the Magnaclean and out from the boiler drain. The boiler flow is shut for this. The main resistance to this would have been any return blockages.
Given that the return pipework flows nearly 50% faster than the flow pipework, would this be accounted for by 'normal' resistance in the HE? Or does this indicate a possibly blocked HE? From my flow measurements, I would guess I can rule out blockages in the return pipework - one of my main concerns. What do you think?
I am running the system again now, having done one reverse flush, one forward flush and another reverse flush.

 

[MikeAustin]

I have cracked it! My boiler is now running constantly at 67C (64C at the pump and with nice hot radiators.
The problem is the location of the boiler - in the garage, which is a bit cold. The flow and return pipes are lagged, but apparently not enough - particularly at the elbows and round the Magnaclean (I have a 'chill' bag round that!). The return was cooling down too much just before the boiler - so that is why it was cycling. I stuffed all around the pipes with extra bits of foam pipe lagging and tied several layers of blanket round everything and it is running fine now. Of course, it looks a mess at the moment so I will try to sort out something a bit more sophisticated in due course.
I was wondering why the boiler's temperature was reducing in the colder weather over the last couple of days. I thought it was something I had done. However, I do think the boiler is a bit inadequate for my purposes. It has taken about 90 minutes to heat up from mains temperature of 11C to the current working temperature - and it was cycling a bit at the beginning.
Anyway, thanks to all you guys for your help!

 

[Steelmasons]

I have cracked it! My boiler is now running constantly at 67C (64C at the pump and with nice hot radiators.
The problem is the location of the boiler - in the garage, which is a bit cold. The flow and return pipes are lagged, but apparently not enough - particularly at the elbows and round the Magnaclean (I have a 'chill' bag round that!). The return was cooling down too much just before the boiler - so that is why it was cycling.

BS...
Never heard so much shyte in all my life...

 

[Aquaheat]

best get the extra lagging and the blankets stocked up on the van then.

Who would of thought that when it's cold you have to put an extra blanket on the boiler to keep warm!!

OP, If you strap a hot water bottle or two to it too it should reach operating temperature quicker than 90 minutes as you will have additional stored heat to increase the return temperature quicker. Basically its like a rubberised thermal store.

After you finish with the heating whip off the bottles and fill the bath. Free hot water!! Puts your boiler into the A++++ band on the ErP ratings too so always good if you are considering selling the house.

Jon

 

[MikeAustin]

The last two posts miss the point of what I said. I am not getting something for nothing here. Imagine if you could cheat the boiler into thinking the return was, say, 5C, hotter. It would fire longer and reach a higher temperature to maintain the 20C differential. My bit of extra lagging (on the return pipework - not the boiler) does actually increase that temperature a little. My system is marginal enough for this to count. I am not saying that there are no other problems with it.
What is apparent is that my investigative actions are influencing the system's behaviour. When I am working on it, the garage door is wide open. The boiler is effectively external. Also, I have the floor boards up in one cool, unused room to inspect the return pipework over about 4 metres. This also reduces return temperature.
I am not saying the system is all functioning perfectly (not with 90 minutes to warm up!), but what I have done was enough to at least lift the maximum boiler temperature.

 

[Steelmasons]

BS.

 

[MikeAustin]

Please stop quoting my post code to me.

 

[Agile]

Sorry to disappoint you but I rather concur with the other recent comments above.

I don't understand any of what you have said about this flushing. I would have liked to see the mains water sent through just the HE without any other pipework and with a high flow rate ideally of over 15 li/min. That would just be to ensure that all the air has been expelled.

Can you tell me what the flow and return temps are on each of the three possible loads are? Engineers like detailed facts!

Unless you know something that I don't, the boiler is mostly concerned with its flow temperature and not its return temperature.

Now, could whatever flushing that you have done have removed air and caused some improvement in performance?

I don't see any reason why the boiler should not be able to give a flow temp of 75 C.

Tony

 

[shambolic]

I take it you have took an FGA reading, a gas rate, a working/inlet gas pressure test, case seal integrity and FSD check after going into the combustion chamber on this boiler?

 

[Steelmasons]

Please stop quoting my post code to me.

Then please refrain from posting BS.
Another keyboard fooked.