Another Viessmann 100 problem

[MikeAustin]

This system gets more perplexing every day! Running HW alone allows the boiler to operate over a reasonable temperature range: 52C/72C. Running one rad only is similar: 47C/70C. Running two rads drops it to 41C/52C. I was running a Wilo Gold RS50 at speed 3.

I renewed the pump to an RS60 (yes, I know it's overkill!)
Running at speed 2 with two rads, boiler runs at 42C/65C.
Running at speed 3 made no difference.
Adding a third rad, boiler runs at 42C/57C.
Adding the HW circuit, boiler runs at 45C/68C.
Adding all rads (9), boiler runs at 41C/60C.

All this is shown on the following graph:
View media item 10321
I think the warmer HW return allows the boiler to run at a higher temperature, while the CH just trickles round. Eventually, CH builds up to temperature because the boiler doesn't short cycle on a warm HW return.

I think there must be a restriction in the CH circuit. I suspect a T-piece. The CH return is (from what I recall) a 15mm into 22mm T-piece. Could a mis-soldered joint like the following explain this behaviour?
View media item 10323

 

[Agile]

Either the flow rate is too low or the boiler output power is too high.

Tony

Thats what I said before I stopped helping you.

To anyone reading what he has just said, the lower figure is NOT the return temperature which he has consistantly refused to give me. Its what the flow falls to at the end of the anticycling delay.

Now you seem to be suggesting that your heating circuit is only in 15 mm !

What you have drawn should not be possible on a properly formed fitting but some cheap ones are not well made.

Tony

 

[MikeAustin]

To anyone reading what he has just said, the lower figure is NOT the return temperature which he has consistantly refused to give me. Its what the flow falls to at the end of the anticycling delay.

Tony, I never said that this (41C) was the return temperature, and I have consistently not had a suitable thermometer to measure it.

Now you seem to be suggesting that your heating circuit is only in 15 mm !

I have not suggested that the heating circuit is 15mm. From what I recall, there is one such T-junction that the installer pointed out under the floorboards. This will be my next check.

It seems to be the case is that the return from the HW is OK, but not from the CH. The former can sometimes allow the latter to function, albeit in a feeble manner.

 

[Agile]

Tony, I never said that this (41C) was the return temperature, and I have consistently not had a suitable thermometer to measure it.

You may not have actually said it, but for any heating engineer its quoting the flow and return temperatures which are to salient temperatures and on which an assessment of your system malfunctions would be based.

If all your CH is fed or returned from a 15 mm pipe then its not surprising that there is a restricted flow and the boiler cuts off so quickly!

Tony

 

[MikeAustin]

If all your CH is fed or returned from a 15 mm pipe then its not surprising that there is a restricted flow and the boiler cuts off so quickly!

Not all my CH is 15mm. Apart from this dubious T-junction, I also have long 15mm runs to some radiators, which is not very smart. However, the system did work (inefficiently) for 3 years. I think the explanation must be because the boiler was operating on a warm HW return (probably poor cylinder insulation). Does that make any sense?

My job now is to address this T-joint and make any 15mm radiator runs as short as possible. This T-joint, for example, comes after a run of several feet of 15mm pipe alongside the 22mm return. I see no practical reason for this arrangement.

 

[Agile]

15 mm is fine for feeding each rad probably up to a distance of about 10 m !

Its limited to about 3-4 kW on a shorter distance of about 3-4 m.

Tony

 

[MikeAustin]

15 mm is fine for feeding each rad probably up to a distance of about 10 m !

Its limited to about 3-4 kW on a shorter distance of about 3-4 m.

Tony

Thanks. I will change whatever pipe runs that I can and which make sense with these figures.

 

[MikeAustin]

Well, one of the installers came round this evening. They are totally baffled by my system! The installers replaced the boiler in 2005 and the cylinder a few weeks back, but they are not familiar with all the pipe work.

The boiler is functioning OK - proved by the HW working. The CH circulates on 2 rads but not on 3. It works to a fashion if HW is switched in as well.

One guy suspects there may be 2 systems joined together with an NRV somewhere - but he said that was clutching at straws. They are embarrassed they cannot work it out and are going away to think about it!

 

[bengasman]

The whole system is clearly poorly designed and poorly installed. You will probably keep having problems until you get somebody in who is properly qualified and knows what he is doing.

 

[Agile]

They are embarrassed they cannot work it out and are going away to think about it!

More likely going to the pub to have a drink and forget about it!

 

[MikeAustin]

They are embarrassed they cannot work it out and are going away to think about it!

More likely going to the pub to have a drink and forget about it!

That's one flow they will not have a problem with!

 

[MikeAustin]

My system is fixed now. It turns out there was crap in the boiler. After a nice long flush through with FX2, it is sorted. This was done one heating loop at a time. The boiler made a terrible noise as all the stuff flushed through. The MagnaClean got MagnaFilthy. Also, for good measure, some pipework was tidied up.
Right from the word go, I though it must be blockages somewhere. But I do have a new controller board and ignition block, which is probably a good idea anyway.

 

[JohnD]

the system has been power-flushed, a MagnaClean unit added,

so how was it still dirty?

 

[MikeAustin]

the system has been power-flushed, a MagnaClean unit added,

so how was it still dirty?

There was sediment that was broken down by the FX2. The MagnaClean trapped it once it was shifted.