Impossible to fix flow problem? 4 grand down so far...

[Old UKPF 99429]

They are not all fully open, although yesterday went round and opened most up. It the past it behaved better with some turned off completely, and with them balanced a bit.

 

[Harvest Fields]

Remember the position of them all by writing them down then open them all fully. Then close them all apart from one and see if that one gets hot all the way back to the boiler then close that one and open another until you get to the last rad

 

[Old UKPF 99429]

Thanks, will do that. I always read start with the one closest to the boiler, but think I need to start with one close to the pump (which is upstairs)? Never know which is first in line. But will shut them all off tonight and try. What would it tell me if it works fine with just one radiator! (which I think it might)...

 

[Harvest Fields]

Thanks, will do that. I always read start with the one closest to the boiler, but think I need to start with one close to the pump (which is upstairs)? Never know which is first in line. But will shut them all off tonight and try. What would it tell me if it works fine with just one radiator! (which I think it might).

Not a lot you need to do each rad one at a time

 

[Best]

You have very slow flow in the circuits and that is why balancing and turning some rads was appearing to help the system work better.
It could be a part blockage or a valve somewhere nearly closed.
It can be in the flow or return and the return being cool proves nothing.
Have any of the tee off connections on the return where old cylinder etc was checked for blockages. A strong magnet out against a copper fitting or pipe will suggest magnetite at that point if the magnet is pulled to the pipe/fitting.

 

[Old UKPF 99429]

Phoned Viessman today and they were adamant that is was a circulation problem (as am I). They even finished my sentence for me, I said "my boiler starts up, the temp increases... " and he said, "and then cuts-out at 50 degress, and goes back down again"... Apparently because it is looking for a temperature difference of 20 degrees between flow and return (as expected).

Best - I had thought that perhaps it could be some of the old piping right next to the boiler where the 3 returns join together, might explain why probalem seems to occur on HW only, HW and DH, and just CH?! Even though they are separate returns...

 

[sammathias]

Send us some pictures of this pipework fella.

 

[Rob Foster]

I like this type of problem, where are U based

 

[ShaunCorbs]

I like this type of problem, where are U based

about 80 miles between you and the op

 

[fixitflav]

Does it not sound normal to have 3 returns converging at the boiler, one comes up out of the screed, and two from upstairs.

There's nothing wrong with that, I've lived in houses with that arrangement. The only thing you need to watch is that all heating returns must be commoned before the HW return comes in, or you can get flow through rads when HW only calling. But it wouldn't cause your symptoms.
For your problem, do you have a 3-port valve or 2 (or more 2-ports)? If the latter, worth making sure none is stuck closed. Though if it's wired correctly the boiler and pump should run only when 1 or more is open, to avoided dead-heading.
Is there a bypass in the system? If that is wide open it might be taking all the flow, with little or none left for the rads.

 

[Rob Foster]

about 80 miles between you and the op

160 miles No bother thats about 2.5 gall of diesel in my stink pot 4 round trip, msg me and I will come and see what can be done.

 

[Old UKPF 99429]

Got a three port valve, that is also new (well, 20 months old and seems to be working fine), it sits 2 inches above the pump. No idea if I have a bypass valve? Sorry, wouldn't know where to look?!

I turned all rads off apart from one earlier, and still no joy. The boiler simply shoots up to 50 then cuts-out, the rad does get a little heat into and would get hot if the boiler would stay on. Also, then turned on a towel rad, same story, rad immediately got warm, but never hot as the boiler wouldn't stay heating... So clearly, the circulation is not good, although the boiler is looking for 20 deg differential between flow and return rather than measuring flow rate I think.

So, it was sort of working, with around 8 rad (some turned off) at the end of winter, slow to heat, but would heat up eventually. And yet now, after being left for 4 months, doesn;t work at all. Not sure what that tells me!

 

[sammathias]

Don't the vitodens have an internal pump and you also have an external pump? Bypass would be fitted after pump before 3 port valve. Need some pictures of your set up...boiler pipework/ pumps etc.

 

[Jim Goodenough]

Have you checked pump valves are both fully open. Very basic suggestion but could be a knackered pump valve??

 

[fixitflav]

No idea if I have a bypass valve? Sorry, wouldn't know where to look?!

The bypass (if there is one) would be from pump discharge to boiler return, in parallel with the load, so if the rad controls throttle down to nothing there is still a flow through the boiler. I believe modern bypass valves don't just throttle but have some flow control feature, so they don't bypass a lot of flow when not necessary.

the boiler is looking for 20 deg differential between flow and return rather than measuring flow rate I think.

I don't understand that, perhaps the experts here can explain. For a given pump flow and boiler design output, the differential is fixed. If that is greater than 20°C, and the boiler is looking for a maximum 20° differential, when that is reached I would expect it to modulate (on a modern boiler) to maintain 20°, but then boiler output is below design. To get design output, need to increase the water flow. As the system heats up, if it's working properly the flow and return temperatures both rise, keeping the 20° differential.
But I don't see any advantage over controlling to a boiler flow (output) temperature, the more traditional way.
There doesn't seem any reason to think your water flow is blocked, and it would have to be severe to give your symptoms, so it looks to me like a boiler control issue. Might be worth discussing with Viessmann, and asking if there are any other options for boiler control.

 

[sammathias]

The boiler always "kettles" on starting up too.

Boiler empty 😎

 

[Old UKPF 99429]

Fixitflav - and that is what I don't get... Seems to me like perhaps I do have a circulation problem a bit but def no blockage as such. And it feels like the boiler is just too picky about the conditions it will operate in. Thinking the only way to tell for sure is putting a flow sensor in, on the return by the boiler? Could just be that I have some circulation, but very limited, the pipes are 35 years old...

 

[Old UKPF 99429]

I also think if I was to put a huge pump in there, it might just work!

 

[fixitflav]

Fixitflav - and that is what I don't get... Seems to me like perhaps I do have a circulation problem a bit but def no blockage as such. And it feels like the boiler is just too picky about the conditions it will operate in. Thinking the only way to tell for sure is putting a flow sensor in, on the return by the boiler? Could just be that I have some circulation, but very limited, the pipes are 35 years old...

Maybe sammathias has a point, it could be as simple as that. What is system pressure? Is there flow from a vent point on the boiler, or the pipework close by?
Where is the expansion vessel connected?
Some photos and sketches would be useful.

 

[Old UKPF 99429]

System pressure is sat around 0.6 bar rising up a bit when hot water is on currently. Can top it up later, didn't think that would make a huge difference? Expansion vessel is connected on the return I think, next to the boiler, will draw it out later... Wasn't sure I understood the question about flow from the vent point on the boiler?