Vaillant EcoTec Plus 428 constant cycling

[divedive]

Hello All,
Bear with me please while I explain my issue. In the meantime I have been trawling this forum to seek answers to my issues and am well aware that the Vaillant 428 has problems.

We have the Vaillant 428 EcoTec plus installed for about 15 years. Its an extended bungalow (we have an upstairs) - 13 rads and a mixture of microbore downstairs and 15mm upstairs. Its open vented and HW is via a Megaflow. I know the boiler is oversized and we have had D.0 set to 18 since the instal. Pump is a Grundfos UPS 15-60. By and large its been fine until recently. About a month or so back we noticed that the pump was a bit noisy and the heating seemed to be less efficient. I bought an Alpha 1 15-60 as a replacement but wasn't happy with the performance - best we found was PP3. I managed to source a new old stock identical UPS pump and fitted that - still quite noisy and I still felt the system wasn't working well.
This week I bit the bullet, and drained the system, cleaned the F&E tank, fitted a Magnaflow Pro filter on the CH return, added some X400 and refilled. Its been a nightmare ever since - couldn't get the rads to heat last night, but seem to have hot water. Today it did eventually warm the rads but they were not hot so I drained again - flow through the hose is pretty decent - flushed through and refilled. Same issues. There seem to be insufficient flow as the boiler is constantly cycling so never runs long enough to get the heating hot.

Either I have a blockage in the system somewhere (seems unlikely as my drain flow was strong and all rads heat albeit very slowly), or an airlock, or the heat exchanger matrix on the boiler is clogged and thats been the root cause of my issue all along.

Contemplating a 25-80 pump to see if that will fix it.

Any help much appreciated!

 

[cross thread]

Can you upload a photo of the pump / valves and the position of the expansion pipe/ cold feed connection

 

[CountryFan]

Did you adjust any radiator valves? X400 is a cleaner, did you add any x100? Are all flow and return valves, including any magnetic filter ones fully open?

 

[divedive]

Expansion pipe and cold feed are in the roof space. Pump valves are in the airing cupboard ground floor - no changes to any rad valves. The only things I’ve done are drain , refill , bleed, and add the magnetic filter ( yes , valves are open on that). Apart from that it’s identical to how it was 2 days ago.

 

[CountryFan]

Is the motorised valve opening correctly?

 

[divedive]

Yep, it is. And to be sure, I took the motor off and turned it manually.

 

[divedive]

Ok, an update. Up early this morning as the house was cold! I have the system running - set d.0 to 6kw to give the boiler a chance to run steady state and put the pump on max, with just a couple of rads open and it ran continuously. I've opened all rads up and all are heating up, although slowly (as expected). This is important to me as it confirms that none of the rads are blocked, although that doesnt mean flow through all pipes / rads is optimal.
I still have a suspicion that flow resistance through the boiler may have increased over the 15 years or so its been in use and that leave little head left over for the pipework/rads , resulting in low flow and hence the cycling. I think its worth trying a bigger pump as its 10mins to fit and the cost is substantially lower than other options!
It may well be that running the boiler with lower d.0 going forward is the way to go - 6kw is plenty for the HW cylinder and we only heat a few of the rooms at a time anyway as we have a Wiser system on all rads, so total rating of those rads is prob less than 5kw.
If I had to chose again, I'd go for a boiler that had a lower resistance matrix and one which didnt apply full heat before modulating....hindsight etc
Any other thoughts/input most welcome!
Cheers
Steve