Vaillant Ecotec Plus 428 S53

[jackthom]

In my open vented Y plan system when heating up only the DHW cylinder with the boiler flow somewhere around 70C the S53 code is regularly appearing if the CH is then turned ON. The system is clean, the pump is healthy (set to max speed) and flow is good even when only DHW is being heated.

I can only think the sudden inrush of cold water at the return is not quickly enough reflected in the flow temperature so the boiler thinks there is too wide a temp difference, ie insufficient flow when the opposite is probably the case.

Has anyone had any similar experience of this, or any suggestions other than ditching the Vaillant and getting an Intergas?

 

[picasso]

The 428 is a fairly big boiler, you could try down rating it depending on how big your system is and see if it helps, a stronger pump would probably help as well.

 

[jackthom]

Yes its output is too much for the house, I’ve honestly no idea what I was thinking when I had it installed 14 years ago.
I have set d0 to 18kW and the boiler behaves fine under all other circumstances.
It’s only ever caught out (when flow is already at higher temperatures) by the rush of cold water when the heating circuit is opened, mostly when the CH has been off for some time.

 

[jackthom]

Eventually solved this with an extra relay to give DHW priority, ie to remove any Heating demand while the cylinder stat is calling for heat. There is now sufficient delay at the end of a DHW run for the boiler contents to be flushed through before it fires up again for heating.

I wonder has anyone here any experience of the more recent Vaillant 418 open vent boilers?

I understand they've now changed over to aluminium heat exchangers and I'm wondering if the waterways are less restrictive, hopefully making S53 much less likely to occur.

 

[fezster]

I didn't see your earlier posts, but I had the exact same issue with my 438. There is no solution to it, afaik. My boiler is correctly sized. The problem occurred with any of my 3 zones (u/s, d/s, DHW) and just as you said, only when one zone was running at very high flow temps and another opened.

One solution would be to put in a logic controller to briefly remove the SL and have the boiler restart its cycle once the cold water has mixed with the hot on the return. I looked into it but it seemed far too complex.

In the end, I went with a combination of low loss header (added benefit for me was I no longer required a commercial pump), which reduced the occurrence of s53, but did not eliminate it totally. Plus I added DHW priority. I seldom observe an s53 now, but if it does occur, the boiler recovers from it, whereas previously it would end up in a perpetual s53 cycle and require manual reset.

 

[jackthom]

I have a VR65 in between the cylinder stat and the boiler which triggers DHW demand by ebus commands. Fortunately when DHW is satisfied the boiler doesn’t ever seem to drop straight into Heating mode without several seconds of no flame. This allows the very hot water in the boiler time to disperse without triggering S53.

The boiler also generally starts very gently in DHW mode and doesn’t do that 70% of full power routine you get at the start of a heating session. It now always heats the cylinder in one continuous burn without overshooting and cycling. I have it limited to 12kW for hot water.

This is a conventional Y plan system except that HW ON isn’t connected to directly fire up the boiler and is used to trigger the VR65 instead ( and to energise the DHW priority relay ).

 

[fezster]

Similar to mine, documented here: https://www.diynot.com/diy/threads/vr65-vr66-for-hw-flow-temp-control.494071/page-2#post-4938942

You're right that when DHW priority finishes, the boiler turns the flame off and then shortly refires in CH mode.

 

[jackthom]

When I was previously using a raspberry pi and ebusd software to boost the flow temperature of the boiler for DHW, I experimented with dropping the flow temp dramatically for a few seconds when heating demand suddenly occurred. Unfortunately, before I could get the boiler to flame off, the 3 port valve was already open to heating and allowing the cold water into the boiler return, so I still couldn’t beat the S53.

As you say it all becomes too complex just to try and solve a problem that Vaillant have caused for us.

 

[fezster]

I'm not sure this is a Vaillant specific issue, though. The 30 degree max differential between flow and return is to protect the heat exchanger. An influx of cold water from another zone would almost certainly exceed this if the flow temp is at 70 degrees at the time - this would be a problem for any boiler manufacturer, I'd imagine.

 

[jackthom]

I'm not sure this is a Vaillant specific issue, though. The 30 degree max differential between flow and return is to protect the heat exchanger. An influx of cold water from another zone would almost certainly exceed this if the flow temp is at 70 degrees at the time - this would be a problem for any boiler manufacturer, I'd imagine.

I just think that an occasional influx of cold water at the return while the flow is at a high temp should be an expected event and the Vaillants could at least allow a little more time before deciding there's possibly a fault.

I'd really like to know if the more recent versions of this boiler are still as prone to throwing an S53.

 

[fezster]

My understanding is that a large differential between flow and return could lead to the hex cracking. So there's zero leeway without the risk of damage. Like I say, I don't think this is vaillant specific - every manufacturer must have a similar safety mechanism.