Vaillant 418 HW cycling / S.7 question

[WillJoiner]

Hello

Recently moved into a 3bed and having some trouble with the HW, perhaps someone here has some advice. This is my first property with an open-vent system/cylinder/etc, it's been...interesting...to learn about.

Vaillant 418 in heat-only mode (only the lower dial controls temp request), around 160L cylinder (can't find any info on the cylinder, just a quick measurement/calculation for the volume), Y-plan with a Grundfos UPS2 15-50/60 pump set to Speed 3, single old Honeywell stat on the cylinder and a single old Honeywell room stat. Siemens RWB29 programmer, 9 older radiators of various sizes and 2 newer towel radiators (one in each bathroom). Terrace house built in the 80s - I imagine this is a standard heating system for this type of property.

Anyway, at first the 418 would only briefly fire and then go straight to s.53. Barely any HW or CH...took about 9 hours of this behaviour to just warm the cylinder so I figured there was a problem. After browsing this forum I learned about the Vaillant diagnostic codes, d.0 specifically, and saw that my 418 was set to max power and I moved it down.

Setting d.0 to 10 and temp dial to 70C sorted out the CH for the most part...nice long burn of about 30min before it then hit an s.7, then another longish burn after, etc. Radiators nice and warm. Looking at d.40 and d.41 shows a 16C difference between them, I think this is about right?

HW on the other hand is still a problem. Asking for HW kicks of a first burn of about 2min and the flow temp ramps up very quickly before the 418 stops and an s.7 happens, then the burns get shorter and shorter until they're maybe 20 sec burns before the 5min s.7 kicks in, over and over. Difference between d.40 and d.41 with a HW request is 5C. It takes around 3 hours to heat the 160L cylinder to 55C with this behaviour when (ideally) I think it should take less than an hour? Dropping d.0 lower than 10 doesn't seem to help either.

Additionally, when HW and CH are both on, the last few radiators on the circuit don't warm up and the 418 still has the short cycles with s.7. I thought that perhaps this was because now there wasn't enough power being generated for both HW and CH with d.0 at 10 so I experimented with moving d.0 back up, but nothing changes.

It seems as though whatever I do with the HW, the 418 short cycles and s.7 pump overrun status shows up. It's as if there's too much power being supplied to the HW circuit/residual heat from the HW circuit can't be dissipated quickly enough whereas the CH circuit is fine. Is this likely to be some sort of circulation issue with the HW, or are there settings on the 418 which also could account for it?

Thank you!

 

[picasso]

It sounds like the cylinder coil is scaled up or perhaps very old and ineffiecent, is there a balancing valve on the return from the cylinder (normally a gate valve with a red handle )?

 

[WillJoiner]

Unfortunately there's no balancing valve that I can identify.

 

[Wannabebetter]

Not sure if this is relevant, but have you checked that the 3 port works ok?

 

[jackthom]

The s53 suggests there is insufficient flow of water through the boiler; it throws this code when there is too great a difference between the flow and return temperatures. The rapid rise in flow temp, even when the water in the rest of the central heating system system is starting from cold, is a classic symptom.

It may be the pump is on the way out or something else is hindering the flow.

The difficulties with heating the hot water cylinder may also have the same cause, plus the cylinder may not be a fast recovery type and unable to transfer the amount of heat being supplied.

 

[WillJoiner]

Thanks much for all the replies - looks like possibilities could include scale on the coil/in the tank, the 3 port valve, and the pump. Easiest ways for a nonprofessional like myself to check these?

For scale can I just draw down water out of the cylinder, pop out the immersion heater and take a look through the top? I assume if the coil or cylinder is badly scaled there's nothing to do but have the whole thing replaced?

For the valve I see a motorized switch thingy on the top of it, I assume this is what opens/closes the valve to direct flow to cylinder/CH/both. There's a metal lever on the switch that is currently in the AUTO position, it looks like you can move it to the right to a position that says MANUAL. How would I manually check if the valve is restricting flow to the cylinder?

Jackthom for the pump do you mean the Grudfos pump in the airing closet? Some Vaillant boilers appear to be able to measure flow rate and display it as one of the diagnostic codes, but my 418 doesn't seem to have this ability, and it doesn't have a pressure display or some of the bits that my previous boilers had. Is there a simple way to measure flow rate, or check the pump in some other fashion?

Sorry for such basic questions, I'm much more comfortable with electrics, plumbing always seemed like Black Magic to me and this is the first time I've needed to understand a bit about it as every other place I lived in it 'just worked'. I suppose I've been lucky!

 

[Mottie]

For scale can I just draw down water out of the cylinder, pop out the immersion heater and take a look through the top? I assume if the coil or cylinder is badly scaled there's nothing to do but have the whole thing replaced?

I'm not a heating expert but I think if you did that, you'd only be able to see the outside of the heating coil - you need to see the inside of it.

 

[WillJoiner]

Ah yes of course, I was thinking about scale out the outside of the coil that might be insulating the coil, but of course that would have nothing to do with flow through the coil. Cheers!

 

[jackthom]

Last time I had similar problems I found our Grundfos pump impeller was clogged with small flakes of magnetite.
You can close the valves at both sides of the pump body and easily dismantle it with an Allen key.
Mine was much quieter and back to working properly after a gentle scrape out and a good flush under the tap.
The state of the pump interior might give you a clue as to how clean the rest of the system is. I invested in a Magnaclean afterwards.