Vaillant Ecotec 418 regular boiler; which hot water cylinder to use?

[pbanks]

I'm trying to find what to look for to get the correct spec. hot water cylinder to work with my Vaillant 418 boiler.
Any recommendations for a conventional indirect open vented hot water cylinder that would be the best spec. heating coil wise to go with the Ecotec 418plus.? The existing tank is approx 1000mm high x 500mm . Its an open vented fully pumped system with 3 port valve and 11 radiators controlled by a Tado wireless control box and wireless rad valves.

I can't seem to find info on how cylinders are measured\& sold, regarding the internal coil\s & how efficient they are at transfering heat to the water. I've seen some that state a kw rating for the cylinder but not sure if thats what I should be looking at??. Most don't seem to say or just say "rapid" or "fast" recovery, what does that mean?.

The system was updated from an older oil fired boiler a couple of years now but I have always had a problem with it cycling when it is only doing the water. I have delved into the boiler settings so that it modulates down to 6kw max output when doing the water, which helps, but it still goes into anti cyling mode after a time. I have come to the conclusion that the coil in the hot water tank can't always take enough heat out of the water before it returns to the boiler & because its return temperature is close to the flow temp the boiler goes into anti cycling mode and shutsdown before starting the same cycle again after 2 mins. etc. etc.



Thanks

 

[ScottishGasMan]

They work best with Vaillants own control system, which can see the exact temperature of the system and set a spesific power output for the boiler to match the cylinder when its running to reduce cycling and improve recovery time.

However if your looking to keep the controls as they are, and assuming the size (volume) of hot water you currently have is ok, then the same size of replacement with as big a coil as you can get is the way to go. if you can get one with an 18kw coil then great, if its 14kw, its still probably much better than the old one still.

The coil ratings are from the boiler running high temp and the cylinder being cold AFAIK, so as the cylinder heats the transfer of heat slows and you get less heat moving into the cylinder, so when the cylinder starts to climb in temperature, even on a big coil it will still cycle in most cases, which is fine as it will have had a straight run for most of the re-heat, but also means that you dont have to worry to much about getting a coil that matches the boiler exactly.

 

[pbanks]

Thanks for your reply,
So from what you say it's normal for the boiler to cycle when the water temperature in the tank gets near it's target temp. I was thinking it shouldn't. I'll have to do some more monitoring on it to see how long its doing the cycling to see if I'm bothering unnecessarily.

I do have the Vaillant65 wiring control centre(orVR66) can't remember now exactly which as I fitted it about 2 years ago. It has the Vaillant NTC cylinder stat installed in the now unused emersion heater stat tube in the tank, this plugs into the VR65 control centre. The VR65 is connected back to the Ecotec 418 boiler on the Ebus signal cable. The Tado control centre connects to the VR65 and signals it to modulate\control the boiler.
It seems to work reasonably well apart from when doing only the water heating and it seems not being able to modulate the boiler down quick enough when it has restarted after having gone into anti cycle mode. So when it fires again & again the return temp flies up within a few seconds to close to the flow temp but the water in the tank hasn't quite reached its target temp.

I was wondering whether changing the hot water cylinder to one with a more efficient coil would stop it doing it as much.
From what you say then the kw rating on a cylinder is the coil rating in the tank. So if it says it's an 18kw coil it can take more heat out of the water going round it than a 14kw one?. That's what I couldn't seem to find info. on and a lot of the specs on cylinders don't even seem to say what the coil in the tank is at all!.
Thanks for your help. It's made me think I need to investigate if I really do have a problem .

 

[Gasguru]

The UK hot water cyclinder manufacturers are dinosaurs compared with much of the world...that's why the UK blocked unvented cylinders until the late 80s to protect them despite a century of unvented before.
The mentality of the industry means they are still producing cylinders to the bare minimum standards and hence a good reason for not disclosing the Kw rating of the calorifier.
Some manufacturers produce higher rated coils but that's often at the expense of small volume cylinders.

All you can do is phone around and see if any are prepared to disclose the info.

FWIW I have a 12kW boiler dedicated to heating an unvented cylinder with a coil rated around 20kW...I get a long burn and the minimum (if any) cycling at the end.
At the other extreme there are cylinders with a single 28mm coil designed for old gravity heating and they will barely dissipate a few kW.

 

[ScottishGasMan]

Have you been into the boiler D.Code menu? Assuming its the older 418, its the i and + button at the same time, and you can scroll through the D.Codes till you get to D.77, press i again, and it will probably be at 18, try changing that to say 7, press and hold i to confirm.

D77 is the power output to hot water, if you have the vaillant wiring center you can use this to alter the power to the cylinder so that its not going to full rate when heating it, although the older 418 boiler have to go into high rate for a moment after igniting to confirm the water flow is ok, so it may still cycle somewhat.

Obviously Vaillant didnt design or approve of any of their own controls for mixing with tado, so I dont know how effective any of this will be but certainly worth a try.

 

[pbanks]

Have you been into the boiler D.Code menu? Assuming its the older 418, its the i and + button at the same time, and you can scroll through the D.Codes till you get to D.77, press i again, and it will probably be at 18, try changing that to say 7, press and hold i to confirm.

D77 is the power output to hot water, if you have the vaillant wiring center you can use this to alter the power to the cylinder so that its not going to full rate when heating it, although the older 418 boiler have to go into high rate for a moment after igniting to confirm the water flow is ok, so it may still cycle somewhat.

Obviously Vaillant didnt design or approve of any of their own controls for mixing with tado, so I dont know how effective any of this will be but certainly worth a try.

HI, yes, I already have D77 set to 5kw so that it doesn't bombard the cylinder when it's doing the water heating, but it still does the cycling when it gets the water near the target temp.
I also previously came across another problem and had to set D70 "water priority" to position the 3 port valve to the water only position, this makes it heat the water first before it will heat the radiatiors. If D70 is set so the 3 port is at mid position it tries to heat both water & rads, BUT because D77 is set to 5kw max the boiler limits it to that thinking it has to because it's doing the water (boiler\system doesn't seem to have the sense to override the 5kw max set in D77 & up the KW when its doing both). And when it's limited to 5kw it never gets the flow hot enough to heat to radiators or the water!. Set D70 to not limit the KW and it works Ok when it's doing both rads and water BUT then if it's only doing the water it pushes 18kw out and continually cycles. It's a catch 22 .

The Tado system does have some intelligent integration with the Vaillant because when the rooms gets close to temp it does modulate the boiler down so that they don't go above the preset room temp set on the inteligent radiator stats or room stat.

Might be like you say that when the boiler fires up after cycling it goes flat out & goes into anti cycling before it's had time to modulate down??.
When you say "although the older 418 boiler have to go into high rate for a moment after igniting"
The boiler was installed in 2011, do you think the later 418's have been changed so they don't go to high rate ?. Will these be software\firmware updates or hardware updates?. Wonder if it can be updated ??.

 

[pbanks]

At the other extreme there are cylinders with a single 28mm coil designed for old gravity heating and they will barely dissipate a few kW.

Think that's what I may have ??. It was bought from one of the DIY stores in the 1990's when I revamped the bathroom & moved the tanks\cylinder etc.
Strange that manufacturers don't readily put the kw rating of the heating coil that's inside the cylinder??.
Must be more important to know what it is in these days of condensing boilers.

 

[ScottishGasMan]

When I'm referring to the older 4 series boilers yours is the type I mean, they were replaced with the new 4 series in 2015, but the new ones are completely different boilers.

Your version of the boiler had a new PCB released in 2014, the older software would hold the boiler on a high rate for ignition for 1minute nearly each time it lit, the newer software would be around 20 seconds. On ignition the high rate can cause the boiler to go over its maximum flow temperature and shut the burner off before it gets a chance to modulate down to a lower fire, which is when the D77 setting may not help much. They were built to have a minimum flow rate through the boiler over 700 litres per hour, it may be that on the older cylinder/pipework that its struggling to achive that which will result in more cycling.

Its a difficult one, as a new cylinder and clearing any potential restrictions to associated pipework would no doubt be beneficial, but not really necessary while the current is still working all be it with a little cycling.

 

[polesapart]

https://www.gledhill.net/uploads/Vented-Cylinders-v2.pdf pp7-9
(other high recovery cylinder manufacturers are available)

The cylinder manufacturers focus is on fast reheat time i.e. minimising storage volume when high volume drawoffs are required: with a 4** series Vaillant any "high" or "fast" recovery cylinder may be a better fit.

 

[Gasguru]

Is the auto bypass valve correctly set? ie. probably no more than a setting 2 for 2 meters head.

 

[pbanks]

Hoping I may have got somewhere -- I've upped D78 on the boiler (storage charging temp limit (target flow temp in storage mode)).I changed it from 76 to 83c . This has allowed the flow temperature to go up when it is doing the water and I think its allowed it to maintain the required differential in temp between the flow and return temps to avoid it cycling.
It was trying to heat the water in the tank to 56c & when it was getting close to this & the flow temp was at its previous max setting of 76c the return temp was coming back at 60c+ . Now it can take the flow temp up to 83c and so it still sees some cooling on the return ??. Seems to work when I was trying it now.
Forgive me if I'm being simplistic but Im not a proffesional

Thanks Gasguru -- I had screwed the bypass valve in so it couldn't send the water round that rather than through tank coil but it hadn't helped. The pump is a Wilo one set to auto so it should be OK.

Thanks to everyone for pointing me in the right direction. A new Gledhill cylinder as suggested may be installed in future.
Fingers crosssed it cuts down on its time taken to complete its heating of the water now.

 

[Gasguru]

The boiler must have a minimum flowrate through the heat exchanger at all times.
Open the bypass up...try it at say 1.5.
I would not advise using the Wilo auto modes....set it to a fixed speed say 2.

 

[Harry Bloomfield]

Hoping I may have got somewhere -- I've upped D78 on the boiler (storage charging temp limit (target flow temp in storage mode)).I changed it from 76 to 83c . This has allowed the flow temperature to go up when it is doing the water and I think its allowed it to maintain the required differential in temp between the flow and return temps to avoid it cycling.

I have a late model 418. When first installed on my old controls, I had a similar problem with it frequent cycling when in HW mode. The problem was, my none Vaillant controls meant both CH and HW flow maximum temperature had to use a single setting. I wanted HW cylinder at 60C, but had to settle for 50C because of the HW cycling.

I soon swapped my old controls for Vaillant, which allowed me to set flow max at 80C for HW/ cylinder still set at 60C - end of the boiler cycling. All works great, except that now it cannot run HW and CH at the same time - if there is a demand for HW the CH has to wait until it is satisfied. The d.70 setting on the later 418 does not exist.

 

[pbanks]

The boiler must have a minimum flowrate through the heat exchanger at all times.
Open the bypass up...try it at say 1.5.
I would not advise using the Wilo auto modes....set it to a fixed speed say 2.

Thanks, I've changed the bypass valve to 1.5 & the Wilo to fixed speed of 2.
Take it you don't rate the auto setting on the Wilo pump . That it just made it regulate itself to the required speed depending on the back pressure??.

 

[Gasguru]

If the pump has a traditional fixed speed setting (constant speed) that would be safest, failing that a constant pressure setting.
But a proportional pressure or dynamic adapt (autoadapt on the Grundfos) could cause allsorts of issues....keep it simple IMHO.

It's no good coupling a pump making it's own decisions to a boiler designed for minimum flowrates containing dubious over complex software that falls over at the drop of a hat.

The savings from eco pumps are insignificant in most homes and the whole push towards these designs has been driven by a German/Dutch market where pumps run 24/7 unlike most properties here.