Vaillant EcoTec 428 configuration issues for Hot Water Only

We have recently had a Vaillant EcoTec Plus 428 installed (open vent, normal hot water tank) and we (myself and the installer) are trying to get it and the system configured. We get good heating performance, but when the heating is off and we only require hot water we are struggling.

The hot water cylinder is directly below the boiler, as cuch the piping run to it is very short, probably around 3m in total (1.5 flow, 1.5 return).

From it's coolest temperature, the boiler fires continuously for about 15/20mins, circulating and modulating to the set temperature (70 degrees). During this time the hot water in the tank reaches 40-45 degrees or so (target 55).

The boiler then drops into a cycle where it fires for 15-20secs before turning off, during the 'fired' time the flow temp quickly rises to the set temp. We know the 'anti-cycling' timer can be configured.

If the anti-cycle time is configured to 10 (so in reality the delay before re-fire is about 2-3mins), the flow temp drops to about 60, before the boiler fires again for 15secs or so. Doing this as long as no hot water is drawn, the tank will reach temp after about an hour during which time the boiler will fire and shut off about 15 times. Seems like that's a lot of firings and I'm sure at a previous house full temp was reached much sooner.

If the anti-cycle time is set much higher to 60 (so the delay before re-fire is about 11/12mins), the flow temp drops to about 50 before refire, but when refire happens the boiler still only fires for 20-30secs or so. Like this the tank hits temp after about 1.75 hours, but has only fired maybe 10-12 times.

The pump overrun is set such that the pump doesn't cut out between the cycles.

However we set it, after the initial firing (that lasts for a decent period), we can't seem to avoid the 'cycling' phase, the boiler won't again bring the flow temp up and hold it there without cutting out. Is this kind of behaviour to be expected? Or does it sound like something is amiss?

'Ranging' the boiler makes no difference in this scenario, it appears for the firing, the boiler doesn't begin to modulate until firing at full power for about 1 minute first. 'Ranging' doesn't appear to effect this initial 1 minute where it sounds like it's firing at the full 28kw power regardless.

Any hints, tips or suggestions gratefully received.

Is the cylinder old or did was it replaced?

Why are you using 628 when 428 would have been the boiler to pick for open vented installation?

DP said:

Is the cylinder old or did was it replaced?

Why are you using 628 when 428 would have been the boiler to pick for open vented installation?

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The cylinder is old but the flow through the heating coil is fine, I had wondered if things would improve with a cylinder better at dispersing the heat, I don't know if they come in different power ratings or how much efficiency they lose as the years go past.

Regarding the boiler, that probably my mistake (typo), it's max power is 28kw and it's definitely the correct model for the open vent, if that makes it the 428, then that's what it is.

It sounds to me as if the boiler is grossly over rated. What is the heat loss calc for your house?

Is this an old cylinder which cannot absorb even the minimum boiler output when its half warm.

It should fire at the max d0 power setting and your installer should measure that not listen to the boiler.

I think you mean your system is sealed but the cylinder is OV.

Tony

Old cylinder coils fail to 'use' the heat produced by the boiler.

It is a rule of thumb that old unlaged 'red jacketed' cylinders should be replaced with factory lagged cylinder that normally come with fast recovery coil.

Also, if there is no 'balancing' of HW zone, then HW zone will take 'all' the water and circulation through the CH zone likely to be poor.

I choose not to throttle the boiler, but many say otherwise.

Agile said:

It sounds to me as if the boiler is grossly over rated. What is the heat loss calc for your house?

Is this an old cylinder which cannot absorb even the minimum boiler output when its half warm.

It should fire at the max d0 power setting and your installer should measure that not listen to the boiler.

I think you mean your system is sealed but the cylinder is OV.

Tony

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Max heat demand is circa 15kw, with a planned extension though that would take it to 19/20, the 418 would be too close to the mark in future and there's no Vaillant in-between.

Installer 'ranged' to 20 and (with his agreement) I have experimented with lower values. The 'sounds like' observation (of fan noise I think) comes from me, not the engineer. I have tried ranging to 6/7, still the same problem.

Pretty sure whole system is open vent unless my definition of open vent is wrong. There are two top up tanks in the loft, and re-filling / top-up is automatic from one of those, ie no filling with filling taps while watching gauge.

DP said:

Old cylinder coils fail to 'use' the heat produced by the boiler.

It is a rule of thumb that old unlaged 'red jacketed' cylinders should be replaced with factory lagged cylinder that normally come with fast recovery coil.

Also, if there is no 'balancing' of HW zone, then HW zone will take 'all' the water and circulation through the CH zone likely to be poor.

I choose not to throttle the boiler, but many say otherwise.

Click to expand...

Cyclinder is not 'red jacketed', it is factory insulated, but I've no idea of age, it came with the house so could be anything from 2 years old to ancient.

One point of note (I think it may be at least having done a little more reading) that I haven't mentioned is that the old cylinder was previously heated via a gravity system. Does this suggest it would NOT contain a fast recovery coil and could as such explain the slow heat up times?

My old house (fully pumped, smaller capacity boiler) would heat a whole cylinder from room temp (ie been off all weekend) to full temperature in about 45 mins.

tommcdonna said:

Installer 'ranged' to 20 and (with his agreement) I have experimented with lower values. The 'sounds like' observation (of fan noise I think) comes from me, not the engineer. I have tried ranging to 6/7, still the same problem.

Pretty sure whole system is open vent unless my definition of open vent is wrong. There are two top up tanks in the loft, and re-filling / top-up is automatic from one of those, ie no filling with filling taps while watching gauge.

Click to expand...

If its open vent the boiler should be a 428 with seperate pump ???

The 428 will only go down to about 9.7 kW which will be far too high for an old gravity coil to absorb. The boiler cannot give less power than about a third of the max which is why sizing the boiler is so important. I would have chosen 418 and added insulation and/or TRVs which I suppose you dont have?

Your installer should have told you about replacing the cylinder. What colour is it? Quite apart from the problems you are having, keeping it is not in accordance with the energy efficiency advice.

You can do a few things to hide your defects, set boiler power to minimum ( until it is insufficient to heat the rads ), later run hot water at the same time as heating, adjust the lockshield on the HW circuit as danny says to balance them and then finish heating HW while the heating is on.

You should set the anticycle to minimum and there is another cycling setting which can be minimised too. But a new cylinder is the real answer.

Tony

Agile said:

tommcdonna said:

Installer 'ranged' to 20 and (with his agreement) I have experimented with lower values. The 'sounds like' observation (of fan noise I think) comes from me, not the engineer. I have tried ranging to 6/7, still the same problem.

Pretty sure whole system is open vent unless my definition of open vent is wrong. There are two top up tanks in the loft, and re-filling / top-up is automatic from one of those, ie no filling with filling taps while watching gauge.

Click to expand...

If its open vent the boiler should be a 428 with seperate pump ???

The 428 will only go down to about 9.7 kW which will be far too high for an old gravity coil to absorb. The boiler cannot give less power than about a third of the max which is why sizing the boiler is so important. I would have chosen 418 and added insulation and/or TRVs which I suppose you dont have?

Your installer should have told you about replacing the cylinder. What colour is it? Quite apart from the problems you are having, keeping it is not in accordance with the energy efficiency advice.

You can do a few things to hide your defects, set boiler power to minimum ( until it is insufficient to heat the rads ), later run hot water at the same time as heating, adjust the lockshield on the HW circuit as danny says to balance them and then finish heating HW while the heating is on.

You should set the anticycle to minimum and there is another cycling setting which can be minimised too. But a new cylinder is the real answer.

Tony

Click to expand...

Thanks, good answers.

The insulation on the cylinder is green-ish I think, the only remnant of the label says 'grade 3'. Only one fitter asked if we wanted to replace the cylinder, we said we didn't as money is tight. The other two quoters didn't even ask, just assumed we'd keep it.

You are right about the config, it's 428, seperate pump. If I'm reading the specs right, the 428 will actually range down to 5.3, the 418 goes to 5.0.

I'm interested in the two anti-cycle settings, we know about d2 (can be set to a minimum of 2, max of 90 I think), but what's the other?

Setting the anticycle to lowest value does get the cylinder hot the quickest. Just seems horrendously in-efficient, though I've no idea if it actually is, it's just a 'feel' thing.

tommcdonna said:

Max heat demand is circa 15kw, with a planned extension though that would take it to 19/20

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How did you calculate these?

D_Hailsham said:

tommcdonna said:

Max heat demand is circa 15kw, with a planned extension though that would take it to 19/20

Click to expand...

How did you calculate these?

Click to expand...

Sum of rad max outputs, plus 3 for hot water cylinder.

tommcdonna said:

Sum of rad max outputs, plus 3 for hot water cylinder.

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But that assumes the radiators are correctly sized!

Use the Boiler Size Calculator to find out what size boiler you need; then add in the rads for the extension (which, hopefully, will be correctly sized for the calculated heat loss ).

Since you have ascertained that the boiler modulation and temp regulation works satisfactorily on central heating, there is probably a flow problem in the cylinder/pipework.

The 4 series likes a good flow round the heat exchanger, the smaller models can give problems in this regard.

I think the modulation/boiler sizing argument is a popular one with a few posters on here, but not one I subscribe to in most instances. As you rightly state, the lowest gas rate on the smallest Vaillant models is so similar to the 428 as to make no difference.

Evening Simond,

When you say you are adjusting the anticyling time, are you playing with 'D2' ?

I suggest you change this to 5 or 6, try that but to explain waht it actually alters is complicated.

D_Hailsham said:

But that assumes the radiators are correctly sized!

Use the Boiler Size Calculator to find out what size boiler you need; then add in the rads for the extension (which, hopefully, will be correctly sized for the calculated heat loss ).

Click to expand...

Well that calculator gives 17kw as the size required, but doesn't take into account having an unheated garage underneath two bedrooms, old double glazing etc. So that seems to backup the sizing choices of both boiler and rads.

I'm not sold on the restricted pipework theory, I'm no expert, but flow AND return temps increase rapidly and there's so little pipework (all of it new) I just can't see it. Thanks for the suggestion though simond.

Beerlover, yes, it's d2 I'm adjusting, I understand what it does and the factors that influence the length of the actual calculated anti-cycle time (seen in d67). Setting d2 to low values gives fastest 'charge' for us.

Several posters have suggested the problem is down to the coil in the cylinder not being capable of transfering enough energy to the water it's storing, I suspected that may be the case but didn't want to lead the thread too much (and was hoping that there might be a few less expensive things to try!).

The only part I'm not sure of is the 'ranging' part. When the boiler does modulate to low powers, it's very quiet. However if I 'range' the boiler to a low value, say 7 or 8, the sound when it fires up is exactly the same as when it's ranged at 20, it's not just my ears playing tricks, it is the same volume etc. Maybe what I'm hearing is a flue fan or something that always starts up at max and runs down after a minute to a lower level?