New Vaillany 428 switched on/off all the time, is it normal

[rickp]

Hi, I hope someone can help as my gas engineer is lost for an explanation. I have just has a new gas boiler fitted in place of my old baxi. The new one is a Vaillant 428 heat only boiler, the system has now also been changed from open vent to a sealed system. I have a magnaclean installed too. Old hot water cylinder was reused.
Problem: when heating water only the boiler will sometimes fire up for around 5 mins before switching off and cooling down, then it will begin a cycle of firing for anything from ONLY 6 seconds to 60 seconds, going off for its 2 min cooling off everytime. All this when the temp in the tank is tepid, This cant be normal, it is taking anything from 1 hour plus to heat a tank of water as it spends twice as much time off, than on. My engineer is lost for an explanation, the only one he has is that as the tank is close to the boiler and the return run is quite short, the water hasn't had a chance to cool off before returning to the boiler. The boiler sensing that the water is more or less the same temp both ends switches off!
The magaclean is fitted to the return side as is the expantion vessel. Its on 1 bar. Boiler performs more or less as I would expect when the heating is on, switching off when the rads are hot. Sometimes it just runs for 10-20 seconds, other days for 5 mins.
Vaillant help line do not want to know as we have not got vaillant cylinder!!
All vaillant helpline offered was "to turn down a valve on the flow side to restrict flow". We did this and the bolier was shutting off after 5 seconds everytime!
Is this a flow restriction problem, air lock, location of the explantion vessel, old tank or have I got a duff boiler or is that how they work.
Any help please
Rick

 

[kevindgas]

check flow & return temps are they too far apart?
has he range rated the boiler to suit your system?

these boilers are normally very reliable and run fairly consistantly
i would be looking at a flow problem with the symptoms you describe

 

[rickp]

Kevin thanks for the reply, my guy checked the flow and return temps over a period of an hour and it appeared as soon as the flow and return were similar temp, the boiler switched off. Yet on Tuesday I watched as the boiler kept going full bore for over 6 mins with temps at both ends the same, no problem. It appears to work correctly for a while then goes back to this shutting down after a few seconds with the tank nowhere near temp. The cylinder stat is set at about 55c, the boiler max setting is 73c.

Not sure what you mean by range rated. I do know this is the first Vaillant he has fitted, told me this after he started.

Vaillant helpdesk said on the phone that it was normal to switch off when both flow and return were similar temp and because the return run was only a few metres from the tank back to the boiler, that was the problem. My engineer disagreed with them. they then told him to run a long 22mm copper loop in the loft on the return to "dissipate" the heat before returning to the boiler. He thought this suggestion mad too.

rick

 

[DP]

Old hot water cylinder was reused

This could be his or your first mistake. You also have not said if the pipes to cylinder are 15mm, 22mm or even 28. No mention has been made of type of controls that have been retained or fitted to comply with BC.
Old cylinders has an indirect coil that does not ‘sink’ the power supplied by the boiler all that well i.e. if the old boiler was rated at 16kwatts, cylinder coil rated at 10kwatts boiler did cycle but not that rapidly when cylinder was cold.

Your boiler is rated at 28kwatts. Initially it is firing to heat cold primary water. Having reached operational temperature, it is shutting down as it is getting too hot, while your cylinder is not keeping pace with it. If you had a 28kwatt coil in the cylinder, your new boiler would stay on, start modulating and hot water would appear at the tap within 5 minutes
If the cylinder is on gravity loop, boiler operation will be worst.
If the system has not been cleaned, what more can I say.

Fitting a new boiler is not just a case of take the old out and bang in a new one. Since that days of Old Baxi, system controls, appliances and operational parameters have moved to a different dimension. What you have done is fitted a Bentley engine to a clapped out JCB

No wonder I do not fit boilers. Doing this kind of work goes beyond the knowledge of just pipe connections.

PS. boiler is switching off because the flow and return are at similar temperature. You will note this is not the case when heating is on. When you switch on heating at the programmer, does HW channel also swiches on?

 

[Agile]

It seems that there are two problems!

You have an old cylinder that can only adsorb about 5-6 kW.

You have a boiler running at 28 kW which is grossly over rated for the application.

Your installer apparently does not know how to calculate your heat loss to find out the required boiler size. Must older three bed semis need about 11-13 kW, newer houses even less.

If your installer had rated the boiler at its minimum it would be closer to the required load and run for a little longer.

I am doubting that he has the Energy Efficiency certificate which all boiler installers should have.

For the forum, I picked up a little snippet at the show yesterday that G-S will not notify boilers unless the installer has the certificate ( even though they dont show it on their card as CORGI used to do. )

The OP may find the installer cannot notify the boiler to Building Control as he should. This would possibly give problems with searches when selling the property.

Tony

 

[rickp]

Thanks for all the advice,
The old tank is 20 years old and was retained as it was in good order, I had 4 engineers round to quote and only 1 of them suggested changing it and that was a throwaway remake, not a recommendation.

The pipework is 22mm , only 15mm near the rads.

Original electronic controls used to call for heat etc, original cylider stat etc.

Whole system was powerflushed prior to install.

The controls allow me to switch c/h on only, so it bypasses the cylinder, this appears to be fine. I suspected the tank maybe the culprit.

What tank do you suggest, I have been looking at a standard Glenhill which seems to heat quickly or are they no better than a standard cylinder. I dont hve a lot of space in the area to work with, current tank is 900 x 450.

Any further suggestions are apprechiated.
Rick

 

[rickp]

Agile, We have a 4 bed detached bungalow, 15 rads, all 4 engineers who came round measured all the rads and recommended the same boiler and size.

I am in the situation I am now in, what do you suggest to help me overcome it.

thanks Rick

 

[Agile]

You have a boiler running at 28 kW which is grossly over rated for the application.

Your installer apparently does not know how to calculate your heat loss to find out the required boiler size. Most older three bed semis need about 11-13 kW, newer houses even less.

If your installer had range rated the boiler and set it at its minimum it would be closer to the required load and run for a little longer.


Tony

Rather than replacing the cylinder at this stage the boiler should be set at or near the minimum power output.

If your nupty installer cannot do that then you could probably do it yourself as its only a software setting explained in the boiler book or get another more competent engineer.

Tony

 

[rickp]

Is it possible to range rate the boiler now and turn it down. How can 4 different gas bolier engineers come up with the same boiler and rating.
Does the cylinder need changing and if so what model?

I know he was fiddling with the controls for ages, resetting things.

I know I can turn down the bolier temp on the front bottom dial. I have just reset this to 60c and still the same thing, shuts off after 35 seonds.TANK TEMP is around 37c at moment.

thanks

Rick

 

[rickp]

sorry Tony, I only got half your reply above.
Rick

 

[Agile]

The boiler is 28 kW whereas your bungalow probably needs about 11-13 kW.

The cylinder probably only adsorbs about 5-6 kW.

The boiler power output setting needs to be reduced to about 10 kW. Look in the book to see how to do that.

So many in this business dont know what they are doing and put in seriously oversized boilers and in your nupty's case dont even know how to range rate them!

Tony

 

[rickp]

Thanks, I'll get him back tomorrow and see if he can turn it down.

Would a new cylinder be helpful too?

Rick

 

[DP]

Rick, I think Tony is missing a bit of essential information when he says the boiler be 'adjusted'. Boiler is, as it stands able to do this down to 5.3kwatts. You need do nothing.

To address the problem of cycling, get your fitter to change the cylinder for a modern fast recovery unit. Some cylinders will heat up in 20 minutes. Look for a cylinder (unvented might be better) that can use the heat the boiler throws at it.

Fitted same boiler and replaced a cylinder many moons ago. When boiler was fired for HW only, for several minutes the flow pipe to cylinder was hot hot and return to boiler was cold. Water from hot tap 5 minutes after boiler was started, was comfortably hot.

 

[rickp]

You might have it there DP, at the moment the returning hot water, moments after firing, is returning to the boiler very hot.

The cylinder seems to take virtually NO heat out of it all all.

Is there any make of cylinder to look for? I have only space for a 900 x 450 normal shaped one.
Rick

 

[DP]

Do a search for indirect cylinders.

McDonald Engineers make one that is fast recovery. A 24kwatt boiler will heat the water in 6 minutes.

They also make a 900x450 that will heat the water in 20 minutes.