Vaillant vcw282 CH not working properly

[Flyingstu]

Hi Guys

I have a Vaillant 282eh boiler about 15 years old . It has over its lifetime had a new DHW exchanger,Main heat exchanger, DHW control valve, and just recently a new Diverter valve. It gets alot of use and I have been reasonably pleased with its performance.

I have a problem of the boiler heating up and cutting out due to the CH overheat Stat cutting in. This appears to be a result of poor CH circulation as radiators are not heating properly and following normal diagnosis would suggest a pump flow problem. The pump is running fine which led me to examine the gas operator modulation (it uses a diaphragm pump system to modulate gas flow). I can modulate gas flow by blowing into the air control line but I dont see any modulation of gas when system is operating normaly (green light on panel comes on okay suggesting diaphragm pump is ok).
DHW system works fine but again I dont see any gas modulation when water flow rate is reduced

I cannot understand why the circulation is poor as the system is clean. I noticed though that there is a system bypass valve shown on the VCW282 schematic. This valve is not mentioned in the fault finding and the CH bypass pipe work appears to link across under the boiler into the diverter valve. If it is leaking it could perhaps be my problem as it would short out the CH circuit causing rapid heating and if I have a gas modulation problem could lead to a rapid overheat

My question is , after all that rambling, is the system bypass valve a function of the diverter valve operation ? Is it possible that the diverter valve is dumping CH water through the bypass pipe?

thanks for any info

 

[Agile]

I am afraid that your brain is over working!

When mending boilers its best to keep things simple which is why women are so good at repairing boilers!

It seems that your boiler is working on full power all the time. Rather than guessing that the parts modulating the power have failed its best to concentrate on the origin of the control !

So the first thing to check is the NTC temperature sensors and next the PCB which evaluates the NTC values.

The NTCs rarely fail on that model but they are the first thing to be checked by measuring the resistance cold!

Tony

 

[Flyingstu]

Thanks Tony

Good advice .... I have measured the cold NTC resistance at 2.3K ohms and it does appear to reduce as it is heated. The troubleshooting chart suggests that if the boiler shuts down when this is grounded then it is NOT the Thermostat PCB . The boiler does shut off when this is grounded but I am not convinced that the NTC is faulty unless the cold resistance value is way off . Is that cold resistance figure about right? ......

 

[Agile]

Many Vaillant NTCs are about 1300 cold and reduce to about 200 hot.

Unfortunately I dont know the value of yours.

Regardless, I would add a 220 ohm resistor across it and see if that caused the boiler to either modulate down or go off completely. If it went out then I would put a 470 instead and see if that caused it to modulate.

In reality I use a pot so that I can quickly simulate any boiler temperature and evaluate the response of the boiler control system.

Your original approach would be a bit like changing the wheels on a car if the engine did not start! With boilers you start at the beginning and check the simple things first as that usually where the fault lies!

Tony

 

[Flyingstu]

Thanks for the info Tony I only had a 200 ohm resistor to hand and that cut out the burner when tried. I will replace the NTC anyway as it is so old.

I think I need to start again as the symptoms are not so clear.

1. The DHW works okay , the boiler keeps heating the water and does not shut down so long as there is a demand for DHW. Primary heating water stabilises at 70 degC.

2. When there is a demand for CH the boiler start as normal and does appear to modulate after 5 seconds to full gas demand BUT after 10 seconds or so and BEFORE there is any significant temperature indication the gas shuts off leaving the pump running and the boiler will not relight for several minutes. When it does relight it repeats the above cycle.

3. The hot water feed pipe to the radiators does get warm but the radiators are luke warm and the return pipe is always cold.

4. I have dismantled the pump and it is showing signs of age and although it runs I am not convinced it is running at full output

5. I have gone through the NTC tests and have got a new one ordered in case I have missed something. The boiler does not run long enough with CH demand to prove if the temperature control system is working.

Tomorrow I will seek out a new pump and fit a new NTC....hmmm

 

[Agile]

I think I need to start again as the symptoms are not so clear.

1. The DHW works okay , the boiler keeps heating the water and does not shut down so long as there is a demand for DHW. Primary heating water stabilises at 70 degC.

What is that meant to mean?

A thermal survey in DHW mode should produce flow and return temperatures of about 55/40° with DHW at 40°C.

It sounds as if you have a flow restriction or a faulty pump.

A "finger test" would diagnose a faulty pump.

From the very few details, I suspect the boiler is working fine on CH apart from a severely restricted flow rate.

A measurement of the flow temp at the sensor will determine if the boiler is controling its temperatures properly and I suspect that it is!

Tony

 

[Flyingstu]

Hi Tony

you were up late !!

The 'primary heating water stabilises at 70 degC' is a reference to the temperature that is shown on the front temperature dial when continuosly drawing hot water, I may not have used the correct term but I meant the system water temperature.

I carried out a little experiment to prove that the gas control is working. I figured that if the circulation is poor due to a weak pump then if I opened the system drain valve at the same time opening the feed such that the water pressure was maintained ( we have a very high mains pressure) I could effectively increase the water circulation rate. A liitle bit non standard perhaps but anyway it worked. The boiler stayed alight on CH and I also could modulate the flame to prove that the gas control was working.

I have ordered a new pump and will have to wait a day or two before I can move further.

Thanks again for your help

 

[Flyingstu]

Oh I forgot to ask

How do you carry out a pump finger test ?

 

[DP]

Flyingstu, 70 degrees indication on temperature gauge would suggest optimum heat transfer during HW mode. You boiler is working well in this mode.

Faulty or weak pump results in system control not working correctly as pump power is used to position system components for HW or CH demand. For instance, the diverter position and servo valve on gas valve are water operated, therefore a duff pump would show up as boiler malfunction. Also, HW demand would not work correctly if pump were indeed faulty. I think you are wasting your money on a new pump.

Check the heat on the left hand pipe (out of the heat exchanger) and also how hot the pipe above the pump is. If they are both at similar temperature, I would be looking at the diverter as being the culprit- even bypass.

Do you have plastic or copper pipes? If plastic, I would be looking at these as they are prone to blocking but first check what I suggest in previous paragraph.

Green light dims to indicate reduction of flame size on burner. If it is staying at full brightness, it is because the primary water temperature is short of demand set on control stat.

Tony's fingure has special sensors fitted to it. You can forget carrying out such test using digit that is uninitiated.

 

[Flyingstu]

Hi Tony

I agree I dont think the pump will fix the problem but it is old and I will replace it as the original is becoming noisy.
I have looked at the pipe work and the CH return pipe connects to the system bypass pipe (painted black) in a junction before entering the diverter valve. This junction is at the back of the boiler and looks as though it could contain a spring loaded bypass valve. Am I in the right place ?
This section also connects to the expansion vessel. The details in the servicing book do not show where the bypass valve is but I think it must be external to the diverter valve.

I will also remove and examine the diverter valve, this was only replaced last month

Cheers

 

[Flyingstu]

Okay I have got to the bottom of it I think

I took out the recently replaced diverter valve and found that part of the valve had broken internaly. This was a supposedly reconditioned part but obiviously had not had its insides replaced.

The valve seat that had broken up is the one that opens when DHW is demanded ie when the valve shifts to the right. This has obiviously been leaking and allowing CH water to be bypassed via the DHW heat exchanger.

Problem now is the place in Wembley where I got the valve is closed for the night. It seems a shame that it is not possible to get the insides of the diverter valve replaced rather than change the whole thing.
Do they do refurbishment kits for these valves?

 

[Agile]

I took out the recently replaced diverter valve and found that part of the valve had broken internaly. This was a supposedly reconditioned part but obiviously had not had its insides replaced.

I hope that this acts as a caution to anyone who tries to use these repaired parts.

However, as that place gives out my name if anyone calls and asks for details of a competent engineer, I have to be nice to them.

They also sell new boilers and new spare parts and are a major supplier of both new and used/reconditioned parts.

Tony

 

[Flyingstu]

Hi guys

Got the new diverter valve from our friends in Wembley this morning and they exchanged the old one without any quibble. I checked before leaving that the spring and check valve were new.

Put it all together and eureka ! we had ignition and the one to be obeyed has been suitably warmed up !!

In hindsight my initial feeligs that there was a short circuit of the CH system was correct but I didnt believe the Diverter valve would be the problem as it had so recently been replaced (cos of a leak in the stuffing box). Water was routing via the DHW heat exchanger all the time even though the diverter diaphragm was moving.

I still will replace the pump when that turns up but it is a releif to get it working again after a frustrating couple of days. Feel I know the boiler alot better though......

Thanks alot for your support....excellent website
cheers Stu

 

[Agile]

Hi Guys

I have a Vaillant 282eh boiler about 15 years old . It has over its lifetime had a new DHW exchanger,Main heat exchanger, DHW control valve, and just recently a new Diverter valve. It gets alot of use and I have been reasonably pleased with its performance.

You have to realise that when someone says a "new" part we expect that thye are telling the truth and it really is new!

The thermal survey would have quickly discovered that the diverter was faulty.

At least I steered you clear of changing all the back end parts!

The finger test is so simple but I will post a new topic so that people can search for it!

Tony