Vaillant Combi Luke Warm Hot Water.

[apick1]

Hello

I have a Vaillant Comi Model VCW GB 242 EH for which I can no longer get the Hot water to increase to a reasonable temperature.

The DHW temperature adjuster produces mildly warm water when turned fully on.

The complete water section was replaced about 6 Months ago so I don't believe there's a fault there.

However when I reduce the Cold water flow into the system by turning off the valve under the boiler I can find a point where the temperature increases as the flow reduces.

Is this a scaled up Heat exchanger and is it worth descaling using chemicals ?

help please.. Tony

 

[DP]

Call in some one who KNOW this boiler (not just pretends to). Good boiler but difficult to repair unless you know how it works.

Hint. Ask the engineer if you can watch while he is working. If he is out of his depth, he will be offended. If he is able, he will 'show off' his knowledge of this excellent boiler.

 

[weargas]

Firstly does the boiler fire for dhw with the ch off? it could just be a diaphragm.
if by "water section" you mean you've had the replacement dhw heat exchanger kit then the problem could be scaling of the new heat exchanger. Are you having to fill the boiler frequently because the indicated pressure on the gauge keeps falling? If so then that is almost certainly the cause of your problems. descaling the heat exchanger will only be a short term fix unless you locate where the pressure loss is coming from and repair the leak. also be sure whoever installed the dhw heat ex kit installed the little diverter valve disc, it helps prevent the heat ex scaling by not allowing water to bypass into it whilst in ch mode. If you have the old plastic dhw heat exchanger then it is likely to be one of 3 problems
1. diaphragm pump.
if this pump fails or doesn't pump strongly enough then it will result in a reduced gas pressure to the burner and a reduction in heat on the dhw. You will need the correct equipment and a knowledge of gas pressure to check this. with the front panel removed look at the 2 indicator lights when the boiler is running. you should have a yellow light first and then a few seconds later a green. the green indicates that the diaphragm pump is activated. If the green light is on but the gas pressure is low (around 2mb) then it's possible that the pump is not running or the tubes are blocked/restricted. It's also possible that one of the tubes has become detached from the pump housing. the diaphragm pump is located within the pcb housing and is in it's own little compartment underneath. on the rear and towards the bottom of the pcb housing you'll have 2 silicon tubes coming out, 1 on the right the other on the left (looking from the front). the left hand pipe is the positive pressure. if it has become detached the diaphragm pump isn't feeding the gas valve and you'll only get low gas pressure. If the green light is not on.......
2. pcb problem
more than likely it's the electronics pcb but there's 4 pcb's in there so unless you've got money to burn and want to try replacing them one by one get someone who knows this boiler to test the pcbs. it can be done there's no need to try and guess which one has failed, vaillant technical will even fax you a walkthrough for testing pcbs.
3. temperature selector
it's not just turned down low is it???????

 

[apick1]

Thanks for the reply, I appreciate you taking the time.

Here's my response:

What I mean by 'Water Section' is the unit that has the DHW temperature control knob on it. This was changed because it had seized and the DHW temperature couldn't be altered.

I've got the DHW temperature turned up as far as it will go which gives 28C at the tap. Reducing the flow to this section using the tap on it I can get up to 40 C with about 60% of the original flow.

Amber and Green indicators are operating fine and boiler is lit all the time DHW is demanded. Both pipes to the diaphram pump are clear and intact.

The system holds its water pressure ok.

I've turned the CH fully up and can get 80C out of it. Does this tell me that the gas pressure is ok ?

The main heat exchanger is clean and the flames clear Blue.
The flame height is about 5mm internal glow with another 4mm burn above. Is this the normal height ?

I've run the system with the CH main outlets turned off (CH electrically off) which I presume forces some flow through the system Bypass valve. This didn't increase DHW temperature.

Using a temp probe from a cold system I couldn't detect any heat going to CH through the diverter valve with CH off and DHW demanded. Therefore I think the diverter valve is ok and it is throwing the microswitches fine.

I've put Sentinal 400 into the system and running the DHW for a while to try and clean.

I thought I'd let it run for a few days, drain and put 200 in. Is this a waste of time ?

The DHW heat exchanger is the plastic type, the screws on the top and bottom are corroded with some deposits around them. I don't think it would stand dismantling to check the internal heat exchanger for blockages/Scaling.

Only the previously mentioned DHW temperature selector section has been renewed over the 10 year boiler life.


Thanks again

Tony

 

[weargas]

I've turned the CH fully up and can get 80C out of it. Does this tell me that the gas pressure is ok ?

no, you don't need as much gas pressure for ch so even on min gas pressure the boiler would reach that temperature

The flame height is about 5mm internal glow with another 4mm burn above. Is this the normal height ?

that would indicate min pressure (relying on your measurements being correct) but you can only be sure by using a manometer.

I've put Sentinal 400 into the system and running the DHW for a while to try and clean.

I thought I'd let it run for a few days, drain and put 200 in. Is this a waste of time ?

it's unlikely to cure it, spend your money on a manometer instead of the chemicals

The DHW heat exchanger is the plastic type, the screws on the top and bottom are corroded with some deposits around them. I don't think it would stand dismantling to check the internal heat exchanger for blockages/Scaling.

don't try to dismantle it, it won't be watertight afterwards. besides that type of heat exchanger doesn't really scale up. the deposits are probably small weeps that have taken up with the heat, it'll eventually start to leak (they all do) and you'll have to replace it with the metal plate heat ex kit


i would say it's yer diaphragm pump based on the info you've given me but it's always best to check the gas pressures to be sure......i'm not a fan of replacing parts until a boiler starts to work, take a little more time to test everything thorougly and only change what needs to be changed.

get someone to turn on the hot tap whilst you watch the flames. does the flame increase in size as the green light comes on? (you should also notice an increase in noise and a slight vibration as the diaphragm pump kicks in.

 

[apick1]

Nothing changes when the green light comes on.

No vibration from the pump or change in flame.

I'll check all the tubes again and see what I can find.

Thanks.

 

[weargas]

if the green light comes on and there's no change in the flames the diaphragm pump is your problem (around £40 something). All this is assuming no-one has messed around with the gas pressures. Has the gas valve adjuster cover still got the factory seal intact? If no-ones adjusted any gas pressure i would change the diaphragm pump.

 

[bradleybear]

Just a quick mention, but you say you changed the DHW water section, thats fine, bit you must make sure you throttle the water flow rate down to the recommended manufacturers flow, I think its 8 l/min on the VCW 242. I have had loads of work on these and would consider myself quite knowledgable on the way they work.
What you really need to do is adjust the spindle height (locknut and adjuster nut) on the top of the water section so it is just making when the tap with the lowest flow rate (normally bath tap) is open full. This spindle does one of two things, it makes the microswitch to fire the boiler for DHW operation, and it also moves the servo valve to enable the diverter valve to be pulled in to allow the water to pass around the calorifier.
You really want to shorten the height of the spindle, so it just makes the cam move over (and make the switch) when DHW is in demand from the tap with the least flow rate. If the spindle is too high, it may not move back over for central heating afterwards. To remove the spindle, there is one screw holding the servo valve in place, then you must ease the servo valve to the right, then forward slightly. There is no need to remove the hydraulic weep tubes for this, and the spindle is dry (i.e. no need to get wet).
This boiler must have the correct flow rate, or the water will not pick up the heat it needs, as it burns a lot less gas compared to other boilers.

And another mention, you will not see the flame going up or down until the water gets warm in the first place, as the boiler will have no need to modulate, the thermistors will not see the need to if the water is not hot, so the diaphragm pump is ok.

WBA FC

 

[apick1]

Checked out the diaphragm pump.

There was no evidence of suction or pressure from the outside.

Romoved unit from main controller, left it connected and switched on system. The pump vibrated as it should do when operating.

This can't be detected without removing it from the unit as it is quiet and similar vibration to the whole system.

I removed the pump completely and found that the inlet and outlet plastic right angle connectors both had cracks where they push on the pump.

Removed both of these and pushed the two internal pipes directly onto the pump. There's plenty of room inside the unit to do this without kinking the pipes.

Connected the unit and it started pumping.

After re assembly the flame is about twice the height with the green light on and the DHW temperature is up to 50C.

The CH performance has somewhat changed also.

I guess the pump has slowly begun to leak through time and I've never noticed the change.

All I can say is many many thanks for the replies and help.

Cheers

Tony

PS. I'm off for a hot powerful shower.

 

[weargas]

I removed the pump completely and found that the inlet and outlet plastic right angle connectors both had cracks where they push on the pump.

I've never come across that before, loads of faulty d pumps but never cracked connectors. It's good to know for the future, it might save someone the cost of a d pump.
See, even those in the profession are still learning.

 

[paddy69]

Sorry to bump a very old post BUT;
I am having a similar problem to this.
I have an old VCW 242 combi compact which has been very little trouble over the years (we have been in the house13 years, boiler ???? years) We have just noticed recently that the hot water is not coming quite just as hot. My wife has had showers on full hot and when I run a bath I only need to put very little cold water in else it becomes too cold.
We have done nothing to the boiler apart from it getting drained, flushed and re-filled at the end of the summer.

It has an old plastic type DHW heat exchanger!

I have checked and the diaphragm pump does operate as the flame height increases BUT is it possible that these get "tired" and lose their oommpphhh!?? Pipes are OK! but I haven't had the pump out. The problem that I read above, with the cracked connectors, must only be obvious when the pump is taken from it's housing???

Is there any way that the water section/diverter valve can actually let cold water seep into the hot stream??? I ask as I have noticed in the past, when wallowing in the bath, and wanting more hot water, that it takes quite a while to draw hot water through and in the process the water can get quite cold before the hot comes??

Any help and clues much appreciated as I would like keep this old girl running for as long a possible so am keen to find out how it all works!

Cheers!

Phil

 

[apick1]

Yes it is an old post and my VCW 242 is still going strong.
My hot water is running at about 48C with the flow control (water temp setting) at about mid way. Stick a thermometer under your tap and see what you'r getting.

Re. The diaphram pump look at the flame with central heating running, adjust the temperature setting to max and then bring it down until the boiler cuts off you should see the flame reduce by at least half. I would guess mine goes from about 5mm to about at least 12mm. If your does the same the diaphram pump is fine.

Best way to check whether your diverter valve is faulty is to turn on the hot water system when the heating has been off and the radiators are cold. If the divertor is stuck the radiators will start getting warm instead of your tap water. If you can touch your pipes coming directly out of the boiler and know which get hot for heating is a better method.

Good luck.
Tony

 

[paddy69]

Thanks Tony.

I will keep fiddling.

I did take the diverter valve apart quite some years ago as it wasn't switching properly. From what I remember we had hot water but no heating! A good clean out and it was right as rain. I didn't replace the diaphragm in there at the time and should have done really!

I have ascertained that the diaphragm pump is doing something as the flames do increase when it kicks in (the green light comes on) I am just not sure whether it is giving all the suck that it should. I should really bite the bullet and pull the pump out and check those little plastic connectors.
I do also intend to check the water temp at the taps but don't think we have a thermometer in the house that will do it!!

Phil

 

[paddy69]

Well, I ran my bath tonight and had my thermometer under the tap.
45'C is the top temp. As far as I know the knob on the water section is turned down to the lowest flow rate possible (it doesn't want to turn now) I have never measured the water temp before!
Phil

 

[paddy69]

YUP, been in to the diaphragm pump. The two plastic elbows were split! As you said, thrown them away and put the plastic pipes straight on.
Funny that, I still have the old type DHW heat exchanger in mine, maybe they don't ALL leak and need changing, yet these plastic bits had split!
Must be a design fault
Phil