Vaillant vcw 242 hot water lukewarm changing heat exch FIXED

[TOMCAT1400]

Hi
Yesterday I fitted a new DHW heat exchanger to this system (vcw 242) as my old plastic one was leaking. Now its a proper Vaillant part and I even changed the divertor valve disc even though there is not much scale up this area.
Whilst I had the valve dismantled I done a mini refurb on it,cleaned it up and greased the diaphragm spindle as it was a little sticky.
Problem now is that the hot water is not hot enough,flow rate is 10L/min but it is only lukewarm.
Tried adjusting hot water temp. valve on boiler but does not make much difference.
Diverter valve appears to be working as ch flow pipe gets hot when dhw is turned off.
I have also cut link w18 on cct board as my boiler is a hybrid (does anyone know why you need to cut this wire when you change heat exchangers ?)
I have a slight water leak from the hot water temp valve on the boiler but it looks like this has been there a while.
Should the replacement heat exchanger offer a similar performance to the old one as the new one is a much smaller sardine can type?
Any help greatly appreciated as I am out of ideas now...

 

[mcniven]

Try turning the pump setting down.

- Maybe, since you've cleaned a few things out the water flow around the system has less restrictions, thus, it is leaving the heat exchanger a lot quicker than previously.

Is the new Heat Exchanger got the same pipe run and sufficient fins as the previous plastic? Is it from the specific manufacturers?

- Having less fins, and a shorter pipe run it is again leaving the Heat Exchanger too quickly.

** Please note, i've never worked on these boilers, and I am only suggesting these for you mate **

 

[TOMCAT1400]

Thanks for the reply but I don't think you can adjust the pump speed on this boiler.(Could be wrong though)
The part I fitted was a genuine maufacturers part which replaced the old design of heat exchanger and came as a kit of parts with new pipes,washers and valve disc and instructions.
I followed instructions to the letter but am puzzled why the hot water is not as hot as before the conversion.
Anyone else got any ideas.

 

[Agile]

Did you measure the DHW flow rate BEFORE replacing the DHW HE ?

Have you measured the gas rate at the meter to see what the input power is? See FAQ or search site for any help.

My best guess is that the power input is low but the flow rate before your work was low so the temperature was higher ???

Tony

 

[Gasguru]

Try turning the pump setting down.

- Maybe, since you've cleaned a few things out the water flow around the system has less restrictions, thus, it is leaving the heat exchanger a lot quicker than previously.

Is the new Heat Exchanger got the same pipe run and sufficient fins as the previous plastic? Is it from the specific manufacturers?

- Having less fins, and a shorter pipe run it is again leaving the Heat Exchanger too quickly.

** Please note, i've never worked on these boilers, and I am only suggesting these for you mate **

This just confuses the OP. Turning a pump down on any combi is not advised and would have an adverse effect.

Leave the pump at MAXIMUM but these Vaillants are normally fixed speed. The plate heat exchanger is likely to be more efficient than the plastic version.

Are you sure the heating flow pipe does not get hot in DHW mode? Measure the gas rate as Tony suggests.

 

[TOMCAT1400]

Thanks for the replies.
Did not measure flow rate before swopping but the hot water was fine even with the leak.

CH flow pipe not getting hot with dhw on so I dont think it's the diverter.

Measured the gas flow and with hot tap full on it takes 49 secs for 1 cu/ft of gas to flow which I reckon equates to 22.28kw..
By the way the boiler is a VCW 242 and not 282 as I stated earlier.

I found a small 3" diameter pressure vessel which I pumped to 5 bar and that seemed to improve the DHW heat,it's connected to the manual DHW temp. valve via copper tubing (what's this used for ?)

Still wondering why you have to cut w18 link in cct board,does this override some thermostat or stop gas modulating ???

Any Vaillant guru's out there?

 

[twgas]

quite possibly turned stop cock on more than it was previously, increasing flow rate and thus reducing temperature.

 

[Gasguru]

22.28Kw is low.....should be 29.7 Kw, you will need to call in a corgi to confirm/reset burner gas pressures. Alternatively there may be another underlying fault.

The small expansion vessel takes up the small extra volume of domestic water as it is heated with the taps off. The water contained in the plate heat exchanger is heated a little even when the boiler is in heating mode. The vessel prevents an excessive (and possibly damaging) pressure rise. It will make no difference to the hot water output. Unless you have used a compressor it is unlikley you will have inflated the vessel. Normally most of the air pressure is lost when disconnecting a normal footpump valve. Most of these vessels have been flat for years with little effect on the boiler.

I've never got to the truth regarding the link cutting but is only applicable to the hybrid models (ie the ones with the hybrid thermostat board).

 

[TOMCAT1400]

Thanks for the replies.
Stop cock is at same position as before the swop, tried varying it but it made no real difference.

I did use a compressor to pump the vessel and yes it has been flat for a while as I remember thinking it was an air bleed a couple of years ago (duh)

Should it be 29.7kw even for vcw 242 ?

Just cant see how simply changing the heat exchanger could cause a problem on the gas side,I can see how the flow rate may be varied as the new exchanger is smaller and possibly less resistant to water flow through it.

Another thing is that I can hold onto the pipe from the boiler into the top of the heat exchanger and although it gets quite hot never unbearably so.

Read on another post about a gas valve test switch which overides modulation,does this boiler have such a switch and if so where?

Even more useful would be a service manual for this boiler,I have checked the Vaillant site but they dont have one on there. Does anyone have a service manual they could email to me..??

 

[Gasguru]

Yep 29.7 Kw.

Have you disturbed the small flexible hoses on the back of the control box? Check thet are both fitted securely (at each end) and not split. These are used to control the burner gas pressure.

 

[TOMCAT1400]

Thanks gasguru

I may have disturbed them as I had the control box pulled forward to get to the w18 link. Is this the clear 5mm pipe that goes to the bottom of the main heat exchanger. Don' remember seeing more than 1...

 

[Gasguru]

Two tubes, one from the combustion chamber and the other to the gas valve.

VCWGB242EH

• Gasguru
• 30 Mar 2008
• 1

Gas valve air pressure tubes

 

[TOMCAT1400]

Thanks for the picture,I can see both the tubes now.

I will try and check these tomorrow as it's getting a bit late now.

Just had a shower and I reckon the flow rate for the same shower temperature is almost half what it was before the swop so it must be due to the dhw not getting hot enough which in turn must be due to either a diverter valve not fully switching to dhw (checked & seems ok) or the new heat exchanger not being as efficient as the old one (no idea how i will find this out) or there is not enough gas getting to the burners (will check tomorrow).

Thanks to everyone who replied,I will let you know the outcome (unless its something stupid I,ve done in which case I will just say it was gremlins and it is ok now )

 

[namsag]

242 IS A 24KW OUT PUT boiler and is about 70% efficent so as gasguru says it should have an input rating of around 30kw check this on HW from strone cold

 

[TOMCAT1400]

I,ve fixed the problem with the lukewarm water.
Basically when I cut the w18 link wire in the control box I had to pull the cct board out an inch or two and when I,ve put the cct board back in one of the pins on the edge of the board has bent and not made contact with the connector.
Boiler is now using 28.73kw instead of the 23.73kw it was using with the fault.
Thanks to all who suggested answers and I,ve attached a photo of the board with the bent pin for anybody who may have a similar problem in the future.

P.S. Can anyone supply me with a copy of the service manual for this boiler.

Untitled

• TOMCAT1400
• 31 Mar 2008
• 2