I have a vaillant VCW GB 242EH. It's about 9 years old and has been serviced every other year. The problem I have with it is that the hot water temperature is now only tepid. The central heating works fine. The servo moves when hot water is drawn, the boiler fires up to full as normal, the servo filter is clear, the servo diaphragm looks sound, the diverter valve seems ok and the diaphragm seems sound, I think the divertrer valve is ok because the radiators don't heat up when running a hot tap for half an hour and the gas pressure is within parameters. Enough information? The corgi guys I've had don't seem to have a clue what's wrong and 1 of them just wants to change the parts 1 by 1 till it works (the bit's that were ok would be non returnable though). Unfortunately the parts all seem to upwards of £100+vat so it would only take a few parts to come to the same cost as a new condensing boiler (cheap one obviously). Could it be the DHW heat exchanger or something as simple as the DHW thermostat (my 2 corgi plumbers think not). Any ideas would be welcome.
vaillant tepid hot water
- Thread starter Smithysmith
- Start date
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Open the hot taps fully (you want at least a bucket of water per minute flowrate) and measure the gas rate (ie how much gas is being consumed). See the FAQ...it will tell you how to calculate this using the gas meter. Come back with the answer.
With the system cold and set to HW only. Turn the tap on and feel the CH flow pipe. If it gets hot you could have a problem with the diverter valve. These diverters work hydraulically and often a blocked tube is the culprit. Monitoring the operation of the head can give a clue to any fault. Does the head move to the right with a call for hot water? How long does it take to get there?
These came with a 'black box' DHW heat exchanger at the front left of the boiler. These tend to leak and block up so that would be something to look at.
A lot have been replaced with a modern plate HE. These sometimes get blocked and can usually be cleaned out.
I can't rule out a sensor fault from here but the good thing by starting with sensors; is that they tend to be relatively cheap 'lets try one of these' components. They can be tested by those corgi's who have access to the necessary information.
There is a flow rate adjuster on the water section [red knob]. I take it that this is set and working correctly? If the water comes through too fast it may not be there long enough for the boiler to heat it up.
Have any of these corgi's that have been round checked the boilers working pressures?
These came with a 'black box' DHW heat exchanger at the front left of the boiler. These tend to leak and block up so that would be something to look at.
A lot have been replaced with a modern plate HE. These sometimes get blocked and can usually be cleaned out.
I can't rule out a sensor fault from here but the good thing by starting with sensors; is that they tend to be relatively cheap 'lets try one of these' components. They can be tested by those corgi's who have access to the necessary information.
There is a flow rate adjuster on the water section [red knob]. I take it that this is set and working correctly? If the water comes through too fast it may not be there long enough for the boiler to heat it up.
Have any of these corgi's that have been round checked the boilers working pressures?
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Thanks gasguru. It took 76 sec per revolution which works out as 14.36. Should it be running at approximately 24? Gas pressure problem?
Slugbabydotcom I don't have a manometer to check the pressure while running the hot water but I'll get that checked. Both the Corgi's that came checked the gas pressure but to be honest I don't know if they checked the working gas pressure. There is the slightest weep at the bottom corner of the DHW heat exchanger but it doesn't drop the pressure in the system (it's a black box). The head at the diverter valve moves to the right almost instantly when there's demand for hot water.
Forgot to mention, that I've got the red knob turned up full but have tried different variations with the same result. If I turn the tap on a trickle it will sometimes get very hot. When I say a trickle I mean the boiler usually turns off it's so low.
Chris R, I unplugged the sensor (DHW thermostat if that's what you meant) and the water temp didn't increase any.
Thanks people.
Regards
Slugbabydotcom I don't have a manometer to check the pressure while running the hot water but I'll get that checked. Both the Corgi's that came checked the gas pressure but to be honest I don't know if they checked the working gas pressure. There is the slightest weep at the bottom corner of the DHW heat exchanger but it doesn't drop the pressure in the system (it's a black box). The head at the diverter valve moves to the right almost instantly when there's demand for hot water.
Forgot to mention, that I've got the red knob turned up full but have tried different variations with the same result. If I turn the tap on a trickle it will sometimes get very hot. When I say a trickle I mean the boiler usually turns off it's so low.
Chris R, I unplugged the sensor (DHW thermostat if that's what you meant) and the water temp didn't increase any.
Thanks people.
Regards
Remove the two large screws holding the grey lower panel on.
Turn off the boiler on the front.
Open a hot tap fully.
Note the green light just to the right of switches.
Turn on the boiler...the burner will light...after a few seconds the green light will come on and you should notice the flames viewed through the glass port hole significantly increase in size. As the light comes on you should also hear a faint buzzing behind the silver control box.
Turn off the boiler on the front.
Open a hot tap fully.
Note the green light just to the right of switches.
Turn on the boiler...the burner will light...after a few seconds the green light will come on and you should notice the flames viewed through the glass port hole significantly increase in size. As the light comes on you should also hear a faint buzzing behind the silver control box.
Turned on the boiler. Amber light to left came on with ignition, green light came on and flames doubled in size to about 1", heard a vibrating buzzing sound with green light that sounded a bit like the fan? Couldn't say for sure if it was coming from the control box though but it does start when the green light comes on and fades very quickly??? Amber and green light stay on.
If your gas measurements are correct the boiler is only consuming half the gas it's supposed to (book rate is around 30Kw).
However if the flames are visibly increasing in size as the green LED comes on it would appear the air pump and gas valve may be operating correctly.
However you mentioned earlier that gas pressures were measured and appeared OK....this does not concur with your gas rate measurement.
Sometimes the air pump pipework can partially fail (splits or the connections break) preventing full burner output but without measuring the actual burner pressure you can't tell.
When you timed the dial at 76 seconds (with hot taps fully open) did the flames stay at full height?
However if the flames are visibly increasing in size as the green LED comes on it would appear the air pump and gas valve may be operating correctly.
However you mentioned earlier that gas pressures were measured and appeared OK....this does not concur with your gas rate measurement.
Sometimes the air pump pipework can partially fail (splits or the connections break) preventing full burner output but without measuring the actual burner pressure you can't tell.
When you timed the dial at 76 seconds (with hot taps fully open) did the flames stay at full height?
The flame stays at the same height continuously. Unfortunately I didn't measure the gas working pressure myself. I did specifically ask the corgi's if there was a problem with the gas pressure though (as both the fault finding guides I have for this model request that it is checked). I'm not corgi registered and suspect that even checking the working pressure will be beyond my allowable remit, unless you can tell me different. I'll see if I can get the pressure double checked again tomorrow as I agree with you that the sums do not add up. The hot taps were fully open when I carried out your instruction. I think I convinced myself that it couldn't be the gas as the central heating works fine. The flames are lower when the central heating is on compared to when I demand hot water, if that makes any sense.
15 Kw will easily heat the typical property a combi is installed in.
But you need the full 30Kw for hot water. Get them to check the burner pressure (should be 5.1 mbar but check the data plate underneath) and gas rate.
But you need the full 30Kw for hot water. Get them to check the burner pressure (should be 5.1 mbar but check the data plate underneath) and gas rate.
I'll do that. Many Thanks.
The corgi is here at the moment at the standing gas pressure is 8.5 and the working pressure is also a steady 8.5. he realizes this is higher than what the book states but thinks it should actually help??? He's checked the diverter valve, servo, central heating side etc and is stumped. He phoned the Vaillant helpline and we were just queued for 40 minutes before hanging up. The corgi is taking the installation manual/fault finding guide away with him to study it and see if he's missed anything. He also phoned a number of his collegues who couldn't come up with any ideas. Could the gas pressure being too high indicate a fault? It didn't budge from 8.5.
Regards
It appears you have got yourself another idiot.
Did he use a digital manometer (pressure gauge)?...perhaps he had it set on the wrong units. The standing gas pressure (ie with no gas flow) is normally between 20 to 30 mbar...it just gives an indication of the incoming gas pressure to the property. If he is only measuring 8.5mbar there is something seriously wrong.
He should measure standing pressure at the meter...this gives an indication of the street pressure..sometimes it can be as low as 14/15 mbar but thats very rare and would warrant further investigation by Scotia (I assume your gas supplier).
The working pressure at the meter with the boiler on should be measured...this should be 21mbar plus/minus 2 mbar.
The working pressure at the boiler inlet should then be measured...should be the W.P measured at the meter minus 1mbar...any larger pressure drop and the gas pipework from meter to boiler is undersized.
Then the burner pressure is measured......8.5mbar does not sound correct. As I recall it should be around 5.1 mbar but check the data plate underneath the combustion chamber.
Under no circumstances should he have left the boiler operating if the high burner pressure resulted in a gas rate (ie the Kw input) exceeding the data plate value by 5%. What figure did he calculate for the Kw input?
Sadly there are few competent corgis to undertake boiler repairs, yesterday I went to a Worcester, the previous repair involved a replacement gas valve. The owner checked the installer was registered at the time. He managed to put the gas valve in back to front and could not therfore setup the minimum and maximum burner pressures. As a result he left the boiler burning way too much gas.
No he didn't use a digital manometer. It was a water filled one which he blew the water level back and forward to test it's accuracy. He checked the gas pressure at the front right hand side nipple below the burner. You are correct that the pressure is factory set at 5.1 and working pressure should be +/- .5 mbar. He didn't check the pressure at any other point apart form the one I've mentioned. Central heating on, hot water running or everything off all read 8.5 and the (water level) didn't budge from 8.5. He did come in and turn the 4 gas rings on full bore then check the boiler pressure again and it was still 8.5 mbar. Hopefully when he eventually gets through to Vaillant things will become clearer. He's suggesting I replace the DHW heat exchanger (2 min job but part is £100) but to be honest I'm just about at the end of my tether and may just install a new condensing boiler, I don't see the point in spending hundreds on parts that aren't broken just to find the cause. The boiler is over 8 years old so maybe that's the best course of action.
I've been time served for over 16 years and never had the financial need to become corgi registered (although not being able to fit a gas hob does drive me insane at times), I've never had to go back to any job of mine in 16 years, I know more about my boiler than the 3 corgi's (2 of which are with major servicing firms) and I would certainly never fit a gas valve back to front. However, I'm sure my latest help wouldn't fit one backwards either and I've known him for some time and I'm sure he is just unfamiliar with my Vaillant. The gas pressure checks do worry me though.
Smile and the world smiles with you.
I'll keep you posted.
Thanks Gasguru.
I've been time served for over 16 years and never had the financial need to become corgi registered (although not being able to fit a gas hob does drive me insane at times), I've never had to go back to any job of mine in 16 years, I know more about my boiler than the 3 corgi's (2 of which are with major servicing firms) and I would certainly never fit a gas valve back to front. However, I'm sure my latest help wouldn't fit one backwards either and I've known him for some time and I'm sure he is just unfamiliar with my Vaillant. The gas pressure checks do worry me though.
Smile and the world smiles with you.
I'll keep you posted.
Thanks Gasguru.
I'm getting nowhere fast with this but here's what I'm going to do. Although the servo and diverter valve seem to be operating okay I'm going to replace them. I've checked the lines for blockages and all is clear. I'm also going to change my DHW heat exchanger as, although the flow isn't restricted, the internal unit seems pretty corroded. And if it's none of these parts I'll stick them on ebay and get most of my cash back. Fingers x'd.
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