Vaillant water heater

Frankie-H

Frankie-H

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Hi Folks. Anybody out there have any experience of the Vaillant wall mounted water heater. Mag 125. I don't know how old it is but it has given us continuous service for the last 16 live aboard years.

It's gas lights properly and the water pressure remains the same but it will not give hot water. It has a tap on the inlet pipe and, briefly, this morning I managed to get acceptably hot water by manually restricting the flow. Any less and the gas goes out. Now it won't even do that.

As always any help will be gratefully accepted.
 
We had one on our previous boat for 20+years and it worked very well and seemed to need little in way of maintenance. I don't remember the model number but it was the boxy one (i.e. Didn't have a rounded base).

I seem to remember that it had a similar problem that was rectified by dismantling the valve at the base. This is a sort of "demand valve" where the water pressure drop operated a valve to turn on main gas flow. If it sticks then pilot remains on but nothing else happens (i.e. No whoosh as main burners go on full). It was on the water side and not the gas side so I was quite happy to work on it.

You could try looking at the base to see if there is an assembly with water in & out. Drain & disconnect these pipes & there is a drain plug on side of the valve. I used to winterise by opening the taps and removing after the initial flow had died.

The valve has five (I think) screwheads visible when looking up from directly underneath. Remove these to remove the cover plate. I'm a bit fuzzy on the details but think there's a rubber diaphragm unside and this can be perished or punctured. I think that the valve can get stuck in one position (think I might have moved it and put a little silicone grease on the shaft).

The screws should have a narrow waist so that they break in freezing weather to avoid damage to the valve. Previous owner had used ordinary ones so I had to get replacements.

Sorry that I can be of more help but hope it provides a starting point or prods. someone else to say why it isn't correct.
 
If its any comparison, my Vaillant combi boiler at home recently refused to give hot water but the central heater was OK. A bit of googling led me to air pressure sensor as being the trigger for the boiler to fire up when the tap is opened. I replaced it (easy) and all is now fine. Brownie points earned as well :)
 
Mistroma said:
We had one on our previous boat for 20+years and it worked very well and seemed to need little in way of maintenance. I don't remember the model number but it was the boxy one (i.e. Didn't have a rounded base).

I seem to remember that it had a similar problem that was rectified by dismantling the valve at the base. This is a sort of "demand valve" where the water pressure drop operated a valve to turn on main gas flow. If it sticks then pilot remains on but nothing else happens (i.e. No whoosh as main burners go on full). It was on the water side and not the gas side so I was quite happy to work on it.

You could try looking at the base to see if there is an assembly with water in & out. Drain & disconnect these pipes & there is a drain plug on side of the valve. I used to winterise by opening the taps and removing after the initial flow had died.

The valve has five (I think) screwheads visible when looking up from directly underneath. Remove these to remove the cover plate. I'm a bit fuzzy on the details but think there's a rubber diaphragm unside and this can be perished or punctured. I think that the valve can get stuck in one position (think I might have moved it and put a little silicone grease on the shaft).

The screws should have a narrow waist so that they break in freezing weather to avoid damage to the valve. Previous owner had used ordinary ones so I had to get replacements.

Sorry that I can be of more help but hope it provides a starting point or prods. someone else to say why it isn't correct.
Click to expand...



Mistroma. Thanks a lot for all that. I will have a look for the diaphragm tomorrow. Our water flow is normal and turns a very satisfying woooshhh of burning gas on. All very normal as far as I can tell but sadly no hot water:(:(
 
I'm with Mistroma and can only add that a lot of the screwheads broke when I was dismantling - replaced from the junk box.
That was the only remedial work I ever did on mine and it was working when I dumped it for a calorifier years later.
 
As described above If the diaphragm is holed or stretched the burner often takes longer to come on or will not come on at all. If you are burning the gas the water must be heating up unless the diaphragm is not lifting the gas valve enough to give you full gas rate. There should be a data badge with the burner pressure on it and this can be tested using a manometer. This is a very common problem with domestic multipoint water heaters. Also as mentioned clean spindle and lubricate spindle and spindle seal with silicone grease.

Pete
 
Frankie-H said:
Mistroma. Thanks a lot for all that. I will have a look for the diaphragm tomorrow. Our water flow is normal and turns a very satisfying woooshhh of burning gas on. All very normal as far as I can tell but sadly no hot water:(:(
Click to expand...

If you can see the burners going quite strongly then the heat exchanger should be getting hot. I can imagine the burners running (not just pilot light) and water coming through cold due to a complete restriction in the exchanger inlet side. i.e. Nothing entering the exchanger but outlet side full open to let all contents boil off. But I think that would just result in no water from hot tap. It should also cause fairly loud noises as the water boils in the exchanger.

Alternatively, the water could be running through so quickly that it doesn't get time to heat. That seems more likely.

Can you turn the central knob on the front from Cold to Hot? I remember that this had the effect of slowing down th flow through the heat exchanger (& water temp. went up). If it seems to have no effect on output from hot tap that points to a problem in that area.

If you can't reduce water flow by turning the knob to full hot then that may be the place to look at first.
 
Mistroma said:
If you can see the burners going quite strongly then the heat exchanger should be getting hot. I can imagine the burners running (not just pilot light) and water coming through cold due to a complete restriction in the exchanger inlet side. i.e. Nothing entering the exchanger but outlet side full open to let all contents boil off. But I think that would just result in no water from hot tap. It should also cause fairly loud noises as the water boils in the exchanger.

Alternatively, the water could be running through so quickly that it doesn't get time to heat. That seems more likely.

Can you turn the central knob on the front from Cold to Hot? I remember that this had the effect of slowing down th flow through the heat exchanger (& water temp. went up). If it seems to have no effect on output from hot tap that points to a problem in that area.

If you can't reduce water flow by turning the knob to full hot then that may be the place to look at first.
Click to expand...



Again my thanks. You were up very late last night !

This is exactly my problem. Water pressure seems normal and flow from the taps seems normal. Ignition is very quick and normal.

I have had the heat regulator out a few times over the years and it is clean and well lubricated. When I turn it all the way to hot, to the right. I can hear a difference in sound of the water running. This seems to be on the maximum restriction. I managed to get a brief but reasonable shower yesterday by restricting the flow further by partially closing the inlet tap at the back of the unit. Just at the point when lack of flow turns off the gas. The water then ran cold, although the gas was still well ignited and I can not duplicate the experiment again.

I think the diaphragm is controlling the gas supply. If this is the case, then I don't want to touch it, in case I damage it, since the gas supply and ignition is normal.

Again thanks for wasting so much time on my problems.
 
As stated above the water usually enters the water section under the diaphragm and with no flow the pressure above and below the diaphragm equalises when you open the tap the pressure above the diaphragm drops the diaphragm moves up and pushes up the spindle which goes usually up through a water seal out of the water section and into the gas section where it lifts the gas valve off it seat against a spring. If it does not lift enough you get full water but not full gas.
Pete
 
Might be off the mark here , cos I have only skimmed the thread.(assuming it's a domestic boiler)
If the boiler is running with the CH working but no hot water when required , the problem is likely to be the diverter valve.
ie. water is heated via the heat exchanger but then passes round the heating circuit cos it has not been diverted to the hot water circuit.

(the energy has to go somewhere or the heat exchanger will meltdown or the boiler will shutdown with an overheat alarm)
 
jakeroyd said:
Might be off the mark here , cos I have only skimmed the thread.(assuming it's a domestic boiler)
If the boiler is running with the CH working but no hot water when required , the problem is likely to be the diverter valve.
ie. water is heated via the heat exchanger but then passes round the heating circuit cos it has not been diverted to the hot water circuit.

(the energy has to go somewhere or the heat exchanger will meltdown or the boiler will shutdown with an overheat alarm)
Click to expand...

The Vaillant is very simple and doesn't have an overheat alarm. I was going along the same lines earlier when I said that if gas burners were working it could really only be far too much water or far too little through exchanger. The heat exchanger is quite small and doesn't take too much to overwhelm the burner output.

I would normally have leant towards resticted flow but my memory of this unit is that it makes quite a lot of noise if flow is restricted. Steam, expansion, condenses in outlet, pressure collapses etc. so pops & bangs.

However, I also agree with the above diagnosis. I already suggested trying to turn to full heat to test this out. You can usually hear the change in flow though he valve. If it only gets a bit hot then valve isn't closing enough and if no change in sound then it isn't working at all. Or you could bite the bullet and take the valve out for repair.
 
Back again folks. I seem to be getting too much flow and I can restrict the flow by partially closing the tap at the back of the unit. This is very critical and changes with the temperature, obviously heat expansion. If I maintain the flow at just past the level where the gas starts to cut out, I can get hot water. I don't think that this is a gas problem. More of a water flow problem. The pressure pump is about 1 year old and has been working perfectly. Does anybody know if it is possible for the pressure to increase inside the pump and therefore increase the water flow to such an extent that it is moving through the heater too quickly.
 
Frankie-H said:
Back again folks. I seem to be getting too much flow and I can restrict the flow by partially closing the tap at the back of the unit. This is very critical and changes with the temperature, obviously heat expansion. If I maintain the flow at just past the level where the gas starts to cut out, I can get hot water. I don't think that this is a gas problem. More of a water flow problem. The pressure pump is about 1 year old and has been working perfectly. Does anybody know if it is possible for the pressure to increase inside the pump and therefore increase the water flow to such an extent that it is moving through the heater too quickly.
Click to expand...

Always difficult to diagnose a problem remotely. Sounds as if you are making some progress in tracking it down. First thing I usually ask is "what's changed" and if you haven't made any changes then the performance of some part has altered.

It's unlikely that the pump has suddenly started pumping a lot more water. It does seem to confirm that something has broken or siezed in the valve. You say that the flow seems critical, too much and it doen't heat, too little and the gas goes out. That suggests the valve is shutting too easily when pressure drops.

Can you push up the gas supply piston by hand when the bottom of the valve & diaphragm have been removed. I seem to remember that it was very difficult to do this and assume that there is a strong spring behind it. If so, perhaps it has broken.

Very much guesswork as I haven't worked on this unit for around 10 years.
 
It's one of two things. Either your gas pressure is low or the diaphragm has a pinhole in it.

If you replace the diaphragm get the complete kit and not just the rubber diaphragm and make sure you remove and replace the mini O-ring on the spindle.
 
Last edited:
We had one, it had no flame fail device and our insurers questioned whether we still had it installed ( it was approx 25 years old and worked perfectly)
Replaced it with a calorifier which is magnificent but was a costly solution!
 
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