Gas Ch Boiler Problem

[ladyalex]

Hello. Any gas boiler experts out there? Or anyone who has had a similar problem and solved it ?
We have a gas boiler. A conventional one, not a condenser type. We have a hot water tank and central heating. The boiler is more than 20 years old but has been regularly 'serviced' and recently had both the hot water pump and the central heating pump replaced.
The problem is that the boiler fires up and gets the rads nice and hot, then everything cools down. The heat gets too low for comfort. Then the boiler fires up again but only for 5 minutes at the maximum, so the rads never get properly hot again.
We have had a procession of British Gas men, often 2 at a time in the house over the last 10 days, all coming up with different 'soutions' none of which work.
They have 'rebalanced' our rads, checked that there is no significant 'sludging' changed the schedule so that the Hot Water heating is not on at the same time as the Central heating, made sure that the rads that we don't need to be on are set as low as possible, given us a new room thermostat....all to no effect. And yes, you have guessed it, they say that our boiler is too small for the system, we need a new boiler and a powerwash. That boiler has kept us warm and toasty for years.

We think that there is a problem with the boiler timing mechanism.

Any ideas anyone ?
Thanks a lot.
:-))

[KARL]

Firstly, I totally agree with what you imply: The boiler size is not the issue at all. I am unclear on just exactly what the "setup" is but is it a "vented" system - is there a header tank on the boiler circuit ? This would be a smallish tank filled through a float valve, positioned somewhere above every part of the system, like in an attic or elsewhere up close to the ceiling - it will be entirely separate from a tank to fill up the hot water tank (which would be of significant size).

Also, it is not clear to me what level/type of controls you have but it sounds like, as in 99% of British homes, there is a timing/rationing/quota device involved. Can I suggest that you set that device to continuous operation (all day and all night) and see what happens, how the system behaves ? Once that is clear add a description to your post and then more informed/detailed advice should become available.

[ladyalex]

Thank you Karl.
We do indeed have a header tank.

[ladyalex]

The system at the moment is essentially on all day (from 06:45 until 23:45) .
The rads are warm enough when we get up around 8-ish but then get cold and never warm up again.
Also, possibly as a result of the gas men fiddling with the system, our hot water temperature is lower than it was. We reported this to them on Friday and they said 'You'll hurt yourself is we put it up higher' when we insisted that we could,not wash our dishes in it far less have a bath, they put it up a bit but not much.
So, everything with the Gas Men is a bit of a battle at the moment. They're coming back tomorrow for another round.

[Etch]

Make and model of the boiler? Not all boilers are the same, the service manual may be available on the internet. My Potterton Netaheat, still going strong after 30 odd years, has a thermostat to control the boiler water temperature. I replaced this on my boiler because I thought it was letting the temperature get too high (opposite to what you have) and made sure that the sensor was bonded to the casing with thermal compound. Perhaps there is a similar thermostat on your boiler which is failing or needs turning up.

[bhg481]

The only thing left is the diverter valve which switches the flow of hot water to either cylinder or radiators. It is usually placed close to the pump and has a lever on it which allows for manual opoeration in case of it sticking, which can happen sometimes at the end of summer when the valve has been pointing towards the cylinder for several months. You can see the lever move so can see which way the water flow is. They are usually set such that when the system comes on the water is heated first and the the radiators only when the water has reached the set temperature.
Incidentally, our radiators won't come warm unless the hot water is on as well. In other words, if we want to boost the house temperature during the daytime we have to switch both hot water and radiators on.

[ladyalex]

It is a Vaillant Thermocompact VU 282E

[bobbinwales]

we had a problem simlar to this caused by the pressure being too Low. A went on youtube and are repair took 2 minutes- we just had to turn a little tap/valve 45-90 degrees (angle not temprature) to let a bit more water flow in for a minute then turn it back. Worked a treat

[Etch]

Ok, an installation and servicing manual is available here (should open the manual at the "Fault finding" page):
https://library.plumbase.co.uk/flipbooks/RE/vaithecvu2821e_23005_t/mobile/index.html#p=44
To rule out a problem with the boiler I would check that the correct indications are showing when the boiler is supposed to be active, and that readings are as expected on the 'Temperature and pressure gauge'. See this page for the location of the latter:
https://library.plumbase.co.uk/flipbooks/RE/vaithecvu2821e_23005_t/mobile/index.html#p=6

[bhg481]

Just to explain the system to you: there are 3 thermostats in your system. One is on the boiler and is probably adjusted by a knob on the boiler somewhere. When the system is "on" the boiler maintains the water inside it at the temperature set, regardless of whether it's needed for the water or radiators. There is also a thermostat strapped to your hot water cylinder which governs the temperature of the water that comes out of your taps. There will be a third thermostat in one of your rooms, usually the living-room or hall; this governs the temperature of that room and, if the system has been properly designed, the temperature in all the other rooms in the house. Sometimes there are thermostatic valves on radiators which limit the temperature in that room only .
The idea is that the boiler "always" contains water which is hotter than that required for the taps. When you turn on the heating the water from the boiler is diverted to the cylinder to heat up the tap water to the temperature set on that thermostat. When the tap water is hot enough the diverter valve sends the boiler water to the radiators until the room thermostat tells it to stop. At that point the pump switches off.
If the hot water or the room cools down the pump should restart and the diverter valve will direct the boiler water in the relevant direction.
In our system, once everything is at the chosen temperature the boiler hardly lights and the radiators are little more than aired.
I am puzzled by the fact that you refer to 2 pumps having been replaced. All the systems I have come across only have 1 pump; is the second pump for a power-shower perhaps?

[Etch]

@bhg481, ladyalex refers to the 2 pumps as "hot water" and "central heating" so it sounds as if, in her system, the supply from the boiler simply splits (without a diverter valve) into H/W and C/H and each circuit is then pumped individually (controlled by its appropriate thermostat).

[bhg481]

Etch - I've never come across a system like that but I an only an amateur.

[ladyalex]

Thanks for responding, everyone.
@Bobbinwales Is yours a Combi boiler ? Ours is not . It is an old conventional one....open vented is the term I believe.

I have now set the Central Heating to "Always on" and will report back.
Thanks, all, it is good of you to help us.

[ladyalex]

Thank you for the links to the Manuals Etch, but they are too complicated for us to understand.....and well above our competence level to follow.

[Etch]

bhg481 - when I installed my system I opted for the single pump/diverter valve configuration that you comprehensively described. However, 'fully pumped using two pumps' is a possible configuration shown in the installation instructions, and 'gravity feed' to the hot water cylinder is yet another. I'm still only an amateur as well though!

[bhg481]

Etch - it's about 40 years since I designed and installed my own system and I don't recall coming across the 2-pump option; perhaps it's a more recent idea. I did have gravity-fed hot water and pumped radiators though - the hot water cylinder was directly above the boiler.

[The Builder]

Dearest Lady Alex,

1. Avoid British Gas.
2. Try and get hold of a proper, independent local gas boilerman.
3. If in doubt... contact Vaillant for guidance. They might possibly
point you towards someone in your area.
4. Avoid British Gas.

One of the first things they should be trying, is replacing any (cheap) thermistor type temperature sensors.
Possibly, the system is going off-heat because it's being told by a faulty sensor that the temperature or maybe the flow rate is Ok, when it really isn't.
... but that's only one suggestion. A decent boilerchap should get to the bottom of it.

[The Builder]

https://www.vaillant.co.uk/homeowners/contact-us/?gclid=CjwKCAiAlNf-BRB_EiwA2osbxbd_8baaWWylAtXk3aLQclXkzyqs0qdNRVmtD4TrxfdH5g0g9md4VRoCUg8QAvD_BwE

[ladyalex]

Thank you Builder.

We do intend to avoid British Gas in future, but at present we are sort of tied into them with an annual service agreement that last until next summer. When it comes to an end we are certainly going to go elsewhere.

Thank you for the links to Vaillant. Lordalex is on to it and also intends asking them for a local tradesman .

Thank goodness we have some electric heaters to tide us over.

[KARL]

Following the description you have provided and your explanation of your timed interval and system behaviour, I would be interested in trying switching off once the system starts to noticeably cool down and within 5 minutes switching it on again. Repeat this exercise if the behaviour (re-heat followed by cool down) suggests that is called for. If the system simply continues cooling down inexorably without warming up at all then abandon the trial. If the system restarts/warms up every time then at least you have a means of keeping warm but a long term solution remains to be found. How long from start-up until heat ceases to be provided (boiler continually off) ?

[KARL]

One incidental thought: Almost all technicians prefer to advise replacing any boiler with problems if it is more than 3-5 years old. This is because a) this provides work plus a mark-up gain on the equipment and materials and b) there is no thinking required to arrive at this recommendation.