Glow-Worm Fuelsaver MKII - Overheating

[mrlynch]

Hi all,

This is my first post on these forums so please excuse me if I miss any important information.

Our boiler (Glow-Worm Fuelsaver MKII) has been like this since we moved into our house back in October and I think its getting to the stage now where we need to finally find a fix or get it replaced.

As far as I am aware the boiler is approximately 25 years old so will need replacing eventually anyways.

If I just put the boiler on for the hot water it will stay on for approximately 2 minutes, overheat and then stay off for quite a few minutes until it is cool enough to come back on.

If I put it on for the central heating it can come on for anything up to 15 minutes before overheating and cutting off.

Over the winter months this wasnt too bad as if we had the heating and hot water on together the boiler would stay on long enough to heat up both.

Although the boiler is better when the central heating is on I wouldnt say the radiators ever got that hot (the house rarely would get up to 22 or 23c even if the heating had been on for hours. Also, the hot water was only mildly hot (you could comfortably run the hot tap over your hand (although without the heating on the hot water only gets warm).

I have tried draining the system, bleeding the radiators, running some descaler for a few weeks and then draining the system but nothing has improved. Oh I also changed the thermostat on the boiler but it turns out the old one was working fine.

I have had one plumber look at it and he was unable to diagnose the problem, his only suggestion was the heat exchanger may be scaled up?

I'm open to trying anything as we have a few months before we will really need a replacement. At the moment we aren't using the boiler, instead we're just using the immersion tank for half hour a night - its nice to have HOT water again

Thanks in advance for any replies, please let me know if I can provide further details.

Many thanks,

Darren

 

[twgas]

firstly does the flow get hot from boiler, to pump, through motorised valves, to cylinder.

 

[mrlynch]

Hi Mick,

Yes the pipes are hot from boiler to pump and from pump to cylinder.

Thanks

 

[mickyg]

Theres lots of possible causes of your problem, so check a few things first.

Check header tank has got water in, and while heating is running does water pump into it from vent pipe??
What is the pump speed set to? Does changing speed change anything?
After boiler has operated for a while check both pipes to see what the temps are like. The return should be slightly cooler than the flow.

 

[mrlynch]

The header tank has got a good amount of water in it. I've been in the loft occasionally when the boiler has been on and only every now and again will a little splash of water come out of the pipe (there isnt a regular flow or trickle) - is this normal?

The pump is operating at maximum speed - I have tried changing it to the two slower settings but this didnt have any effect

Of the two pipes coming out the top of the boiler, one is very hot (too hot to touch) and the other is hot (although you can keep you hand on it).

Thanks

 

[mickyg]

The header tank has got a good amount of water in it. I've been in the loft occasionally when the boiler has been on and only every now and again will a little splash of water come out of the pipe (there isnt a regular flow or trickle) - is this normal?

It shouldn't really do this, but could be just as simple as not having the vent pipe high enough, or a partial blockage in the pipe where your vent and cold feed pipe connect to, or they could just be wrongly configured.

The pump is operating at maximum speed - I have tried changing it to the two slower settings but this didnt have any effect

Of the two pipes coming out the top of the boiler, one is very hot (too hot to touch) and the other is hot (although you can keep you hand on it).

When a central heating system is running properly with a standard efficiency boiler, ideally you want around 11c difference between the flow and return and this shouldn't be too much of a problem with just the hot water on. It sounds as if you have a restriction in flow. it would be very difficult to suggest what the exact cause is.

Common causes would be:-

Pump--although it might appear to be spinning, it could be blocked or impeller could be damaged etc.. pump valves can also "drop" or get blocked

cold feed pipe-- can commonly block where it connects to the ch pipe.

honeywell motorised vaves(silver box)--ball inside can swell and cause restriction.

heat exchanger--can get full of crud and block(normally this would be accompanied by noises--kettling, banging, knocking etc..)

this is just common stuff, which I would check first if the problem isn't more obvious.

some of this stuff is probably beyond the average diyer, so you might wanna try and get someone in, maybe someone better than last time!

 

[mrlynch]

Thanks for the reply mickyg, i'll definitely try and look into all of these possibilities. I've done quite a bit of research into it and most of them seem to agree with your points.

The problem seems very likely to be poor circulation in the system and the two most likely causes as you suggest seem to be:

1) Dodgy pump
2) Heat Exchanger (the boiler does whistle like a kettle before it overheats and clicks off)

The pump I can probably do myself however I think the heat exchanger will need a pro

Does anyone know of a good or can recommended a plumber in Kent (i'm based near Maidstone).

Thanks

 

[eddyheadbutt]

Hi Darren just came across your post. did you ever get to the bottom of your boiler problems. Reading your description it sounds similar as the problems we are having. cheers tony

 

[mrlynch]

Hi Tony we ended up going for a new boiler in the end, and with the last couple of winters it was the best decision we ever made

As for the exact cause of the problem it may have been down to a poor connection in the controller which was causing the pumps to work in reverse, the new boiler had this issue until they rewired the controller.