Grant Vortex Oil Boiler Exploding

[rcg733]

Wonder if anybody has any ideas?

I have an external Grant Vortex Pro Oil Boiler which worked nicely for a few months, but now it is the habit of just boiling the water within itself.

Last night at 1130pm, the pressure release vessel did it's job and sprayed boiling water around the kitchen. The overheat cutout also tripped on the boiler.

The obvious problem to me is that the thermostat in the boiler had gone, but that has been replaced and we still have the problem.

Any Ideas?

[guindilias]

Is the pump working OK?

[rcg733]

Yes the pumps appear to be working fine - but I guess this happens when the rad's have shut themselves down with the TRV's

Surely the boiler should stop heating itself up when it gets to the set temperature ( eg 65C), not just keep burning until the water has been vapourised

Well that's what I would hope...

[Alucidnation]

Spraying boiling water around the kitchen??

That should be vented outside.

You should get an engineer in to sort that regardless.

[rcg733]

It is all vented to drain outside, but it can't cope with the volume of steam being generated by the boiler, that blows the pressure relief.

Is it normal for a boiler to just turn it's contents to steam?

[clockworks]

rcg733 said:

- but I guess this happens when the rad's have shut themselves down with the TRV's

Do you have an automatic bypass valve? Is it working and set correctly? (I assume it's an unvented system)

[rcg733]

Yes it is an unvented system.

Not sure about automatic by-pass valve - I would presume we have whatever is needed.

Would the lack of any of these things cause the boiler to simply boil it's contents?

Surely the boiler has a thermostat so that once the water within it reaches temperature, it stops providing heat?

[guindilias]

It should do, yes - normally stuffed into a little pocket in the water jacket. Summat like this - https://www.ebay.co.uk/itm/Grant-boiler-thermostat...

[clockworks]

With all the TRVs closed, and no bypass, the water in the boiler heat exchanger will stop circulating. Maybe there's enough heat in the metalwork to cause the water to boil, even if the thermostat has shut the burner down.

Try removing one of the TRV heads (to create a bypass)?

[rcg733]

Thanks Guindilias - Yes, we replaced the thermostat, and during normal operation, the boiler fired and when then stop providing heat when the water back into the house hit 55C.

All was well, then last night, off it went again, after steam was released the output from the boiler was 92+C!

Whilst I accept that there may be lag caused by heat retained in the metal which my heat the water within the boiler above the 55C setting, surely, regardless of loops elsewhere etc, the boiler should stop providing more heat which ends up with the water boiling!

I guess my first job is to remove the TRV on the By-Pass rad, but even if that stops the steam, it still appears to me that the boiler is pretty seriously mal-functioning - but obviously I am no expert, hence my post!

[Black_S3]

You should always have one rad without a TVR?

[clockworks]

If the burner is still running after the thermostat has told it to stop, then it does have a problem. Does it actually carry on running though?

[King Herald]

Black_S3 said:

You should always have one rad without a TVR?

This. I was told that usually the bathroom rad is left on all the time, no TRV or any other skullduggery.

[Black_S3]

King Herald said:

This. I was told that usually the bathroom rad is left on all the time, no TRV or any other skullduggery.

I thought it was meant to be the room with the thermostat in it?

[guindilias]

That just eliminates the need for an auto bypass valve, though - if you have TRVs everywhere, the bypass valve is effectively your "bathroom radiator". It should recirculate the hot water into the boiler until it reaches thermostat cut-out temp, then the burner should stop firing.

[eldar]

Black_S3 said:

I thought it was meant to be the room with the thermostat in it?

That is the case.. Avoids the thermostat being over ridden by the TRV.

[fastbikes76]

The no TRV's is a red herring in this case, The VTX range does not have pump over run unless its the Pro combi, even then it will work without any bypass or TRV's. Not ideal but will still work without overheating.

The PRV opening and dumping is not temp controlled, it is pressure controlled so will blow open at 2.5 bar pressure or more. You need to make sure you have 1 bar pressure cold and see what pressure it rises to when hot. If you have the correct amount of expansion on the system it shouldn't really go over 2bar ever.

The control stat is range rated at 60* min and 85* maximum, the high limit overheat stat is rated at 111*s so if it was purely a overheat issue it would just go out on the limit stat rather than blow out the PRV. With latent heat from immediate shut down, I've never seen them go much above 95degrees standing. I suspect you have no air pressure in your expansion vessel (1 Bar air and then add 1 bar water to the system) or you have a whole lot of air in the which is turning to steam in the top of the boiler and creating enough pressure to open PRV.

I would drop all the pressure out the system and top up to 0.5 bar, run the system and see how high it goes. Also make sure you turn the boiler control stat to minimum. When the boiler cuts out on the control stat turn it up to max and see if the burner cuts back in. If the burner cuts back in when you turn the stat back up to max it means the control stat is working ok.

[rcg733]

clockworks said:

If the burner is still running after the thermostat has told it to stop, then it does have a problem. Does it actually carry on running though?

What other rational explanation is there for the water within the boiler turning to steam?

There was still steam hissing out of the top of the boiler 3 hours after it had cut off last week!

Of course, when you sit and watch it, it is as good as gold, but then 24 hours later it blows again.