Condensation in back boiler flue sensor

6 Oct 2009
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United Kingdom
We have a problem with a replacement back boiler. Its a Glow Worm Miami 4.

Everytime the boiler isn't used for a while the flue sensor device appears to get blocked by condensation. Removing the silicone tube and sucking/blowing clear allows the boiler to be reset.

Is this common? Any simple fix?

It is definately condensation and can be seen through the clear tube at the bottom of the steel tube. After the initial bend from the top of the pipe it is a continous fall to the sensing device. Would a slightly longer silicone tube that allowed a slight dip before the end allow condensation to be held there and evapourated by the heat from the boiler before it reached the sensor?
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I am guessing that your boiler might have a fan and this is a fan proving switch.

In that case its a safety device and should not be modified. Nor is it a DIY job to work on your boiler!

Its not a boiler we see very often.

It might be worth calling the manufacturer however as its probably as a result of an installation fault it would be best to have it checked by a gas registered engineer first.

No, no fancy fan asisted stuff. A simple gas back boiler with copex flue.

From what I understand gas back boilers were more or less outlawed a few years ago in favour of more effecient kitchen/combi boilers?.

As a result the major players sold of their existing stock at much less than cost. Ie this one was bought for under £200 delivered direct from the manf.

Fitted and tested by a corgi guy I am no longer in touch with.

Over to you guys :LOL:
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blimey, a bbu and presumably fire front for less than £200!

From memory the last baxi I installed was about £1100 plus the copex. Current prices for the HE baxi about £2k plus.
As far as I know the Miami was a Radiant Convector Fire.

The Oxysafe flame sensing & flue blockage
safety system.
For your safety, this appliance is fitted with a flue blockage
safety device which will shut down the appliance in the event of
abnormal flue conditions. This device is NOT a substitute for
an independently mounted Carbon Monoxide detector.
The device will also automatically shut off the gas supply to the
fire if the pilot flame goes out due to lack of oxygen or for any
other reason.
If this device starts to repeatedly shut off the gas, get expert
This device incorporates a probe which senses that the heat
from the pilot flame is correct. If this probe is cool, the device
will prevent any gas flow unless the control knob is held down
at the ignition position.
If, for any reason, the flames go out when the fire is hot or if the
fire is turned off when hot, always wait at least three minutes
before attempting to relight.
I would guess this plastic tube is part of the oxy pilot to shut the boiler off in the case of spillage.

I would expect its an installation or flue fault as I cannot imagine they would market a design with an inherent problem like that.

As its a safety device the boiler should be given a proper service check by a gas registered engineer who will test the flue with smoke.

It is a 54/4. The complete bbu and gas fire was £199 inc del and arrived on a Worscester banded wagon via the plumb centre. This was in April this year. The boiler has been fine since then, but only worked on hot water so far. There is a Co2 detecter fitted in the hall next to it. Never triggered so far.

It is fitted in a small/med sized bungalow with a coal fire as the main heat provider, (the old lad was a miner).

These units were sold off as due to effeciency issues, they could no longer be fitted to any property without some sort of dispensation, ie, no other fitment type was possible, such as in period properties etc.

Flue was tested back then with a smoke cartridge and no spillage.
did you get exemption to fit that then or is it illegal?
Sometimes on really cold still nights with very little wind the oxypilot will kick in (we're have really freezing cold nights here at the moment, are you having the same?) mate but you do have to get the flue checked just in case.
Not particularly cold, quite windy tho'.

Regarding legality this is a supply/efficiency issue not safety. Were any of you pro guys aware of this? The unit was fitted by a registered chap so was proven safe at the time, whether technically he was allowed to fit it obviously is a question to be asked, but so is the issue of which was safer, the old,18 year unit, or a brand spanker with the latest safety mods?? ;)

Next step will be to get the flue checked. What is meant by a damp flue? This is a copex jobey in an ex coal fire chimbly.
If it hasn't been used for a while.

This device incorporates a probe which senses that the heat
from the pilot flame is correct. If this probe is cool, the device
will prevent any gas flow unless the control knob is held down
at the ignition position.
Thats not the way I thought an oxypilot worked.

As far as I am aware an exemption was granted for back boilers from April 2005 when condensing boilers were required.

But that might have been for a limited period of perhaps two years.

So fitting a BBU this year may not have met the Building Regulations.


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