Is a combi boiler safe with NO mains cold water? (Urgent)

[Lycanfox]

Thanks in advance for any replies to this.

Basically this morning a hot tap would not turn off and was gushing hot water. I couldn't find an isolation valve for the tap so had to turn off all the water via the main stopcock. The only worry is a plumber I rung said that without the cold water going to our combi boiler, the heat exchange could melt itself and cause a lot of damage so we had to turn our heating off.

Now correct me if i'm wrong but surely as long as the hot water setting is turned off on the boiler, the boiler should have two different heat exchangers so the hot water one would not melt due to not having any water to heat if it was off, and the heating exchanger works on a closed loop so should also be ok?

Thanks again for any replies, the last thing I want is to destroy my boiler in such a cold winter.

Adam

 

[Millsy 82]

think lame is saying that any water left in dhw side will expand in the heatex from the ch side as its dualpass and this could cause probs unless a tap is opened, as op says hot tap is stuck open there will be no probs imho

I still think hes sprouting rubbish as its usually full of water at mains pressure so I cant see the problem the boiler has an overheat which will stop it boiling and the heat exchanger is designed to with stand the pressure of mains water.

 

[dancinplumba]

see your point but stat will only kick in if ch water gets too hot imo

 

[tom609]

hi everyone i'm new to the site i agree with dancin on this will be ok

 

[Reg Man]

Overheatstat will kick in on this boiler, at 80degrees "with any luck". But this is a distressing fault condition temperature for the poor old heat exchanger. Why take the risk? Could right off the boiler for the sake of waiting for a tap repair.

 

[lame plumber]

I still think hes sprouting rubbish as its usually full of water at mains pressure so I cant see the problem the boiler has an overheat which will stop it boiling and the heat exchanger is designed to with stand the pressure of mains water.

but you just hit the nail on the head there, mains is off as the tap is leaking so its not at mains pressure, so water can boil at 100 deg c and lack of water can let it boil, produce steam which can be quite high pressure which can also damage the o rings and cause leaks. Hence why all low water content boiler have overrun to cool the hex to prevent hot spots etc cooking them. So the only one spouting rubbish is milsy

 

[Nostrum]

How can high pressure steam be produced when the tap is open

 

[lame plumber]

How can high pressure steam be produced when the tap is open

bit of water left in the pipework (clutching at straws here ), but the main issue is you could fairly easily cause damage when there no need to if you dont use the boiler, why risk it??

 

[dancinplumba]

hi everyone i'm new to the site i agree with dancin on this will be ok

hi tom introduce yourself here New Member Introductions

 

[dancinplumba]

but you just hit the nail on the head there, mains is off as the tap is leaking so its not at mains pressure, so water can boil at 100 deg c and lack of water can let it boil, produce steam which can be quite high pressure which can also damage the o rings and cause leaks. Hence why all low water content boiler have overrun to cool the hex to prevent hot spots etc cooking them. So the only one spouting rubbish is milsy

the ch side is still full of water and will dissipate the latent heat in the hex

 

[Reg Man]

How can high pressure steam be produced when the tap is open

No it will just boil the heat exchanger dry. Oh we still have the overheat stat. I wonder what the boilers potentiometer will think of this.

 

[lame plumber]

would any of the leave it its ok, who have a ferrolli like this at home, pop out and set it up to see what happens in this situ and let us know pse

 

[dancinplumba]

would any of the leave it its ok, who have a ferrolli like this at home, pop out and set it up to see what happens in this situ and let us know pse

or a junior

 

[stevetheplumber]

its safe to leave the heating running even with no mains feed it will only fier for heating and the water flow keeps the hex at normal temps and pump will over run to dissipate het when boiler goes of

 

[lame plumber]

forgot, plumbers dont work on sundays unless callouts £200 plus :sleeping:

 

[Millsy 82]

There is no way that the boiler firing up for heating is going to cause any damage to the heat exchanger because the DHW side is empty The pump will run and this will take the heat out of the boiler. Yes there is still going to be water in the DHW side but at zero pressure it will not build up so drastically that it is going to cause damage to the heat ex. As for the OH stat it will also cut the boiler out on the DHW side of things as well as the heating it is a safety feature that will shut the boiler off when heat exchanger gets to a set temperature. It will not do any thing to the potentiometer as every OH stat I have ever come across in the 14 years I have worked in this trade work the same they go from closed to open circuit.

I have left many peoples heating running in the past whilst I have had the water off changing taps etc and surprise surprise I have never killed anybodys heat exchanger. I would suggest Lame plumber that you do not comment on things you are 100% sure of as in this case you are definately sprouting rubbish.