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

[lame plumber]

we can all have an opinion, but there's no need to be rude, after all forums are for discussion and everyone has had different experience of boilers and problems and because of issues I have had, esp with a ferrolli in the past I offered my opinion such that the bloke posting a query could make a choice. Like I said, I've experienced a ferrolli doing just this when I failed to bleed a pump thro recently, it only wrecked a couple of o rings but when chris from ferrolli turned up, he was decent enough to point out loads of his work involves repairing boilers where the hex is boiled and o rings blown and flow valves melted, and with that damage pcbs etc knackered. This is being done regularly on new installs so there is the potential to do the same with this boiler if your luck runs out so why risk it. So as you tried to say, you shouldnt comment on things you are not 100% sure of, but in this case I'm 98% sure so I think I can offer an opinion without being told I'm sprouting, sorry spouting a lot of rubbish. If you want to comment, please feel free, its an open forum but dont get arsy as its not really needed. After all the guy only wants an opinion to make his choice! be it yours or mine or someone elses

 

[Billy Bob Bob]

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.

sorry but your wrong,end of argument,as said this boiler has a twin pass heat exchanger,there is a risk that remnants of water left in the domestic side can cause issues if the ch is used,now who has the balls to argue differently with me about boiler design??? i am not taking anyones side but this is FACT

 

[dancinplumba]

we can all have an opinion, but there's no need to be rude, after all forums are for discussion and everyone has had different experience of boilers and problems and because of issues I have had, esp with a ferrolli in the past I offered my opinion such that the bloke posting a query could make a choice. Like I said, I've experienced a ferrolli doing just this when I failed to bleed a pump thro recently, it only wrecked a couple of o rings but when chris from ferrolli turned up, he was decent enough to point out loads of his work involves repairing boilers where the hex is boiled and o rings blown and flow valves melted, and with that damage pcbs etc knackered. This is being done regularly on new installs so there is the potential to do the same with this boiler if your luck runs out so why risk it. So as you tried to say, you shouldnt comment on things you are not 100% sure of, but in this case I'm 98% sure so I think I can offer an opinion without being told I'm sprouting, sorry spouting a lot of rubbish. If you want to comment, please feel free, its an open forum but dont get arsy as its not really needed. After all the guy only wants an opinion to make his choice! be it yours or mine or someone elses

nothing to do with the dhw side then ??

 

[dancinplumba]

sorry but your wrong,end of argument,as said this boiler has a twin pass heat exchanger,there is a risk that remnants of water left in the domestic side can cause issues if the ch is used,now who has the balls to argue differently with me about boiler design??? i am not taking anyones side but this is FACT

how and why ?

 

[Nostrum]

Ok, so whats the difference between this and the heat exchanger on the DHW side being full as you would find in normal operating circumstances? So for example, the heating has been on all night and no one has run the tap. Late at night the tap is run. The water in the DHW side does not get anywhere near boiling point otherwise it would be lethal as it comes out the tap (the volume of water contained within the heat exchanger all night)

The reason is because the heating side is dissapating the heat, which is exactly the reason why the heat exchangers are joined. If there were 2 completely seperate heat exchangers being primarily and directly heated by the burner then this would occur. Remember this is a heat exchanger and its primary purpose id to be able to transfer heat efficiently. Having a twin pass means the 2 act like a big heat sink to one another.

The reason (in my opinion) lame plumber, you had said problem is because you had no flow in either DHW or heating, therefore no way of dissipating that heat, the boiler cannot react quick enough to shut down so the static water boils (or as near as)

 

[Billy Bob Bob]

how and why ?

the domestic circuit runs through but not in direct contact with the primary circuit(ch) the hex is a direct heat design,as apposed to the plate heat set up that is a indirect heat design,take a polystyrene cup,put a tiny amount of water in it and place your gas lamp on full and torch the bottom,you then get some idea of what your doing to a twin pass heat exchanger when running it in heating mode with no water in the secondary (dhw) circuit only in this cup the water quickly evaporates and the cup melts,in a boiler the small amount of water turns to steam melting o rings,the plastic gubbins of the flow switch,and very quickly the copper tubes within the heat exchanger

 

[chris watkins]

the domestic circuit runs through but not in direct contact with the primary circuit(ch) the hex is a direct heat design,as apposed to the plate heat set up that is a indirect heat design,take a polystyrene cup,put a tiny amount of water in it and place your gas lamp on full and torch the bottom,you then get some idea of what your doing to a twin pass heat exchanger when running it in heating mode with no water in the secondary (dhw) circuit only in this cup the water quickly evaporates and the cup melts,in a boiler the small amount of water turns to steam melting o rings,the plastic gubbins of the flow switch,and very quickly the copper tubes within the heat exchanger

Still don't understand the above ?

Long & short of it is - do we think that a boiler manufacturer (even Ferroli) would design a combi boiler that if the mains water supply was interrupted due to say, water board turning off or a burst pipe, when the heating was on & it would cause damage or failure of heat exchanger ? I don't think so ! I can just see the claims rolling in now, Although not an every day occurrence common enough for the industry to hear about it.

 

[Billy Bob Bob]

i give up

 

[lame plumber]

:77:........

 

[dancinplumba]

lets see if op comes back tomorrow and says because of the advice given here my heatex is buggered ? any bets ?

 

[dancinplumba]

:77:........

YouTube

 

[Nostrum]

Lets all just agree to disagree but we were right and you were wrong? Sounds fair to me

 

[dancinplumba]

Lets all just agree to disagree but we were right and you were wrong? Sounds fair to me

here here

 

[Reg Man]

Lets get this straight, has anyone checked the MI's? I have. This boiler does not have a seperate plate heat exchanger. Its heat exchangers HTG + DHW are both located above an atmospheric burner. So when it calls for HTG it also heats the DHW side regardless of DHW demand. So if the DHW side become empty through evaporation because the water is turned off then the consequences could cause damage.

Agree with everyone if it had a seperate plate heat exchanger heated from the main HTG heat exchanger through a diverter valve. Which is the normal for all your Vaillants, Worchesters ect, but not for an old ferrolli Modena.

Also agree that the overheat would probably protect it. but would not rely on it as thats your last line of safety device.

 

[DirksPlumbing]

I am with Dancing. Lets think what can happen:
This boiler would be designed in a way that the water in the HW heatex pipe could get overheated (significantly past 90odd degC) at any situation. The user would open the HW tap, the pressure would drop, the water in the heatex would turn in milliseconds into steam at the usual expansion rate. Lets say the tap is just near the boiler.
1st: the customer could loose his hearing because of gases leaving at above 1bar.
2nd: the customer can suffer severe steam burns.
3rd: the boiler internals might not cope with the rapid pressure increase and the customer gets hit by boiler(parts).

Next thought: How much water is in the HW heatex and how long would that take to superheat at 30kW input. Lets say there would be demand for half an hour. We assume the HW would be functional but not in use. What temperatures do you expect?

I would say this boiler is safe to use without HW. If not then you have to expect it to be unsafe with HW working too.