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GAS Question - Open Flue vs Room Sealed

View the thread, titled "GAS Question - Open Flue vs Room Sealed" which is posted in Gas Engineers Forum on UK Plumbers Forums.

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jaydebruyne

jaydebruyne

Plumbers Arms member
Plumber
Gas Engineer
Just to reiterate, Croppy said it was cool to ask gas questions in here as I'm doing my gas at the mo.
Well, I've passed all my exams and practicals, I just need to do my portfolio.

Anyway..

How do you tell if, say a boiler for example, is room sealed or open flued (if the open flued boiler doesn't have a draught diverter)? Are there visual signs or is it just a case of getting the manual?
 
Cheers for the link Stani.. So looking at the terminal will be a good visual clue?

I understand everything on the link you posted, but I want to know what the visual clues are telling an open flued boiler from a room sealed boiler, rather than the technical aspects of various types of flues.
 
I will probably be shot down for this but, most balanced room sealed flues exit directly behind the appliance, using a square section flue, open flued appliances use a chimney, and fanned flues are circular and usually 100/125mm diameter.
 
Ahhh ok, cool.. 100/125mm refers to the air intake & flue gas pipe diameters (respectively), right? So I would have to core a 127mm hole?

Sorry for dumbdumb questions but they don;t teach you these things at the course...

Cheers Stani
 
Not necessary, some are 100mm dia. Some are 125mm, sorry for that I was trying, not very well, to say that the PF dia could be either size,
 
stani said:
Not necessary, some are 100mm dia. Some are 125mm, sorry for that I was trying, not very well, to say that the PF dia could be either size,
Click to expand...

haha ok.. so I've seen it written on a gas safety today (by worcester bosch) that a flue terminal needs replacing 60/100. Is this meaning what I said above?
 
stani said:
I will probably be shot down for this but, most balanced room sealed flues exit directly behind the appliance, using a square section flue, open flued appliances use a chimney, and fanned flues are circular and usually 100/125mm diameter.
Click to expand...

Sorry - last question I swear!!!! So the square balanced room sealed flues are not fan assisted, but natural draught?
 
jaydebruyne said:
haha ok.. so I've seen it written on a gas safety today (by worcester bosch) that a flue terminal needs replacing 60/100. Is this meaning what I said above?
Click to expand...
As Stani says but more often than not they will be concentric (exhaust pipe inside air inlet pipe) but not always sometimes they can be run separately with the correct manufacturers kits then they are about 50 - 60 each.
stani said:
Yes, 60mm exhaust and 100 mm air intake
Click to expand...
 
solutions said:
As Stani says but more often than not they will be concentric (exhaust pipe inside air inlet pipe) but not always sometimes they can be run separately with the correct manufacturers kits then they are about 50 - 60 each.
Click to expand...

Yeah, Ive assisted on a couple boiler installs where the exhaust and air intake were separate 2" grey GRP pipes.. I think it was a Keston if my memory serves me correctly..
 
jaydebruyne said:
Sorry - last question I swear!!!! So the square balanced room sealed flues are not fan assisted, but natural draught?
Click to expand...
If you mean the large square ones that look like tipped over hamster cages then yes but there are small square / rectangular fan assisted flues that look like miniature versions of tge ballanced flue ones. I think the potterton netaheat had one if I recall.
 
solutions said:
If you mean the large square ones that look like tipped over hamster cages then yes but there are small square / rectangular fan assisted flues that look like miniature versions of tge ballanced flue ones. I think the potterton netaheat had one if I recall.
Click to expand...

I've seen those, had to drain down to fit a new TRV which was an old potterton with the small rectangular flue at the back.. So that is fan assisted. GADDDAMMMIT! Large & square = natural draught, small & rectangular = fan assisted. I haven't seen any of those natural draught flues yet I don't think
 
top: natural draught balanced room sealed
bottom: fan assisted room sealed

Flues.jpg
 
just to confuse you even more there were some fanned flues that were not room sealed one was a vokera unlikley youll see one nowdays
The main difference between open flued and room sealed is where the boiler gets its air supply from Room sealed collect the air from outside the building but open flued take air from the room they are situated in and expel the products of combustion(POC) outside
The large square balanced flue terminals are actually air in and fumes out, to make the terminals smaller and therefore the hole needed to put the flue through smaller they introduced a fan
the earlier fans didnt work in hot air so the fan was used to draw cold air in which makes the boiler positive presure which can cause problems with case seals leaking and pushing poc into the room as fan technology was improved they then used the fan to expell the hot air which also had the benifit of making the boiler negative presure so if seals failed air was drawn into the boiler which is obviously safer
the newest boilers blow air and gas mixed through the burners and then through the flue hope this is of some help
 
Last edited:
No, they look to close together, or is it 2 photos, on my iPad with a cracked screen so pictures aren't always great.
 
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