Unavoidable cold bridge

rampantbunny

I've a balanced flue gas fire planned for installation in my build.
The flue runs horizontally across my ceiling (strapped to the hollow-core) and exits through side wall of house.

For a while now I've noticed that condensation builds up on the underside of the flue and drips to the floor. At some point I'll be putting plasterboard under the flue, but the condensation build-up concerns me.

What can I do to combat this build-up and to stop my plasterboard from getting soaked?
I'd ask the registered gas installer that installed the flue, but he injured himself on another job and looks to have retired from duty (i.e. doesn't take any calls).

gctest50

rampantbunny wrote: »

........ At some point I'll be putting plasterboard under the flue, but the ...........

before you go installing you need to ask an RGII about boxing in flues

this kinda thing :

http://www.nhbc.co.uk/Builders/Technicaladviceandsupport/Gasboilersfluesinvoidssafetyalert/

http://www.nhbc.co.uk/Builders/Technicaladviceandsupport/Gasboilersfluesinvoidssafetyalert/

http://www.rgii.ie/_fileupload/installer%20files/Technical%20Info/111221%20Concealed%20Extended%20Flues%20Technical%20Bulletin%20Final-a.pdf

rampantbunny

gctest50 wrote: »

before you go installing you need to ask an RGII about boxing in flues

this kinda thing :

http://www.nhbc.co.uk/Builders/Technicaladviceandsupport/Gasboilersfluesinvoidssafetyalert/

http://www.nhbc.co.uk/Builders/Technicaladviceandsupport/Gasboilersfluesinvoidssafetyalert/

http://www.rgii.ie/_fileupload/installer%20files/Technical%20Info/111221%20Concealed%20Extended%20Flues%20Technical%20Bulletin%20Final-a.pdf

Original RGII installer specified what was and was not to be done re. enclosing the flue so all is OK on that front. We never discussed/thought of condensation issues.

Have another RGII contact in coming week, but want to have some idea what the possibilities are before having conversation.

The Sidewards Man

Get a qualified RGI to solve this issue, too many unqualified RGIs around.

rampantbunny

The Sidewards Man wrote: »

Get a qualified RGI to solve this issue, too many unqualified RGIs around.

A (very helpful) RGII auditor provided a short-list of RGII installers. I picked an installer from this list so I'd have to think his credentials were good, even if his ability to answer the phone is bit slack.

lownhard

Aren't you supposed to have inspection hatches on these flues every few meters. Check that out. I moved my gas boiler away from plant room to an outside wall because of this

rampantbunny

lownhard wrote: »

Aren't you supposed to have inspection hatches on these flues every few meters. Check that out. I moved my gas boiler away from plant room to an outside wall because of this

You are, but I don't think access hatches are going to help with condensation build-up (unless this provides ventilation also, which I also don't think is another of their purposes).

rampantbunny

Just in case anyone (other than myself) is interested, I spoke with an RGII member and he wasn't familiar with the issue of condensation build up on the internal duct runs i.e. he had never come across it himself personally.

I asked if the duct could be wrapped with insulation and he reckoned it would do no harm. So, that's what I'll be doing...not wrapping with wool, but will need to find something that won't soak moisture. In the meantime I'll be chatting to another RGII installer later to finish off the installation to get another opinion.

west79

rampantbunny wrote: »

Just in case anyone (other than myself) is interested, I spoke with an RGII member and he wasn't familiar with the issue of condensation build up on the internal duct runs i.e. he had never come across it himself personally.

I asked if the duct could be wrapped with insulation and he reckoned it would do no harm. So, that's what I'll be doing...not wrapping with wool, but will need to find something that won't soak moisture. In the meantime I'll be chatting to another RGII installer later to finish off the installation to get another opinion.

Check the installation manual for the appliance. It should give the max length of a balanced flue for the appliance. It may not be allowed to have such a long horizontal flue.
The condensation is only present because there is moisture in the house. At it is under construction there is water everywhere. The surface of the flue is like a mirror in a bathroom full of steam. The water is drawn to it. This issue might resolve itself as the house dries out and heating and ventilation is working properly

rampantbunny

west79 wrote: »

It may not be allowed to have such a long horizontal flue.

I don't think I mentioned what length the horizontal flue run is, but I understand your point. Again, an RGII installer was appointed to install the system. He wouldn't have touched it if it wasn't within manufacturers tolerances. We've gone a little off-topic re. flue lengths, access hatches etc.

west79 wrote: »

The condensation is only present because there is moisture in the house. At it is under construction there is water everywhere. The surface of the flue is like a mirror in a bathroom full of steam. The water is drawn to it. This issue might resolve itself as the house dries out and heating and ventilation is working properly

This might be the case west79, but it is the 'might resolve itself' that is cause for concern. When the house is complete and all systems working as expected, if I still see condensation, then it's too late. I don't want to go ripping a ceiling to fix at that stage.

With respect to the build up of condensation, the way I understand it; when the gas appliance is not in use the flue cannot help but be colder than the room it is located in, due to the coldness from the outside firstly being conducted in through the metal (the cold bridge), and secondly because cold air has to flow in to supply the system (convection). If the air inside the building is warmer (and it has to be or I'd be better off living outside), then any moisture in the air will condense on the colder flue. The ventilation system will not completely dry the air in the house so there will be some moisture so therefore I think this condensation is unavoidable. Insulating the flue is not to keep it warm in this instance but to keep the warmth away from it.
If the fire is in-use, the warm air expelled will heat the inner ducting; the cold incoming air will hit this surface and presumably condense, but will dry off because of the heat emitted by the inner duct. Plus this moisture is inside the ducting, not on the outer surface.

The above is all about what I think should happen. Need a pro to properly advise, but in the meantime, I'm here wondering if anyone else has seen this.

ht9zni1gs28crp

Its of no help to a Flue but I see condensation on the intake pipe for our MHRVC on really cold days, and that's our passive house running currently around the 49-51% Humidity internally. So its a real and present danger that your right to get on top of it now...

I guess the Pro needs to either install a twin walled insulated flue or provide the data that will show the exhaust heart will dissipate any moisture that gets near it. If it were me id be looking for an insulated option. I'm not fully au fait on Flues though so I could be wrong...

rampantbunny

miller_63 wrote: »

Its of no help to a Flue but I see condensation on the intake pipe for our MHRVC on really cold days, and that's our passive house running currently around the 49-51% Humidity internally. So its a real and present danger that your right to get on top of it now...

I guess the Pro needs to either install a twin walled insulated flue or provide the data that will show the exhaust heart will dissipate any moisture that gets near it. If it were me id be looking for an insulated option. I'm not fully au fait on Flues though so I could be wrong...

It's a balanced flue currently priced at over 100GBP per meter. A balanced flue has two concentric ducts. The inner duct for expelling exhaust gases, and the outer for the intake of air to fuel the fire. The incoming air regulates the temp of the outgoing hot exhaust and the installer reckons the outer ring will not be uncomfortable to touch (I was originally concerned re. plasterboard ceilings becoming affected/cracked and I also questioned the flue coming into contact with the pumped bead within the cavity).

I haven't seen an option for a balanced flue that has an insulated outer shell, but if they do exist, I'm sure they are prohibitively expensive. Plus I'd have to get rid of the existing duct, not to mention the labour of taking it out.
I think at this point, wrapping with something is the best option, but not wool. If I thought it would be a neat job, I'd put fire rated expanding foam around the duct, but expanding foam and neat don't go together in confined spaces.

Have you insulated your MHRV intake ducts?
Mine are installed inside a warm attic but I've been advised to lag them in any case.

ht9zni1gs28crp

I'm insulating the ducts as we speak. It was a 'see how you get on' situation..