Gas boiler in attic - Now getting lots of condensation ?

[philbean]

I posted this query on the HVAC reddit but as thats largely a US site I thought I might have better luck here....



My dad has a 1960s semi.
For years he had an old 'behind-the-gas-fire-in the living-room' type boiler, venting up the chimney. With a hot water tank in the loft (attic).

This packed up about 6 years ago and they removed the fire and boiler and water tank, and replaced with a new combi boiler in the attic where the tank used to be.

All working fine, but we noticed that the attic was getting a little damp. They've always used it for storage and now started to notice cardboard boxes were getting a bit soft, almost soggy.

We ignored it (I know !) and its just got worse and worse. Now its obviously very damp up there. Anything not sealed in airtight storage tubs is getting ruined. There's visible mould on the bit of carpet he laid up there and on suitcases etc.

Good loft insulation on the floor of the loft. Nothing on the ceiling of the loft (underside of the roof).

There are vent holes cut into the overhang under the gutter - they've been there years, but are still all clear and still 'venting'. He definitely had no damp issue up there before the boiler was moved.

Any idea what's going on ? Advice ?

We assume the boiler being up there is the issue, but why? Was something missed when it was installed or something ??

 

[Deleted member 120897]

Can you provide photos? This will help us to look for something that has maybe been missed. For the time being I would stop using the Combi boiler as a precaution, and you may be best getting a gas safe registered engineer in.

 

[philbean]

I don't think there's anything wrong with the boiler. Its been serviced every year for 5 years by registered engineers.
I'm just thinking there's just something to do with the ventilation in the loft that's been missed/overlooked.

 

[Deleted member 120897]

This is true, but with you saying it’s only happened since the boiler has been installed, it’s a modern room sealed Combi boiler, so no requirement for additional ventilation.

 

[Gasmk1]

two things could the insulation be causing the roof to sweat or is there a leak when it rains and the warm roof space is causing it to evaporate and condense on the underside of the roof

 

[Chuck]

It's unlikely to be PoC (flue gas) leaking into the roof space but the possibility should be considered and ruled out first.

In addition to the excellent suggestions made already I'd add to the list:

It could be the removal of the DHW tank in the roof space that's the significant change. If this was old it could have been leaking several hundred watts into the space and this might have been enough to keep it dry.

Also check for leaks from the warm humid air in the house, particularly the bathroom, into the roof, e.g. via holes that were left open or created when the pipework was changed or around the access hatch.

You have some vents that work, but the question is: do you have sufficient area of them?

 

[Deleted member 120897]

Other factors I’ve since thought of:

• Is the boiler flue vertical?
• Since the removal of the bbu (boiler behind the fire) - is the fire still in place, if not, is the chimney ventilated?

 

[Ric2013]

If the boiler is making the loft warmer, the problem could be this, just to develop/explain Gasmk1's good idea a bit further.

Some part of the brickwork or other element (such as a chimney) is damp. Penetrating damp, a leak, whatever. Without the boiler, the RH is high, but the air has little capacity to hold moisture. Thus the absolute humidity (the actual quantity of moisture per cubic metre) is low. If the boiler warms the air, the air has more capacity to absorb moisture and the absolute humidity rises as the RH tends to stay the same.

However, the absolute humidity is higher than before while the felt remains at the same low temperature so now it is easier for the air, when it hits the felt, to be cooled below dewpoint (i.e. bringing the RH up to beyond 100%) and resulting in condensation.

Also, if your old unused chimney is either blocked or uncapped (should be slightly vented and with a raincap of some sort), that will bring moisture in!

 

[Timmy D]

Any bathroom extractor fan ducting running through the loft?

It could have accidentally been disconnected/damaged.

If it’s getting that wet, there’s got to be a source for the water/moisture.

 

[gmartine]

A condensate leak somewhere, is it pumped or gravity assisted?

I know they should but would servicing engineers really check it's integrity.