Please help me understand my MVHR if you can!

[haddock]

Good morning, I've been doing some experiments today trying to understand how my MVHR is supposed to work and wondered if the experts here might be able to help.

 

I say supposed to work as I'm having a lot of water dripping from the ceiling around skylights, where moist air is coming into contact with colder surfaces:

 

 

In the garage there's a Brink Renovent Medium which I have running at 290m3/h and a Brink Allure gas heating system which blasts hot air through floor vents around the house:

 

 

 

As well as the floor vents in every room, there are three ceiling grilles, one in each of the three bathrooms. You can see a floor vent and a bathroom ceiling grille in this pic:

 

 

There's a much larger grille at ceiling height in the main living space:

 

 

My highly scientific experiments with cigarette smoke and a hygrometer have determined the following:

 

1. Each floor vent blows out a lot of air, and there are a lot of them, maybe 15 to 20. The total flow rate must be quite impressive. The heating fan can be set to blow air all the time or only when it's actually heating. When the heating is active, the air coming out of these vents is very low humidity. I can't discern any air coming from them when only the MVHR unit is running, but the output from the MVHR would be so thinly distributed anyway. The metal grilles do feel surprisingly cold when only the MVHR is running, so perhaps fresh air is trickling out.

 

2. The three ceiling grilles in the bathrooms suck in air all the time the MVHR is running, but the one in the main bathroom is barely working. The air flow at these vents is the same regardless of whether the heating is running.

 

3. The large grille in the living space has really surprised me this morning. It blows out air when only the MVHR is running, but sucks it in when the heating fan is also on. In both cases, the flow rate is high. I did wonder where the heating got its air input from.

 

This large grille is right where all the condensation damage is occurring (see previous pic), so it seems key to find out whether the living space is supposed to be ventilated by air flowing in or out of that grille. I can then configure the heating fan to stay on all the time, or only when actually heating (which is not very often as the house has a massive solar gain).

 

If you're still with me, here's a floor plan:

 

Possibility A: Air is supposed to flow into the large living space grille (heating fan on all the time)

In this case, the air would primarily be moving from the floor vents in the living space, right to left as shown in the plan, and into the large grille. 

 

Possibility B: Air is supposed to flow out of the large living space grille (heating fan only on when heating)

In this case, air emerges from the large grille in the living space and eventually makes its way to the three bathrooms where it's extracted. If this is how it's supposed to work, I'm not surprised there are problems because (a) the only thing sucking air across the long skylight in the living space is the grille in the main bathroom at the top of the plan, and it isn't working properly, and (b) what if the doors are closed??

 

I have water dripping from the ceiling today and the girlfriend just got dripped on, so I really need to get this sorted out! Haven't had the heating fan on for weeks so if (A) is how it's supposed to work then that'll be why.

 

If you made it this far, thank you!

 

Max

[Mr Punter]

The water damage is either weather ingress or condensation.  If it is weather, you need to look at the roof and rooflights.  If condensation you need to look at insulation and vapour barrier.  I don't think your heating and ventilation systems are to blame.  Where is the house located?

[jack]

11 minutes ago, haddock said:

In the garage there's a Brink Renovent Medium which I have running at 290m3/h

 

My first question is why you're running it at this rate.

 

My house is 289m², with high ceiling downstairs (so a lot of volume), populated by two adults, two kids and a dog and we run ours at 100m³/h. In fact, for the first year I ran it at 50m³/h and that was fine too.

 

As for the water, are you certain it isn't a leak around the skylight? That seems a much more likely scenario to me.

[haddock]

41 minutes ago, Mr Punter said:

The water damage is either weather ingress or condensation.  If it is weather, you need to look at the roof and rooflights.  If condensation you need to look at insulation and vapour barrier.  I don't think your heating and ventilation systems are to blame.  Where is the house located?

 

The house is located in sunny Manchester. We do have some dry spells here and there isn't any correlation between rain and the problem, so that's one reason why I'm thinking it's condensation. (There is a strong correlation between outside temperature and the problem however.) I've also had hosepipes running on the roof for long periods through dry spells without any effect.

 

It's been ongoing for three years now so the damage you're seeing has been very gradual. I can't remember the figure but the airtightness test on the house was apparently very impressive, so surely I'd see symptoms like this if the MVHR wasn't working as it should? There is definitely an insulation problem however — part of the skylight support has a long metal flange that runs continuously from the interior to the exterior. Up on the roof on a freezing night it is warm to the touch when all the other glass and metalwork is icy cold.

 

52 minutes ago, jack said:

 

My first question is why you're running it at this rate.

 

Just because I think I have a condensation problem and I presume running it too high isn't going to cause any trouble(?) Once the issue is resolved, I'll see how low I can take it without causing a problem.

[Declan52]

Are the pipes going to the vent where the damage is visible insulated??

With warmer air going through these pipes if they aren't insulated the warm air going through cold pipes will definitely result in condensation. Which can leak out causing your damage. 

Can you remove the ceiling vent and put your hand up to see if it's damp in the pipe. 

[Jeremy Harris]

It's also possible that it's condensation from thermal bridge around the skylight, especially as water vapour is lighter than air (it's why clouds float) and so any condensation is more likely to occur at any cold spot high up.

 

In the recent weather I doubt that either the temperature or humidity conditions, even at night, would be enough to cause condensation of this sort, though, and anyway, there should be a vapour control layer (VCL) that's specifically there to prevent moist, warm air from inside the house getting into the structure.  The VCL should be right behind the internal plasterboard, and have no holes in it - care being needed to seal around it where there are things like light fittings inset into the ceiling.

 

 

 

 

[haddock]

15 minutes ago, JSHarris said:

there should be a vapour control layer (VCL) that's specifically there to prevent moist, warm air from inside the house getting into the structure.  The VCL should be right behind the internal plasterboard, and have no holes in it - care being needed to seal around it where there are things like light fittings inset into the ceiling.

 

Here's what the flange that's acting as a thermal bridge looks like from inside the room. Only a small part of it is exposed and as you can see the upper surface gets mouldy from the wet. I think a lot more condensation forms on the underside of the flange caused by moist air rising via the ceiling void but I don't have any direct evidence of that.

 

[haddock]

54 minutes ago, JSHarris said:

The VCL should be right behind the internal plasterboard, and have no holes in it - care being needed to seal around it where there are things like light fittings inset into the ceiling.

 

There’s nothing to stop air rising through the light fittings and into the ceiling void — that’s maybe 20 ingress points just in the main living space. Would the VCL be something that would rest over the top of the light fittings then? Not sure what form it takes but I doubt there’s any practical way of retrofitting one. 

[Jeremy Harris]

Yes, there should be a VCL fitted somewhere between the inner surfaces and the insulation, to prevent warm, moist, air being able to penetrate up and cause interstitial condensation.  Ideally the VCL should be as close to the rear of the inner surface as practical.  For example, here's a photo of our roof (we have insulation between the rafters, rather than above the ceiling) showing the VCL - it's a plastic sheet that's there to prevent warm moist air reaching any area that could be cool enough to cause condensation to form:

 

 

This is an area in the services room upstairs where there's no plasterboard fitted, but everywhere else is the same; the plasterboard is screwed to those 50 x 50 battens/.

 

Where the VCL meets window or door frames it's taped to them, behind where the plasterboard fits, to maintain a vapour-tight seal.

[Mr Punter]

It seems that you have eliminated weather ingress and so this is condensation.  Could you remove an area of damaged plasterboard to expose the rooflight / roof interface, roof structure, insulation, vcl (if fitted) etc.?

[Declan52]

Take out the downlight and put your phone up and take a few pics. Use the timer function on your phone to get the pics.

[jack]

2 hours ago, haddock said:

Just because I think I have a condensation problem and I presume running it too high isn't going to cause any trouble(?) Once the issue is resolved, I'll see how low I can take it without causing a problem.

 

Fair enough, and a good idea when you have general moisture problems that you want to help dry out. However, if condensation was caused by the MVHR as you originally thought might be the case, then running it faster would potentially make it worse. 

 

Do you have any idea where the MVHR ducts run in the ceiling? You have damage on both long sides of the rooflight, but it seems very unlikely that there are MVHR ducts on both those sides. Can't rule it out, but some other form of leak or condensation seems likely.

 

You mention it dripping out today. Even if it's a form of condensation, I can't see how you could have so much of it at current temps and humidity in Manchester (20 deg C and 37% humidity right now, accordingly to Google).

 

Phone up through a downlight hole seems a good way forward. Try and see if you can take a video with the phone's light on.

[haddock]

3 minutes ago, Declan52 said:

Take out the downlight and put your phone up and take a few pics. Use the timer function on your phone to get the pics.

 

Have spent many hours doing that over the last few years and got the hang of taking a good video while clinging to the bottom of the phone through the downlight hole! This video shows the water pooling and the vertical plasterboard that goes up to the top of the skylight where the flange is: https://www.icloud.com/sharedalbum/#B0R5oqs3qi9Dql

 

Anyone prepared to cast a vote on whether possibility (A) or (B) is how the system is supposed to ventilate the main space?

[haddock]

14 minutes ago, jack said:

Do you have any idea where the MVHR ducts run in the ceiling? You have damage on both long sides of the rooflight, but it seems very unlikely that there are MVHR ducts on both those sides. Can't rule it out, but some other form of leak or condensation seems likely.

 

 

The ducts run from the large grille directly away from where the problem is — there's definitely no ducting anywhere near the skylight. There's also an occasional problem with water forming in the ducts (so many problems!) but the water from that ends up pooling inside the MVHR itself.

[ProDave]

Video won't play for me on either computer or phone (probably because they are not i thingies.

 

What is directly above where the pool of water is? it should be obvious what it is dripping from.

[haddock]

6 minutes ago, ProDave said:

Video won't play for me on either computer or phone (probably because they are not i thingies.

 

What is directly above where the pool of water is? it should be obvious what it is dripping from.

 

Sorry for the iThing only link, thought Apple had moved beyond that. Here's it is on youtube. What's above is the skylight including the aforementioned thermal bridge:

 

 

[Mr Punter]

It looks as if you have a warm roof with the vcl and insulation above the OSB.  I think you have correctly identified the issue and the rooflight upstands are creating a thermal bridge.  What is the thing in the puddle?

[haddock]

Just now, Mr Punter said:

What is the thing in the puddle?

 

It's a Honeywell leak detector — I have put a couple of them up there to monitor progress as I look for solutions. As well as updating me on leaks (pooled water) they also report temperature and humidity. I'm sat in the pub right now and it's telling me it's currently 26.1 degrees in the ceiling void, 48% humidity. It's 25.0 degrees at the thermostat in the room below. Up at the very top of the room at the skylight next to where the thermal bridge is, it's only 19.6 degrees and 56% humidity.

 

 https://www.honeywelluk.com/products/Connected-Smart-Products/Honeywell-Home/W1-Water-Leak-Detector/

[jack]

26 minutes ago, haddock said:

The ducts run from the large grille directly away from where the problem is — there's definitely no ducting anywhere near the skylight. There's also an occasional problem with water forming in the ducts (so many problems!) but the water from that ends up pooling inside the MVHR itself.

 

Okay, thanks. I think we'd ideally separate out the MVHR issue from the condensation issue, if we're convinced they're not related.

[jack]

2 minutes ago, haddock said:

 

It's a Honeywell leak detector — I have put a couple of them up there to monitor progress as I look for solutions. As well as updating me on leaks (pooled water) they also report temperature and humidity. I'm sat in the pub right now and it's telling me it's currently 26.1 degrees in the ceiling void, 48% humidity. It's 25.0 degrees at the thermostat in the room below. Up at the very top of the room at the skylight next to where the thermal bridge is, it's only 19.6 degrees and 56% humidity.

 

 https://www.honeywelluk.com/products/Connected-Smart-Products/Honeywell-Home/W1-Water-Leak-Detector/

This is from the Wikipedia article on dewpoint - might be of assistance in figuring out what surface and air temperature combos would need to be involved to be causing condensation at your measured humidity levels.

 

 

 

 

Given the condition of the plasterboard, I'd consider taking a couple of sections out so you can get in and have a good look. Having that amount of water sitting around can't be good.

 

[Mr Punter]

If it is the cold bridge and condensation you need to remedy with insulation and vapour control.  The plasterboard is damaged beyond repair, so you may as well remove it.  I am surprised not to see damp on the vertical board that is on the left but there may be a void between the board and the flange, so perhaps the warm moist air enters the void, condenses on the flange / upstand and runs down to collect on the ceiling.

[Declan52]

How is the roof built up insulation wise?? None there in that video so is it above the OSB or is there none.

[haddock]

1 hour ago, Declan52 said:

How is the roof built up insulation wise?? None there in that video so is it above the OSB or is there none.

 

It's Structural Insulated Panels (SIPs). I guess that OSB that's visible is the actual SIP(?) https://en.wikipedia.org/wiki/Structural_insulated_panel The water drips off the OSB directly onto the ceiling below, which is why the vertical plasterboard is mostly unaffected.

[ProDave]

12 minutes ago, haddock said:

 

It's Structural Insulated Panels (SIPs). I guess that OSB that's visible is the actual SIP(?) https://en.wikipedia.org/wiki/Structural_insulated_panel The water drips off the OSB directly onto the ceiling below, which is why the vertical plasterboard is mostly unaffected.

I am astonished that give the summertime temperatures, that any surface is cold enough for moisture to condense.  It must be part of the cold window detail where it is condensing then finding it's way through the SIP panel at a joint perhaps.

 

[Jeremy Harris]

Doesn't look like there is any vapour control layer at all, and if this is a high point in the house it's quite possible that it's interstitial condensation caused by the absence of any VCL.  I'm as surprised as the others above that this is happening in our current weather conditions, as the temperature has been too high, even at night, for condensation to form at the sort of humidity levels we've had.  However, if there is no VCL at all (which I have to say seems highly likely, from the video and photos) then it may well be that very warm and humid air from somewhere like a bathroom, shower room, kitchen or utility room may be entering that void and then condensing out. 

 

It doesn't take much - we had severe condensation in the loft of our old house from a very tiny (not visible when looking up from floor level) crack around the edge of the wall to ceiling joint.  That was enough to let warm moist air into the loft, which then condensed out on the underside of the sarking felt, dripped down onto the insulation (which was that non-itch stuff wrapped in a polythene sleeve) and then the brown puddles would trickle down and make stains in the Kitchen ceiling, so 6 metres or so away from the bathroom.  The fix was to create an airtight vapour barrier under the loft insulation and over the whole of the bathroom ceiling, using plastic sheeting, tape and sealant.  That, together with better ventilation in the cold loft space, completely fixed the problem.  Interestingly, the problem only appeared after I increased the level of loft insulation, which made the loft a lot cooler.