We bought a (not ideally) converted, damp, barn in West Cornwall a couple of years ago and have been getting work done on it since. To date this has been getting the cement pointing replaced with lime, grouting the building with lime slurry (7 tons!) and painting with Beek BS+, ensuring the slate cills are a good fit, fitting an ASHP (wet UFH was already fitted, run off bottled gas), new Windows/ doors from Russelltimbertech and a Wood burning stove.
The layout is reverse level with vaulted ceilings upstairs and no real space between the ceilings & floors. There are two rooms upstairs (living & kitchen) with three beds, hall and a bathroom downstairs. Granite steps lead from the living room to the garden. The entire building is rendered internally in cement.
As there was evidence of condensation inside I was interested in considering MVHR, but couldn't work out how we could fit it due to the layout of the building and lack of space to run ducting.
We have now got planning to build an ensuite and balcony on the back which has got me thinking again. Due to the position of the extension if an MVHR unit is housed in it ducts could easily run to the new bathroom, kitchen, living room and two bedrooms (less than 2m runs). We could also run an external duct, hidden by the balcony to the other bath and bedroom. Ideally this would be a single (extract) run, unfortunately the bedroom involved is the one showing most signs of condensation.
The floor area will be 105m2 and volume approx 270m3.
I realise that it is not an ideal situation for MVHR but would welcome opinions as to whether fitting it would be a reasonably efficient way of managing air quality in the house.

Is the building reasonably airtight to make MVHR worthwhile? And is it well-insulated enough that heating the air is a substantial fraction of the heating bill?

If not, it may be you are better off with MV without the HR, at which point it doesn't need to be centralised and can just use one or several fans to achieve suitable airflow. But you will want to heat the incoming air unless it is well-mixed.

The house won't be particularly airtight as I would be amazed if any attention was paid to that when it was converted in the early '90's. On the upside I am sure it could be worse as the solid floor, new windows/doors & rendered walls must help to a degree. The stove draws ok & has no external air supply.
One difficulty will be getting ducts through the walls - two feet of granite!
I realise it is not an ideal state of affairs but am wondering if managing the internal humidity via MVHR would a) work and b) be more cost effective than running the heating at a low level when the house is unoccupied.

Posted By: djhIt's the mechanical ventilation that controls the humidity, not the heat recovery.

but having warm air controls reduces mould even better than lower humidity alone!

Posted By: djhThe only point of heat recovery is to save energy

Grief that just shows that:

Posted By: djhI've always only been interested in an insulated, airtight building

When you extract (MEV) you suck cold air in - but from where - the answer in a bit leaky houses tends to be around the windows, just where it is already coldest - I have some lovely pics of my house where the mold is all around my windows extending at the top to the wall ceiling join (big cold bridge). Unless you put ruddy great vents everywhere, which I am sure you are not advocating....

With central PIV you have a gale of cold air entering the house at one point - a big no from me.

Rooms tend to be coldest in corners etc - with HR you tend to circulate air much more (because of the design of input vents) and create a flow throughout the room (by careful positioning of vents). Also input vents placed by a window can even out the temps and prevent mould even though theoretically you are lowering the avg temp in the room - ie the input air is warming the coldest surfaces even though it might only be 80% of the extracted air's temp - bearing in mind extracted air is from kitchen and bathrooms which are generally a fair bit warmer than bedrooms.

I set my fan speeds to create a slight over pressure and yes whenever there is a wind it will overcome the very modest HR fans overpressure but that's only a proportion of the day.

Finally we don't really have any way of discussing what air tightness is when it is not tested or of judging whether it is 'worth' it or not. Rendered walls, solid floors, new roof, new windows and doors - sounds like it would be 'reasonably air tight to me.

My house isn't particularly airtight, ants can get in in one of my renovated windows and in through the roof somewhere else but with solid rendered walls, solid floor, new roof covering, lots of new doors and windows it isn't 'poor' either. I can tell you that MVHR is the best thing since sliced bread in such a house.

Posted By: Gotanewlife
When you extract (MEV) you suck cold air in - but from where - the answer in a bit leaky houses tends to be around the windows, just where it is already coldest - I have some lovely pics of my house where the mold is all around my windows extending at the top to the wall ceiling join (big cold bridge). Unless you put ruddy great vents everywhere, which I am sure you are not advocating....

With central PIV you have a gale of cold air entering the house at one point - a big no from me.

My house isn't particularly airtight, ants can get in in one of my renovated windows and in through the roof somewhere else but with solid rendered walls, solid floor, new roof covering, lots of new doors and windows it isn't 'poor' either. I can tell you that MVHR is the best thing since sliced bread in such a house.

Our bungalow is not airtight, it even has a few drafts, but I am very glad we put in MVHR as it solved our condensation problems within hours of being turned on. The quotes for PIV where not much less, as the complex layout would require at least two PIV systems, being a bungalow it was easy to fit the MVHR ducts in the loft.
(Single room MVHR systems in the bathroom and kitchen may have work OK and are now easier to get.)

My mother in law has a PIV system, I have also fitted one to a property I rent out. In both cases there is no “gale of cold air” as the system if fitted correctly mixes in incoming air at ceiling level. Both of the buildings benefit greatly from the PIV systems. I think they are better than MEV, as you are not sucking in cold air next to the cold windows.

I suspect the received wisdom against heat recovery in less air tight buildings isn't really comparing like with like.

Ie they're probably saying it isn't worth fitting sophisticated, expensive systems that might be appropriate for a well sealed building (many hundreds to thousands of pounds).

I would agree with that. However there are much cheaper and simpler systems and if you compared them with simple mechanical ventilation the conclusion might be different.

My DIY system wasn't much more than £300 for everything and I'm pretty happy with it, but spending much more I don't think would have been worth while.

Ed

Posted By: nick1cWould it matter that one of the bedrooms has an extract rather than an input duct?

It depends where it was getting its air from. If the air was coming in from outside, through a living room, down a hall and into the bedroom then I'd say no it wouldn't matter. Not ideal but likely to work alright, unless you have people who stay up late smoking and drinking in the living room whilst others are using the bedroom. But if the bedroom was getting its air from an adjacent bathroom, then I'd say yes it would matter.

Posted By: nick1cThe bathroom and bedroom in question are adjacent, but the input air would be from the living room and other beds.

I think you'd need to make sure the bathroom has a larger extract rate than the bedroom, to make sure the bedroom doesn't pull air from the bathroom. Other than that and making sure you've got enough supply air for whichever rooms it's coming from as well as the bedroom, I think you would be OK. Living rooms and bedrooms are not used much at the same time, but other bedrooms are of course.

Nick
Has replacing the old pointing with lime made any difference?

You could try force ventilating that passageway where you had some damp and seeing if it makes a difference before you commit to a more expensive MVHR unit. Either a single room MVHR or just an extractor fan rigged up somehow.