MJNewton

That sort of thing doesn't bother me. It HAUNTS me!! 😂

Looks lovely - well done. Perfect time of year to be completing a room like that!

If you do run the MVHR (I would - find it great for drying washing so the dehumidfying effect is definitely there) keep the filters in place. They protect the heat exchanger from becoming fouled up.

Yeah, and suggests there's something wrong with the figures (or something external that is skewing them).

If the calculation focuses on how much the supply temperature has risen it doesn't consider where the source of that heat comes from, or rather assumes it is all from heat transferred from the extracted air. The reality is that some heat may have come from the air surrounding the box itself, the contribution of which will be greater with a poorly insulated unit than a well insulated one. Similarly with one that leaks air compared to one that is airtight. Hence poor units get 'rewarded' (as in appear better) than better ones using such a calculation. If you take an alternative approach that isn't based on supply temperature but factors in exhaust temperature (i.e. it views the unit from the opposite direction in terms of heat transfer), then that same heat leakage that artificially increases the supply temperature is picked up by the increase in exhaust temperature and results in a decrease in calculated efficiency. Thus poor units now get 'punished' for any leakage that exists.

I was going to comment on that actually as I am not quite sure how that has happened. The temperature sensors haven't been swapped round have they?

Forgive me sounding so negative but if you take the simple ( Supply - Intake ) / ( Extract - Intake ) calculation method that gives ( 13.4 - 12.1 ) / ( 14.4 - 12.1 ) = 57% which isn't all that high? Better than 0% of course so heat is being recovered, but it's not really in the ball park (85%+) that is realistically achievable with these units.

In our case whilst we wanted it low profile we weren't after flush (there's nearly a brick height drop on the outside anyway so benefits would be limited). I think we've got around 10-15mm above finished floor level. It might be technically slightly less actually given the finishing strip over the edge of the wooden flooring. I do remember being worried about threshold height and at what point would it be too high. Part of the subsequent thought process was to become better informed through first-hand experience so I set about measuring existing door threshold heights - which had never caused any issue - and couldn't believe how high some of them were! I suppose your brain unconsciously deals with that sort of thing. Bit different between a front door and a wide glass opening to the garden of course but it did remind me not to worry too much about such things. I still did though.

I seem to recall our sliders had a gap of 10mm all round. They did remark that it was one of the squarest openings they'd ever measured so may well have taken a tighter tolerance than usual!

It looks great. and very neat! The sort of thing deserving of a perspex lid and internal lights... 😆 As mentioned above though it does seem rather on the small side, or am I being tricked by the perspective? Maximising contact time in the HEX is what's important; it doesn't matter how big the unit itself is. You will likely find that as your air flows increase the efficiency will drop - there's only a certain amount of transfer capacity available. You mentioned about sourcing fans; what are you using at the moment and are they representative flow-wise of what you are ultimately seeking?

They are usually centrifugal EC fans which tend to be manufactured as OEM for inclusion into other supplier's products and so aren't quite so readily available directly to end users. They can be found though (examples) but there are a whole bunch of design parameters you'd need to know to narrow the search down e.g. airflow, noise, size, control etc.

Yeah, if the unit is in a warm room then the supply-referenced calculation can result in apparent higher efficiency figures because the heat from the room warms a poorly insulated unit up and thus in turn creates a higher supply temperature too. With yours being in the attic though you won't be picking up that 'extra' heat if the ambient temoertaure is closer to the that of the intake. Internal thermal bridges will still lead to differences in results against an extract-based calculation to though. An average of both methods is arguably the best overall approach if you're after a single figure.

The labels are a little confusing, but I think I can work them out by their relative values. One way of calculating efficiency of the heat exchanger (although it's not perfect as a poorly insulated unit will result in higher figures if it allows heat from the room in) is simply: ( Supply - Intake ) / ( Extract - Intake ) Thus, with your figures: ( 15.0 - 12.8 ) / ( 16.7 - 12.8 ) = 0.56 56% efficiency isn't great given that >90% is readily achievable with off-the-shelf systems but I'd say it seems pretty good for something homemade though - it's certainly working!

Simple economics of supply and demand, ecomies of scale etc. Square/flat(?) insulation is used absolutely everywhere - floors, walls, roofs - and in large amounts. Tapered insulation is, I assume, only found in flat roofs. And even then very rarely given that firrings and standard insulation does the job perfectly fine in 99.9% of cases. So what does the 0.1% represent? Dunno. Maybe the need for minimising height buildup, or minimising installation time?

I would expect the cost of timber firrings + standard insulation boards to be considerably less than tapered insulation boards alone.

I must admit it is always a concern when price isn't up front. It usuallly means it'll be significantly more than what you want/expect it to be!

Saw this and thought it might be of interest!

With the right filtration things might be different, but mine are just standard G4 filters so wouldnt do anything for smoke.

It will handle such pollutants, but only insofar that it will indeed happily draw them in and distribute them around your house! Unfortunately, likely more effectively than if you were just passively ventilated. A continuous controlled supply of fresh air was one of the motivations for us retrofitting MVHR and so it was to much concern when our (friendly and perfectly pleasant) neighbour opted to fit a woodburner... Panic set in and fearing certain death from them lighting the thing up I rigged up an air quality sensor inside the supply manifold which, if a particular threshold of PM1/2.5/10 was reached, would 'trick' the MVHR unit in to frost mode which keeps the extract going but cuts the supply for 10 minutes before reassessing the situation. It works really well and since fitting it we've never once noticed even the slightest whiff of smoke even when conditions (wind in the wrong direction, overly damp wood etc) might otherwise have potentially caused us to do so. However, and this is probably he biggest point I'd like to make, not only did I fit an air quality sensor inside the supply manifold but put one inside the house and that has made me realise that we almost certainly create as much pollutants (assuming all particulate matter of a given size is equally harmful?) ourselves every day from cooking than what the neighbour's woodburner some distance from our MVHR inlet might do and so have really become quite chilled about the matter!

With regards to the design detail why is there ventilation? You wouldn't normally have it in a warm roof construction.

We've got an extract above the hob and it works well - helps ensure steam and cooking smells are removed at source. Did think we might have an issue with airborne grease and so fitted a washable sponge filter inside the vent but we don't do all that much frying so I wonder how much we really require it. The extract is double ducted and we have two single ducted supplies at opposite ends of the open plan kitchen/diner/lounge.

Yeah, absolutely agree with this. Fitting weep vents is trivial, and even fitting cavity trays is pretty straightforward for a brickie. They get a lot of practice for retrofitting trays when building extensions so it won't phase them at all.

Although I've returned to a connected can after well over a year without use and it worked as if it was only yesterday. I must admit I was very surprised though so probably a stroke of luck rather than an anecdote to be relied upon.

You've mentioned nail holes a couple of times yet all the discussion seems to be about the membrane material. If the water is coming in through the nail holes, and only the nail holes, then what does the material have to do with it? A hole is a hole and of course is never going to be waterproof. The mitigation for this is, and always has been, ensuring the membrane is drooped between rafters (keeps any water that finds its way through away from the rafters and nail holes) and of course having a layer of tiles on top (which practically keeps all the water away in the first place).

It is perfectly fine for thin cutting discs as the shaft flange supports sufficient (if not the whole) depth of the hole. It is only thicker discs - grinding discs in particular - that benefit from the flange on the nut.