A little bit of anti-MVHR heresy : it'll do us all good.

[ToughButterCup]

When you've built to near passivhaus standard, and for 6 months of the year your 2.5meter ' picture' window is almost always wide open: why fit MVHR?

Hot June: window wide - yes it was a bit warm but who cares? Wet July, window in the wind shadow, so always open. September storms - yes its shut.  When its cold but still, and there's bright bright sunshine, its open. So no point in using the MVHR. Put the oven on and the window gets opened. Christmas and the kids around - wide open.

 

Whassa point if the MVHR  is going to be off half a year? Even when its cold the two bedroom windows (the suicide doors on the first floor) are put on tilt,the heat from the two of us is sufficient to warm the room a good deal. There's no draft (or if there is its not noticeable) 

 

Yes I have to fit it. If I  just drill trickle vents in the windows, @craig will cry.

But, on the basis of living in the house a couple of years without it,  I'm not as much of a fan of MVHR  as I once was.

It'll be the last thing we fit : second hand if I can get it.  Grrrrr.

 

 

 

[craig]

Vent the slider, I’m fine with that. Open and then drop it down in position #happydays 

 

Vent slot in windows 🤬🤬🤬🤬😂😂😂

[ProDave]

If you have windows open in summer you could turn it off.  But ours runs all the time.

 

A good test of an air tight house, is you can open one door or one window on a windy day and you won't get a draught coming in our out.

[SteamyTea]

You don't get hayfever then.

Or live in a noisy place.

[JohnMo]

We are the opposite, windows and doord shut all day, if the sun's out underfloor cooling on (powered by PV).  Basically keep the heat out and don't let it in.  Once the temps drop outside or the inside exceeds the outside due to the oversized glazing, then a couple windows at front are opened and rear door opened for a blow through.  MVHR on all the time.

[Dave Jones]

10 hours ago, SteamyTea said:

You don't get hayfever then.

Or live in a noisy place.

 

even a broken clock is correct once a day!

 

Have to agree, we have allergy sufferers so will be a godsend for them next summer. Will still use the large sliders as and when just let the system do its thing.

[ToughButterCup]

Is the argument that MVHR is for allergy sufferers? 

 

I accept that  our situation - countryside, sheltered - is not universal. And that MVHR is a good idea. 

 

But after the great privilege of living in a Passivhaus (nearly) WITHOUT MVHR for two years I now realise - given that we will use it for 5 months a year - that MVHR is a very much costlier item than I thought. 

 

Now, if I could rent an MVHR system for the Final Inspection and hand it back afterwards, I would. 

[JohnMo]

There is a lot I like about humidity controlled MEV system. One fan on extract only, but only when required in the area required. Humidity controlled inlets and extract points, fan speed increasing only when required.

 

Way cheaper to install and run when compared to MVHR, but offset by slightly increased heating costs.

[dpmiller]

where is the massive expense in MVHR? Or have you not put ducting in already at this point?

 

If you've built airtight enough I wouldn't be so sure just having a window open will actually give you the ventilation you require; gotta have crossflow and then you're into draughts...

[ToughButterCup]

7 minutes ago, dpmiller said:

where is the massive expense in MVHR? Or have you not put ducting in already at this point?

 

If you've built airtight enough I wouldn't be so sure just having a window open will actually give you the ventilation you require; gotta have crossflow and then you're into draughts...

 

MVHR first fit already in place. 

As I write this post the large slider is open, and both suicide doors upstairs are on tilt. There's no apparent draft, (but there is one I'm sure).  And I'm sure that as I get (even) older, I'll feel that more.

 

Just about to have a shower in the wet room : window wide open, door shut - yes there's a draft under the door . Floor dries itself in a few minutes. 

 

My point is that there's an obvious advantage to the application of MVHR systems. But the cost - for us i.e. not in general -  is effectively double what it is for many ; that is those people who need to use MVHR because of local characteristics. 

 

The accident of not being able to afford MVHR (yet) has been an excellent opportunity to see how the house performs WITHOUT MVHR. And I'm saying that in our case, the house performs very well without it.

 

• So I'm hinting that others might like consider delaying the second-fit of their MVHR system until they see how their house performs without it. 

 

In recognition of  @SteamyTea's  consistent reference to data-based and evidence - based design, I've now got a couple of years of temperature and RH data. (Using 5 Sensor Push  data recorders) . Time to have a look at that data soon, I suspect.

[SteamyTea]

2 minutes ago, ToughButterCup said:

I've now got a couple of years of temperature and RH data. (Using 5 Sensor Push  data recorders) . Time to have a look at that data soon, I suspect.

Get on with it.

Do you have external i.e. weather data for the same period, and heating data?

[dnb]

1 hour ago, ToughButterCup said:

Now, if I could rent an MVHR system for the Final Inspection and hand it back afterwards, I would

You could borrow mine for a couple of weeks. It would help me with procrastination points on the house not finished thread...

 

I will need it back because the house is really bad for co2 and heat build up when we work in there and there isn't a good through draft.

Edited August 23, 2023 by dnb

[JohnMo]

The point of MVHR is in an airtight house you need ventilation.

 

When it's cold your windows are shut, you have ventilation your house doesn't turn green inside.

 

When it's hot outside your house stays cooler for longer than it would otherwise. Windows open means hot air comes inside.

 

You recover the heat otherwise lost through ventilation. So heating costs are lower.

 

If you don't want MVHR build a leaky house to min building regs, pay for big heating bills for ever.

[ToughButterCup]

@JohnMo : I accept all your arguments above - more or less.

 

In my particular situation   I find to my annoyance and surprise that the house performs brilliantly on its own without help from MVHR.

 

Re: more or less (above)

• The only time all our windows have been shut is when we've been on holiday. Otherwise at least two are open all the time.
• We don't mind heat - SWMBO was totally happy in the recent warmth - even at night. I couldn't care less.
• Our heating season is very short indeed : a couple of cheap Aldi heaters on for an hour or so, and electric underfloor heating in both wet rooms: oh and one of those warm blankets
• As @craig says above, we can crack the large slider open a few mm - and lock it in that open position.

@dnb, I'll be down for that in a year or two. Loads to get on with - especially as I've had a nudge from @SteamyTea about the data handling. Paths and carparks first.

 

@SteamyTea - no outside data - but the university Met Dept is across the quad  from where Debbie is ( 2miles away) . I'll go and ask if they have local data (bound to)

As to heat input - no data. But we've just had a SMART meter (SMETS 2) installed - and we know exactly how much PV we've injected into our 14 kW SunAmp. (from the inverter and the myEnergi kit). Annoying number of days in June that we exported to the grid. Grrrrr.

[Kelvin]

Having stayed in a few houses with MVHR the first thing you notice is how pleasant the atmosphere  is. Cool and fresh. We are still building our house obviously but as I’ve been going around sealing the house up I’ve noticed how stuffy it’s quickly become. Cross ventilation is easy in our house given the way it’s laid out however if I were to rely solely on opening windows I’d spend my days running around opening and closing them due to the rain unless you automated that. If you go out for the day and shut the house up doesn’t it become a bit stuffy again? Ours does but it is still drying out. 

[ToughButterCup]

@Kelvin   We have a large slider that sits under the protection of a protected open area ( 4.2m by 2m : we call it the Winter garden. Kevin WotsHisFace from Grand designs calls that sort of area a Breeze Corridor)  See the image above.  So far the rain has reached that window a handful of times in a couple of years. 

 

If we go out - as opposed to go away for a bit - we leave one of the suicide doors (see image - top left) on tilt and maybe the Velux on the latch. Even in bad weather locking the slider open a few mm would not allow any rain in the house.

 

Yes, if we went out for the day and closed everything up it would be stuffy.

 

Cat fart lingers worst of all.

[Iceverge]

@ToughButterCup

 

I suspect you have plenty of cat swinging space per person.

 

It's fallicy to equate a house with 2 occupant's, high ceilings and an open plan compared to a house with tiny rooms and a large family even though they will have the same designed ventilation. 

 

I really liked the active system like Aereco's DCV with the humidity controlled vents. However it was dearer than our dumb MVHR system. 

 

 

 

 

 

 

 

 

 

 

[ToughButterCup]

31 minutes ago, Iceverge said:

...

It's fallacy to equate a house with 2 occupant's, high ceilings and an open plan compared to a house with tiny rooms and a large family even though they will have the same designed ventilation. 

...

 

Quite. Which is why I stress (above) '... in our particular situation...' 

Even though we are  'light-and-airy' we still need MVHR.

But I now realise that for 5 to 6 months of the year the system will be switched off. 

 

If I can't take a joke, I should not have started this build should I ? Keep smiling and get on with the next job.

 

[Redoctober]

@ToughButterCup Thanks for posting your thoughts - MVHR systems are clearly of benefit in the right settings. That said, there is life without such a system and this type of thread will help people come to their own decsions as to whether to install one or not. We chose not to and like you, we do not feel we are missing out on anything in particular. 

 

The decising factor for us was upfront capital investment, ongoing maintainance and adjustments [ perhaps] and advice / guidance from a popular house builder, who also chose not to install in their houses. 

[JohnMo]

17 minutes ago, Redoctober said:

That said, there is life without such a system and this type of thread will help people come to their own decsions as to whether to install one or not

Not really correct - if you have an airtightness better than 3, building regs in Scotland (maybe England also) demand you have balanced mechanical inlet and extract ventilation system. So you have little or no choice in the decision.

[Iceverge]

It'd be interesting to see the CO2 and RH stats for the house. 

 

I suspect it'd be fine due to the low occupancy rates and the propensity to keep windows open and happily exist a few degrees colder. 

 

Also, if I recall correctly, you do have an active ventilation system in the form of a stove. A completely room sealed 5kw stove will pull about 14m³ of air through an hour assuming it only uses half the O2 available in the air. 

 

[SteamyTea]

10 minutes ago, Iceverge said:

A completely room sealed 5kw stove will pull about 14m³ of air through an hour assuming it only uses half the O2 available in the air

Another way to look at it is that for every kilo of timber burnt, about kilos of air needs to be combined.

That is roughly 10m³ of air.

Domestic wood burners are not very efficient though.

 

[ToughButterCup]

54 minutes ago, Redoctober said:

...

The deciding factor for us was upfront capital investment, ongoing maintenance and adjustments

...

 

I can't afford a system either - yet.

But I will be able to at some stage. I hope by then, performance will have gone up and price come down for older systems ( they already perform well enough for us ). 

 

Or maybe we could rent one for the duration of the last BC visit

[Dave Jones]

20 minutes ago, ToughButterCup said:

 

I can't afford a system either - yet.

But I will be able to at some stage. I hope by then, performance will have gone up and price come down for older systems ( they already perform well enough for us ). 

 

Or maybe we could rent one for the duration of the last BC visit

 

its a bit like saying cant afford a roof. its a decision. To me a well insulated, airtight house without MVHR should not be issued with a completion cert as its not fit for use.

[Redoctober]

22 minutes ago, Dave Jones said:

 

its a bit like saying cant afford a roof. its a decision. To me a well insulated, airtight house without MVHR should not be issued with a completion cert as its not fit for use.

 

A roof? Not quite - That said you are right to say that a well insulated house should have one and that is why the regs set a marker at 3 [ in scotland anyway] -

 

In simple terms isn't like buying a car with or without aircon? Having aircon in a car has many advantages but those who don't have it still make do, knowing that their "lot" could be better. But on balance, they prefer to go without or do so due to budget restrictions.🤷‍♂️