MVHR and windows open

[Trw144]

Is there an issue with leaving mvhr on with the windows open? On a glorious day like today, I ve got the patio doors wide open and I ve l ft he mvhr on. Should I be switching it off in this instance or is it fine (bar wasting a bit of energy on having the fan running)?

[Jeremy Harris]

No problem at all, other than wasting a small amount of energy, which if you've got a PV array is of no consequence. 

[Redoctober]

An interesting point and the whole MVHR issues is one I have been considering for some time. I have come to the decision not to install one.  Although the house will be well insulated,  we are not looking to be as air tight as others may expect and we do like to have the windows open.

 

This appears not to be an issue for the reasons @JSHarris says, but for me, I just can't see the point of having a system running in the background which is, in all intents and purposes there to provide "clean" "healthy" air, in an air tight enclosure, and then have the windows open. Yes the "energy" used may be negligible but it is still wasted energy.

 

Could it not just be turned off during the Spring / summer months?

 

"Tin hat at the ready !"

 

PW.

 

[Jeremy Harris]

You can turn it off whenever you like, it's primarily there to provide clean ventilation air and recover heat that would otherwise be wasted, but there's no point in having it on when you don't need either heat recovery or ventilation.  For us it's not worth getting up to push the off button, as any time we might want it off it will be running from the excess PV generation, so there's zero running cost in just leaving it on all the time.

[Trw144]

I think I might programme it onto the home automation so I can switch it off/on at the touch of a button, or possibly have it off during the daytime in the summer months and on during the night when we are unlikely to have windows open. 

[PeterW]

38 minutes ago, Trw144 said:

I think I might programme it onto the home automation so I can switch it off/on at the touch of a button, or possibly have it off during the daytime in the summer months and on during the night when we are unlikely to have windows open. 

 

Does it not have summer bypass ..? If so then night time is probably when you want it on so you can get cooler filtered air into the house. 

[Jeremy Harris]

This time of the year there is a pretty wide diurnal temperature variation; last week I had to scrape frost of the car windows in the morning, yet we had a lot of sunshine during the day.  I suspect that there is still a worthwhile benefit from having the MVHR on at night, even if you turn it off during the day, when you have doors or windows open.

[joe90]

I still think that the MVHR should be run like a heating system, the thermostat tells the heating when it's needed, I.e.sensors ( co and RH) switch the MVHR on when it's needed, or manually over ride it when you decide. I think I have been told the co sensor is expensive. ( I too like having windows open in summer and can't see the reason for using electricity ( even if a small amount) when it's not required. Also the whole unit will last longer, not need servicing as much if used less. 

[Jeremy Harris]

Non-Dispersive Infra Red (NDIR) CO₂ sensors (the very reliable and accurate ones) are around £80 - £100.  I have a few that I bought for around £20 each, that were surplus, sold off when a building monitor manufacturer went bust, that I use to monitor the house CO₂ level 24/7, with the data logged to a USB stick.  It would be easy to add a system to switch the MVHR on and off, but I can't really see any benefit for us.  For most of the year (like now) we generate far more electricity than we use, and we get paid the same whether we use it or not, so the few tens of watts the MVHR uses makes no difference at all during the day, and at night the chances are we'd want the MVHR on anyway.

[Simplysimon]

@Redoctober, one of the biggest problem we will see in the future is 'fairly well' insulated and airtight houses with no proper ventilation, poor design with cold bridging and may as well throw in poor build quality! in fact many children are growing up with ailments due to this.

if you have a warm house and very little ventilation, condensation will go somewhere, usually a cold corner with no air movement. new houses with trickle vents will not be sufficient to do the job properly. it is fine if the windows are opened in all seasons, though the majority of people won't open windows having paid to heat the house, you may be different.

it may cost more to fit an mvhr, however, the house will be properly ventilated and minimise problems in the future.

[Bitpipe]

We had the sliders wide open today and the mvhr running, I don't see an issue and the energy loss is minimal.

 

MVHR is still pumping fresh air into the rooms that are not naturally ventilated and extracting foul air from the bathrooms, so not a total waste of effort, although I accept that the efficiency literally goes out the window when its open.

 

MVHR comes into its own in the winter when you need fresh air but don't want to let heat out.

 

Also in the summer when the external air is warm, we will be keeping all doors and windows shut to minimise overheating but will have fresh air in the house and will use summer bypass and nighttime purging to keep the house cool.

 

We noticed this effect last summer just as the house was being finished - keeping the east blinds down in the morning and all windows closed kept the house nice and cool even on a very hot day. The decorators took some convincing and prior to this, kept opening all the doors and windows and then wondering why the house was so warm...

[Trw144]

2 hours ago, PeterW said:

 

Does it not have summer bypass ..? If so then night time is probably when you want it on so you can get cooler filtered air into the house. 

 

Yes it does, hence my same thought of bringing it on at night and cool the house.

[TerryE]

8 hours ago, Redoctober said:

@JSHarrisI have come to the decision not to install one  ... Yes the "energy" used may be negligible but it is still wasted energy.

 

There's a fundamental catch-22 with this logic.  If you want a reasonably well ventilated house in winter, the heat losses from air exchange without HR are about half your total heating bill -- or you half your heating bill by having heat recovery. Of course, the alternative is have a pretty airtight house without mechanical ventilation, and just put up with the stuffy air, humidity, mildew, etc.

[Declan52]

You have to provide ventilation at a specific rate set out by whatever version of the building regs you are covered by. So depending on how airtight your build is you will have to provide some sort of ventilation , trickle vents , stack , purge mhrv, to get your house passed. The choice is up to you but it makes more sense to me do pick the option than recovers some heat as well.

[Tom's Barn]

My vent axis units state on the label - do not turn off - however from what I am reading it would seem it is ok to turn off when not needed. Do your units have the same warning?

[jack]

4 minutes ago, Tom's Barn said:

My vent axis units state on the label - do not turn off - however from what I am reading it would seem it is ok to turn off when not needed. Do your units have the same warning?

 

I don't recall seeing any such warning on my (Brink) machine or in the instruction manual.

 

Personally I don't like the idea of it being turned off for long periods of time. Sounds like a recipe for stale air and condensation, and everything that goes along with those. 

 

I never turn ours off, but if I did I'd probably have it come on for a few minutes every few hours to keep the air inside the pipework fresh.

[JohnMo]

No particular warning on my Titon units.

 

My thoughts are. For what it saves why switch off and risk not switching on when you have closed the windows.

 

Mvhr sucks in moisture from wet rooms, so be definition the extract side is wet or damp.  Switching off for a prolonged period would allow these damp areas to generate bugs, mould etc. Neither is healthy for the unit or you.

[Tom's Barn]

Thank you for your comments on this, I agree with the approach of keeping them on. However, what I have done is adjusted the fan speed to lower setting plus I have reduced the humidity threshold. Now I am saving on electricity, plus should there be an increase in moisture the fans will increase to ensure that remains under the 55% threshold.

 

For summer it can stay in this setup and will be complimented by opening windows on tilt mode for cooling.

 

As always thanks for your thoughts on this. @JohnMo @jack

[mjc55]

Resurrecting an old thread here but it does focus on something we have been thinking about.

 

We have always (40+ years) slept with the window open at night (apart from the odd occasion when the rain was horizontal!) and embarking on our build leads me to question how this will work with MVHR?

 

We stayed in "passive" house last weekend house-sitting for our Son's friends.  I have to admit to waking up and really missing the "fresh air" from an open window.  It did feel a bit stuffy.

 

We are still at the planning stage and although we want our build to be passive, we will not be going for any certification for it.

 

I know when we went to the passive house to sort out staying a few weeks ago the owner was very keen that the door was not open for too long wen we came in and did say that he did spend some time looking at the weather for the next day so he can anticipate changes in temperature and react accordingly with their system (combined ASHP and MVHR).  They don't have any form of UFH and they have large windows to the East of the property which take advantage of natural heating from that direction in the earlier parts of the day,

 

I do understand, to a degree, the arguments that passive house proponents put forward re the advantages, not least the running costs nut do have problems about how we live in that sort of environment.

 

 

[ProDave]

We don't weather watch, everything just controls itself with the magic of thermostats.

 

We often have our bedroom window open, we like a cool bedroom. No issues.

[Russdl]

3 hours ago, mjc55 said:

missing the "fresh air" from an open window.  It did feel a bit stuffy

That shouldn’t have been the case, perhaps the MVHR isn’t set up correctly?
 

3 hours ago, mjc55 said:

do have problems about how we live in that sort of environment.

You would live exactly how you wanted to live.
 

If you want a window or door open, just open them however,  if you’re trying to keep the place warm through the colder months by the power of sunshine alone then don’t leave them open for too long or you will fail. 

 

Low running costs result from the investment in the structure and the setup and should only impact the internal air quality and comfort in a positive way. 

Edited April 13 by Russdl

[Adrian Walker]

3 hours ago, mjc55 said:

Resurrecting an old thread here but it does focus on something we have been thinking about.

 

We have always (40+ years) slept with the window open at night (apart from the odd occasion when the rain was horizontal!) and embarking on our build leads me to question how this will work with MVHR?

 

We stayed in "passive" house last weekend house-sitting for our Son's friends.  I have to admit to waking up and really missing the "fresh air" from an open window.  It did feel a bit stuffy.

 

 

 

 

 

 

It shouldn't feel stuffy at all.  If this is the case then it hasn't been commissioned correctly IMHO.  A CO2 reading is the quickest way to confirm this. Every home should have one.

Edited April 13 by Adrian Walker

[JohnMo]

2 hours ago, mjc55 said:

stayed in "passive" house

 

4 hours ago, mjc55 said:

have to admit to waking up and really missing the "fresh air" from an open window.  It did feel a bit stuffy.

Sounds like the ventilation rates are not that good. If correctly set the air in the bedrooms should always feel fresh.

 

We do get hot in our bedroom, so we just open the window as we feel.  Our heating or ventilation doesn't get touched on a day to day basis. In fact ventilation doesn't ever get touched.

[ProDave]

4 hours ago, mjc55 said:

We stayed in "passive" house last weekend house-sitting for our Son's friends.  I have to admit to waking up and really missing the "fresh air" from an open window.  It did feel a bit stuffy.

Another point, I stayed in a passive house many years ago before I knew much about them.  And like you, thought it was stuffy.  The house was empty and we were offered it as workers accommodation for a few days.  It was well into the stay that we realised the mvhr unit was turned off, probably because the house was empty before we were given the keys.

[Iceverge]

We leave the window in our bedroom for much of the summer.

 

I support we could turn the MVHR off but it's so cheap to run and as it guarantees fresh air regardless we don't bother. 

 

Despite my statz-esque approach to kids leaving the entrance door open I try  to remember a house is there entirely for our benefit and and enjoyment and not as a science project.