MVHR settings for a less airtight house

[pstunt]

I am wondering if and how, when working out the required extract and intake values, the air tightness of the building is taken into consideration. 
 
In my simple mind would there not be less intake air required for a house with a poorer airtightness score ? If so how is this accounted for ?

My situation is this, our airtightness for our conversion was not what we hoped for, about 4. Many reasons for this and some of them will be sorted. We have not had our MVHR properly balanced and formally commissioned yet and probably won’t. The building inspector wasn’t the slightest bit interested and is ready for sign off once we have produced other standard certificates.

 

I  am about to now embark on trying to balance the system myself but was wondering if the airtightness score should be considered in anyway ?

 

We have been living in the house for a few months and I love the quality of the air inside, although with the airtightness score I appreciate it’s not an ideal scenario but sometimes it does feel like there is too much air coming in through the vents. Even though I have crudely measured them using a home made device, similar to others on this forum but probably with far less skill and precision.

[Savage87]

I'm interested in the answer as well, I have just balanced my system which was installed 4 years ago and the house is a bit leaky. I think there a bit too much ventilation TBH with current building regs. I had it set to 25-50% less and it was fine for years, boosting only when there was high humidity in the kitchen or bathroom.

 

[IanR]

7 hours ago, pstunt said:

I am wondering if and how, when working out the required extract and intake values, the air tightness of the building is taken into consideration. 

~~~
My situation is this, our airtightness for our conversion was not what we hoped for, about 4

For reasonable air quality, you don't need to have your MVHR on as you are above 3m³/m².h@50Pa of natural infiltration. Had you been below then perhaps BC would have wanted to know more about the MVHR.

 

I guess though that as you've installed MVHR you haven't got any other form of extraction to outside for your wet-rooms, in which case you'd need the MVHR on for boost.

 

Can your MVHR be configured to be on but with 0% fan speed, if so I'd set it up like that for most of the time and just run the fans for boost.

 

Since you don't need the MVHR for air quality, then having it running 24 hours a day will incur you extra losses as you are ventilating more than you need and loosing energy when you do.

Edited January 12 by IanR

[pstunt]

4 minutes ago, IanR said:

For reasonable air quality, you don't need to have your MVHR on as you are above 3m³/m².h@50Pa of natural infiltration. Had you been below then perhaps BC would have wanted to know more about the MVHR.

 

I guess though that as you've installed MVHR you haven't got any other form of extraction to outside for your wet-rooms, in which case you'd need the MVHR on for boost.

 

Can your MVHR be configured to be on but with 0% fan speed, if so I'd set it up like that for most of the time and just run the fans for boost.

 

Since you don't need the MVHR for air quality, then having it running 24 hours a day will incur you extra losses as you are ventilating more than you need and loosing energy when you do.

I felt it was more of a case that BC didnt know much about or certainly didnt have any interest in the MVHR. He had a very common sense approach. The air tightness test occurred after his visit so didn't factor into his reasoning. 

 

The Unit is a Vent Axia Advanced, and I was thinking along the same lines of what you suggest but I still want the fan to bring some air in but perhaps not as much. The Vent Axia allows you to tweak the various levels but I was thinking of leaving the extraction rate at about the same level it currently is. 

[JohnMo]

19 minutes ago, IanR said:

Since you don't need the MVHR for air quality, then having it running 24 hours a day will incur you extra losses as you then ventilation more than you need and loosing energy when you do.

Very good point

 

8 minutes ago, pstunt said:

but I still want the fan to bring some air in but perhaps not as much. The Vent Axia allows you to tweak the various levels but I was thinking of leaving the extraction rate at about the same level it currently is.

If you have unbalanced flow, the MVHR efficiency drops like a stone. A 90%+ efficiency, could easily drop to 50% or less, which is almost a waste of time running, as you are using twice the electric of a MEV system.

 

As @IanR say run when you need, but do so with balance flows.

 

Or just run all the time, at very low flow rates.  Get a CO2 monitor stick in your bedroom and monitor.  If you go over 1000ppm at night increase the rate. If you settle under 800ppm slow the rates down.

[pstunt]

4 minutes ago, JohnMo said:

21 minutes ago, pstunt said:

but I still want the fan to bring some air in but perhaps not as much. The Vent Axia allows you to tweak the various levels but I was thinking of leaving the extraction rate at about the same level it currently is.

If you have unbalanced flow, the MVHR efficiency drops like a stone. A 90%+ efficiency, could easily drop to 50% or less, which is almost a waste of time running, as you are using twice the electric of a MEV system.

 

As @IanR say run when you need, but do so with balance flows.

 

Or just run all the time, at very low flow rates.  Get a CO2 monitor stick in your bedroom and monitor.  If you go over 1000ppm at night increase the rate. If you settle under 800ppm slow the rates down.

Thanks for the feedback. You are all highlighting my lack of understanding, so this is all appreciated. I will take the time to balance the system as best I can, then look to run it at the lower setting. I haven't tried how running at a lower setting works with the extraction boost in the bathrooms yet but I guess the humidity sensor will just handle that and boost it accordingly. Will try it over the weekend.

[Andehh]

4 is still a good amount below average for new builds In the uk, not great by buildhub standards but better then average!!

 

Two £1.5m local to us were 5 5 and 6.9... No MVHR, but didn't stop them selling.

[pstunt]

36 minutes ago, Andehh said:

4 is still a good amount below average for new builds In the uk, not great by buildhub standards but better then average!!

 

Two £1.5m local to us were 5 5 and 6.9... No MVHR, but didn't stop them selling.

Well that’s a little reassuring, even though I was hoping for buildhub standards.

[Tumble]

I live in a trickle-vent house, built before airtight regulations came in, but don’t feel we get sufficient ventilation. I’ve got a CO2 monitor and it regularly goes over 1000 in our lounge and requires significant window opening to reduce. We also get a lot of condensation on windows, bathrooms, etc. 
 

So while we aren’t airtight I’m installing a MVHR to hopefully improve the air quality (and then where opportunities come up I will improve airtightness). I plan to set the background ventilation rate to be sufficient to keep CO2 in check, and then boost to the regs requirements. 
 

In summary, I hope to find the MVHR system very useful even in a non-airtight house (as is supported by other posts on this forum). 
 

Keeping the flows balanced makes sense to me in terms of reducing drawing in of external-temperature air. If it extracted more than it supplied then cold outdoor air would have to make up the difference. Keeping it balanced means the only outdoor air which will want to come in will be wind driven. 

[JohnMo]

19 minutes ago, Tumble said:

trickle-vent house

If you have intermittent fans, just replace with dMEV fans, they run at a low rate, almost silent, they will pull the air through the trickle vents and give cross flow ventilation. Then replace the normal trickle vents with humidity activated ones, so you vent through the trickle vents when needed. Least impact on heat loss, no need to pull your house apart to install.

[Coanda]

I'm pretty new on the mvhr scene and while  I am researching, I am running a temporary set up which is not powerful enough, but better than nothing.

 

My house, I thought was quite leaky but RH and co2 levels were much higher than they should have been so obviously more air was needed.

 

I think my mvhr won't have to work as hard as one in a tighter house, but is still necessary.

 

 

[JohnMo]

A quick comparison of different solutions to humidity control. To give people an option, as MVHR can be difficult to install as a retrofit.

 

Two solutions implemented - MVHR and dMEV. 

 

Outside currently is 0 degs and 82% humidity.

 

House has MVHR, inside humidity currently is 41%, and 21 degs. 

 

Summer house, dMEV, inside humidity currently is 40%, and 19.3 degs.

 

MVHR isn't the be all, end all solution, different solutions are sometimes easier to implement and give similar results, especially in less airtight properties.