Sizing MVHR

[sonicboom]

My property has a floor area of 100m2, and volume of approx 480m3. I have a PIV and before the air tightening noticed  about 35l/s is more than enough for the property. The question I would like to ask is, how do I select the proper size unit. I was thinking of going for a Brink Flair 450 but not sure if that'll be enough.

[JohnMo]

1 minute ago, sonicboom said:

My property has a floor area of 100m2, and volume of approx 480m3. I have a PIV and before the air tightening noticed  about 35l/s is more than enough for the property. The question I would like to ask is, how do I select the proper size unit. I was thinking of going for a Brink Flair 450 but not sure if that'll be enough.

4.8m tall rooms, is that correct?

 

How airtight is your house?

[sonicboom]

5 minutes ago, JohnMo said:

4.8m tall rooms, is that correct?

 

How airtight is your house?

^{Sorry, 2 floors, each 2.4m.}

[SteamyTea]

10 minutes ago, sonicboom said:

volume of approx 480m3. I have a PIV and before the air tightening noticed  about 35l/s is more than enough for the property

That is about 0.25 ACH.

As @JohnMo says, how leaky is the rest of the building.

[sonicboom]

Sorry, missed @JohnMo airtightness question - long day.

Its extremely air tight, to the point the PIV is not really working any more.

[Nickfromwales]

3 hours ago, sonicboom said:

Sorry, missed @JohnMo airtightness question - long day.

Its extremely air tight, to the point the PIV is not really working any more.

Respectfully, you need to do a test and get the results. If it’s worse than you think then maybe a 450 won’t do it, if it’s much better (sub 1.0 ACH) then the 450 may suffice. 

 

That said I’d still go for a 600 if you’re not counting every penny    

[Iceverge]

How many people in the house?

 

BALLPARK numbers here.....

 

30m³ per person per hr, normal flow rate for an MVHR unit is about 30%

 

So if a Brink 450 is I assume 450m³/hr in a nominal setup then it'll do 4.5 people. 

 

If you have 6 people get a Brink 600 (if even such a thing exists). 

 

 

 

 

 

[Eric]

On 23/12/2024 at 21:40, Iceverge said:

If you have 6 people get a Brink 600 (if even such a thing exists).

 

It does exist, I have one

[sonicboom]

22 hours ago, Nickfromwales said:

Respectfully, you need to do a test and get the results. If it’s worse than you think then maybe a 450 won’t do it, if it’s much better (sub 1.0 ACH) then the 450 may suffice. 

 

That said I’d still go for a 600 if you’re not counting every penny    

What test is this an how is it performed ?

[sonicboom]

22 hours ago, Iceverge said:

How many people in the house?

 

BALLPARK numbers here.....

 

30m³ per person per hr, normal flow rate for an MVHR unit is about 30%

 

So if a Brink 450 is I assume 450m³/hr in a nominal setup then it'll do 4.5 people. 

 

If you have 6 people get a Brink 600 (if even such a thing exists). 

 

 

 

 

 

There are 5 people in the house at the moment, but it'll drop to 4 in the next year or two. Interesting you say 30m3 since the PIV worked very well at close to that rate too (120m3/hr for 5 people). Although I would prefer to run it lower and reactively increase it based on humidity since not everyone is at home all the time. Good idea ?

Edited Tuesday at 20:16 by sonicboom

[Iceverge]

Not worth the hassle, just set an average rate to maintain an acceptable overall air quality. 

 

The house will have enough volume to buffer the peaks and troughs. 

 

 

 

[JohnMo]

12 hours ago, sonicboom said:

would prefer to run it lower and reactively increase it based on humidity since not everyone is at home all the time. Good idea

Why bother, gains are not worth the bother. Most mvhr will have a setback mode. This reduces the fan speed from current to about half current speed. I just put ours on set back during the winter, to stop the air drying too much. Then in spring put the speed back to normal.

[MikeSharp01]

13 hours ago, sonicboom said:

What test is this an how is it performed ?

Usually by putting a fan in your front door opening and pressurising the house to +/- 50 pascals while calculating the air flow through the fan when holding that pressure. knowing the volume of the house you can then work out the air tightness relative to the building control levels and the natural leakage rate you can expect. This then gives you the air changes per hour and you can very simply size the MVHR unit from there. I would look also at the Titon units, which we have and think much of, made in UK and have simple to use yet sophisticated control features. 

[JohnMo]

43 minutes ago, MikeSharp01 said:

would look also at the Titon units, which we have and think much of, made in UK and have simple to use yet sophisticated control features. 

+1

 

For total flow rates they are easy enough.  Just total floor area, multiply by room height up to a max of 2.5m, multiply by 0.3. apply same to extract and supply.

 

You need an air test to see if you benefit from MVHR, 3m³/m² at 50Pa. Will cost £100 to 200.

 

But I would dump PIV and replace with dMEV (Greenwood CIV 2 or 3) and have self modulating trickle vents, only on dry rooms, seal and vents in wet rooms. Easy to do, very little in the way of heat loss, as it's all done on demand with very low background flow. Whole system for pennies to the pound compared to MVHR.

 

 

[Iceverge]

36 minutes ago, JohnMo said:

+1

 

For total flow rates they are easy enough.  Just total floor area, multiply by room height up to a max of 2.5m, multiply by 0.3. apply same to extract and supply.

 

You need an air test to see if you benefit from MVHR, 3m³/m² at 50Pa. Will cost £100 to 200.

 

But I would dump PIV and replace with dMEV (Greenwood CIV 2 or 3) and have self modulating trickle vents, only on dry rooms, seal and vents in wet rooms. Easy to do, very little in the way of heat loss, as it's all done on demand with very low background flow. Whole system for pennies to the pound compared to MVHR.

 

 

 

I'd double on this. 

 

They're a straight swap for most bathroom extractors. Almost silent, cheap, fit and forget.

 

MVHR does take a certain amount of know how and servicing. 

 

The filters need doing a couple of times a year. 

 

The exchanger could do with a wash every few years and the bearings in the motor will wear out too. There's probably £100/year of maintenance when you take this into account so unless you're in a passive class house and know what you're doing a dMEV is a very easy recommendation. 

 

 

 

 

 

 

 

 

[Nickfromwales]

On 25/12/2024 at 11:32, Iceverge said:

 

I'd double on this. 

 

They're a straight swap for most bathroom extractors. Almost silent, cheap, fit and forget.

 

MVHR does take a certain amount of know how and servicing. 

 

The filters need doing a couple of times a year. 

 

The exchanger could do with a wash every few years and the bearings in the motor will wear out too. There's probably £100/year of maintenance when you take this into account so unless you're in a passive class house and know what you're doing a dMEV is a very easy recommendation. 

All “mechanical” systems require the same, just a big difference between caretaking a single point of service vs multiples of. I wouldn’t want such a statement to frighten anyone away from MVHR, as it is the best solution, in a suitably airtight house, hands down.

Filter cleaning / replacement in a quality MVHR unit is a 10-15 min job at worst; sub 5 minute job if they’re simply being replaced. Brink filters can be changed in sub 60 seconds, just swing open the front access door and slide them in/out.

Through-wall single point MVHR units are reported to be more grief than they contribute, but PIV and dMEV are things I have the least interest / experience with, due to near constant cold air influx in winter with zero heat recovery (AFAIK).

[JohnMo]

3 hours ago, Nickfromwales said:

due to near constant cold air influx in winter with zero heat recovery (AFAIK)

PIV yes agree. dMEV or MEV that is setup to only do minimal airflow, unless humidity levels demand otherwise, coupled with demand response trickle vents, not so sure there is that much in it.  Only a local fan boosts, not the whole system, for example. Filter replacement time/cost zero. One set of air movement fans - half the running cost. Install in wet rooms, utilise the holes that most likely exist in those room already (core drill if needed).  No ducts to retrofit for dMEV, extract only for MEV.

 

Retrofit an MVHR correctly - rip house apart, make air tight or install in house not really suited to MVHR because it leaks like a seize, loads of cost negative heat loss benefit (heating costs you more because of the additional ventilation provided by MVHR). OP is retrofit because he doesn't like PIV.

[john0wingnut]

16 hours ago, JohnMo said:

PIV yes agree. dMEV or MEV that is setup to only do minimal airflow, unless humidity levels demand otherwise, coupled with demand response trickle vents, not so sure there is that much in it.  Only a local fan boosts, not the whole system, for example. Filter replacement time/cost zero. One set of air movement fans - half the running cost. Install in wet rooms, utilise the holes that most likely exist in those room already (core drill if needed).  No ducts to retrofit for dMEV, extract only for MEV.

 

Retrofit an MVHR correctly - rip house apart, make air tight or install in house not really suited to MVHR because it leaks like a seize, loads of cost negative heat loss benefit (heating costs you more because of the additional ventilation provided by MVHR). OP is retrofit because he doesn't like PIV.

Might be a silly question, but could one not dial back the MVHR to provide the minimal amount of ventilation required? Thus not over ventilating if it is a leaky house?

 

I am having this exact debate in my head at the minute; currently have trickle vents and 4 dMev fans (3 wet rooms and 1 kitchen). Co2 levels are fine but humidity is constantly 56-60% and various rooms are getting a fair amount of condensation / mold on the window frames / reveals.

 

Bungalow is 170m2 and trickle rate on the fans is 11 l/s x4, so only just under 51 l/s required by BR. 

 

So I was contemplating putting MVHR in as I know that brings the humidity down.

[JohnMo]

39 minutes ago, john0wingnut said:

am having this exact debate in my head at the minute; currently have trickle vents and 4 dMev fans (3 wet rooms and 1 kitchen). Co2 levels are fine but humidity is constantly 56-60% and various rooms are getting a fair amount of condensation / mold on the window frames / reveals

Are trickle vents open?

Do you have door under cuts to allow cross ventilation?

41 minutes ago, john0wingnut said:

are getting a fair amount of condensation / mold on the window frames / reveals.

 

Have you checked you don't just have an open void around the window frame, causing a huge cold bridge? That is the normal cause of that issue, bodge window installation, not filled the gap with a good expanding foam, just some cheap filler pieces hiding a hole.

 

You NEED to find the route cause. MVHR isn't the magic fix.

45 minutes ago, john0wingnut said:

Might be a silly question, but could one not dial back the MVHR to provide the minimal amount of ventilation required

If so what's the point of all the work and upheaval?

[john0wingnut]

23 minutes ago, JohnMo said:

Are trickle vents open?

Do you have door under cuts to allow cross ventilation?

Have you checked you don't just have an open void around the window frame, causing a huge cold bridge? That is the normal cause of that issue, bodge window installation, not filled the gap with a good expanding foam, just some cheap filler pieces hiding a hole.

 

You NEED to find the route cause. MVHR isn't the magic fix.

If so what's the point of all the work and upheaval?

All trickle vents are open.

Doors are undercut by 15mm.

I should have also said glazing also has condensation.

 

Windows are on the to do list and I will ensure are done correctly, but you could be right as it could be empty behind the silicone currently.

 

I am running a dehumidifier to keep humidity in check which is 250w an hour, plus heat loss via dMEV, so my thought process was the MVHR would bring down humidity, recover heat and cost less to run than the humidifier currently.

 

I do accept there is a fair amount of work involved and initial outlay though.

 

BPC also stock the duco focus box now which is essentially demand controlled MEV, which I was also considering, but once you add in all the required control valves on top of the fan box, your looking at the cost of a decent MVHR unit.

 

 

[JohnMo]

30 minutes ago, john0wingnut said:

thought process was the MVHR would bring down humidity

Screen shot, shows outside humidity, house humidity with MVHR and summer house (pod) with dMEV. The days below were above freezing.

 

This shot it was colder, with days well below zero, so house and pod humidity drop.

 

dMEV has no different humidity than MVHR, within a couple of percent.

 

I would spend time/money resolving cold bridging. You want decent install, and decent frames. If you have argon filled double glazed with a good frame U_w should be about 1.2.

 

[sonicboom]

On 25/12/2024 at 09:57, MikeSharp01 said:

Usually by putting a fan in your front door opening and pressurising the house to +/- 50 pascals while calculating the air flow through the fan when holding that pressure. knowing the volume of the house you can then work out the air tightness relative to the building control levels and the natural leakage rate you can expect. This then gives you the air changes per hour and you can very simply size the MVHR unit from there. I would look also at the Titon units, which we have and think much of, made in UK and have simple to use yet sophisticated control features. 

Are there any specific fan systems designed for this ?

[Iceverge]

18 minutes ago, sonicboom said:

Are there any specific fan systems designed for this ?

 

Yes, calibrated ones. Expensive though. 

 

I made my own to depressurise and find leaks in the house from a car rad fan and some OSB for about €40. 

 

There's a thread of how to do it somewhere.