How to select correct size MVHR given my m3/h?

[gc100]

Hello all,

 

I've calculated I need  170 m3/h to satisfy part F requirement under normal background ventilation (47 l/s for my 156m2). I'm aiming for 180 m3/h just to make sure under normal conditions regs are covered as this value also covers the extraction rate needed on boost (which is 133 m3/h). ( calcs here ). This should give me an ACH of 0.42 which seems about right from my understanding.

 

Looking at various units they all seem to state their max rate - but how do I size them against my background rate needed?

 

Thanks 

Edited April 15, 2020 by gc100

[Conor]

My neighbour who designs and installs MVHRs, says aim for 50% normal operating range. Feels the building reg values are far too low. But, if I followed his recommendation I'd need two 300mh3 units for my 300m² build.... So that's not happening!

[Jeremy Harris]

Our experience, mirrored by others here, I think, has been that the ventilation rate given in Part F is higher than needed, rather than lower.  We turned our MVHR down a bit, whilst monitoring air quality (RH and CO₂ concentration) and haven't seen any detectable change from doing this.  We can set four speeds on the MVHR, and run it on 2 (about 30%) most of the time, with 3 (about 70%) being the normal boost and 1 (about 10%) being a very low setting for times we go away.   I don't think we've ever run it at speed 4 (100%) although it may ramp up to that automatically when the integral heat pump is running.

 

 

[epsilonGreedy]

Joining the dots here, the rule of thumb seems to be to size the MVHR box @ 100% throughput to the Build Regs ACH requirement and post sign-off turn the MVHR down by 50% or more according to personal preference and/or RH/CO2 measurement.

 

Does this sound right? @Jeremy Harris

[Jeremy Harris]

1 minute ago, epsilonGreedy said:

Joining the dots here, the rule of thumb seems to be to size the MVHR box @ 100% throughput to the Build Regs ACH requirement and post sign-off turn the MVHR down by 50% or more according to personal preference and/or RH/CO2 measurement.

 

Does this sound right? @Jeremy Harris

 

 

More or less, although it's handy to have a bit of spare boost capacity, I think.  IIRC our MVHR is currently running at a rate that's about 70% of the background rate given by Part F, which seems to be OK, but there are only two of us in a 130m² house.  The same size house with more occupants may well need a bit more ventilation, and my guess is that the Part F rates may be set for a typical mass house builder type house and occupation density.

[JFDIY]

Think you have to be careful, because the unit may quote it can deliver say 300m3 per hour, but that's at no or low static pressure loss, once you get the ducts in the pressure loss might mean you struggle to get close to it. i.e. if you have long duct runs the furthest, it will need the system to run at a higher static pressure to overcome the frictional losses and mean the total available supply is lower due to the static pressure imparted by throttling down the vents closest to the unit.

 

I over sized by about 25% as it's easier to throttle something back, it will run quieter and for longer by being less stressed.

 

 

[Jeremy Harris]

Just now, JFDIY said:

Think you have to be careful, because the unit may quote it can deliver say 300m3 per hour, but that's at no or low static pressure loss, once you get the ducts in the pressure loss might mean you struggle to get close to it. i.e. if you have long duct runs the furthest, it will need the system to run at a higher static pressure to overcome the frictional losses and mean the total available supply is lower due to the static pressure imparted by throttling down the vents closest to the unit.

 

I over sized by about 25% as it's easier to throttle something back, it will run quieter and for longer by being less stressed.

 

 

 

 

Yes, it's essential to read the pressure/volume flow rate curve data on the MVHR spec sheet to get this right.  Not hard to do, though, as there are a few online calculators for estimating duct flow resistance.

[epsilonGreedy]

Thanks both @Jeremy Harris& @JFDIY

 

Based on feedback the updated rule of thumb might be:

1. Size MVHR unit at 100% throughput to Build Regs ACH.
2. Add 25% for reserve boost and to account for vent throttling.
3. Add another percentage if long duct runs were required.
4. Add another percentage in the case of full family occupancy.

In practice I will delegate MVHR planning to that nice keenly-priced Irish company I met at the NEC and also favoured by many here. I will use this thread to cross check their recommendation.

 

Question: What length of ducting qualifies as a long run? +10m maybe?

[JFDIY]

10 minutes ago, epsilonGreedy said:

Question: What length of ducting qualifies as a long run? +10m maybe?

 

I would say that's about right, I've got some that are getting on for 30m, but I don't expect a lot from them. The system was primarily put in to serve the rooms best that are sited close to the unit. 

I think if I had a pro design it, they'd have used two smaller units, but I wanted to avoid the potential nose in the bedroom where the second unit would have had to go.

 

It really isn't that hard, if you're using radial, double up extract pipes and double supply on long runs or to big rooms.

[gc100]

I've got a length of 25m to cover to the kitchen area which has the highest extract value. Given what everyone is saying here, I think I best aim for 50% , so in my case a unit that can handle 300 to 350 m3/h even though I only need 170 m3/h for regs.

 

Just to clarify a point though? ; I keep seeing in some places there is a minimum requirement for 0.5 ACH for the whole building , but I don't see anything in the part F regs that talks about ACH at all. Is this a Scottish building regs thing perhaps? 

[JFDIY]

There's something about 0.3l/s per M2 of floor area in part F which equates to something like 1.08m3 per m2 of total internal floor area if memory serves as a minimum.

 

Even this is considered to be too high on here, but will depend upon occupancy and use.

 

You should aim to demonstrate this for building control purposes

 

 

[gc100]

14 minutes ago, JFDIY said:

There's something about 0.3l/s per M2 of floor area in part F which equates to something like 1.08m3 per m2 of total internal floor area if memory serves as a minimum.

 

Even this is considered to be too high on here, but will depend upon occupancy and use.

 

You should aim to demonstrate this for building control purposes

 

 

 

Yes that gives me my 169 m3/h, which equates to 0.39 ACH for the whole house. 

[Jeremy Harris]

The volume of our house is about 338m³, and the MVHR (allowing for duct loss) can deliver about 290m³/hour, so at full boost it ventilates at around 0.86 ACH.  The relationship between fan speed and flow rate is non-linear, so at speed 2 (about 30% fan speed) the ventilation rate is about 0.35 ACH, and we use this setting pretty much all the time as the background rate.

[dpmiller]

4 hours ago, epsilonGreedy said:

In practice I will delegate MVHR planning to that nice keenly-priced Irish company I met at the NEC and also favoured by many here.

 

Irish? you trying to start a war??????

[Jeremy Harris]

5 hours ago, epsilonGreedy said:

In practice I will delegate MVHR planning to that nice keenly-priced Irish company I met at the NEC and also favoured by many here.

 

I thought you were completely opposed to the idea of good insulation, airtightness, installing MVHR etc, and considered all this energy-saving stuff to be unecessary?

[epsilonGreedy]

44 minutes ago, dpmiller said:

 

Irish? you trying to start a war??????

 

Well he had Irish accent. A bit of googling leads me to believe I was talking to https://www.bpcventilation.com/contacts in Co Antrim. Is that North of the border? I have only sailed to Kinsale and around the Fastnet Rock before.

[Jeremy Harris]

13 minutes ago, epsilonGreedy said:

 

Well he had Irish accent. A bit of googling leads me to believe I was talking to https://www.bpcventilation.com/contacts in Co Antrim. Is that North of the border? I have only sailed to Kinsale and around the Fastnet Rock before.

 

 

Rather importantly, Co. Antrim is in the United Kingdom, a distinction that carries a fair bit of significance. . .

[epsilonGreedy]

22 minutes ago, Jeremy Harris said:

 

I thought you were completely opposed to the idea of good insulation, airtightness, installing MVHR etc, and considered all this energy-saving stuff to be unnecessary?

 

 

Why would you think that? I am politically opposed to wrecking Western civilization in an irrational response to dubious global warming science but saving money makes sense. I happen to believe a faithful implementation of current house building thermal regs leads to a decent home that is thermally far better than the average UK housing stock. Chasing higher performance introduces new problems that lead to expensive solar shading retrofits, hot bedrooms, heat removal technology and humidity regulation technology.

 

I might get to a low B EPC with a little extra effort above basic regs.

 

I also suspect that experiencing less than 1 degree of temperature variation across 24 hours is bad for human health.  

[iansr]

On 15/04/2020 at 21:37, gc100 said:

Hello all,

 

I've calculated I need  170 m3/h to satisfy part F requirement under normal background ventilation (47 l/s for my 156m2). I'm aiming for 180 m3/h just to make sure under normal conditions regs are covered as this value also covers the extraction rate needed on boost (which is 133 m3/h). ( calcs here ). This should give me an ACH of 0.42 which seems about right from my understanding.

 

Looking at various units they all seem to state their max rate - but how do I size them against my background rate needed?

 

Thanks 

Did you create the spreadsheet with your calcs yourself or is there a template someplace? 

[Conor]

I used this one and tweaked it a bit

 

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14912

 

[iansr]

Thanks for that ?

[CalvinHobbes]

On 16/04/2020 at 12:12, epsilonGreedy said:

Thanks both @Jeremy Harris& @JFDIY

 

Based on feedback the updated rule of thumb might be:

1. Size MVHR unit at 100% throughput to Build Regs ACH.
2. Add 25% for reserve boost and to account for vent throttling.
3. Add another percentage if long duct runs were required.
4. Add another percentage in the case of full family occupancy.

In practice I will delegate MVHR planning to that nice keenly-priced Irish company I met at the NEC and also favoured by many here. I will use this thread to cross check their recommendation.

 

Question: What length of ducting qualifies as a long run? +10m maybe?

Which company? Will do same .

[Iceverge]

I think they're referring to BPC ventilation.