MVHR how much does boost actually increase flow rates?

[happyfolk]

Hi everyone,

 

I was wondering if someone (maybe someone keen with an anemometer) has measured what difference boost mode makes to supply and extract with their MVHR.

 

I have a bog standard Vent Axia that I run on either 50% or boost which is 75%

according to the initial commissioning boost can 2x the air supply. Seems a bit punchy!  I generally have the windows open and I'm not too confident this system is working.

 

Was wondering if these numbers make sense?

 

thanks everyone!

 

[Mr Punter]

Often the standard flow rates are far lower.  You may find that 30% is plenty.

[JohnMo]

Boost should be about 25% more than normal flow.

 

Double the flow rate in the bedrooms is excessive. Doesn't look that well commissioned. Did the flow rates actually get setup with a flow meter?

[happyfolk]

thanks !! Interesting that boost should be about 25% more airflow. Not 100% more like on the cert 

You would assume the flow rates were setup properly at commissioning but you never really know.

 

I have tried to get it commissioned again but not a single company I contacted wants to work with this design.  The inflow/extract points are not just mushroom style but ski-sloped (to make it look pretty i guess). Even the company recommended by Vent Axia do not work with this style.  There is exactly one mushroom style vent but that's in the utility cupboard, wonder if it's worth measuring that as a baseline for the rest of the setup.

 

For the rest (see pic): I guess you would block the whole opening to take a reading.  Any ideas how you would go about measuring these flow rates?

 

Feels a waste having to have the windows open 

 

many thanks!

 

[Blooda]

Initial thoughts....  I cannot understand how the living room continuous to boost only goes up 8% yet the bed rooms 100%. [well I can....] 

Can you show the extract figures as well. ? 

 

Is the outlet in here ?

 

When you say you need to open the windows ? why ? 

 

 

 

[ProDave]

I can't find the figures form mine, but the actual continuous normal rate I have set a little lower that what BR may say to ensure it is totally silent, and practice suggests it is giving perfectly good ventilation.  I have the boost set as fast as it will go and it is noisy.  The boost is many times the trickle rate but I can't find my figures to say exactly what it is.  Why would you not want the boost as fast as it possibly can go?  *

 

*  Well that is true to clear moist air during and after a shower where we run it at full speed boost.  But we also have a mid speed option used when cooking to give an in between rate to improve kitchen ventilation while not sounding like a jet engine.  That is often running for longer than "bathroom boost" and we don't want it that noisy.

[SteamyTea]

What are you measuring mass flow rate against?

A dial position, a linear scale, a numerical display, or the power to run the fan?

I think the power is the cube root of the mass flow rate.

[happyfolk]

@ProDave sounds like you're setup is doing you well.  Does it ventilate entirely, or do you still open windows? Sounds like your full boost is called "purge" mode on my system which runs 100% for a set time, is indeed a bit noisy.   

@Blooda yes your red arrow; the outlet is in there with the AC grills, all lumped together, can't use a cone style anemometer to measure.  I open the windows for ventilation sadly.  Here's the extract and inflow figures.  Regular continuous was at 50% and boost 75%.  I guess it passed the BR on boost 75% for the extract 

Do you think these figures make sense?

thanks everyone!

  

[ProDave]

5 minutes ago, happyfolk said:

@ProDave sounds like you're setup is doing you well.  Does it ventilate entirely, or do you still open windows? Sounds like your full boost is called "purge" mode on my system which runs 100% for a set time, is indeed a bit noisy

Yes, in winter we keep all the windows shut and there are never problems with stale air or condensation.

 

In the summer we often keep windows open so we can hear the wildlife and the trickle of the water in the burn.

[JohnMo]

If you "need" to open windows to ventilate with MVHR there is something not right.

 

My boost is 25% over the trickle vent rate, the MVHR has humidity sensors and very rare for it to auto boost. I will occasionally boost if doing a fry up or feel it needs after a long shower.

 

To measure flow across those grilles you need to take velocity readings, say 10 across the grille and the average to numbers, the use the cross sectional area to get to flow rate. Or make a cardboard filler piece with a hole in center to close of the opening in the wall. Then you can use a normal flow meter to get the flow rate.

 

Was that level of detail followed?

 

With the grille how do you change the flow rate, do you have orifice plates at the manifold end?

[Blooda]

I am going to hazard a guess here that this has not been commissioned correctly.  The figures are too good/.  And the living area increase vs the bed rooms increase gives it away.  

 

What model of Vent Axia is it ?

Do you have a control box ?

Can you get to the MVHR easily ?

 

 

[happyfolk]

@Blooda Really, you think it wasn't commissioned correctly, that was my fear. I thought such things were happening 10 years ago, not now! What makes you think the figures are too good?

Unit: Vent Axia 443319 Sentinel Kinetic

Yes I can access the unit for maintenance. But not the ducting 

 

@JohnMo Thanks very much for the detail on how to take the velocity.  The only information I have is the equipment used was a Testo 417 Anemometer with 100mm Vane. The method is not detailed.

I will ask the person responsible for the ductwork if there are orifice plates to control the flow rates.

Last time I spoke with them, they wanted to charge a full days rate to measure the extract and inflows, because of the type of apertures! But then I heard nothing back from them  

 

[dpmiller]

2 hours ago, happyfolk said:

What makes you think the figures are too good?

supply and extract being *exactly* the same on boost certainly doesn't pass my sniff test. Airflow is a pain in the a*se to measure and set up...

[Marvin]

This was the requirement from the manufacturers for our home:

Normal rate 30 Boost 37 litres per second.

 

 

[Renegade105]

@happyfolk Hi, ok so the 417 with 100mm vane and a hood is NOT the best at getting reliable readings, far too much error. You need a powered flow hood. Might look the same but totally different. You can hire it if you wish (BSRIA) to do the balancing yourself.

Finally, I can’t see where the supply vent is…is it a linear vent which has been plastered in that recess in the ceiling? Can you get access to the cutwork, perhaps there is a mass flow damper upstream of the vent that you could take flow readings with using a manometer.

 

Good luck

 

 

[dpmiller]

^ yep. The error on the smaller vane units is massive, even before you put a hood on them. Back when I was doing this daily, only Airflow Developments EDRA/ RVT vanes would cut the mustard and nothing hot-wire under £2k was worth looking at.

Until... Testo brought out the 405.