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Ventilation Rates Issue


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I've finally got round to balancing the MVHR and am more than a bit worried by the figures I'm getting.

 

I've followed @JSHarris excellent write up (and spreadsheet) and am using a Testo 405i in a 1m length of 100mm diameter straight duct with a cone to go over the inlets/extracts. The system is a Vent Axia Kinetic High Flow (max flow rate 666m3/h) and the house has a floor area of about 305m2 with high ceilings upstairs - I'm guessing at a total volume of about 1000m3.

 

Buildings Regs is looking for 92l/s (trickle rate) but on Boost (50%), I'm getting just 62.5l/s extract (about 225m3/h) and 57l/s inlet (205m3/h) with all vents open between 50% and 75%. Opening the vents more changed the balance but didn't appreciably change the total flow rate. I've attached the work-in-progress spreadsheet for reference

 

mvhr.thumb.jpg.2353d495b1cde194f2d73b4f8600c492.jpg

 

Now, I do need to replace the filters and I'm sure that doing so will improve matters, but surely not by the amount required!

 

Am I doing something wrong? I know I can fudge the figures but at this stage that's not something I'm looking to do. Starting to panic a little now if I'm honest... :(

 

 

 

 

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The results look odd, as on boost I would have expected the MVHR to be able to run pretty close to it's rated maximum, probably at least 550 to 600m³/hr, allowing for some duct losses. 

 

Either the measurements are out, or something seems to be restricting the flow a great deal.  The sums seem correct in the spreadsheet, in that the velocities do equate to the right flow rates, but the boost rates seem low to me.

 

Our MVHR has a lower maximum capacity,  about 470m³/hr, but the flow rates I measured were a lot higher than yours.  For example, our kitchen extract (which has a double duct run to get the required high flow rate) gave a velocity of 4.04m/s on full boost, which equates to a flow rate of 31.73l/s, way over the 13l/s the regs require.  Our system gives a total ventilation rate in normal background ventilation mode of 45.79l/s and a total ventilation rate in boost mode of 115.3l/s.  In boost mode it's running at a total flow rate of 415 m³/hr. so is not meeting its max rating, but that's down to the flow resistance of the ducts -  I'm pretty sure they specify the maximum flow rate for a fairly low flow resistance system.

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Hmmm... something's not right, as you say.

 

Before I get to butchering up my equipment (!), I'm using a reducing cone into a 1m length of 100mm duct - the anemometer is probably about 90cm from the vent. Could this length of duct potentially be causing a problem?

 

Also, I've just re-tested as follows:

  • all vents fully open.
    • Extract rate = 80.6 l/s
    • Inlet rate = 59.9 l/s
  • ran the system without any filters to see if that might have an effect. I only measured the extract side as I ran out of time
    • Extract rate = 97.3 l/s

So it's looking like the dirty filters are having an effect although replacing them won't solve the problem.

 

Would you expect to see a proportional increase in flow rate with boost speed? Just thinking that 59.9 l/s is roughly 290 m3/hr (at 50% boost), which is about half the 666 m3/hr quoted, allowing for losses.

 

One final thing, I suppose it could be the anemometer itself although I'm not sure how I'd go about getting it tested

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10 hours ago, PeterW said:

Can you disconnect the main duct from the unit and check what’s being put out at the unit itself ..??

Not that easily no (it's all taped, sealed etc) , although I suppose I could just cut a slit in the flexi duct and then tape it up afterwards. 

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Bit of an update...

 

I spoke with BPC toadying and they've suggested a slightly different approach which I'll certainly try. One of the things they said was that having all the vents open could actually decrease the overall flow due to less back-pressure. I was quite surprised at that but every day is a school day, as they say! Also, I had quite a few windows open (as it was very warm here) and apparently that does have quite a large effect, so will need to address that next time. It'll probably be the weekend before I can get on to it again and I'll give the new approach a go before I try anything more 'serious'!

 

 

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Having windows open is likely to have a pretty big effect, especially if there is any breeze blowing.  Sadly the "having all the vents open causing a loss of back pressure which reduces flow rate" is pure and utter BS, and I'd have expected a lot better of an apparently well respected company. 

 

All you need to do is look at the fan curves that are published in the data for any MVHR to see that this is patently untrue - I suggest they read the manuals for the products they sell, familiarise themselves with the relationship between fan RPM, flow rate and pressure and then re-think the advice they give to customers in future...

 

By way of an example, here is the pressure vs flow rate fan curve for the 50% lower capacity version of our Genvex, which has a pretty typical backward-blade centrifugal fans fitted to either side:

 

1321957657_GenvexPremium1fancurve.jpg.3645ff361331286590bbce173fd64904.jpg

 

As you can see, as back pressure increases, flow rate decreases, and this is a common characteristic for every type of fan or propeller. 

 

 

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From all I've heard they are a pretty good company, but saying that any fan-driven system needs back pressure to improve flow rate is a pretty fundamental misunderstanding.  There is an issue the other way around, when back pressure gets too high, but that's caused by fan blade stall, and causes the fan RPM to increase, the fan power to decrease and the flow rate to drop to a very low level.  Maybe they got confused.

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Could there be a control system that attempts to anticipate when the windows are open (exceptionally low back pressure) and reduces fan speed? I have seen a heating systems that tries and identify when a window is open and reduces the heating load to save energy. Just a guess.

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Well, I'm going to start on this again now - wish me luck!

 

Does anyone have a template of a commissioning certificate they'd care to share? Just want to ensure I capture what is necessary and no more! Going for sign-off next week and I'm sure they're going to pick up on our stairs - don't want it to be the MVHR too!!!

 

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Ok, so commissioning re-done - better than before but still not great. The figures below are for extract boost at 50% and inlet boost at 60%.

 

The snug is showing a very low flow rate despite the vent being well open and being the closest to the MVHR, so I'm guessing there is a problem with the ducting there. Unfortunately it's too late to do anything about that now :(

 

The house is about 305m2 so we need to get 92 l/s as the background rate. I didn't try it, but I think I'd only be anywhere near that figure at around 70% inlet boost!!!

 

Think I'm a bit stuck now in terms of BC tbh although in reality the house is fine as we have no condensation issues, bathrooms clear easily and the house doesn't feel stuffy. 

 

mvhr-comm.thumb.jpg.396d85aa3c7dc7c1a40b696789d10d2e.jpg

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TBH, the building regs background ventilation rate is, IMHO, way too high.  I run ours a lot lower and we find that the air quality is very good indeed.  I have a suspicion that they set the 0.3 l/s per m2 figure following some advice with regard to venting volatile organics from the house furnishings, and I think someone made an error somewhere.

 

I'd try to get the intake/exhaust rates a bit closer, just for better heat exchanger efficiency, then fudge the data to satisfy building control.  I strongly suspect that you wouldn't be the first to do this...

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Thanks for the reply Jeremy. Yes, looking at it I do think I need to get those figures closer. What would be the best approach for this - increase the inlet speed relative to the exhaust or adjust the vents?

 

Sorry to be cheeky, but do you have a template for your cert that you could share? I'm assuming you self-certified?

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I did self-certify, this is the report I submitted to building control, who happily accepted it, even though it doesn't follow the guidance (I only found the guidance after I'd sent them this) Mechanical Ventilation and Heat Recovery system Test Report.pdf

 

I'd increase the inlet vents, and if this doesn't do it, increase the inlet fan speed, but try the vents first, as that's the lowest energy loss option.

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  • 2 years later...

@worldwidewebs

Hi there, can you offer any advice?

I have the same Vent Axia from BPC in a similar size property.

About 270sqm and need to achieve 80l/s flow rate for the size of house.

Extract is achievable but even on 100% boost struggling to meet the supply targets especially in the sitting room (20l/s)

Did you find a solution?

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14 hours ago, JamesP said:

@worldwidewebs

Hi there, can you offer any advice?

I have the same Vent Axia from BPC in a similar size property.

About 270sqm and need to achieve 80l/s flow rate for the size of house.

Extract is achievable but even on 100% boost struggling to meet the supply targets especially in the sitting room (20l/s)

Did you find a solution?

 

That 80L/s flow rate target is surely for the whole house though, not individual rooms?!

Edited by MJNewton
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2 hours ago, MJNewton said:

 

That 80L/s flow rate target is surely for the whole house though, not individual rooms?!

Yes the 80l/s is for the whole house.  The 20 l/s for the sitting room seems unachievable even running at 100%.

The Vent Axia Kinetic Plus has a maximum of 500m3/h.

Do I add both the supply and extract m3/h flow rate as this would exceed the capability of the MVHR.

 

Airflow Rates.jpg

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9 minutes ago, JamesP said:

Yes the 80l/s is for the whole house.  The 20 l/s for the sitting room seems unachievable even running at 100%.

 

Okay, understood. When you said '20 L/s' I thought you meant that's what you were getting and you were trying to get it up to 80 L/s (for that one room alone). So what are you getting?

 

Quote

Do I add both the supply and extract m3/h flow rate as this would exceed the capability of the MVHR.

 

No, it's the balanced volume *throughput* that's of interest i.e. your 80 L/s target is 80 L/s supply and 80 L/s extract.

 

P.S. Try and stick to one set of units; it gets really confusing swapping between the two!

Edited by MJNewton
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@MJNewton P.S. Try and stick to one set of units; it gets really confusing swapping between the two!

 

Will do. Found it all a bit confusing.

Target is 80 L/s for each.

At 100% boost I have :

Extract 93.30 L/s

Supply 76.65 L/s

So it is possible with reducing the extract but might miss the 20 L/s for the sitting room.

Its very close. I think the Vent Axia is near its limitations.

 

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I'm wondering if it's the relatively high number of wet rooms that is pushing the overall extract requirement to be quite high, and this in turn is forcing the supply rooms to be higher than otherwise needed to ensure an overall balance is maintained. Taking the specific 20 L/s for the sitting room in insolation that's quite a high flow. I suppose it is the room that would likely see the most number of simultaneous occupants but even so; try not to focus on what the figures say as the real world is a lot more forgiving and has to be given the imperfections of the system as a whole.

Edited by MJNewton
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