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Problems balancing system


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Hi all,

 

I've been trying to balance the system today, and am struggling.

 

House is approx. 95m2, on 1 level only.  5 supply, 3 extract valves.  MHRV unit is a Vent Axia Sentinel Kinetic BH.

 

Supply (1 75mm pipe to each plenum)

I can get this to balance at 60% boost easily.

 

Extract (2x 75mm supply pipes to kitchen and bathroom, 1 to Utility.  Pipe runs are all very short, no more than 4m) 

Won't balance.  When I get the system to 65% boost I can get the total required airflow, but I cannot reduce the bathroom and utility plenums to their required values, and cannot increase the kitchen to it's required value.  At 100%, there is plenty of flow everywhere.

 

Do you think I need some sort of restrictor inside the plenum to reduce the output?  I am using these plenums: https://www.bpcventilation.com/90-plenum-round-pipe-airflex

 

Also, for a house of this size, should I need boost levels so high?  The unit is spec'd to be good for up to 150m2 of floor area.

 

Feeling a little deflated, as all if the installation wen't very well generally.  I have also ensured there are no air leaks and foil taped up every join on the 5" side.

 

Any tips or guidance much appreciated as ever.

 

Tim

 

 

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I assume everyone has gone outside to enjoy the weather, but hopeful that one of the resident experts will be about again soon.

 

I'll try BPC again tomorrow, but whenever I've called the person who deals with these things isn't there ? 

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  • 3 weeks later...

Still not had much luck with this.... I don't feel the unit should need to run at 80% on boost for a house the size of mine.

 

BPC looked at the rates they provided, and said they were wrong and re-did them, requiring a slightly higher overall rate!  They also said I needed to set the Cooker Hood ''Normal' and 'Boost@ settings to 100%, even without a hood fitted - maybe a useful note for others with a Vent Axia unit.

 

Hopefully at the weekend, I will try and balance it up at 90%, or 80% of BR requirements and see how I go.  Reality seems to be nobody wants to run their systems at BR rates anyway!

 

Tim

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Things are looking a bit more positive now!

 

I can get the system roughly balanced, with the system at 60% boost extract, and at 90% of BPC's recommended flow rates.

 

I've asked BPC to supply me with a proper restrictor for the utility plenum, as this is the one I was struggling to get to a low enough rate, to get the others to a high enough rate.

 

Tim

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  • 1 month later...

Finally got the parts, and a bit more time on this today.  I've pretty much got it balanced - it's definitely a good work out, and a little frustrating as every adjustment changes all remaining valves.

 

I'm just a little unsure as to why it's balanced with the supply at 50%, and the extract at 70%?!  This seems like a massive difference, and I'm not aware of any leaks.  Does anyone have any thoughts as to why this may be?  I wonder if having 2 of 3 extract plenums with 2 connections has some sort of influence?

 

Would like to get this project marked as complete now!

 

Thanks

Tim

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51 minutes ago, phatboy said:

Finally got the parts, and a bit more time on this today.  I've pretty much got it balanced - it's definitely a good work out, and a little frustrating as every adjustment changes all remaining valves.

 

I'm just a little unsure as to why it's balanced with the supply at 50%, and the extract at 70%?!  This seems like a massive difference, and I'm not aware of any leaks.  Does anyone have any thoughts as to why this may be?  I wonder if having 2 of 3 extract plenums with 2 connections has some sort of influence?

 

Would like to get this project marked as complete now!

 

Thanks

Tim

 

 

It'll just be down to the difference in the restriction between the two sides.  I think ours has about a 9% correction via the extract and supply fan speed adjustments.

 

To minimise this difference the duct system really needs to be designed so that the total air flow resistance of each side is exactly equal, but this is pretty difficult to do in practice, as it's unlikely that duct lengths, number of bends etc will be that well matched, plus there will normally be more supply terminals than extract terminals, so the extract side often needs to work a bit harder to provide balanced airflow on both sides.

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