Balancing MVHR system - how to?

[readiescards]

So finally everything is in place:

* mvhr unit up and running

* terminals and valves installed

* house pretty much as air tight as it will ever be

* Anemometer Tesla 405 borrowed with extra large valve covers - big thanks @JSHarris

 

So to work on balancing. Any tips on how best to approach this?

* Screw everything shut and open on longest extract one first get its flow rate right and then open another set it and go back to first readjust?

* Set all valves at middleish point and randomly run around house?

* Concentrate on extract first? Concentrate on fresh air first? pick and mix?

 

Any other tips?

* Place chairs under every valve before starting?

* Get 5 tall mates from the pub?

* wack the unit up to max and slowly decrease?

* count the number of turns needed to add/remove 1litre/sec?

 

Bit concerned this could take all day and night if I just run around the house randomly with the meter!

 

Thanks in advance, Paul

 

 

 

 

[Jeremy Harris]

I would start will all ducts/valves about half way open.  There is a lot of interaction when adjusting rates - as you close one valve to reduce the rate the flow rate at all other terminals on that run will start to increase.

 

This makes the process a bit of a pain, as you end up adjusting every room several times.

 

I started with the kitchen, as that had the highest flow rate requirement, and so would tend to cause the least upset to other rooms.  I also did all the extract rooms first, as building regs only really stipulates extract rates for individual rooms.  The individual room extract rates in the regs are actually the maximum figures the system needs to provide, not the background figures, so turn the MVHR up to boost for these initial room settings.

 

Once you have at least 13l/s from the kitchen, next move to the bathrooms and utility room, and adjust those to get at least 8l/s.  Then do the WCs, and adjust for 6l/s.  After doing this, go around again and check that you still have at least these rates from each of the extracts -  some will have changed because of the adjustments throwing off the main plenum pressure.  You will probably have too high a flow rate everywhere, so I would advice opening up the valve to increase the flow rates whilst keeping them in proportion with the regs requirement for each designated extract room (kitchen, bathrooms, utility room, WCs).  If you have fitted the toilet smell extractor system I came up with, then just ignore the flow from that - it isn't officially part of the ventilation requirement.

 

Once you have the system extract rates looking in proportion at boost (and bear in mind that they will probably be way over the requirement at this stage) then start measuring the fresh air inlets, also on boost (doing it this way will highlight the main duct loss differences). Adjust these to achieve two objectives.  You want the sum of the fresh air flow to be as close as possible to the sum of the extract air flow, plus you want to adjust the fresh air flow so that the most frequently used rooms have the higher flow rates.  There is nothing in the regs about fresh air flow rates, so use your own judgement here.  For example, you will want the largest and most often used rooms to have the highest flow rates, the smallest and least often used rooms to have the lowest flow rates.  The main thing is to adjust the fresh air total flow to match, as closely as possible to the extract total flow.

 

After doing this, go around again and check all the extract flow rates.  The chances are they may have changed, and if so then adjust them to make sure they all exceed the building regs limit and match the fresh air feeds.  Then go back and double check the fresh air feeds, these may need slight adjustment to get the system balanced at boost.  The chances are that you will find at this stage that the system is pretty close to being balanced.

 

Next, reduce the MVHR to the background ventilation rate setting (this is specified as speed 2 on our system, but see my note later).  Go around as before and measure the flow rate on all the extract terminals and note the total extract flow rate (the sum of the flow rates from all the terminals).  Do the same for the fresh air feed terminals.  The total figures for each should match.  If they don't, then adjust only the fresh air feed flow rates to get them to match, do not adjust the extract rates. Note the total extract flow rate with the MVHR system on the background ventilation setting and make sure that it exceeds the continuous ventilation rate requirement in Part F, which is calculated from the house total floor area (the total must exceed 0.3l/s per m² of net internal floor area for the whole house).  With luck you should find that the ventilation rate exceeds this figure with ease.  If it doesn't, then go back to the start, turn the MVHR to boost and go around opening up all the extract ducts, starting with the kitchen, (this is why I said at the start it was a good idea to open up all the extract ducts to maintain the ratio and exceed the building regs extract figure), then go though the process again.

 

Once you have the system so that it meets the building regs Part F Table 5.1a max extract rates from the specified rooms, and is in balance at the background ventilation rate of 0.3l/s/m² of floor area, the job is done.  However, you may well find that the background ventilation rate is too high in practice, and that as a consequence the MVHR is using more power than it needs to in order to keep the air in the house fresh.  Our experience is that turning down the background ventilation rate has had no detrimental impact at all, so I now run ours at speed 1, with the fan speeds around 25% of full speed.  It's worth noting that some MVHR units have the option of being able to individually adjust the extract and fresh air fan speeds independently in the set up menu.  This is useful for fine tuning, but is detrimental to efficiency if used as a shortcut way to try and balance the system.  I have our system set to 28% extract and 25% fresh air fan speeds, to fine tune it, but would suggest that anything more than a 5% difference really needs to be adjusted out in the ducting/terminals if possible.

Edited September 2, 2017 by JSHarris

[ragg987]

Nice writeup.

 

I have a timer on ours and humidity sensor in bathrooms.

 

At night it slows to approx 0.2 ACH. At daytime we have it at 0.25 to 0.3 ACH.

 

The humidity sensor will boost to up to 0.6 ACH, which is close to regs for our house.

 

So far so good, not sure if those rates will be OK in the cold when condensation becomes a bigger factor, so awaiting winter to assess this.

[readiescards]

So I'm in the middle of doing my balancing having waited for the chippy to finish the final air-tightness and have just realized my log burner is not yet fitted so I have a gapping 8" hole to the outside at the moment.  I'll bung it up and start again but how much do I need to be concerned about doing the balancing with the final air-tightness?  I don't want to hold up the next person in the BuildHub Loan Tool queue!

[Jeremy Harris]

I assume that you have two big holes to outside for the woodburner, not just one?

 

If you don't have a balanced and room sealed air feed to the woodburner then the MVHR will be running massively out of balance, especially when the woodburner is actually lit.  When running, with no sealed external air feed, the woodburner will extract far more air than the MVHR in all probability.

 

So, if you don't have an external air feed, for the woodburner I'd definitely say you need to fit one.  You also definitely need to temporarily seal up the flue when balancing the MVHR.

 

Once you've fitted the woodburner, with a balanced external air feed, and good seals around the air feed, the woodburner doors and the flue, then the MVHR balance shouldn;t be affected as long as the woodburner doors are closed and sealed.

Edited September 10, 2017 by JSHarris

[ProDave]

Here is my thoughts.

 

All terminal vents want to be as wide open as possible, the theory being anything that restricts flow, could create noise.

 

Although I have not done mine yet, that will be my starting point, all vents wide open, with perhaps those feeding the small bedroom closed down a bit to start with, and perhaps the second biggest bedroom closed down just a little because it has a very short run.

 

A question that will affect balance no doubt. Do you balance the system with all doors closed, relying on the air gap under the doors, or do you balance it with all doors open?

[readiescards]

29 minutes ago, JSHarris said:

I assume that you have two big holes to outside for the woodburner, not just one?

 

If you don't have a balanced and room sealed air feed to the woodburner

 

Apologies, yes I have two big holes (just the air feed one is already temporarily blocked) and the log burner is a full air tight model (or claims to be) when it's doors are closed.

 

8 minutes ago, ProDave said:

Do you balance the system with all doors closed, relying on the air gap under the doors, or do you balance it with all doors open?

Ditto I also wondered this particularly on the rooms with extracts. Or is it best to simple do the balancing with the doors in their most typical position? Or is it not important as the door air gaps are larger than the duct c/s area?

 

 

Edited September 10, 2017 by readiescards
Grammar

[readiescards]

Sorry another MVHR balancing question!

 

It is blowing a gale outside, lovely and quiet inside but that means the MVHR external vents are subject to the blustering winds - must I wait for the winds to drop for balancing?  The vents are on the same wall, 3 metres apart at the same height protected by metal cowls.

[Jeremy Harris]

Best to start with all vents/ducts about half way open initially, if you can, and see what the readings look like.  It's rare that you will want any wide open, and starting from the mid point tends to make things a bit faster to adjust.  If you have a system with restrictor rings in the plenum chambers, then you have to start with them all open, and it's one of the things that makes adjusting a system like this a bit of a pain.  The flip side is that any terminal noise is lower, as the restrictions are well away from the rooms.

 

In theory, all the doors should be shut, but I found that it didn't make a noticeable difference, and it was a lot easier to just leave them all open as you're running around from room to room!

[Jeremy Harris]

1 minute ago, readiescards said:

Sorry another MVHR balancing question!

 

It is blowing a gale outside, lovely and quiet inside but that means the MVHR external vents are subject to the blustering winds - must I wait for the winds to drop for balancing?  The vents are on the same wall, 3 metres apart at the same height protected by metal cowls.

 

 

I had this problem, and found that it was difficult to get steady readings with the wind gusting outside.  I ended up averaging several readings to try and compensate for the variations due to wind gusts.  If you can do this job on a still day it's quicker and easier.

[Bitpipe]

Was just about to start my second round of testing and stumbled on this thread - thanks @JSHarris as ever

 

One question, we fitted two extracts to our kitchen, one equidistant between the hob and ovens (which are opposite each other) and one in the corner - this was advised by BPC so I went with it.

 

As it is, both are giving me just over 14l/s at boost (50% fan speed).

 

 

 

Edited September 12, 2017 by Bitpipe

[Jeremy Harris]

It's the total extract from the room at boost that needs to be met for Part F, table 5.1a, so as long as the sum of both extracts exceeds the minimum (13 l/s for a kitchen) then you're fine.

 

In practice, I've found that turning the trickle ventilation rate down to well below the whole house rate in the regs (the 0.3l/s/m² rule) seems fine, as long as you can boost the system up when needed to clear steamy bathrooms, cooking smells etc. 

[Bitpipe]

57 minutes ago, JSHarris said:

In practice, I've found that turning the trickle ventilation rate down to well below the whole house rate in the regs (the 0.3l/s/m² rule) seems fine, as long as you can boost the system up when needed to clear steamy bathrooms, cooking smells etc. 

 

Meeting the whole house rate is my challenge - with a relatively large internal floor area of 343m2, I needed to crank the supply fan to 40% today which feels very high. Once I get the correct measurements, I'm going to reduce it back to 30% in a few weeks to see if theres a noticeable difference in air quality.

 

Question - for rooms in roof, can the effective floor area be reduced due to the reduced room volume? This would help the numbers somewhat.

 

As luck would have it, the only room that ever feels stuffy is our study where we both work (and have laptops, printers etc). I'm getting the lowest readings from this vent - it is on the opposite side to the house to the MVHR but on the GF so not the longest run but must have the most bends in the run compared to others.

 

On our distribution box, 10 ducts terminate at the front side and four at the rear, these rear ones introduce an additional sharpish bend to the duct. I didn't pay too much attention as to which duct went where but now wising that I'd checked to put the shorter runs to the rear where the additional bend would not make much difference. Something I could possibly change in the future if needs be.
 

Next challenge is balancing the extract which initially was about 20% less than supply at normal speed.

 

Just to ensure that my maths are correct, I'm using a vane type anemometer and the cone I've constructed completely covers the measuring head. Do I use the internal area of the measuring tool (dia is 57mm) or the internal area of the cone where it meets the tool (dia about 65mm)?

[Jeremy Harris]

The area used should be the swept area of the vane, when the vane type anemometer is fitted tight to the cone, as that's the area the air is really flowing through.

 

The whole house ventilation rate requirement in part F can be a struggle for a larger house, and, iMHO, is way OTT - I run our system at around half that rate and it's fine, far better air quality than the old house.  In terms of area, then you may be able to argue that it's the actual room external wall/ceiling areas, and exclude the unventilated spaces, like the semi-sealed eaves areas in room in roof houses.  It's probably not strictly OK, but my experience was that building control weren't too fussed about detail.

[Alex C]

Reading this with interest as it is also my next job. You use cross section at sensor end, this is why most ones you can buy have 100mm dia head and then taper out to make the maths easy.

[jack]

12 hours ago, JSHarris said:

The whole house ventilation rate requirement in part F can be a struggle for a larger house, and, iMHO, is way OTT - I run our system at around half that rate and it's fine, far better air quality than the old house.  

 

Yep. We run our unit at the absolute minimum setting when it's just the family in the house. That's about 60% of the mandated rate, and it's perfectly fine. 

[Jeremy Harris]

12 hours ago, Alex C said:

Reading this with interest as it is also my next job. You use cross section at sensor end, this is why most ones you can buy have 100mm dia head and then taper out to make the maths easy.

 

 

It's one reason I opted to use 100mm bore duct as the flow straightener and measurement section for the unit I made up, with adapters at the end that fits to the terminal of increased diameter.  Because a hot wire anemometer doesn't interfere with the flow very much, I calculated the cross sectional area at the measurement point, assuming that the tip of the hot wire probe was in the centre of the measurement duct.  In practice, the small reduction in cross sectional area caused by the probe is pretty small, but I felt that if I was trying to make accurate measurements I should allow for it, as it clearly does reduce the cross section at that point, and so, from Bernoulli, it must increase the local velocity at that point for any given flow rate.

[BrianB]

Hi Many thanks for the many positive and helpful responses. I should also have mentioned that our house is a 2 storey, with an independent MVHR for each level. This was done due to economics and space saving in not having to run multiple ducting for the whole house from a single point.

I was thinking of treating each MVHR system individually, and setting each one up, whilst the other MVHR unit was switched off. Then when both units were individually balanced, switch them both on together and hey presto......they should both be balanced? Any thoughts on this method?

[dpmiller]

the "off" unit would need it's ducts sealing off too...