Constant volume fans - a worthy differentiator?

[MJNewton]

I am looking to see what MVHR units could suit our 120m² house and am being drawn towards the Xpelair Natural Air 180. One feature that seems to set it apart from others in this price bracket (~£1k) is the use of constant volume fans. These, if my understanding is correct, are fans with a known relationship between RPM, current draw and flow rate thus enabling closed-loop control of actual flow rate in real time despite varying external factors. Sure enough the controls for trickle, boost and purge rates on this unit are all set in m³/hr rather than RPM or %power.

 

This sounds like a useful feature to have, not only for initial commissioning to make the whole-house supply vs extract rates balanced but to also help ensure the desired performance levels remain constant over time despite gradual filter fouling, varying wind pressures etc? I am assuming that the ability of the unit to keep supply and extract rates balanced may also aid efficiency if (or, rather, given!) my house is likely far from airtight by helping minimise pressure-induced air ingress/egress via routes other than the MVHR?

 

I am wondering what the panel's views are of constant volume fans and whether they are a worthy differentiator between competing products or am I better off remaining focused on other aspects of a unit's specification e.g. efficiency, noise etc? Does anyone have experience of the Xpelair 180 in particular?

Edited February 21, 2018 by MJNewton

[MJNewton]

During further research into constant volume fans I stumbled across this article written by someone at the Passive House Institute on the benefits of automatic long-term balanced operation through the use of them. They do seem a very worthwhile feature so unless I find some other aspect of the Xpelair unit to discount its selection I think I'm going to go with it. There is a caveat in the article about the extra benefits of calibration once in the installed state so I will endeavour to compared the measured total flow rate of each terminal with that measured by the unit to see if any permanent offset adjustment is warranted (and/or indeed just to see how accurate the measurement and control really is).

Edited February 21, 2018 by MJNewton

[SteamyTea]

I have often wondered why they do not have automatic balancing as standard.  Would make installation, commissioning and maintenance dead easy.

[Jeremy Harris]

33 minutes ago, SteamyTea said:

I have often wondered why they do not have automatic balancing as standard.  Would make installation, commissioning and maintenance dead easy.

 

Sadly it doesn't really help much with balancing on the house side, all it does is balance total airflow rates in the unit, which is a good thing once the system has been balanced and set up as it compensates for natural dynamic pressure variations to some extent, like the wind, doors and windows being opened and closed, etc. 

 

There are some self-balancing MVHR units around, but they don't remove the need to check and adjust for every room, and they can make balancing the room flow rates a harder task, as the constantly varying fan speed as any adjustment is made is yet another variable in an iterative process that is already a bit tedious, I think.

[dpmiller]

55 minutes ago, JSHarris said:

 

Sadly it doesn't really help much with balancing on the house side, all it does is balance total airflow rates in the unit, which is a good thing once the system has been balanced and set up as it compensates for natural dynamic pressure variations to some extent, like the wind, doors and windows being opened and closed, etc. 

 

There are some self-balancing MVHR units around, but they don't remove the need to check and adjust for every room, and they can make balancing the room flow rates a harder task, as the constantly varying fan speed as any adjustment is made is yet another variable in an iterative process that is already a bit tedious, I think.

I'd agree with this. You'd really want to be able to lock out the feedback while doing duct balancing.

[SteamyTea]

Could some software not be developed. You set the flow rates for each room/area, have a total overall airflow for the building, then let the unit adjust itself.

Would not be easy, but it would be self correcting though its life.  So may well be better overall.

I am sure that motorised valves and the rest of the hardware need not be that expensive.

More a case of just doing it, rather than thinking up reasons not to do it.

[Jeremy Harris]

3 minutes ago, SteamyTea said:

Could some software not be developed. You set the flow rates for each room/area, have a total overall airflow for the building, then let the unit adjust itself.

Would not be easy, but it would be self correcting though its life.  So may well be better overall.

I am sure that motorised valves and the rest of the hardware need not be that expensive.

More a case of just doing it, rather than thinking up reasons not to do it.

 

There's no real driving need for it though.  Once the system has been balanced, it tends to just stay balanced permanently, with the exception of external effects on the overall balance.  The self-balancing units around get around the latter problem, and adding greater complexity just to avoid the need to do a one-off room balancing exercising doesn't seem worth it to me.  All the motorised air valves would spend their entire life, after the initial setting up event, in the same position.

[MJNewton]

Without closed-loop control I would expect the supply and extract rates to become unbalanced over time by virtue of the filter fouling rate being different, assuming that the fresh air and exhaust filters tend to be removing different types and quantity of particulate matter (eg. pollen and other outdoor air detritus for the former; dust, lint and dead skin for the latter?).

 

Of course, regular filter changes will help mitigate this but flow resistance will inevitably have increased prior to this happening. 

Edited February 22, 2018 by MJNewton

[SteamyTea]

23 minutes ago, JSHarris said:

after the initial setting up event, in the same position.

Except that houses change usage.  Doors are left open, as are windows, extensions are added, kids move out, guests come to stay.  Sometimes a long bath or shower is taken, sometimes just a minor wee.

I am trying to think up a decent driving analogy.  Is there any point in a hatchback, or people carrier with it's adaptability.  Why not just a van with two two seats.  That would do most people.

[Jeremy Harris]

I checked ours a few times after commissioning, over a period of around a year.  Nothing changed, the flow readings for each room were the same as they had been.  Opening and closing internal doors seems to have no measurable effect either, probably because the flow rates are pretty low - ours is set to change all the air in the house in a bit over two hours.

 

I'm not convinced there is a problem to solve here at all, with the possible exception that auto-balancing the net inlet and exhaust flow rate to account for the wind (far and away the biggest cause of variation, I found) might be very slightly advantageous.  I'd personally stick with the simplest arrangement possible, as I can't see there being any significant, perhaps not even any measurable, advantage in having anything more complex.

[richi]

2 hours ago, JSHarris said:

Once the system has been balanced, it tends to just stay balanced permanently.

Unless someone else fiddles with it without your knowledge ;-)

[Jeremy Harris]

43 minutes ago, richi said:

Unless someone else fiddles with it without your knowledge ;-)

 

Hard to fiddle with restrictor rings fitted inside the distribution plenum chambers though!

[SteamyTea]

I am not so sure that there is no problem.

It is possibly true that in a very well built and managed building variations can be managed effectively.

But in a chaotic building i.e. 4 kids and a wife, or maybe a scrapyard in the back garden automatic control could be advantageous.

To give you an idea yesterday was sunny.  I opened my back windows a crack to draw in some warm air from the front.  This works well and saves heating the north east facing kitchen where I live.

I forgot that I had done this as the wind was not too strong yesterday.  Got home from work, turned the heating on as it was chilly.  Had a shower, cup of tea and went to bed.

Because my fan heater had tuned off at 21°C, I forgot to turn it off at the mains.

This morning I had a lovely warm house.  Only cost me 7 kWh.

Now if I had MVHR with automatic balancing and control, it would have known that I had the windows open a crack as it would probably not have managed to balance itself properly and issued a warning, or just shut down the area totally.

It is hard to quantify without creating a decent model, but at £1/day for that sort of mistake, it is not out of the question financially.

 

I also think that MVHR could be split into separate units.  One for the clean side i.e. bedrooms, livingrooms, halls and passageways, and another for the smelly side i.e. bathrooms, toilets and kitchens/utility rooms.

Just like space heating and DHW should be split, I think there are gains to be had doing the same with MVHR as you can pick units to suit the different characteristics.

 

[le-cerveau]

5 minutes ago, SteamyTea said:

I also think that MVHR could be split into separate units.  One for the clean side i.e. bedrooms, livingrooms, halls and passageways, and another for the smelly side i.e. bathrooms, toilets and kitchens/utility rooms.

You have basically split the extracts from the supplies!

If you have separate systems you need both in both, otherwise there will be no heat exchange!

The whole point of MVHR is to extract the smelly stuff (moist usually) and import the clean stuff (going through a heat exchanger so no smell crossover).

 

I have 2 systems buth both extract from smelly and import to clean.  One does the kitchen, laundry, WC and 1 x ensuite, the other Bathroom, 3 x en-suites and Airing Cupboard.

 

Also to get my systems balanced (in=out) I had a 27% difference (decrease for input) on one and 11% difference (decrease for input) for the other, this was due to static pressures in the system.  Without AI a suspect an auto balance system would assume differences from setup are due to filter blockages/wind differences/.... otherwise you would get constant alarms for nothing.

[Jeremy Harris]

13 minutes ago, SteamyTea said:

I also think that MVHR could be split into separate units.  One for the clean side i.e. bedrooms, livingrooms, halls and passageways, and another for the smelly side i.e. bathrooms, toilets and kitchens/utility rooms.

Just like space heating and DHW should be split, I think there are gains to be had doing the same with MVHR as you can pick units to suit the different characteristics.

 

 

It is already a split like this, there is complete isolation between the extract from bathrooms, toilets, kitchen and utility rooms  and the fresh air side that feeds all the other rooms.

[SteamyTea]

I shall get back onto this later as I am a bit tied up at moment.

I may not have explained myself very well when I said

24 minutes ago, SteamyTea said:

I also think that MVHR could be split into separate units.