MVHR Duct Design

[TerryE]

We did a lot of trades ourselves in our selfbuild.  Doing the MVHR was one of the simplest, I feel.  I did the basic design myself and sent the stuff to Gary at BPC, who confirmed that the design was fine and he gave us a complete parts list that we would need; we then ordered everything from him: BPC get another recommendation from us.

 

Semi-rigid 75mm pipe work is easy to deploy in Posi-joists so long as there are two of you working together, and yes you will need to double up some runs such as the kitchen and any longer runs.  We've had the system running for over a year now with absolutely no problems, and now that the build has been signed off like others we are thinking of dialling back the flow rate slightly.

 

We placed our plenum manifold chambers in a small cupboard off or landing -- a nice central location adjacent to the room where the MVHR unit itself was positioned -- so that the runs work pretty balanced.  Planning out the "Clapham Junction" into the plenums was fun.  If you can then space out the inlet and outlet on the same wall about 4m apart or more.  Having them on the same wall minimises wind gusting effects.

 

Also make sure to install your foulwater pipework and MVHR ducting before the plumber and the electrician get to work. They can work around 75mm ducting but not v.v.

Edited February 17, 2019 by TerryE

[Triassic]

54 minutes ago, TerryE said:

f you can then space out the inlet and outlet on the same wall about 4m apart or more

That’s the bit I’m struggling with, I don’t have an accessible 4m wall. The wall adjacent to the unit is in a sheltered, South facing position, but only about 3m in length. To get them spaced at 4m or more, I’d have to have the  inlet on adjacent walls, but still in the sheltered position. 

[Jeremy Harris]

4 minutes ago, Triassic said:

That’s the bit I’m struggling with, I don’t have an accessible 4m wall. The wall adjacent to the unit is in a sheltered, South facing position, but only about 3m in length. To get them spaced at 4m or more, I’d have to have the  inlet on adjacent walls, but still in the sheltered position. 

 

If you can get the terminals spaced horizontally from each other by anything over a couple of metres it would probably be fine.  Alternatively, you could look at through-roof terminals, perhaps.

[TerryE]

Yup, the main thing that you want is a reasonable spatial separation with the two vents on the same flow surface.  The spatial separation just needs to be enough to stop any material recirculation.  Using the same flow surface mitigates the pumping effects of high wind.

[Triassic]

One of the MVHR units suggestions by BPV is the Domus HRX-2D. Has anyone used it, are they any good?

[Triassic]

Following a conversation with a MVHR supplier this afternoon about whether to include storage spaces, such as a walk on wardrobe , a cloakroom and hallway, Apparently some suppliers do and some don’t.

 

I was wondering what other have done. Did you include such spaces in the calculations or not?

[lizzie]

we have an extract in the walk in wardrobe.

 

Our hall is semi open plan so is covered from other areas.  We have extracts in guest w.c, both bathrooms, plant room, utility room and walk in wardrobe plus 2 in kitchen area.  We are approx 200sq m single storey house 8 extracts and 6 air inlets.

[Jeremy Harris]

No point at all in putting ventilation in any space that's either non-habitable or not a source of smells/moisture, IMHO.  Fit extracts in the rooms that need it, so kitchens, bathrooms, WCs and utility rooms, and fit supplies in all living rooms and bedrooms.

 

As far as the calculations are concerned, then yes, all the air volume within the insulated and airtight space needs to be included, so that includes built-in cupboards, service areas etc, although there's usually no point in sticking terminals in those spaces.

 

It generally makes life a lot easier, when balancing the system during commissioning, if the number of extracts is roughly equal to, or perhaps one, or maybe two, less than, the number of supply terminals. This is because the extract rate from the kitchen, bathrooms, WC etc will be a fair bit higher than supply to any other room.  Having more extracts than fresh air supply terminals means upping the flow rate through the supply terminals with the risk of flow noise in rooms that you'd normally wish to be a bit quieter (bedrooms and living rooms), so isn't a good idea, really. 

[Triassic]

9 hours ago, JSHarris said:

As far as the calculations are concerned, then yes, all the air volume within the insulated and airtight space needs to be included, so that includes built-in cupboards, service areas etc, although there's usually no point in sticking terminals in those spaces.

Thanks for confirming this, my calculations are based on the total floor area. However, given we have cathedral ceilings in every room with a roof above, it’s resulted in us having a rather high volume house and the need for two MVHR units.  I only budgeted for one unit based on a floor area figure from the Architect (clearly wrong!), hence to desire to double check everything before spending money on double the amount of kit.

[TerryE]

On 18/02/2019 at 22:01, JSHarris said:

there's usually no point in sticking terminals in those spaces

 

We have adjacent walk-in wardrobe and ensuite off our master bedroom.  We did put a small high wall vent between the walk-in wardrobe the ensuite, so there is (some) airflow path between the inlet in the bedroom and the extract in the ensuite.  Not a lot of flow but just enough to keep the ensuite smelling nice and fresh.

Edited February 19, 2019 by TerryE

[Roundtuit]

Not sure if it's been mentioned, but my understanding is that commissioning should be done on 'boost' settings, so that might influence the size of the system, and noise levels during normal operation should be much reduced.

[lizzie]

When they come to swap my exchanger for the enthalpy unit they are going to recalibrate my airflows as I am using the F7 filters and manual says there is a loss of pressure when using these so it needs to be redone.   I feel like Ive got quite high airflow as it is so I’m going to ask engineer to go through it with me properly, I think I must have the system on high a setting judging by airflow.....my desk is right under a vent and its uncomfortable.

[Jeremy Harris]

If you can feel the airflow in the area where a terminal is then that suggests that the flow rate from that terminal is too high.  We have a couple of low level fresh air supply terminals and you can't feel anything coming from them unless you put your hand close to them, when you can just about feel air coming from them.

[lizzie]

1 hour ago, JSHarris said:

If you can feel the airflow in the area where a terminal is then that suggests that the flow rate from that terminal is too high.  We have a couple of low level fresh air supply terminals and you can't feel anything coming from them unless you put your hand close to them, when you can just about feel air coming from them.

Thank you @JSHarris I can feel quite a lot of air from all of them.  I thought it shouldnt be so fierce. Will have a chat with engineer about it.  This is new company I have found not orginal installer. 

[Andehh]

On 19/02/2019 at 07:32, Triassic said:

Thanks for confirming this, my calculations are based on the total floor area. However, given we have cathedral ceilings in every room with a roof above, it’s resulted in us having a rather high volume house and the need for two MVHR units.  I only budgeted for one unit based on a floor area figure from the Architect (clearly wrong!), hence to desire to double check everything before spending money on double the amount of kit.

 

Sorry for reviving an old thread, but we are in the same boat here. Being recommended two small systems, when I would much prefer one larger system.

 

How did you get on in the end? Did you go with two smaller systems?

[TerryE]

IMO, a single correctly-sized system is always best and far easier to balance. Yes, there are circumstances where a split system might make sense for example a main house with a detached annexe, but not if you've got a single well connected living space of many rooms. If you are not careful and the two systems will just fight each other, and be really difficult to balance. 

[JohnMo]

Practical experience of two systems.

 

Commissioning easy as a single system, as long as each unit is balance, same going in as going out and the system as a whole as a whole has the same coming in as going out.  The air doesn't care which unit it is extracted by.  No fighting experience at time with the two systems.

 

Reliability, we had an issue with one unit that required a new fan motor.  The other ran and provided enough ventilation for the house.

 

Two sets of filters required, an additional ongoing to expense.

 

Electric costs same or lower, as you can position the units to reduce duct lengths and have a lower system pressure, the fans run lower on their performance curve so use less electric. 

 

Boost, generally only our small unit is on boost from showers etc.  The large unit is rarely in boost.  Reducing ventilation heat loss.  A single unit would boost the whole house ventilation.

[Triassic]

On 22/08/2022 at 08:00, Andehh said:

 

Sorry for reviving an old thread, but we are in the same boat here. Being recommended two small systems, when I would much prefer one larger system.

 

How did you get on in the end? Did you go with two smaller systems?

In the end we went for a single large system and it’s working fine. The original two system approach was probably suffering from some over engineering.