MVHR - Initial Design Critique

[F113tch]

Hi All,

 

I have completed an initial design of my MVHR for my New-Build with the following philosophy / design criteria:

 

• One unit fitted as centrally as possible (within the garage) for cost, ease of set-up, as well as lack of options for a second unit due to vaulted ceilings throughout the property.  (Note:  I am aware that ducts passing from garage in to living area will require fire collars/stopping).
• Radial system design to avoid crosstalk.
• Large diameter pipes (90mm?) to reduce velocity and therefore noise.
• 2 x ducts used for longer runs.  (indicated by a "2" on the intake/extract positions on attached design)
• The position of the intakes and extracts have been positioned where it is possible to get duct runs so may not be 100% ideal but I think are a reasonable compromise.

 

Please feel free to critique my attached design or the above design philosophy!

 

Questions I have please:

1. With 90mm ducting, at what distance should I consider 2 x ducts to an extract/intake point, if at all, please?
2. What ducting would you suggest using?
3. Is there any issue with having intake and/or extracts on walls rather than ceilings please?
4. I have had poor service from BPC with them not responding to several emails.  Any other suggestions for suppliers please?
5. Other than fire stopping, is there anything I need to consider with putting the MVHR unit in the garage please?
6. Recommendations on unit manufacturers please?  (Zehnder appear to be a popular choice, but stock seems limited at present!?)
7. If 2 x ducts are used for longer runs do they go to separate intake/extract points or to the same point please?
8. Anything else you think I should consider please?

 

Many thanks for your help,

Steve.

 

MVHR Design v1.pdf

[Conor]

Some general points:

 

1. Outlets should be in the furthest corner from the door. You've several that are close to the doors.

2. No need for outlets in communal spaces like hallways.

3. Run the ducts as straight as you can. You've lots of bends and long routes. Any particular reason for this?

 

To you specific questions:

1. I was told 10m or more should need two ducts. However, I used two in my kitchen even though it's only 5m, reason being I will have much higher flow rates there.

2. I used lindab. No issues.

3. Not ideal as you want the air to flow along the surface of the ceiling, but not a major issue. I've a couple like that. Just ensure its a fair bit down the wall, not right in at the ceiling. Think I left a good 200mm gap from edge of plenum to ceiling.

4. Had great service from BPC. Also reasonable service from Lindab that have a range of MVHRs but also supply everything else. Their returns conditions are terrible so make sure you get the right stuff from the start.

5. Why do you need fire stopping? Is that a BC requirement? If it's outside the thermal envelope of the house, everything needs to be well insulated. Also consider noise attenuators.

6. Seems plenty about. Paul, Zehnder being frequently reccomended here.

7. Single outlet, but it'll have two dict connections on it instead.

8. Keep bathroom extract points close to the showers.

Edited March 31, 2022 by Conor

[Marvin]

I think inlets also need to be furthest distance from the outlet, so, furthest from the internal door in the bedrooms.  

[Marvin]

 

this sort of thing re positions of inlets and outlets

[Conor]

Another thing that helps with design. Try to visualise the air flow through the house. You want "stale" air to flow out of habitable rooms and travel via communal routes to extract points. You don't want two extract points competing for air coming from a single outlet, you. end up drawing air in to a supply room. rather than out. Somebody spotted this issue in our build and I quickly changed a dressing room to extract so we'd have better air flow out of our master bedroom.

[Beckside]

I used a Renson Enduro Delta 380 unit and also used 90mm semi rigid ducting. I ran double ducts to a single outlet in kitchen and utility. 13 extracts and 13 supply. It has built in humidity, c02 and particle sensors, no need for any manual boost buttons. We cannot hear it unless on the highest setting, level 4, which it has never actually used. It moves to level 3 circa 60 secs after turning on a shower. I used 180mm dis eps ducts from the unit to outside, and to the distribution plenums. Used HB+ ducts and boxes, Renson supply and extract outlets. The Renson unit is app controlled, but only when in range of the Wi-Fi it is connected to. 

[JohnMo]

If it easier to route you can have supply valves near a door, but you have to use coanda supply valves, which are wall mounted near the ceiling.  These will throw the air across the room, as the air coming out sticks to the ceiling for approx 4 to 6m.

 

You have baths and showers, the extract valve should be either above the shower or bath which ever is likely to be used most, but position away from doors or windows.

 

All your downstairs supply nozzles are too close the windows and doors.

 

Our longest run in 90mm is about 15m, any more than that you are running the mvhr harder to get the flow rates required and would require 2 supply pipes.

 

Your kitchen extract, I would move further from the hob, otherwise it could get grease laden quite quickly.

 

As said place an extract to make sure it is most likely to remove moisture from the room and not get short circuited, so away from windows and doors.

 

Place supply nozzles so the air has to move across the room to be extracted, again away from doors and windows. 

 

Use cross flow through hallways rather than direct ventilation.

 

I found it much easier to install one larger unit and second small unit, this reduced pipe length quite a bit.  I have used unsilenced plenums.

 

 

 

[Bitpipe]

Great advice above, especially this...

 

1 minute ago, Conor said:

Another thing that helps with design. Try to visualise the air flow through the house. You want "stale" air to flow out of habitable rooms and travel via communal routes to extract points. You don't want two extract points competing for air coming from a single outlet, you. end up drawing air in to a supply room. rather than out. Somebody spotted this issue in our build and I quickly changed a dressing room to extract so we'd have better air flow out of our master bedroom.

 

My MVHR design & install process had a few distinct phases.. BTW this was all trial and error for me, made loads of mistakes along the way.

 

1) Airflow strategy: Effectively a refinement of what you've done above but using the suggested principals above.Start with a supply per habitable room and an extract per 'wet' room (kitchen, WC, bathroom etc) Forget about ducting right now but draw arrows on how air will go from supply to extract on your floor plans, navigating doors etc. You should then spot any 'dead' spots and address. Also think about additional extracts near a recirculating hob extractor for example if you find that more than a few metres from the general kitchen extract (as we did).

 

2)Then think on how you'll join these together and do duct runs to the intake and extract manifolds - irrespective of length we used two runs per extract and one per supply. Retrospectively we should have doubled up ducting on a few very long supply runs but not a big deal really. Duct routing can be diagonal through web joists (shortest path) but will need to navigate around steels and other structural elements. They also have a fixed bend radius which can come into play when you have a lot of them coming together at the manifold. By this stage you should have an idea on total duct length. It usually comes in 50m lengths so round up and maybe add an extra one for unforeseen circumstances. You'll also understand how many ports your intake and extract manifold will need (one for each length of duct) and where these will live. You then need be able to calculate the wide bore steel duct, including bends, angles etc to get from your manifolds to teh MVHR and from your MVHR to external  intake / exhaust points. These last two will need insulated as cold air from outside will be coming in one and out the other so they will attract condensation.

 

3) you're more or less ready to order ducting materials. Steel duct will need adhesive and tape for the joins while the flexible duct just slides into position and is secured with supplied clips or catches in the plenums / manifolds with o-rings making the seals airtight. Silicone lubricant spray is a must to make that an easy job. Builders strap is handy for supporting all kinds of ducting if there is not sufficient local support (like a web joist).

 

4) At this stage you should have an awareness as to whether steels  need penetrations pre-fabricated or where there may be a clash of space with other services (usually foul pipes). Sometime you only realise this when the structure is up which is why having some redundancy in your duct supply is useful.

 

5) I've not considered any electrical aspects here - your unit will need a 13A supply (plug top or isolated) and you may need to run cable for a secondary wired controller. Boost switches etc can be taken care of by your electrician. You'll also need to run condensate from the unit to a foul drain.

 

6) install should go to plan but as above, it's easy to miss an obstruction and need to go a longer way round so be prepared for that. This is also where you finalise position of the plenums (wall or ceiling) as structural considerations may dictate that - make sure plenums are installed with the duct ports pointing in the direction that the duct will be coming from to avoid sharp bends. 

 

7) once all plenums are in place (and I faffed about a lot with this) start to run duct from the manifold area to the duct. I found this much easier that the other way round. I used to screw a plank across a doorway and through the centre of a spool BEFORE cutting the ties on the spool so that it could unroll somewhat easily. If you just open the spool, it will knot up very quickly. Equally, pre-cutting lengths is asking for trouble as you risk them being too short or having too much waste. Once the end of the duct is secured in the plenum, cut a generous length next to manifold and move to the next one. You just need to label ducts as intake or extract at the manifold end as a minimum (I used red and blue sharpies). Not a whole lot of use labelling per room. You can then trim each duct and connect to the relevant manifold - this was harder that it sounds as too long or too short and you won't make a good connection. Some trial and error here (obv going from too long and trimming incrementally until perfect fit).

 

'8) Ducting the wide bore stuff is a different game - four connections to make and depends on where your external intake, extract, manifolds & unit are. Leave room to insulate the external ducts.

 

9) At this point you're done from a first fix point of view. Cut plenums flush to the plasterboard to make life easy for your plasterer and pop in the duct covers. You probably won't mount the MVHR unit itself until second fix.

 

Good luck!

 

[F113tch]

2 hours ago, Conor said:

Some general points:

 

1. Outlets should be in the furthest corner from the door. You've several that are close to the doors.

2. No need for outlets in communal spaces like hallways.

3. Run the ducts as straight as you can. You've lots of bends and long routes. Any particular reason for this?

 

To you specific questions:

1. I was told 10m or more should need two ducts. However, I used two in my kitchen even though it's only 5m, reason being I will have much higher flow rates there.

2. I used lindab. No issues.

3. Not ideal as you want the air to flow along the surface of the ceiling, but not a major issue. I've a couple like that. Just ensure its a fair bit down the wall, not right in at the ceiling. Think I left a good 200mm gap from edge of plenum to ceiling.

4. Had great service from BPC. Also reasonable service from Lindab that have a range of MVHRs but also supply everything else. Their returns conditions are terrible so make sure you get the right stuff from the start.

5. Why do you need fire stopping? Is that a BC requirement? If it's outside the thermal envelope of the house, everything needs to be well insulated. Also consider noise attenuators.

6. Seems plenty about. Paul, Zehnder being frequently reccomended here.

7. Single outlet, but it'll have two dict connections on it instead.

8. Keep bathroom extract points close to the showers.

Hi Conor,

 

Thanks for your reply.

 

The upstairs outlets were positioned to shorten the duct routes as far as possible, as with the bedrooms having vaulted ceilings, the ducts will be routed along the ridge above the bed and I was/am concerned about noise.  Am I being overly concerned about noise?  If so, I will modify the design.

The hallway is probably not going to be screened off from the dining area so will be one big open-plan room bro put a inlet in there.

The routes taken are generally following the beams (I have block & beam floor construction) downstairs and are where I can hide ductwork upstairs.

 

On your other points:

1.  What size duct work are you using please?

5.  I was told that I would require fire stopping but will check again.

8.  I was told to keep extract points away from the shower area, which seemed a little counterintuitive.  I take it you've had no issues with the points near to your shower?

 

Thanks again, Steve.

[F113tch]

35 minutes ago, JohnMo said:

If it easier to route you can have supply valves near a door, but you have to use coanda supply valves, which are wall mounted near the ceiling.  These will throw the air across the room, as the air coming out sticks to the ceiling for approx 4 to 6m.

 

You have baths and showers, the extract valve should be either above the shower or bath which ever is likely to be used most, but position away from doors or windows.

 

All your downstairs supply nozzles are too close the windows and doors.

 

Our longest run in 90mm is about 15m, any more than that you are running the mvhr harder to get the flow rates required and would require 2 supply pipes.

 

Your kitchen extract, I would move further from the hob, otherwise it could get grease laden quite quickly.

 

As said place an extract to make sure it is most likely to remove moisture from the room and not get short circuited, so away from windows and doors.

 

Place supply nozzles so the air has to move across the room to be extracted, again away from doors and windows. 

 

Use cross flow through hallways rather than direct ventilation.

 

I found it much easier to install one larger unit and second small unit, this reduced pipe length quite a bit.  I have used unsilenced plenums.

 

 

 

 

Hi and thanks for your reply.

 

The showers will definitely get used the most so I will reposition extracts accordingly.  Should they be within the shower cubicle footprint please?

 

Are my supply nozzles too close to the internal or external doors (or both!)  please?

 

If you do not plan on opening windows, and they are not fitted with trickle vents, is there an issue with positioning supply or extract nozzles next to windows please?

 

I am fitting a local, surface mounted kitchen extract unit to try and capture smells.  The ceiling height above the hob is >4m so not sure if grease will be an issue?  What do you think?

 

A second unit would certainly help with the length of ductwork but I have no obvious place to install it.

 

Thanks again, Steve.

[JohnMo]

1 minute ago, F113tch said:

 

Hi and thanks for your reply.

 

The showers will definitely get used the most so I will reposition extracts accordingly.  Should they be within the shower cubicle footprint please?

 

That's where I installed mine, had no issues in the last six months of use.

 

Are my supply nozzles too close to the internal or external doors (or both!)  please?

 

Both

 

If you do not plan on opening windows, and they are not fitted with trickle vents, is there an issue with positioning supply or extract nozzles next to windows please?

Possibly not

1 minute ago, F113tch said:

 

I am fitting a local, surface mounted kitchen extract unit to try and capture smells.  The ceiling height above the hob is >4m so not sure if grease will be an issue?  What do you think? 

 

That should be ok, but grease filter are not that efficient.

 

A second unit would certainly help with the length of ductwork but I have no obvious place to install it.

 

Thanks again, Steve.

 

[F113tch]

47 minutes ago, Bitpipe said:

Great advice above, especially this...

 

 

My MVHR design & install process had a few distinct phases.. BTW this was all trial and error for me, made loads of mistakes along the way.

 

1) Airflow strategy: Effectively a refinement of what you've done above but using the suggested principals above.Start with a supply per habitable room and an extract per 'wet' room (kitchen, WC, bathroom etc) Forget about ducting right now but draw arrows on how air will go from supply to extract on your floor plans, navigating doors etc. You should then spot any 'dead' spots and address. Also think about additional extracts near a recirculating hob extractor for example if you find that more than a few metres from the general kitchen extract (as we did).

 

2)Then think on how you'll join these together and do duct runs to the intake and extract manifolds - irrespective of length we used two runs per extract and one per supply. Retrospectively we should have doubled up ducting on a few very long supply runs but not a big deal really. Duct routing can be diagonal through web joists (shortest path) but will need to navigate around steels and other structural elements. They also have a fixed bend radius which can come into play when you have a lot of them coming together at the manifold. By this stage you should have an idea on total duct length. It usually comes in 50m lengths so round up and maybe add an extra one for unforeseen circumstances. You'll also understand how many ports your intake and extract manifold will need (one for each length of duct) and where these will live. You then need be able to calculate the wide bore steel duct, including bends, angles etc to get from your manifolds to teh MVHR and from your MVHR to external  intake / exhaust points. These last two will need insulated as cold air from outside will be coming in one and out the other so they will attract condensation.

 

3) you're more or less ready to order ducting materials. Steel duct will need adhesive and tape for the joins while the flexible duct just slides into position and is secured with supplied clips or catches in the plenums / manifolds with o-rings making the seals airtight. Silicone lubricant spray is a must to make that an easy job. Builders strap is handy for supporting all kinds of ducting if there is not sufficient local support (like a web joist).

 

4) At this stage you should have an awareness as to whether steels  need penetrations pre-fabricated or where there may be a clash of space with other services (usually foul pipes). Sometime you only realise this when the structure is up which is why having some redundancy in your duct supply is useful.

 

5) I've not considered any electrical aspects here - your unit will need a 13A supply (plug top or isolated) and you may need to run cable for a secondary wired controller. Boost switches etc can be taken care of by your electrician. You'll also need to run condensate from the unit to a foul drain.

 

6) install should go to plan but as above, it's easy to miss an obstruction and need to go a longer way round so be prepared for that. This is also where you finalise position of the plenums (wall or ceiling) as structural considerations may dictate that - make sure plenums are installed with the duct ports pointing in the direction that the duct will be coming from to avoid sharp bends. 

 

7) once all plenums are in place (and I faffed about a lot with this) start to run duct from the manifold area to the duct. I found this much easier that the other way round. I used to screw a plank across a doorway and through the centre of a spool BEFORE cutting the ties on the spool so that it could unroll somewhat easily. If you just open the spool, it will knot up very quickly. Equally, pre-cutting lengths is asking for trouble as you risk them being too short or having too much waste. Once the end of the duct is secured in the plenum, cut a generous length next to manifold and move to the next one. You just need to label ducts as intake or extract at the manifold end as a minimum (I used red and blue sharpies). Not a whole lot of use labelling per room. You can then trim each duct and connect to the relevant manifold - this was harder that it sounds as too long or too short and you won't make a good connection. Some trial and error here (obv going from too long and trimming incrementally until perfect fit).

 

'8) Ducting the wide bore stuff is a different game - four connections to make and depends on where your external intake, extract, manifolds & unit are. Leave room to insulate the external ducts.

 

9) At this point you're done from a first fix point of view. Cut plenums flush to the plasterboard to make life easy for your plasterer and pop in the duct covers. You probably won't mount the MVHR unit itself until second fix.

 

Good luck!

 

Thanks for the great advice above.

 

With vaulted ceilings and block and beam construction the duct routes are proving a bit tricky!  

 

I will update my provisional design to v2 for further comments.

 

Thanks again, Steve.

[F113tch]

Thanks again for the replies.

 

Please find attached version 2 for further comment.  Hopefully this has captured most of the comments where it is possible.  I have removed 2 supplies from downstairs as I think they were redundant when considering air-flow.

 

Let me know your thoughts please!

 

Steve.

MVHR Design v2.pdf

[Bitpipe]

18 minutes ago, F113tch said:

Thanks for the great advice above.

 

With vaulted ceilings and block and beam construction the duct routes are proving a bit tricky!  

 

I will update my provisional design to v2 for further comments.

 

Thanks again, Steve.

 

You are welcome. I learned it all the hard way  

[Bitpipe]

2 minutes ago, F113tch said:

Thanks again for the replies.

 

Please find attached version 2 for further comment.  Hopefully this has captured most of the comments where it is possible.  I have removed 2 supplies from downstairs as I think they were redundant when considering air-flow.

 

Let me know your thoughts please!

 

Steve.

MVHR Design v2.pdf 310.43 kB · 0 downloads

 

Not sure this was addressed above but is you hob extract venting directly to outside or is it recirculating?

 

If the former then that will seriously unbalance your MVHR system and may compromise your airtightness such that the MVHR is not effective.

 

You still have a few dead zones which may be troublesome.

[F113tch]

2 hours ago, Bitpipe said:

 

Not sure this was addressed above but is you hob extract venting directly to outside or is it recirculating?

 

If the former then that will seriously unbalance your MVHR system and may compromise your airtightness such that the MVHR is not effective.

 

You still have a few dead zones which may be troublesome.

Hi,

 

It was planned to vent the cooker hob to the outside... but it can be changed if it will cause issues?

 

Where are the dead zones please?

[7dayworker]

On 31/03/2022 at 16:46, Bitpipe said:

Great advice above, especially this...

 

 

My MVHR design & install process had a few distinct phases.. BTW this was all trial and error for me, made loads of mistakes along the way.

 

1) Airflow strategy: Effectively a refinement of what you've done above but using the suggested principals above.Start with a supply per habitable room and an extract per 'wet' room (kitchen, WC, bathroom etc) Forget about ducting right now but draw arrows on how air will go from supply to extract on your floor plans, navigating doors etc. You should then spot any 'dead' spots and address. Also think about additional extracts near a recirculating hob extractor for example if you find that more than a few metres from the general kitchen extract (as we did).

 

2)Then think on how you'll join these together and do duct runs to the intake and extract manifolds - irrespective of length we used two runs per extract and one per supply. Retrospectively we should have doubled up ducting on a few very long supply runs but not a big deal really. Duct routing can be diagonal through web joists (shortest path) but will need to navigate around steels and other structural elements. They also have a fixed bend radius which can come into play when you have a lot of them coming together at the manifold. By this stage you should have an idea on total duct length. It usually comes in 50m lengths so round up and maybe add an extra one for unforeseen circumstances. You'll also understand how many ports your intake and extract manifold will need (one for each length of duct) and where these will live. You then need be able to calculate the wide bore steel duct, including bends, angles etc to get from your manifolds to teh MVHR and from your MVHR to external  intake / exhaust points. These last two will need insulated as cold air from outside will be coming in one and out the other so they will attract condensation.

 

3) you're more or less ready to order ducting materials. Steel duct will need adhesive and tape for the joins while the flexible duct just slides into position and is secured with supplied clips or catches in the plenums / manifolds with o-rings making the seals airtight. Silicone lubricant spray is a must to make that an easy job. Builders strap is handy for supporting all kinds of ducting if there is not sufficient local support (like a web joist).

 

4) At this stage you should have an awareness as to whether steels  need penetrations pre-fabricated or where there may be a clash of space with other services (usually foul pipes). Sometime you only realise this when the structure is up which is why having some redundancy in your duct supply is useful.

 

5) I've not considered any electrical aspects here - your unit will need a 13A supply (plug top or isolated) and you may need to run cable for a secondary wired controller. Boost switches etc can be taken care of by your electrician. You'll also need to run condensate from the unit to a foul drain.

 

6) install should go to plan but as above, it's easy to miss an obstruction and need to go a longer way round so be prepared for that. This is also where you finalise position of the plenums (wall or ceiling) as structural considerations may dictate that - make sure plenums are installed with the duct ports pointing in the direction that the duct will be coming from to avoid sharp bends. 

 

7) once all plenums are in place (and I faffed about a lot with this) start to run duct from the manifold area to the duct. I found this much easier that the other way round. I used to screw a plank across a doorway and through the centre of a spool BEFORE cutting the ties on the spool so that it could unroll somewhat easily. If you just open the spool, it will knot up very quickly. Equally, pre-cutting lengths is asking for trouble as you risk them being too short or having too much waste. Once the end of the duct is secured in the plenum, cut a generous length next to manifold and move to the next one. You just need to label ducts as intake or extract at the manifold end as a minimum (I used red and blue sharpies). Not a whole lot of use labelling per room. You can then trim each duct and connect to the relevant manifold - this was harder that it sounds as too long or too short and you won't make a good connection. Some trial and error here (obv going from too long and trimming incrementally until perfect fit).

 

'8) Ducting the wide bore stuff is a different game - four connections to make and depends on where your external intake, extract, manifolds & unit are. Leave room to insulate the external ducts.

 

9) At this point you're done from a first fix point of view. Cut plenums flush to the plasterboard to make life easy for your plasterer and pop in the duct covers. You probably won't mount the MVHR unit itself until second fix.

 

Good luck!

 

Thanks so much. 
 

that ducting has been puzzling me and also avoiding dead zones for smell in open plan kitchen/diner .

there has been reports of loud fan noise if the pump is overworking and siting it to avoid noise and get good outside air temperature ..ie not the hot summer air or coldest north winter air . Any ideas on your design on these and the size of pump. Also sealing noise transimission  through large holes for ducts seems a problem to attend to.

[Bitpipe]

2 minutes ago, 7dayworker said:

Thanks so much. 
 

that ducting has been puzzling me and also avoiding dead zones for smell in open plan kitchen/diner .

there has been reports of loud fan noise if the pump is overworking and siting it to avoid noise and get good outside air temperature ..ie not the hot summer air or coldest north winter air . Any ideas on your design on these and the size of pump.

 

 

Not sure what you mean by pump.

 

An appropriately sized MVHR unit should be more or less silent (i.e. noticeable noise at the room vents) on normal operation (30% fan speed) and only noticeable, but not annoying, on boost (50%).

 

The MVHR unit itself will make some noise so if located in the loft etc then you may need to isolate it using rubber feet etc to prevent vibrational noise to the room below.

 

MVHR units tend to be classified by sqm of the area they are servicing but there are other metrics you can use to size one appropriately such that 30% fan speed or thereabouts meets your minimum BR requirement.

 

Makes sense to locate your unit as centrally as possible in the house to even out the duct runs and to make balancing easier but mine is in the plant room on one corner of the house and it works fine.

 

WRT external intake / extract location - the air temp will be broadly the same but for balance you want both vents on same side of house and a min 1m apart

 

2 minutes ago, 7dayworker said:

Also sealing noise transimission  through large holes for ducts seems a problem to attend to.

 

 Our ducting runs horizontally through the web joists and through 75mm floor deck holes in a few places. Given it's all behind plasterboard, not sure where the noise transmission is happening?

[7dayworker]

Thanks . I will ensure that the unit is placed on rubber legs