MVHR plan questions

[oliviaaa999]

Hello!

I wanted to check about MVHR systems, I have attached the plan we received and I just want to triple confirm everything before we go ahead with purchasing! We will be self-installing. 

We chose MVHR mostly for the fresh air aspect, I really don't like having windows open so we would be using it to freshen air inside as well as removing humidity and the risk of mould. Heat retention would be a huge bonus, but we are not expecting it due to not being a self-build with fully airtight walls/ceilings. If there are any alternatives to MVHR which would be more suitable we’re open to suggestions!

It is a retrofitted 60s bungalow. We have attempted airtightness as much as possible (airtight membrane under the flooring, sealed around gaps in/out of the envelope such as pipes and back boxes, etc). We have also avoided a letter box in the front door, got a recirculating fan for the kitchen, left a 10mm gap under all internal doors and had no external ventilation in bathrooms. We haven't replaced the windows yet as we need the MVHR system before we can get windows without trickle vents, but that will be our next step.

The unit will be in a cold loft, we’d like to insulate it eventually but it will depend on budget and practicality (issues with condensation etc). We can insulate the unit and the ducting/the layer between the floor of the loft and the boarding for the loft if that would work as an alternative?

Our current questions are:

1) Can you use an extract with a grease filter in the kitchen or will it still grease up the vents? I have seen some mixed comments on this but our current recirculating extractor fan isn't quite effective enough for ventilation.

2) Is one supply enough for the kitchen/hallway/living room area? This space is relatively open plan with no doors

3) Can you feel a draft/circulation, and if so, is there anything we can do to reduce this?
4) What is the easiest way to insulate the ducts in a cold loft? Would we do a ‘layer’ underneath the loft boarding with the ducts and insulation? 

5) What is the best place to have the unit? Roof/floor/wall mounted? Central of the house?

Any other comments or opinions are more than welcome! Thank you so much

MVHR plan.pdf

[JohnMo]

To be frank if you don't know your airtightness and it's better than 3m³/m² at 50Pa I really wouldn't bother.

 

I would do keep it simple

dMEV in every wet room. No trickle vents in this rooms. I would choose Greenwood CV2 or CV3 only as these are silent in use and automatically boost intelligently -raising humidity.

 

Either wall or window trickle vents in all dry rooms, but they should be humidity activated. 

 

This will sweep humidity from rooms in use only via ventilated only, minimising heat loss. Pennies a year to run no filters, no ducts to run etc.

[Redbeard]

Definitely not in the roof unless it is insulated at rafter level. Performing heat exchange in the coldest part of a house is not a good plan. You *could* construct an 'MVHR Shed' - a highly-insulated box for the MVHR unit to sit in inside an otherwise cold loft.

 

Agree with @JohnMo  that  if you have much over 3 m3/m2/hr (or air-changes per hour - sometimes the scales converge and sometimes they don't - depends on the form factor) MVHR is a very expensive way of ventilating. OTOH I do know of people who have still gone ahead on air-quality grounds for specific health reasons.

[Mike]

7 hours ago, oliviaaa999 said:

1) Can you use an extract with a grease filter in the kitchen or will it still grease up the vents? I have seen some mixed comments on this but our current recirculating extractor fan isn't quite effective enough for ventilation.

Like MVHR units, it's easy to stick a fan in a box and call it a cooker hood, but that doesn't make it effective. Choose one with a high European 'Grease Filtering Effectiveness' classification - see  https://forum.buildhub.org.uk/topic/42370-cooker-hood-with-its-own-mvhr/.

 

It's still a good idea though to have a filter on the kitchen extract, but it can be a concealed 'sock' type behind a regular extract (a good idea on every extract). The only reason I know to use the 'visible' type in your spec, is that it jogs people's memory to change the filter when they see it's ugliness.

 

7 hours ago, oliviaaa999 said:

2) Is one supply enough for the kitchen/hallway/living room area? This space is relatively open plan with no doors

It looks like there may be 2 on the drawing already - if that's what the large dots are. The 'Single duct supply valve' symbols are in strange locations, so I think they're only intended to show you which type go in which room. Could be clearer...

 

7 hours ago, oliviaaa999 said:

3) Can you feel a draft/circulation, and if so, is there anything we can do to reduce this?

There are a few special circumstances where it's possible (that you're not going to encounter if you're following a good design & specification), but no risk at all if you use a ceiling vent that uses the Coanda Effect (Zehnder Luna, if you're sticking to that brand).

 

7 hours ago, oliviaaa999 said:

4) What is the easiest way to insulate the ducts in a cold loft? Would we do a ‘layer’ underneath the loft boarding with the ducts and insulation? 

Keep them underneath the loft insulation.

 

7 hours ago, oliviaaa999 said:

5) What is the best place to have the unit? Roof/floor/wall mounted? Central of the house?

As already recommended, find somewhere within the heated house envelope. In a larger house you might have a plant room, but a utility room or kitchen would be OK if you can spare the wall space. If not, then there are some models that hang horizontally from a solidly constructed ceiling. For example the Zehnder EVO range, if it's available in the UK and you want to stick with Zehnder.

 

Edited August 14 by Mike

[Mike]

36 minutes ago, Mike said:

7 hours ago, oliviaaa999 said:

2) Is one supply enough for the kitchen/hallway/living room area? This space is relatively open plan with no doors

It looks like there may be 2 on the drawing already - if that's what the large dots are. The 'Single duct supply valve' symbols are in strange locations, so I think they're only intended to show you which type go in which room. Could be clearer...

Just spotted that the duct runs in the attic go to some inappropriate locations too - you don't want the standard vents they've specified that close to the walls. I can't think of any good reason why they've shown it the way they have.

 

You'd be better using the 'big dot' locations as the starting point, then consider how that fits in with your light fittings / showers / other features & come back to us for feedback.

[oliviaaa999]

On 14/08/2025 at 15:29, JohnMo said:

To be frank if you don't know your airtightness and it's better than 3m³/m² at 50Pa I really wouldn't bother.

 

I would do keep it simple

dMEV in every wet room. No trickle vents in this rooms. I would choose Greenwood CV2 or CV3 only as these are silent in use and automatically boost intelligently -raising humidity.

 

Either wall or window trickle vents in all dry rooms, but they should be humidity activated. 

 

This will sweep humidity from rooms in use only via ventilated only, minimising heat loss. Pennies a year to run no filters, no ducts to run etc.

We're not sure how we can figure out the airtightness as we haven't changed over to new windows yet (still have the original leaky windows), and won't be able to change them until we know if we're going ahead with MVHR or not (aka whether we have trickle vents or not). 

 

We're definitely open to alternative solutions, with the dMEV and humidity trickle vents is this purely for humidity or will it also allow fresh air in? General air quality was something we were hoping for as we don't really open doors or windows! The plan was for fresh air circulated throughout the day, cool air at night in summer and heat retention air in winter - not sure if it works that well though?

I'd definitely prefer a simpler solution - I just don't want to regret our choice either way! 

 

Thank you!

[oliviaaa999]

On 14/08/2025 at 18:07, Redbeard said:

Definitely not in the roof unless it is insulated at rafter level. Performing heat exchange in the coldest part of a house is not a good plan. You *could* construct an 'MVHR Shed' - a highly-insulated box for the MVHR unit to sit in inside an otherwise cold loft.

 

Agree with @JohnMo  that  if you have much over 3 m3/m2/hr (or air-changes per hour - sometimes the scales converge and sometimes they don't - depends on the form factor) MVHR is a very expensive way of ventilating. OTOH I do know of people who have still gone ahead on air-quality grounds for specific health reasons.

The MVHR shed was our plan for now!

I see, I had thought MVHR was expensive upfront but not bad for running costs but is that only if the air-tight aspect is efficient? We were hoping that as it's a bungalow it would be somewhat simple to install, our plan was for fresh air circulated throughout the day, cool air at night in summer and heat retention air in winter. We haven't done an air-tight test yet as we have the old leaky windows, but we're open to ideas!

Thank you!

[JohnMo]

Unless your house is pretty airtight you have uncontrolled ventilation anyway, this heat loss will always be there, you cannot control it. Add controlled ventilation over and above that, you start to add to any ventilation heat losses. MVHR is really designed to make airtight houses healthy to live in, with the added benefit of recovering much of the heat test would otherwise be lost. But it isn't a cheap option. Saving made in a not airtight house house are so small you will never see a return on your money, and the uncontrolled ventilation is still there as well.

 

dMEV is pretty simple to install cheap to run, and if you implement humidity controlled trickle vents your additional heat loss is pretty small. Don't bank on MVHR doing much cooling at night air flow rates are way too small, it's just ventilation removing humidity and replacing with fresh air, all ventilation systems are the same if done correctly, MVHR is just mandatory for airtight houses, because it extracts and supplies air at the same time. (The heat recovery bit isn't mandatory, but balanced supply and extract is).