MVHR newbie

[vala]

Can anyone help with a design for MVHR in my home?

I've attached a floor plan of both floors with 'x' where ducting could be fed to. Size of ducting for the first floor, can be what's suited as not restriction to size in the loft space, however on the ground floor I'm restricted to a max height of 75-100mm due to UFH already being installed between the first floor joists.

Location of the main unit could go in the loft against an external wall for venting in/out through there instead of the roof.

Thanks

[BotusBuild]

What's the significance, if any, of the different coloured crosses?

You could consider rigid ducting (125-200mm as required) from the loft to manifolds from which you run flexible ducts at 75mm diameter to the various vents. Obviously different manifolds for exhaust and supply.

Edited February 4 by BotusBuild

[BotusBuild]

[Nickfromwales]

  On 04/02/2025 at 19:41, vala said:

Can anyone help with a design for MVHR in my home?

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Can you update us on the house type / construction / airtight etc please?

 

  On 04/02/2025 at 19:41, vala said:

due to UFH already being installed between the first floor joists.

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Are these not in thin spreader plates directly below the deck boards (P5 Egger boards)?

[vala]

  On 04/02/2025 at 20:39, BotusBuild said:

What's the significance, if any, of the different coloured crosses?

You could consider rigid ducting (125-200mm as required) from the loft to manifolds from which you run flexible ducts at 75mm diameter to the various vents. Obviously different manifolds for exhaust and supply.

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Apologies I should have been clearer.

in ground floor, floor plan;

blue ‘x’ is a small opening from ground floor through to the loft. It’s open at present as it’s where my flow and return for first floor UFH go up, along with h/w.

blue arrows are direction of joists.

green ‘x’ is location where a grille could definitely go.

green ‘x?’ is a possibility.

 

on first floor plan;

blue ‘x’ is the location of the gap from loft to ground floor.

red ‘x’ is where the bathroom grille would have to go. This is due to bathroom ceiling is solid surface and grooves for extractor already routered in.

green ‘x’ is where definite grilles could go, with no restrictions on duct size as that would be in loft.

[vala]

  On 04/02/2025 at 20:58, Nickfromwales said:

Can you update us on the house type / construction / airtight etc please?

 

Are these not in thin spreader plates directly below the deck boards (P5 Egger boards)?

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house is 1970’s semi detached. Has 55-60mm cavities which have recently been filled with eps beads. Loft has 250–300mm insulation. New aluminium windows and doors, all installed with me508 membrane, air seal foam, compriband.

 

ufh is at 135mm centres and sits on 100mm PIR and covered with 25-30mm dry screed. On top of this is t&g boards and aside from the bathroom this is topped with thermal stream underlay and a low tog carpet (total tog 1.8). In the bathroom 6mm hardie is on top of the t&g boards, then ditra mat, then tiles.

[BotusBuild]

What type of joists do you have? Solid, i-joist, web joist (asin picture above)?

Do you have open  access to the joists still or will you have to remove plasterboard etc. to fit the ducting? The former we all hope

[vala]

Joists are solid.

i will have access from the ground floor as existing plasterboard all coming down.

so currently due to the floor build up for the first floor UFH I have approx 75mm space left, however I could increase this to 100mm with some baton.

[JohnMo]

I would do a simple cascade system, with supply ducts to bedrooms only. No need to supply to dining lounge, as air would overflow from bedrooms to stairs and down to kitchen.  Extract will be kitchen and bathroom only.

 

Supply rates

bed 1 and 2 say 35m³h and bed 3 25m³h. So total supply is 95m³h. If you need more flow up the flow rates slightly.

 

Extract rates

Kitchen would extract 50 and bathroom 45m³h.

 

45m³h of the supplied air would be taken by bathroom, the rest would go down the stairs wash through the living room and out of the kitchen extract.

 

Whole house would be be fully ventilated. Only one easy run duct is needed. Do the ducts in 90mm and run 2 to the kitchen to a 2x90mm x 125mm plenum.

 

 

Edited February 4 by JohnMo

[Marko]

Just send your floor plans to BPC Ventilation and they'll do it for you and provide you with flow rates. That gives you a good starting point. 

 

Just an FYI from what I've learnt along the way (some may disagree/correct me): Kitchen extract should ideally be located above/adjacent to the sink, not the stove. All plenums should have a minimum clearance of 300-400mm from walls. If your floor joist permits, try to locate supply valves above/adjacent to the radiators in the bedrooms and other living spaces. Apparently, this helps with convection etc. They say locating the unit in a warm room rather than a cold loft is better in terms of efficiency. I've not had any negative experiences yet though. Don't underestimate the work involved if it's a retrofit and you are slightly OCD.  

Edited February 7 by Marko

[JamesP]

  On 07/02/2025 at 13:29, Marko said:

Just send your floor plans to BPC Ventilation and they'll do it for you and provide you with flow rates. That gives you a good starting point. 

  

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I recommend you also include the volume, ceiling heights etc. Our system was very undersized due to 3,4 and 5 metre ceilings which was not available on floor plans.

[JohnMo]

  On 07/02/2025 at 14:14, JamesP said:

recommend you also include the volume, ceiling heights etc. Our system was very undersized due to 3,4 and 5 metre ceilings which was not available on floor plans

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But don't need to include heights generally, English regs are based on floor area, Scottish floor area and height upto 3.0m, Passivhaus I believe floor area and height up to 2.5m.

 

A very good summary is linked below

 

https://www.paulheatrecovery.co.uk/information-on-domestic-ventilation-and-mvhr/mvhr-info-for-architects-and-designers/

[joth]

  On 04/02/2025 at 23:08, JohnMo said:

I would do a simple cascade system, with supply ducts to bedrooms only. No need to supply to dining lounge, as air would overflow from bedrooms to stairs and down to kitchen.  Extract will be kitchen and bathroom only.

 

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Out of interest, how would this pass building regs, if replacing all windows and for new without trickle vents, and generally improving airtightness?

 

My reading of part F is, in this scenario (a "cat B" retrofit with MVHR) every habitable room needs to demonstrate it has sufficient supply ventilation distributed proportionally to the volume of said room. To me, without putting a supply vent in the living room and balancing it, that's a very tricky requirement to demonstrate compliance with. (of course, it's unlikely any BI will care, but that's a very different conversation)

 

  On 07/02/2025 at 14:59, JohnMo said:

English regs are based on floor area,

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Again my reading of part F is it cares about volumes not floor area for MVHR:

[JohnMo]

  On 07/02/2025 at 15:39, joth said:

Again my reading of part F is it cares about volumes not floor area for MVHR:

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Have you attached the correct document, nothing mentioned volume.

 

  On 07/02/2025 at 15:39, joth said:

every habitable room needs to demonstrate it has sufficient supply ventilation distributed proportionally

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You can demonstrate via "through flow". The alternative is to under ventilation of bedrooms. You can demonstrate exactly the same way as PIV system.

 

  On 07/02/2025 at 15:39, joth said:

generally improving airtightness

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Unless you are getting really good airtightness, why waste your time and money on MVHR.

 

This is a design done for my  house per building regs, by Paul Heat Recovery.

 

Notice the volume Though m³/h column.

[joth]

  On 07/02/2025 at 16:34, JohnMo said:

  On 07/02/2025 at 15:39, joth said:

Again my reading of part F is it cares about volumes not floor area for MVHR:

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Have you attached the correct document, nothing mentioned volume.

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Same document, with the word Volume highlighted

 

  On 07/02/2025 at 16:34, JohnMo said:

Notice the volume Though m³/h column.

 

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Thank you - that makes sense. Turn the "common sense" argument into a balance of flows on a spreadsheet. Makes a lot of sense for open plan areas especially. 

 

  On 07/02/2025 at 16:34, JohnMo said:

Unless you are getting really good airtightness, why waste your time and money on MVHR.

 

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Well this is a very different conversation, for the purposes of this thread so it sounded like MVHR is as a given

 

One view I've seen is where people are doing a lot of internal remodeling they have "one chance" to put MVHR in now, even if airtightness is not good enough to justify it they will do anyway on an optimistic/aspirational view that over time window/door/exterior works etc will gradually improve the performance. They "grow into it" if you will. (Much like using an oversized R290 ASHP with undersized rads today, on basis they need a boiler replacement now, and will get better efficiency of it in future as we gradually improve rads, insulation etc)

 

But regarding needing really good airtightness, that's not necessarily true. The long held rule of thumb was 3 ACH is the break even for MVHR, but even that seems pretty achievable for many houses. But there's plenty folks argue that's too stringent and it will financially payback even at higher levels of air permeability

https://www.passivhaustrust.org.uk/UserFiles/File/research papers/MVHR/2020.04.27-The Case for MVHR-v7.pdf

And in any case, if folks care most about air quality and want to spend their money on MVHR without consideration for the capital cost payback, it's a valid choice in many more homes.

 

Edited February 7 by joth

[JohnMo]

  On 07/02/2025 at 17:16, joth said:

Same document, with the word Volume highlighted

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But only say distributing the flow proportionally by volume. The volume flow is generally 0.3l/s per m² or how many people wet rooms etc which ever the greatest.

 

Trouble with the regs you either over ventilate or under. There are 2 plus dog in our house, turned the flow rates way down and still in winter the humidity is on the edge of over ventilation after a couple of cold days.

 

If the regs didn't say I had to balanced supply and extract ventilation, (Scottish rules for airtight better than 3), would have just installed demand controlled MEV. Would have had self actuating inlet vents.

[JamesP]

@JohnMo@joth Thanks for your input 

I appreciate a lot of  homes have an average ceiling height but ours has nearly double the volume and the MVHR supplied could not meet the requirements to satisfy building regs even with the fan at maximum. 

I did not discover this until near completion.

I had to be a little creative in the end to keep the BC happy. It works fine.

[JohnMo]

  On 07/02/2025 at 18:25, JamesP said:

height but ours has nearly double the volume

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So has ours, currently sat in a 6m high lounge. Average height of other rooms 3.5m. Think my ventilation rate at the moment is about 0.2ACH, based floor area x 2.5m. And I am looking reduce that.

 

Do you monitor CO2 and humidity? I would be careful you aren't over ventilating. Low humidity isn't good, skin and chest issues for two.

[vala]

Thanks for all the replies.

I've fired off the floorpans to a few companies and will see what they come back with.

If the supply vents could come out of the wall I could run a duct beneath the ceiling (above the wall units), from the blue x on the ground floor and in the direction of the joists. Attached an image to make it clearer. Green x would be the supply vents.

 

As for an extract vent by the sink (reply by @Marko), I possibly could get a vent beneath ceiling and above the wall units/extractor, then come up into the ceiling and over to the sink, but would depend on the height needed within the ceiling void for the extract vent.

Don't think I'll be able to get a extract vent to the WC on the GF so maybe just go for a standalone unit in there.

Edited February 7 by vala