MVHR upgrade from MEV

[James_H]

Hello all.

My six year old house was fitted with MEV by the developer but I’m planning to ‘upgrade’ to full MVHR.  Although much MHVR goes on new build, plus some retrofit during refurbishment of older properties, I haven’t seen anything online about anyone upgrading existing MEV in this way.

As background, my parents longed to do a self build, with MVHR oddly enough being one of the things mentioned a lot beforehand.  It was installed in 2010/11, so I am familiar with such a system in practice.  On buying my house new, the showhouses did have MVHR, but unfortunately all the others only have MEV.

My original thinking was that I already have one half the of system in place, though perhaps I already get half the benefits.  So while we get fresh air, humidity at reasonable levels and minimal condensation in the bathrooms, it galls me that trickle vents were cut into the frames of the triple glazed windows for supply and while I have two fans spinning 24/7, the heated warm air is just being pumped out.  The extraction is not optimal – kitchen cooking smells permeate the whole house with the door closed, whether on boost or not.  For some reason, only now are we getting some black mould in the en-suite bathroom.

I have read up plenty since and so – as things stand – currently plan to go ahead.  I’m also going ahead with an extension – highly glazed – and will need the SAP calc, so this is one of the few possible improvements I can make to the existing house if needed.  (Loft insulation, firstly to building regs, though even better to the design spec, would be cheaper and easier so happening as well).  

The airtightness test result when new was 4.4 m3 /m2 hr @ 50Pa.  Fairly average for recent new builds but certainly not spectacular.  It’s below the figure where it should give some benefit, but above a level I expect some might think it’s not worth it.  The extension will be built to a better standard, while there are some gaping holes in the existing house I am aware of and have filled or will do.  Plus sealing up the trickle vents.  Though at this sort of airtightness, I can’t justify a top end system professionally installed, it just will not pay back.  Though I have acquired a good value unit and plan to do much work myself to limit expense.

I don’t think I’ll necessarily use all that is currently in place.  Rigid ducting was used for the extract and is not especially well sealed at joins where visible.  The ceiling extract ducts were generally placed near the doorway, not at the opposite corner.  The two existing roof extract terminals (there are two separate MEV units) are on the southern elevation but have insufficient free area to repurpose one as a supply and leave the other as extract.

The manual boost switches might be the few parts I reuse.  There is power to one of the MEVs that would also be the loft location – admittedly unheated – for the MVHR unit.  I do have useful behind-shower voids at the front and back to enable semi rigid supply to all the ground floor rooms that need it, and back from the extract rooms.  The doors already have sufficient gaps underneath.
The reason for posting here is to ask a few queries on what I’m still unsure of.  And if it does go ahead, perhaps be one of the few resources to document an MEV to MVHR upgrade.  Savings are difficult to quantify at present. 

I would appreciate some hints on the following:

1. Which of these positions, or something else entirely, would you put the external supply and extract terminals?  A (Red): North roof through roof terminals, B (Gold): North roof soffits (might be easiest to install but need to watch the free area), C (blue):  East gable end above the slightly lower roof. Overall I am trying to avoid the SW prevailing wind, be on the same elevation, far enough apart and be shaded in summer.
2. I have seen a number of posts of DIY commissioning and producing the certificate for building control.  As an alternative, there are a number of firms that can do testing, but is there a competent persons scheme for any of these testers?  Reason I ask is that on speaking to building control they said it would be usual for installers to self certify as would be done for gas / electricity, in which case there is no BC fee.  Though I have not found a similar scheme.  If I have to pay the building control fee anyway, then I would probably test myself.
3. Is this really worth going ahead with, or pulling the plug now?  I am hopeful of getting this completed for under £2k all in for the parts, my labour counted as zero.  One possible supplier for the large glazed doors for the extension wanted over £900 to add trickle ventilation.  I’m not going with that firm, but whoever I use I avoid the need for those vents.  Overall, I would hope to minimise the ventilation losses but have difficulty quantifying the payback.  Non-financially, I would see benefits from input filtration (due to hay fever) and soundproofing from sealing the old vents.

 

Sorry for the long post, and thanks in advance for any suggestions.

 

[Bitpipe]

7 hours ago, James_H said:

 

I would appreciate some hints on the following:

1. Which of these positions, or something else entirely, would you put the external supply and extract terminals?  A (Red): North roof through roof terminals, B (Gold): North roof soffits (might be easiest to install but need to watch the free area), C (blue):  East gable end above the slightly lower roof. Overall I am trying to avoid the SW prevailing wind, be on the same elevation, far enough apart and be shaded in summer.
2. I have seen a number of posts of DIY commissioning and producing the certificate for building control.  As an alternative, there are a number of firms that can do testing, but is there a competent persons scheme for any of these testers?  Reason I ask is that on speaking to building control they said it would be usual for installers to self certify as would be done for gas / electricity, in which case there is no BC fee.  Though I have not found a similar scheme.  If I have to pay the building control fee anyway, then I would probably test myself.
3. Is this really worth going ahead with, or pulling the plug now?  I am hopeful of getting this completed for under £2k all in for the parts, my labour counted as zero.  One possible supplier for the large glazed doors for the extension wanted over £900 to add trickle ventilation.  I’m not going with that firm, but whoever I use I avoid the need for those vents.  Overall, I would hope to minimise the ventilation losses but have difficulty quantifying the payback.  Non-financially, I would see benefits from input filtration (due to hay fever) and soundproofing from sealing the old vents.

 

Sorry for the long post, and thanks in advance for any suggestions.

 

 

You seem to have made a thorough consideration of the pros and cons but how are you planning to get the new ducting in situ - pulling the ceilings or from floors above?

 

The 75mm ribbed duct that many of us has used is flexible but a bit unwieldy, especially in long lengths. It was fussy to get it in place in a bare frame (and before other services went in) so you may have more challenges.

 

In my experience, the external vents are somewhat dictated by the position of the unit and how much rigid wider bore (in my case 180mm) duct you can practically run. In an unheated space, it all needs insulated too (I insulated mine even though it is in the plant room as there was a lot of condensation on the cold intake).

 

Commissioning is entirely self do-able and you'll probably do a better job than an 'expert' who's only there for a few hours. It's just trial and error adjustment of ceiling vents & fan control with a little bit of maths to balance in/out flows and meet the min extraction rates for regs.

[James_H]

Thanks for the reply.  For ground floor rooms, I would lift the carpet and cut temporary holes in the floor above.  I am fortunate in that the joist directions run towards those shower voids.  I was planning to use the 75mm semi rigid and realise that the semi unrigid nature might mean fund and games to thread it through.  

 

The MVHR unit I bought (Vent Axia Sentinel Kinetic Plus B) only asks the inlet and extract are at least 1.5m apart.  I could manage further with any of those options, also chosen because the unit would go on that gable wall of the larger loft.  Yes, I would need to insulate.

[Bitpipe]

12 hours ago, James_H said:

Thanks for the reply.  For ground floor rooms, I would lift the carpet and cut temporary holes in the floor above.  I am fortunate in that the joist directions run towards those shower voids.  I was planning to use the 75mm semi rigid and realise that the semi unrigid nature might mean fund and games to thread it through.  

 

The MVHR unit I bought (Vent Axia Sentinel Kinetic Plus B) only asks the inlet and extract are at least 1.5m apart.  I could manage further with any of those options, also chosen because the unit would go on that gable wall of the larger loft.  Yes, I would need to insulate.

 

Crossing joists will be your challenge, hopefully you can avoid that.

 

Are you using new plenums (or whatever the name is for the unit that terminates the duct in the room and that the ceiling vent attaches to? They are quite deep but can be trimmed as required - you'll probably need to cut the hole in the ceiling from above once you know where you want it and then cut it flush on the room side. The ceiling cowls have a generous overlap so hide a good 1/2" or so around the plenum.

 

Get a big can of silicone spray as it makes sliding in the duct & rubber washer much easier - you will probably have to experiment a bit on how much slack to leave in the duct to allow for this.