Implication of not using MVHR

[Indy]

Current plans are for a masonry build and SAP calcs include MVHR + cooling option. Not passive house but BR specs and airtightness target of 3 (I think).

 

After speaking to a local builder, we’re leaning towards a timber frame build (MBC maybe) for speed mainly.

 

Wanted to understand what the implications would be if I decided not to go for an MVHR system – mainly for cost. It seems to be adding about £10k+ to the build cost and to be fair – I’d rather use it for an air con system. Not seeing the payback apart from the comfort option and my personal feeling is that the comfort I get from having heating/cooling/dehumidification from an air con system is going to work better for me.

 

What would it mean for the windows – do I have to add trickle vents or is there another way to overcome this requirement in a generally airtight build?

 

I assume not going for MVHR would trigger a new as built SAP calc to be done at the end of the build, but as long as we achieve the same or higher in terms of overall efficiency – that should be ok?

[IanR]

38 minutes ago, Indy said:

Current plans are for a masonry build and SAP calcs include MVHR + cooling option. Not passive house but BR specs and airtightness target of 3 (I think).

 

You'll need to be worse than 3m³/m².h@50Pa, with trickle vents closed, otherwise BC should require you to have a mechanical ventilation system. Without mechanical ventilation you will need trickle vents in windows.

 

38 minutes ago, Indy said:

Not seeing the payback apart from the comfort option and my personal feeling is that the comfort I get from having heating/cooling/dehumidification from an air con system is going to work better for me.

 

There's no payback directly on MVHR, but, MVHR enables a < 3m³/m².h@50Pa infiltration rate. It's the low infiltration rate that provides the payback on the MVHR. ie. the reduced energy loss from, say, having a 0.5m³/m².h@50Pa infiltration rate pays for the MVHR.

Edited Monday at 12:24 by IanR

[JohnMo]

10 minutes ago, Indy said:

MVHR system – mainly for cost. It seems to be adding about £10k+

Good case of shopping around, I have 2x MVHR units both from eBay, and think I was a total of just over £2kwith all additional materials installed. But I did design and install myself.

 

Other alternatives are

 

MEV or dMEV, both need trickle vents, but this should be self acting (automatic), the MEV or dMEV fans should run at a low rate and automatically boost or increased extract rate based on humidity and or CO2. 

 

As mentioned if you test 3m³ per m² or better you may have to retrofit balanced supply and extract.

 

45 minutes ago, Indy said:

MVHR + cooling option

Expensive option, can be pretty rubbish as flow rates aren't really high enough. If you go this option you need to be able to run the MVHR at full speed to get any useful cooling.

 

Ventilation is basically just ventilation, it's pretty rubbish at anything else. 

 

You have to do an as-built sap anyway. Increased energy usage due to additional heating my also drive to complete other things like PV you may have not budgeted for.

 

My advise do MVHR, look around and shop around, understand the design of it and requirements. Dump cooling and any heating option.

 

Not sure of your ventilation requirements but here is a unit after 10 seconds on eBay - less than £500. Titon are a good make, have good tech support in UK and UK made, with full spares support.

 

https://www.ebay.co.uk/itm/405573660870?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=eo-y_vbiseu&sssrc=4429486&ssuid=8KgwDlAbTJS&var=&widget_ver=artemis&media=COPY

[Bonner]

MVHR is one of the best things I did, wouldn’t like to live in a house without one. Self installed for less than £2k. 

[nod]

It goes show what nonsense the air tests are We scored 2 having no idea of this change They simply put it down as 3 like most of the large scale builds 

[Indy]

£2k sounds virtually impossible unless I source odds and sods off eBay myself - which implies I would know what's needed. Even the most basic unit from BPC without cooling was £4200 (Vent Axia) which then climbs up to £5800 for Zenhnder. That's the parts only without install.

 

[JohnMo]

3 minutes ago, Indy said:

£2k sounds virtually impossible unless I source odds and sods off eBay myself - which implies I would know what's needed. Even the most basic unit from BPC without cooling was £4200 (Vent Axia) which then climbs up to £5800 for Zenhnder. That's the parts only without install.

 

I just pointed you towards an MVHR unit, about £1k off full retail. I bought everything else from Paul Heat Recovery and EpicAir. Everything except MVHR units is either Zender or Ubbink, about £1500 plus my 2x MVHR units (£550 for the two in total delivered)

[crispy_wafer]

we are coming in around 3k for our mvhr.  Could have been more if I wanted plaster in terminals, could have been less if I had a central design rather than in the loft with 2*75mm runs to all terminals.  I don't know the scale of your build, but I'd baulk if someone quoted 10k to me!

 

Like many on here I have no loyalty or trust in any one company, shop about to secure savings, I'm pretty sure there's a thread somewhere in here where many have shared the companies they'd used for parts and bits.

Edited Monday at 14:35 by crispy_wafer

[JohnMo]

18 minutes ago, crispy_wafer said:

but I'd baulk if someone quoted 10k

My first quote was £10k, hence I started to learn quickly.

[Bonner]

4 hours ago, Indy said:

£2k sounds virtually impossible unless I source odds and sods off eBay myself - which implies I would know what's needed. Even the most basic unit from BPC without cooling was £4200 (Vent Axia) which then climbs up to £5800 for Zenhnder. That's the parts only without install.

 

I bought a brand new unit was about £750, gone up a bit in the last three years. Rigid plastic ducting and valves were another £500. The MVHR is very basic but does exactly what it needs to, runs continuously at 30% speed with occasional manual boost. Of course you can spend more on sensors and auto controls but even a basic unit is infinitely better than trickle vents. https://www.ventilationland.co.uk/en_GB/p/itho-daalderop-hru-350-eco-unit-hr-rft-high-rise-350m3-h/9795/

[Iceverge]

In the real world mechanical ventilation is a must if you value the quality of the air you're ingesting. I would never suggest anything else as a sensible option. 

 

MEV works well, but to have some control over the infiltration, you'll need humidity sensitivity wall vents (about £60 each).

 

Say you have one per bedroom and living room maybe 7 in total=£420

 

Then add dMEV in every room with a tap. Maybe £70 x 5 = £350. 

 

A little bit of additional wiring and fitting and you'd have a well performing ventilation system for £1000. 

 

For a 200m² house it'll cost roughly 3000kWh of extra energy to be used annually.  Say £250 extra annually on heating. Electric will be much the same. 

 

There'll be minimal servicing but allow maybe £70 per year for a dMEV needing replacing. 

 

£320 per year for a 200m² house isn't too bad.

 

20 year cost £7400 

 

A bit of shopping around will see you buy a new MVHR unit and the bits for say £4k. Allow £2k for install for some reasonably handed builders. It'll cost you £40 per year in filters and the bearings will need to be done every 4. Allow £100 per year in maintenance. 

 

25 year cost £8000. 

 

There's very little in it long term in £££. Break even is about 20-25 years I'd estimate. 

 

 

 

Edited Monday at 21:35 by Iceverge

[Iceverge]

There's more to consider with MVHR too.

 

1. Noise from outside.

2. Filtered incoming air

3. Heat load ( how big a boiler you need) 

4. The stability of the internal temperature. 

5. Drafts. 

6. Insects in the house. 

 

Hard to put a more accurate price on these. However as I posted elsewhere recently Airtightness and MVHR are far more important than extra insulation so perhaps they might be a better place to trim if you're really stuck for cash. 

 

[Mike]

16 minutes ago, Iceverge said:

There's more to consider with MVHR too.

 

1. Noise from outside.

2. Filtered incoming air

3. Heat load ( how big a boiler you need) 

4. The stability of the internal temperature. 

5. Drafts. 

6. Insects in the house. 

Not to mention

7. Avoiding the need for extractor fans

8. Eliminating the risk of condensation & mould

9. Improved indoor air quality due to removal of indoor pollutants - CO₂, VOCs...

 

10 hours ago, Indy said:

airtightness target of 3

That's really not very ambitions. Using the advice elsewhere on this site you should be able to get much lower than that.

 

7 hours ago, JohnMo said:

£5800 for Zenhnder.

That doesn't sound too unreasonable for the parts; they do good quality kit. If you're practical and have the time, then it's not that difficult to fit it yourself. But even a cheap unit is better than none (and could be upgraded in the future).

 

10 hours ago, Indy said:

Not seeing the payback

What's the payback on your kitchen units / bathroom suite / floor coverings?

 

[Iceverge]

1 hour ago, Mike said:

Avoiding the need for extractor fans

8. Eliminating the risk of condensation & mould

9. Improved indoor air quality due to removal of indoor pollutants - CO₂, VOCs...

 

I think these are pretty much covered with dMEV too which was what I was recommending as a starting point and I estimate comparable long term costs to MVHR with less up front initially.

 

Hole in the wall insulation is really just hope for the best and I don't think anyone should consider it for a house in 2025. 

 

I'm certainly an advocate of MVHR though. 

 

 

 

[G and J]

A slight sidestep topic wise but I’ve quotes for a designed and commissioned MVHR system and could clearly save money by doing as others have in here, but…

 

Our BCO has stated they require the MVHR to be commissioned by a suitable professional.  Is there a sensible way round that?

[Bonner]

I told them that I am a qualified engineer which they accepted (I am a chartered mechanical engineer but knew bugger all about MVHR before I started!). Was going to do a report but they never asked for one.

[JohnMo]

7 hours ago, G and J said:

Our BCO has stated they require the MVHR to be commissioned by a suitable professional.  Is there a sensible way round that?

Ours said similar, but the requirements are to be commissioned with instruments with a valid certificate of calibration and a certificate to be issued.

 

They guy who did our air test, did the MVHR final commissioning and issued certificate with calibration certificate details. I did everything else, design, purchase and install.

[JamesP]

8 hours ago, G and J said:

A slight sidestep topic wise but I’ve quotes for a designed and commissioned MVHR system and could clearly save money by doing as others have in here, but…

 

Our BCO has stated they require the MVHR to be commissioned by a suitable professional.  Is there a sensible way round that?

Most self builders on the forum had no issues with their BCO and MVHR. 

 

My BCO rejected my first self certifying report as the Anemometer had no proof of being calibrated, (used the Buildhub anemometer) I  hired this : https://www.inlec.com/testo-417-vane-anemometer-with-flow-hoods 

 

Just looked through past emails and found this reply from the BC. 

 

I have checked the ventilation commissioning paperwork and this needs to be undertaken in accordance with the below paragraphs from Approved Document F.

 Air flow rate testing and commissioning of ventilation systems4.42 The Regulations require:• mechanical ventilation systems to be commissioned (where they can be tested and adjusted) to provide adequate ventilation and a commissioning notice to be given to the BCB;• air flow rates for mechanical ventilation systems in new dwellings to be measured and a notice to be given to the BCB.4.43 For dwellings, the procedures approved by the Secretary of State for measurement of air flow rates and for commissioning of mechanical ventilation systems are in Sections 2 and 3 of the Domestic ventilation compliance guide. Section 5 of the guide contains air flow measurement test and commissioning sheets which should be completed by the person responsible for commissioning.

[G and J]

51 minutes ago, JohnMo said:

Ours said similar, but the requirements are to be commissioned with instruments with a valid certificate of calibration and a certificate to be issued.

 

They guy who did our air test, did the MVHR final commissioning and issued certificate with calibration certificate details. I did everything else, design, purchase and install.

Did they charge an awful lot extra to do the MVHR commissioning and certification?

[JohnMo]

14 minutes ago, G and J said:

Did they charge an awful lot extra to do the MVHR commissioning and certification?

£200

[G and J]

2 minutes ago, JohnMo said:

£200

I spot a change of plan looming on the horizon.  
 

It all depends how quickly we sell our house.   If we go under offer quickly (a nice problem to have) then I won’t have time to faff around and I therefore expect we'll use a design and commission package.  If the house lingers on the market for ages then there is scope for me to follow your lead and save lots.

[JohnMo]

Just now, G and J said:

depends how quickly we sell our house

Why do you need to sell you house to design the MVHR? Ideal first step is know how big a unit you need, then look out for a bargain 

[G and J]

41 minutes ago, JohnMo said:

Why do you need to sell you house to design the MVHR? Ideal first step is know how big a unit you need, then look out for a bargain 

I started down that path months ago and got some way, partly to make me a bit more of an informed buyer.   But now my attention span is one of many constrained budget lines.  Plus I have to constantly fight my ‘I must do everything’ obsession.

[JohnMo]

1 hour ago, G and J said:

Plus I have to constantly fight my ‘I must do everything’ obsession

I know that feeling. I eventually went down the path of what I can do well. Plasterboard - no, too heavy, walls ok but not ceiling. So got someone else to do it all. Joint Taping, the same.

Electrics - no

Second fix joinery - no. 

Roofing - no, likely to kill myself.

Windows - no one company does it all 

Everything else was fair game.

[Dave Jones]

On 10/03/2025 at 11:45, Indy said:

Current plans are for a masonry build and SAP calcs include MVHR + cooling option. Not passive house but BR specs and airtightness target of 3 (I think).

 

After speaking to a local builder, we’re leaning towards a timber frame build (MBC maybe) for speed mainly.

 

Wanted to understand what the implications would be if I decided not to go for an MVHR system – mainly for cost. It seems to be adding about £10k+ to the build cost and to be fair – I’d rather use it for an air con system. Not seeing the payback apart from the comfort option and my personal feeling is that the comfort I get from having heating/cooling/dehumidification from an air con system is going to work better for me.

 

What would it mean for the windows – do I have to add trickle vents or is there another way to overcome this requirement in a generally airtight build?

 

I assume not going for MVHR would trigger a new as built SAP calc to be done at the end of the build, but as long as we achieve the same or higher in terms of overall efficiency – that should be ok?

 

if your build is 3 month stasrt to finish timber frame is worth the premium if not your wasting the double cost of a timber frame over a standard build.

 

that wasted cash can buy a lot of 2nd fix items.