General: MVHR unit opinions

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- CommentAuthorArtiglio
- CommentTimeJan 27th 2021
Good evening all

I’m lookingminto an MVHR system for a conversion i’m doing, it’ll be a diy install and commission. One of the companies I’ve approached for system design has come up with a list of alternative system components. Part of which is the main unit, the offered options being

Domus Hrx
Vent Axia Sentinel
Komfovent Domekt

Whilst any are affordable on the basis of getting a decent build quality and basics, (a basic kitchen will suffice until savings allow something better etc) I’m not wanting to squander the budget unnecessarily.

Any advice / experiences would be appreciated
- CommentAuthordjh
- CommentTimeJan 27th 2021
I've heard of Vent Axia. What are the relative costs and specs?
- CommentAuthortony
- CommentTimeJan 27th 2021
I like Boulder, metal case, never any problems, economic, available
- CommentAuthorMike1
- CommentTimeJan 28th 2021 edited
The Passivhaus Institute provide a database of independently tested units at https://database.passivehouse.com/en/components/list/ventilation_small - and personally I'd choose one of those.

Komfovent Domekt does feature on it (heat recovery rate 78 %, specific electric power 0.4 Wh/m3), the others don't. The best units have a heat recovery rate of over 90% while using less electrical power.
- CommentAuthordjh
- CommentTimeJan 28th 2021
Posted By: Mike1The Passivhaus Institute provide a database of independently tested units at https://database.passivehouse.com/en/components/list/ventilation_small - and personally I'd choose one of those.

For myself, I would second that recommendation. We have a Brink unit that we're happy with.
- CommentAuthorDominic Cooney
- CommentTimeJan 28th 2021
We have the Vent Axia Sentinel, but isn't connected and up an running yet so I can't comment.
- CommentAuthorArtiglio
- CommentTimeJan 29th 2021
Thanks for the replies , even more to look into.
- CommentAuthorMike1
- CommentTimeJan 29th 2021
Posted By: Artiglioeven more to look into

There's always more to look into :)
- CommentAuthorArtiglio
- CommentTimeJan 30th 2021
Most certainly is and usually just as you’ve whittled things down to what you thought was a final choice.
- CommentAuthorSimon Still
- CommentTimeFeb 1st 2021
Vent Axia Sentinel has been running for 5 years here

It works. it still works. Case is plastic but it's in a cupboard so it's not visible and doesn't get any wear and tear.

There's really not much in these units - I'm guessing they're all basically the same construction
- 2 large fans
- a removable heat exchanger unit
- a load of polystyrene that it all mounts to (because you want all the internals insulated)
- some electronics to control fan speed

Looking at the materials they're staggeringly expensive (the Senintal is about £1k) compared to something like a washing machine, tumble dryer or gas boiler and if/when they become mass market I'd guess the price will go through the floor.
- CommentAuthorMike1
- CommentTimeFeb 2nd 2021 edited
Posted By: Simon StillThere's really not much in these units - I'm guessing they're all basically the same construction

Looking at the materials they're staggeringly expensive (the Senintal is about £1k)

True, but they also vary considerably in efficiency and energy use. Some that is no doubt due to the components - low-energy fans and a high-efficiency heat exchanger. But precision and build quality must be significant factors, to ensure that air doesn't bypass the filter, leak from the inflow to the outflow or escape through the casing. Then there are additions such as the type of summer bypass (if any).

Sounds simple, but I can't think of a mass market extract fan manufacturer that can make a high quality durable extract fan that doesn't start to rattle or pack up after of a few years. Consequently I wouldn't be in a rush to buy an MVHR unit from one either. Particularly if they avoid having it independently tested by the Passihaus Institute.
- CommentAuthorWillInAberdeen
- CommentTimeFeb 2nd 2021 edited
Noticed that one of the units in the OP sells for £1700 in UK but for the Euros equivalent of £1300 in some other countries. I struggle to believe the postage is £400 so it seems like the market is charging whatever the market will bear to pay. If buyers pay inflated prices, they only help to inflate them further. The usual cure is for a new entrant to enter the marketplace and undercut all the others, let's hope that happens soon!

Edit: another option to consider is to use several smaller units (£250ish each) distributed around the house so that each serves one wet room and one dry. Hopefully this will work out cheaper for us as for our requirements it adds up cheaper price than one bigger unit with manifolds, and avoids some very awkward duct runs back to a single central unit.
- CommentAuthorArtiglio
- CommentTimeFeb 2nd 2021
Evening WiA

The multiple small unit idea was the first proposed, but the building is listed and whilst the permissions allow for fairly extensive internal works the council is keen to keep the exterior looking as close to its original as is practical, so putting multiple vents throughbthe walls is a bit of a non starter especially as several are flint and as such makes each hole a bit of a mission.
The system is already being split into 2 as the building is in 2 fairly distinct parts and getting ducting between them was going to be problematic ( especially as this was not,considered from the outset)
- CommentAuthordjh
- CommentTimeFeb 2nd 2021
Posted By: WillInAberdeenit seems like the market is charging whatever the market will bear to pay

Twas ever thus

I wrote to one robot vacuum company expressing displeasure at UK prices versus those elsewhere and their response was exactly "it's a separate market"!
- CommentAuthorjms452
- CommentTimeFeb 6th 2021
Nuaire have sub 1k units with a metal box and up to 95% efficiency.

https://www.nuaire.co.uk/product-list-page/mrxbox-eco

We have a rebadged older version of this that's still ok after 9 years (although we only run it for 6 months of the year)
- CommentAuthordjh
- CommentTimeFeb 6th 2021
Posted By: jms452Nuaire have sub 1k units with a metal box and up to 95% efficiency.

They seem to be getting better. A few thoughts on those models:
- I would definitely recommend getting the AB models (auto summer bypass) that's very useful.
- before buying I would check that it's possible to fit better than G3 filters. G3 is very basic. We use G3 on the exhaust just to protet the heat exchanger from house dust. We use a G4 and a G3 on the intake - the G3 catches large crap such as carne flies in season and the G4 filters smaller crap a bit. Both fit in the slot together. Pollen filters are about the same thickness as a G4 if you need/want to use those.
- I didn't spot anything that says how the frost protection works, which makes my cynical mind suspect it's not a separate pre-heater as required by PHI, nor reflected in test results I think, so that's another thing to consider
- CommentAuthorEd Davies
- CommentTimeFeb 6th 2021
Posted By: djh- I didn't spot anything that says how the frost protection works, which makes my cynical mind suspect it's not a separate pre-heater as required by PHI, nor reflected in test results I think, so that's another thing to consider
The installation manual says:

5.6 Frost Protection

In the event of the intake air temperature at the unit dropping below the predetermined set point (-5°C as standard) the supply fan will reduce to minimum speed, once the temperature rises above the set point the fan will return to its commissioned speed.
I.e., overwhelm any frost by blasting it with greater quantities of extract air so sucking more air in through any other leakage in the house.

Posted By: djh- I would definitely recommend getting the AB models (auto summer bypass) that's very useful.
Depends where you live. Maximum outside air temperature here last year was 21°C.
- CommentAuthorcjard
- CommentTimeFeb 6th 2021
I've a VentAxia Sentinel Kinetic PlusB, and I've no complaints - I've also got 4 cheapo digital thermometers off ebay that I have yet to fit to it so I can see what overall temp exchange it manages (it's only been installed for 4 years, I'll get round to it) but the most important job it does is keep the other half happy with the way the house doesn't smell..
- CommentAuthordjh
- CommentTimeFeb 6th 2021
Posted By: Ed DaviesI.e., overwhelm any frost by blasting it with greater quantities of extract air so sucking more air in through any other leakage in the house.

Ah, thanks, that's what I suspected. That's specifically not allowed for PHI-certified units. There are articles on the web explaining the reason, I believe.

Depends where you live. Maximum outside air temperature here last year was 21°C.

Ouch! I'm not sure I'd want to live somewhere with a purely heating climate to be honest.
- CommentAuthorSimon Still
- CommentTimeFeb 15th 2021
While we're on this, would be grateful for any suggestions. We have a VentAxia Sentinel Kinetic Plus. The 150mm input/output inlets from the unit are on top and have just a few cm of 'upstand' to which you attach the pipes to outside for extract and input.

Our pipes are 150mm rigid plastic, which was insulated with rigid polystyrene. then I stuffed the ceiling space that the pipes run in with insulation too.

In the cupboard where the MVHR unit sits the final metre or so between the MVHR unit and the rigid pipe is done in foil flexipipe with an fibreglass wrap. Final connection in Flexi was advised so pipes didn't have to line up precisely and also to isolate noise from the room vents (connections to manifolds are the same).

The issue is the inlet pipe - in really cold weather this must get some condensation on it that then runs over the top of the MVHR unit and collects on the floor (or in a plastic tub if it's not been moved by other stuff stored in the cupboard)

Any suggestions as to how to improve this final connection? it seems however I attached a pipe that final few cm (and the plastic inlet on the machine itself) will be very poorly insulated and get some condensation.
- CommentAuthorbhommels
- CommentTimeFeb 15th 2021
Posted By: djhA few thoughts on those models:
- I would definitely recommend getting the AB models (auto summer bypass) that's very useful.
- before buying I would check that it's possible to fit better than G3 filters. G3 is very basic. We use G3 on the exhaust just to protet the heat exchanger from house dust. We use a G4 and a G3 on the intake - the G3 catches large crap such as carne flies in season and the G4 filters smaller crap a bit. Both fit in the slot together. Pollen filters are about the same thickness as a G4 if you need/want to use those.

+1
Our Paul Focus 200 allows for F7 filters which are another step up from G4. Since fitting F7s one member of the household could stop using anti-allergy medication.
Well worth the extra power consumption and a recalibration of the flow rates that fitting finer filters requires. To protect the F7 from clogging up with coarse crud I left the G3 filter in place as it also protects the defroster.
- CommentAuthorWillInAberdeen
- CommentTimeFeb 15th 2021 edited
Edit: replying to Simon, post crossed with bhommels

Where is the cupboard? Presumably somewhere inside the house, where the absolute humidity is greater than it is in the outdoors air, hence the condensation? Is the room a utility or kitchen, which is a bit humid anyway?

Bit left field, but could you reduce the humidity locally in the cupboard, by making a tiny hole in the duct that carries warmed fresh outdoor air from the mhrv to the house. This dry air will leak into the cupboard and surround the cold duct, which is at or above the dewpoint of the outdoors air. Eventually the dry air will leak out of the cupboard into the room, where it will help with the ventilation and then be recycled back to the mhrv.
- CommentAuthordjh
- CommentTimeFeb 15th 2021 edited
Also cross-posted. Also replying to Simon.

https://www.roofingsuperstore.co.uk/product/ubbink-150mm-insulated-duct-2m.html and associated products are good and easy to use. They may be flexible enough to get in place by bending/stretching etc, or you could cut a length at an angle so you could fit both ends in place and then rejoin the angled cut with some glue and duct tape. We secured the connection to the MVHR inlets with very large jubilee clips. Cable ties would probably work as well.
- CommentAuthordjh
- CommentTimeFeb 15th 2021
Posted By: WillInAberdeenBit left field, but could you reduce the humidity locally in the cupboard, by making a tiny hole in the duct that carries warmed fresh outdoor air from the mhrv to the house. This dry air will leak into the cupboard and surround the cold duct, which is at or above the dewpoint of the outdoors air. Eventually the dry air will leak out of the cupboard into the room, where it will help with the ventilation and then be recycled back to the mhrv

The plant room where the MVHR is situated is normally plumbed with an extract terminal rather than a supply terminal, FWIW.
- CommentAuthordjh
- CommentTimeFeb 15th 2021 edited
Posted By: bhommelsWell worth the extra power consumption and a recalibration of the flow rates that fitting finer filters requires. To protect the F7 from clogging up with coarse crud I left the G3 filter in place as it also protects the defroster.

Just to say that if the MVHR unit has constant volume fans then there's no recalibration of flow rates required, but there is extra power draw of course. I'm glad I'm not totally mad using a G3 as a pre-filter to capture the big stuff
- CommentAuthorWillInAberdeen
- CommentTimeFeb 15th 2021 edited
Posted By: djhThe plant room where the MVHR is situated is normally plumbed with an extract terminal rather than a supply terminal, FWIW

Yes, that's the problem here. It's drawing in more-humid air from the house, with a dewpoint greater than the outdoors temperature, so it inevitably condenses on the duct. Simon needs to surround the duct with air that has a dewpoint no greater than the outdoors temperature
=> supply some warmed dry outdoors air to that location.

Simon described it as a cupboard rather than a plant room, so I'm sure he'll clarify where it is, like I asked.
- CommentAuthorSimon Still
- CommentTimeFeb 15th 2021 edited
Posted By: djhAlso cross-posted. Also replying to Simon.http:///newforum/extensions/Vanillacons/smilies/standard/bigsmile.gif" alt="" title="" >

https://www.roofingsuperstore.co.uk/product/ubbink-150mm-insulated-duct-2m.html" rel="nofollow" >https://www.roofingsuperstore.co.uk/product/ubbink-150mm-insulated-duct-2m.htmland associated products are good and easy to use. They may be flexible enough to get in place by bending/stretching etc, or you could cut a length at an angle so you could fit both ends in place and then rejoin the angled cut with some glue and duct tape. We secured the connection to the MVHR inlets with very large jubilee clips. Cable ties would probably work as well.

That's interesting. I guess I'd only really need to worry about the incoming air as that's the only one that causes an issue. Might be worth a try. Could redo the connections to the manifolds and try to add attenuators as well
- CommentAuthorWillInAberdeen
- CommentTimeFeb 15th 2021 edited
Our single room mhrv has been running in condensing mode. One of these grows in a day or so.
- CommentAuthorSimon Still
- CommentTimeFeb 15th 2021 edited
Posted By: WillInAberdeen
Posted By: djhThe plant room where the MVHR is situated is normally plumbed with an extract terminal rather than a supply terminal, FWIW

Yes, that's the problem here. It's drawing in more-humid air from the house, with a dewpoint greater than the outdoors temperature, so it inevitably condenses on the duct. Simon needs to surround the duct with air that has a dewpoint no greater than the outdoors temperature
=> supply some warmed dry outdoors air to that location.

Actually, our MVHR is in an odd space - it's in a cupboard in our basement without any extract or supply. Our basement is mostly my partner's furniture restoration workshop. It's inside the thermal envelope (but the UFH is switched off so it's usually a little cooler than the rest of the house) and the external door gets opened a few times most days allowing a lot of external air in. The pipes for an inlet and extract within the room (given it's size) are in place but sealed off and not connected because she creates a fair bit of dust. The rest of the house was balanced with these vents sealed. Theres a door to the rest of the house but should be no positive or negative pressure between the workshop and the house.
- CommentAuthorGreenPaddy
- CommentTimeFeb 15th 2021
Duct insulation.

this stuff - already split
https://www.bes.co.uk/heating-ventilation/ventilation/ducting/ducting/domus-easipipe-insulation-1-m-long-x-150-mm-dark-grey-22424/

As you'll know, the intake AND exhaust is essentially external air temperature (exhaust slightly warmer), so you can't avoid condensation on the duct skin in winter, unless insulated. You ought to have the same level of insulation on the ducts as you have in your walls/loft etc, since it's a piece of the outside world, inside your house.

Start with the moisture (reasonably) impermeable insulation as above, and seal as best you can, to stop any moisture (room air) getting to the duct wall. Then add lots more insulation, to prevent heat loss - wrapping with wools, or building a rigid insulation box around the duct, or adding increasing layers of tubular insulation eg

https://www.airconspares.com/insulation/pipe-insulation/rockwool/rockwool-40mm-thick-219mm-bore-foil-faced-pipe-insulation-x-1-metre

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General: MVHR unit opinions