MVHR and log burner

[IanR]

22 minutes ago, Tetrarch said:

The 1914 leaky house has now been mostly demolished and rebuilt, significantly more airtight than it ever was

 

Unless your airtightness is better than 3m³/m².h @ 50Pa you don't need mechanical ventilation, adding MVHR will just increase your ventilation more than is required, lose additional energy (recovering 80% - 90% at the cost of running the fans) and cost you maintenance on the filters.

Kitchen and bathroom extraction is far simpler.

Edited June 28 by IanR

[JohnMo]

5 minutes ago, IanR said:

Kitchen and bathroom extraction is far simpler.

Greenwood CV2GIP in each room done, silent, smart humidity boost activation, 3 wires is all they need.

[SuperPav]

As a counter to this discussion, and from someone in a very not-passivhaus house (but OK insulation!), we have MVHR, and a log burner. 

 

The log burner is in a room with a supply vent. 

 

I agree with all the above points about it defeating the purposes of an MVHR in a airtight house, but for us the MVHR is more than about heat recovery. It runs on the lowest setting, so is quiet, but it provides good background ventilation to closed habitable rooms, which (at least subjectively) means the air doesn't seem stale in any room (which can't be said for previous houses we've had) and even the completely enclosed tiny wetroom bathroom we've got dries out wonderfully. We also didn't want window trickle vents and lots of bathroom/kitchen extracts, so I'm still happy with the MVHR, even if it costs us money to run. Arguably we could've fitted a positive ventilation system for some of the above benefits but there really isn't that much in it in terms of cost (all was DIY'd, the MVHR unit was a good deal.) Probably further offset by the fact we have free wood to burn!

 

 

Having said all that, if you're after optimal thermal performance, then lose the WBS.

[Tetrarch]

On 28/06/2024 at 15:52, SuperPav said:

As a counter to this discussion, and from someone in a very not-passivhaus house (but OK insulation!), we have MVHR, and a log burner. 

 

The log burner is in a room with a supply vent. 

 

I agree with all the above points about it defeating the purposes of an MVHR in a airtight house, but for us the MVHR is more than about heat recovery. It runs on the lowest setting, so is quiet, but it provides good background ventilation to closed habitable rooms, which (at least subjectively) means the air doesn't seem stale in any room (which can't be said for previous houses we've had) and even the completely enclosed tiny wetroom bathroom we've got dries out wonderfully. We also didn't want window trickle vents and lots of bathroom/kitchen extracts, so I'm still happy with the MVHR, even if it costs us money to run.

 

Completely agree with everything in your second paragraph.

 

Can you tell me what kind of "supply vent" you bought?

 

Regards

 

Tet

[SteamyTea]

On 27/06/2024 at 11:31, Tom said:

This pretty much makes the MVHR pointless

Or a stove pointless.

[saveasteading]

There is a some overstatement here perhaps, that one or the other is pointless.

That what people here have decided to do is the only right decision.

It will depend so much on the building design, lifestyle and location.

 

The extremes may be a passivhaus in an urban situation, or a  conversion in a rural situation. Most of us are in between.

 

For anyone who doesn't know. A modern wbs takes air by duct from outdoors and draws nothing from the room.

I think either or both options could be justified for the right circumstances.

[ProDave]

MVHR is NOT all about air tightness of a house, at least not just about how air tight you can make the fabric of the building.

 

If i take the example of my last house, my first self build.  I don't believe the basic fabric was leaky.  But because it did NOT have mvhr, instead it had:

Kitchen cooker hood exhausting through a 100mm hole.

Utility room extract fan, venting through a 100mm hole.

4 bathrooms, each with it's own extract fan, exiting through a 100mm hole.

A stove that drew it's combustion air from the room, and for that it had an air inlet vent built into the hearth do admit outside air into the room directly behind the stove.  and because it was not room sealed, the stove flue effectively was vented to the room.

We also had a standard letterbox in the front door, a cat flap, and every window had a trickle ventilator which even when "shut" I doubt were particularly air tight.

 

That is what a "standard" house has as a collection of big holes to let air in and out uncontrolled.

 

Fit mvhr and we have NONE of those.  So regardless of what air tightness test you get, simply eliminating those collection of huge holes to be replaced by one inlet and one outlet and an mvhr system is bound to be a big improvement.

 

Of course because everything was thought about this time, the WBS is room sealed drawing all it's combustion air directly from outside.

[Nickfromwales]

The only elephant I see, is if the airflow out of the WBS is in excess of the power / flow of the fan in the MVHR unit, then there is a strong likelihood that 'smells' from bathrooms / kitchens 'could' make their way into the room that the WBS is in, but the stink from the WBS would probably overwhelm that anyways.

 

Seems kinda moot now anyways as the OP has installed the kit and is 'happy'.

 

The likely natural infiltration of this dwelling probably renders everything commented on above a complete waste of time anyways, as MVHR has no place in anything other than an airtight structure of 1.0 ACH or less.........  

[saveasteading]

8 hours ago, Nickfromwales said:

If the airflow out of the WBS is in excess of the power / flow of the fan in the MVHR unit,

In a sealed circuit wbs there is no air intake  from the room.

[IanR]

38 minutes ago, saveasteading said:

In a sealed circuit wbs there is no air intake  from the room.

The  Riva 76 mentioned in the OP takes its air from the room. If it were a room-sealed WBS, this would be a different discussion.

 

On 28/06/2024 at 15:52, SuperPav said:

but for us the MVHR is more than about heat recovery.

 

I'm not sure I understand this point. If there's sufficient natural ventilation to not require MVHR, then the heat recovery is ineffective. The HR is only achieved on the additional, unrequired ventilation, so the MVHR increases the loses of the house and will require additional heat input to mitigate.

 

On 28/06/2024 at 15:52, SuperPav said:

I'm still happy with the MVHR, even if it costs us money to run.

 

Which is completely fine, but there are lower cost alternatives, especially for retrofits.

 

MVHR in itself does not provide any energy saving, it's an enabler for high airtightness (<3m³/m².h @ 50Pa). It's the airtightness that brings about the energy saving. As a relatively expensive install (especially retrofit) and with continuing day-to-day running costs it's not easy to achieve a return on the investment.

 

9 hours ago, Nickfromwales said:

as MVHR has no place in anything other than an airtight structure of 1.0 ACH or less.........  

 

This aligns with my thoughts, MVHR starts to make sense with an airtightness of approx. 1.5m³/m².h @ 50Pa (~1 ACH). Unless you plan to get to this level then hitting 3m³/m².h @ 50Pa with trickle vents and individual dMEVs (with HR if you like) in wet rooms, is the more cost effective option.

Edited July 1 by IanR

[JohnMo]

25 minutes ago, IanR said:

MVHR starts to make sense with an airtightness of approx. 1.5m³/m².h @ 50Pa (~1 ACH). Unless you plan to get to this level then hitting 3m³/m².h @ 50Pa with trickle vents and individual dMEVs

Trouble with Scotland MVHR is mandatory for an airtightness of 3 or below. If you hit 2.9 you have to install MVHR before BC sign off.

 

But have to say MEV or dMEV can be made to be incredibly efficient with self modulating trickle vents and dMEV or MEV altering flow rates based on demand. Very low running costs, no filters, no heat recovery but on an otherwise well insulated house running costs are going to low anyway from a heating perspective.

[Kelvin]

Yep and I know of people who have ‘purposely’ achieved slightly higher than 3 to avoid putting it in. The consequence of this requirement in Scotland is if you make any reasonable attempt at air tightness you’ll be below 3 so therefore you’re better to do it properly and aim for below 1 or plan to be above 3. I was told of a situation where someone achieved just over 1, couldn’t afford the MVHR system as had run out of money so put holes in the building to get it back above 3.  
 

 

[JohnMo]

1 hour ago, Kelvin said:

couldn’t afford the MVHR system as had run out of money so put holes in the building to get it back above 3.  

It does make it all a nonsense, possibly refilled all holes after leak test, then will the first to complain they have mould everywhere.

[Kelvin]

31 minutes ago, JohnMo said:

It does make it all a nonsense, possibly refilled all holes after leak test, then will the first to complain they have mould everywhere.

The builder told me that they’d been in the house a year before putting holes in the building so already had a condensation problem and early signs of mould. Pretty awful situation. If that wasn’t bad enough they had employed a QS at great expense and still ran out of money. It was his suggestion to put holes in the building. 

[ProDave]

1 hour ago, Kelvin said:

Yep and I know of people who have ‘purposely’ achieved slightly higher than 3 to avoid putting it in. The consequence of this requirement in Scotland is if you make any reasonable attempt at air tightness you’ll be below 3 so therefore you’re better to do it properly and aim for below 1 or plan to be above 3. I was told of a situation where someone achieved just over 1, couldn’t afford the MVHR system as had run out of money so put holes in the building to get it back above 3.  
 

 

I am very surprised that if you have all the "normal" holes in a building as listed above for individual extraction and trickle vents that he achieved an air test of less than 3.  Surely it would be easy to open a couple of windows a crack just to get a worse air test than make permanent unwanted holes.

[Kelvin]

They set out to build as airtight as possible and fit MVHR so few penetrations through the walls, windows didn’t have trickle vents etc. They ran out of money so didn’t fit the MVHR and then started having condensation problems.  This was the suggested solution from the QS of all people included breaking the window seals. Their intention was to get it back above three permanently not just to get it past BC.
 

I’m sure I posted about this at the time. The kit erectors left my house to go to this house to make the necessary changes to get it above 3. They weren’t that happy about it but were doing what they were being told to do. 

[Tetrarch]

Hi 

 

Just by way of explanation. The project has evolved significantly since it started. It is mostly new now eith a brand new insulated room.The diagram below shows the house as it is now

Black = Insulated timber frame or BC-complaint 0.17 solid walls

Orange = 1970s cavity wall

Grey = 1914 solid wall

Red = Added insulation

 

In my opinion the house should be very airtight.

 

 

The fireplace is a retained feature - it was something we wanted to keep and the WBS is as much an aesthetic choice as a secondary heat source

 

 

 

Regards

 

Tet

[ProDave]

56 minutes ago, Kelvin said:

They set out to build as airtight as possible and fit MVHR so few penetrations through the walls, windows didn’t have trickle vents etc. They ran out of money so didn’t fit the MVHR and then started having condensation problems.  This was the suggested solution from the QS of all people included breaking the window seals. Their intention was to get it back above three permanently not just to get it past BC.

I am surprised BC passed it without trickle ventilators or mvhr.

[IanR]

56 minutes ago, Tetrarch said:

In my opinion the house should be very airtight.

 

"very" will mean different things to different people, but assuming you expect it to be better than 5m³/m².h @ 50 Pa then you need to install the additional ventilation which for your unit needs to be equivalent to a 10cm x 5cm clear hole to outside.

 

 

With that additional ventilation installed, you'll then need to perform the the "full spillage test " they mention, with all extracts and ventilation running on max. If it passes Building Control should not have an issue.

Edited July 1 by IanR

[Kelvin]

Reading between the lines it hadn’t got final sign off but they had been granted temporary occupancy.  I was getting all this third hand from two builders so who knows so I suspect I didn’t have the full story. 
 

It’s why I was surprised when I applied for the temporary occupancy approval that  it was initially rejected because they wanted the house completely finished as per the warrant given other houses I was aware of that had loads left to do compared to us. Different councils. 

[JohnMo]

7 minutes ago, Kelvin said:

when I applied for the temporary occupancy approval

I never bothered asking, wasn't really aware I had to. But started paying council tax that day. No one seemed to care, so didn't ask questions.

[Kelvin]

The BCO regularly drove past the house as on his route. Illegal to move in without approval etc. He has been really easy to deal with so keeping him on-side made sense. That includes getting me out of a rather large hole the groundsworker caused me! 

Edited July 1 by Kelvin

[Tetrarch]

1 hour ago, IanR said:

 

"very" will mean different things to different people, but assuming you expect it to be better than 5m³/m².h @ 50 Pa then you need to install the additional ventilation which for your unit needs to be equivalent to a 10cm x 5cm clear hole to outside.

 

 

With that additional ventilation installed, you'll then need to perform the the "full spillage test " they mention, with all extracts and ventilation running on max. If it passes Building Control should not have an issue.

 

Thank you - that is terrific information. I fully expect that the fire will only be used sparingly. 

 

Is it possible to use something like this as a manual vent? I can locate this directly behind the fire to make it invisible, but reachable when it is being used

 

https://www.ebay.co.uk/itm/385765701438?chn=ps&norover=1&mkevt=1&mkrid=710-153316-527457-8&mkcid=2&itemid=385765701438&targetid=4584757337645565&device=c&mktype=&googleloc=&poi=&campaignid=554427144&mkgroupid=1298524317493059&rlsatarget=pla-4584757337645565&abcId=9313282&merchantid=87779&msclkid=67fb7f5074de1dcfd3bb19d2d3237291

 

Regards

 

Tet

[IanR]

4 minutes ago, Tetrarch said:

Is it possible to use something like this as a manual vent? I can locate this directly behind the fire to make it invisible, but reachable when it is being used

 

I don't believe you could install a vent that could be closed off.

[saveasteading]

16 minutes ago, Tetrarch said:

something like this as a manual vent?

You need a pipe from outside directly into a purpose made socket in the wbs.