Extension + existing house: dMVHR or the full works?

[rco]

Hi folks,

 

After some advice. We're building a ~50m²/125m³ open plan pitched-ceiling extension on the side of our 1950s two storey detached house in Scotland (downstairs 70m², upstairs 49m²). Ventilation in the existing house is rubbish, and condensation/damp is a regular occurrence: particularly upstairs. We've been gradually improving the insulation & airtightness on the existing house, and the extension is well-designed. We've had a pseudo-MVHR on another house on the other side of the world and it certainly kept the house dry, so I'm investigating options here. Drawings are attached, hopefully gives a reasonable idea.

 

I'm getting a design done for a full MVHR — I think there's a sane way to get ducts from the ground floor ceilings to the attic through the existing accessible eaves, and then locate the unit in the (floored & cold) attic at the gable end. Without boring through joists. But the indicative quote was £8K+VAT with us self-installing.

 

The other option I have thought of is two dMVHR, something like the Prana 210G ERP PRO-mini in the extension (we have enough wall thickness), and another dMVHR for the upstairs, maybe a small ducted unit serving the bedrooms and located in the attic. And forget about downstairs. Is something like the Prana commercial series what we should be looking at? What other options are worth considering?

 

dMVHR will obviously work out a lot cheaper upfront and quicker, but will we be constantly disappointed?

 

Thanks in advance, Rob

buildhub-1.pdf

[jayc89]

We're installing MVHR in our renovation. We've managed to find a couple of areas quite centrally that can be used as service voids to drop ducting down from the loft, where the unit will be, to the ground floor rooms. Our joist directions are also helpful to allow us to run that ducting between rather than over. On one side, we're able to drop down through an airing cupboard and the other side we're just going to make a stud wall between a bedroom and ensuite slightly wider to accommodate the ducts. 

If you're seriously considering insulation and airtightness, service voids will likely have been thought of anyway, to avoid penetrating your airtightness layer, so perhaps this could be an option for you too?

[gambo]

I'm about to make sketch a plan of the house and send it to companies for quotes but I'm expecting £2-3k for the full kit (without labour) for a 4 bedroom house. I don't know yet what the best way is to lay the ducting downstair but I'm thinking about using walls upstairs. One of the rooms downstair can be tricky but it's neither living room nor kitchen, so I'm thinking about single room mvhr unit as a backup if it causes lots of issues.

[JohnMo]

Look in to BluMartin Freeair100 and FreshR, look at the principle of operation, how they cascade the air movement. You can apply the same principles to any MVHR system. 

 

Also look into Coanda effect supply nozzle, these will allow air to follow the ceiling to move from one side of the room to the other. Making supply nozzle placement easier.

[rco]

Hi all, 

 

Thanks for the replies

 

Quote

We've managed to find a couple of areas quite centrally that can be used as service voids to drop ducting down from the loft, where the unit will be, to the ground floor rooms. Our joist directions are also helpful to allow us to run that ducting between rather than over. 

 

Yes, the "middle" of our house is a stairwell/landing & a chimney... but more nastily, the joists all run front-to-back, so to get to the middle is always across joists. Hence the idea of running it between joists to the eaves and up to the attic that way. _Potentially_ I'm estimating the longest run would be a supply: from the MVHR unit 8.5m horizontally, a 90° bend, down 1m, a 45° bend, down 1.5m, a 90° bend, and then 6.5m horizontally (total 18m). That excludes any distribution boxes in the middle. Is that getting ridiculously long & windy, or fairly normal for a multi-storey house?
 

Quote

 

I'm about to make sketch a plan of the house and send it to companies for quotes but I'm expecting £2-3k for the full kit (without labour) for a 4 bedroom house.

 

 

 

Who are you getting quotes from? That seems... cheap to me? We're soon to be a 4 bedroom, 2 bathroom house with 3x living spaces (1 is big). The indicative number was from Paul Heat Recovery, I guess it could be gold-plated, and I can certainly push back on it.

 

Quote

Look in to BluMartin Freeair100 and FreshR, look at the principle of operation, how they cascade the air movement. 

 

Yes, the Fresh-R/Freeair/Prana stuff all seems Our house (upstairs, where the issues are) has very little wall space. If a unit could be connected out through a sloped tiled roof it'd be a possibility, otherwise we're limited in where we can locate it — but I haven't seen any dMVHR units with that option except maybe the Prana commercial ones?

 

Thanks all! Any more suggestions welcome.

[jayc89]

I suspect you'd want to double up on that sort of length. We have a 13m run which I'll double up. 

 

How about 2x units, one for the existing side and one for the extension?

[JohnMo]

2 hours ago, rco said:

Fresh-R/Freeair/Prana

You have missed my point. Also by the way the Prana unit is not the same as the Fresh-R or Freeair.

 

Neither Fresh-R or Freeair are dMVHR units. You need to study how they work, they are full MVHR units less the huge amounts of duct normally required. Making very practical for retrofitting. You can use the same design principles using a basic MVHR unit in your loft or cupboard. And cascading air from one place to another.

[rco]

Quote

Neither Fresh-R or Freeair are dMVHR units. You need to study how they work, they are full MVHR units less the huge amounts of duct normally required. Making very practical for retrofitting. You can use the same design principles using a basic MVHR unit in your loft or cupboard. And cascading air from one place to another.

 

Apologies — Freeair is described on the web as a "decentralised MVHR", and Fresh-R was discussed here in a decentralised MVHR topic 🙂 I appreciate both of them can have extra extract ducting added — though IMO neither of them particularly well explain as to whether that's an "as-well" as the main extract or an "instead-of": the blueMartin video at least suggests "as-well". I like the idea of the BluMartin Transfer & Fresh-R-Forward cascade units for making these all-in-one units go further still.

 

As I said initially, main issues for the condensation is in the upstairs of the existing house, so maybe it's a case of getting a small "full" MVHR unit in the attic and just ducting that together sensibly (through ceilings & under insulation); then having a blueMartin Freeair100 / Fresh-R in the extension space and calling it done.

 

[jayc89]

1 hour ago, JohnMo said:

You have missed my point. Also by the way the Prana unit is not the same as the Fresh-R or Freeair.

 

Neither Fresh-R or Freeair are dMVHR units. You need to study how they work, they are full MVHR units less the huge amounts of duct normally required. Making very practical for retrofitting. You can use the same design principles using a basic MVHR unit in your loft or cupboard. And cascading air from one place to another.

 

Are you talking about a single Fresh-R unit and using Fresh-r Forward units to push stale air from room to room towards it?

[JohnMo]

There are also units that have ducts also.

 

Here is the plan from fresh-r for my house.  Didn't use in the end, but liked concept. Two units, 3 pipes and one forwarding fan, for 192m2