dMVHR - paired or single units

[JamesPa]

Im looking at fitting two or three dMVHR units to my extended 1930s house, which suffers in some areas from poor ventilation.  I've ruled out whole-house MVHR (too much disruption), extract only/passive intake (don't want to make the heat loss worse) and PIV (targets the wrong areas if fed from loft, which seems to be the norm to re-use otherwise wasted heat).

 

There seem to be three principal types/operating modes from DMVHR

 

1. Units which operate in pairs, in theory forcing air through the house, each unit alternating between extract and intake

2. Units which alone, alternating between intake and extract

3. Units which operate alone, with simultaneous intake and extract 

 

Logically (1) seems potentially the best, (2) the worst and (3) somewhere in between.

 

What is the general view, and are there any other considerations or things I should be aware of?

Edited May 15, 2023 by JamesPa

[HughF]

An interesting topic and something that I'm also looking into as we start on our rear extension, into which I don't want to add trickle vented windows.

 

There is a lot of product on the market now, and it's hard to figure out the best option.

[JohnMo]

The only concern with these units is do any of them move enough air around so you get a proper through flow of air, is it just moving a 6m volume of air in to and out of the house. Working on a dilution principle instead of an air exchange principle.

 

Read up on cascade MVHR, this use a central supply or two, then extract in suitable location to get proper cross flow of air.

 

Also when you do all the calcs, condition based MEV isn't much worse than MVHR. You need to factor in install cost, buying the hardware, running cost, filter cost etc. It only allows air exchange if it's needed based on humidity, not all the time.

 

Plenty of nay sayers on here, but I did all the calcs for our house and it was pretty good, but our airtightness and building control, means we have to have a balanced ventilation system, so it had to be MVHR.

[JamesPa]

On 17/05/2023 at 16:31, JohnMo said:

The only concern with these units is do any of them move enough air around so you get a proper through flow of air, is it just moving a 6m volume of air in to and out of the house. Working on a dilution principle instead of an air exchange principle.

 

Read up on cascade MVHR, this use a central supply or two, then extract in suitable location to get proper cross flow of air.

 

Also when you do all the calcs, condition based MEV isn't much worse than MVHR. You need to factor in install cost, buying the hardware, running cost, filter cost etc. It only allows air exchange if it's needed based on humidity, not all the time.

 

Plenty of nay sayers on here, but I did all the calcs for our house and it was pretty good, but our airtightness and building control, means we have to have a balanced ventilation system, so it had to be MVHR.

Thanks for this.  Is cascade mvhr the fresh-r system with fans between rooms.  I was looking at that and as it happens it might suit my layout.  Does anyone else do something similar as fas as you know?

[JohnMo]

Fresh-R is one, but you can do it with a normal MVHR unit also. You could use something like a PIV supply nozzle connected to a standard MVHR as a single supply point. With extracts in wet rooms and where required transfer fans.

 

The other thing to look at is coanda terminals, for supply and extract. They work by the velocity of the air sticking to a surface (ceilings), so you can have a supply one side of a room (even at a doorway or air exit point), air coming out of the nozzle will travel approx 4-6m then come down, travel back across the room, down the corridor to the nearest extract point. The air when it comes down is at room temperature, the air stream is wide so velocity is low, no draft. I have them in lounge and all bedrooms, duct lengths are shorter compared to a normal nozzle.