Whole House MVHR

[Bodgitnscarper]

Has anyone any experience of doing a DIY install of a mechanical ventilation and heat recovery unit and can make recommendations for a suitable unit?
House is 4 bed semi with 2 bathrooms, cloakroom, utility & fish room.

Thanks.

 

[stem]

I've installed a couple in two previous properties a bit smaller than yours (1 bed and 3 bed) The units I had were made by Rega and the last one was installed about 15 years ago. They were OK, but I don't have experience of anything else to compare them with in a domestic sense, (I look after some Mitsubishi Lossnay units at work which are pretty impressive but they are industrial and cost a small fortune!)

So here are a few things I learned:

Design is the key. Air entering the property from the inlets needs to be able to flow freely throughout the property to the extraction points. So, the positioning of the inlet and outlet points is vital, otherwise only small areas will be ventilated properly. The temptation is to run the duct where it's easy, but that may mean that the vents are not located in the best places.

In particular the bathroom extraction point should be diagonally opposite the door to the room where the replacement air will be coming in from.

I added a switch to turn off the inlet air fan in the summer when I didn't want heat recovery. During the summer it acted just as an extraction system and also saved energy by not running a second fan.

The property needs to be as airtight as possible so that the only movement of air entering and leaving the property through the system. Open windows is a 'no no'. (Except for during the summer when the inlet fan is switched off.)

For the air to flow freely through the building internal doors need to allow air to pass unrestricted from room to room, so I had to trim a few mm from the bottom of a couple of doors that had a tight fit on a fitted carpet.

With the first system, I left the fresh air inlet open in the loft as recommended by the Rega instructions. However, because air was being continually removed from the loft and cold air was coming in from around the eves to replace it, the loft got extremely cold. So, to avoid freezing pipes, I installed an intake duct to the outside. But because of being a bit lazy, I located it a bit too close to the outlet, so that it picked up some of the kitchen smells and returned them to the intake. A swift relocation soon sorted it out.

Do I have one now?... No, because Mrs Stem insists on having some windows open 24/7 even throughout the winter! My current property also has 2 chimneys which would provide a source of air external to the HRU. Also, whilst we found the HRU great for general use, we did find that at times of heavy kitchen use, the system didn't really provide sufficient extraction over the hob.

Hope my observations help.

 

[Bodgitnscarper]

Definitely helpful stem thank you I was considering a whole house system thinking it would be efficient due to the heat recovery, rather than just ejecting warm air straight out. However, mine is an ex local authority Wimpey no fines house not renowned for their insulating properties so I'm not sure if fitting this would be like peeing in the wind. I do have a chimney and the mrs is after a posh high efficiency type real flame gas fire so I don't know how that affect our plans either? Thanks.

 

[stem]

No house is completely airtight, which is probably a good job otherwise we would all suffocate! but it makes sense for as much of the airflow as possible to be through the heat recovery system.

With regard to the fire, if it is open, it would probably be an advantage over an 'extract only' system which may draw air down the chimney along with some fumes. If there is a separate air supply coming in from a HRU, then I would expect it to be less likely to do so. However, the high efficiency fires are usually 'room sealed' so wouldn't present an issue.

 

[Bodgitnscarper]

Thanks Stem.
With regard to running the ductwork, would it have to be as big as 150mm diameter to go from the loft into the downstairs areas? I was thinking of creating a void where soil pipe would go if I end up with an internal stack but obviously want to keep the false wall thickness to a minimum.

 

[stem]

The main ducts were 150mm dia. For one installation, I ran both ducts through the airing cupboard, and in the second installation, the kitchen extract duct went through the airing cupboard and the inlet duct though a built in wardrobe. Once the inlet duct was between the floors, it went to a distribution box with 4 smaller connections, probably about 80mm (ish) that then went to the individual inlet grilles in each room.

 

[Bodgitnscarper]

Thanks that's helpful. I am planning built in wardrobes to all the bedrooms and using these as a route down had not occurred to me.

 

[John D v2.0]

I got a couple of cheap airflow duplexvent dv72 online and fitted one upstairs and one down to reduce on pipe work.
I raised up the kitchen units very slightly so I had space for the ducting underneath. Also high wall cupboards in the living room can go on top.
Some people do drop ceilings but that's beyond me. Upstairs was no problem with the loft space, make sure the supply and extract pipes are under the loft insulation.
Just make sure the pipes are big enough.