Heat Recovery Ventilation System

[jim574783]

I am interested in installing a loft based heat recovery ventilation system for my two bedroom house.

The main reason is to provide a fresh air supply to the two bedrooms. With the windows shut, air quality is poor in these rooms, giving us an unpleasant nights sleep. Opening the windows at night is not an option due to noise outside.

Below is an outline of the plan I have come up with. I am not an expert in ventilation systems, so I am aware it might have some problems. However, I have done a lot of thinking and research to come up with it, so it should be a starting point at least.

If you have any suggestions, advice, criticisms, etc, that would be excellent, please reply.


Video tour of the house focusing on relevant areas

Diagrams


1.) Fresh air would come into the system from outside through an air brick at the top of the street facing wall of the house (see video at 0:00, diagram 1) into an attractive looking metal u-bend duct in the front bedroom (see video at 00:58, diagram 2) through which the fresh air would travel up into the loft. From there, through some rigid duct and into the HRV unit. The reason for this route is :

i.) I have been told that the roof tiles are likely to contain asbestos, so I wouldn't want to take air in through a special tile or cowl in the
roof. I know this fear may be irrational, but I don't want to take any risks with the air we will be breathing in our house.

ii.) The alley at the side of the house (see video at 00:12) is quite damp, dirty, no breeze and the neighbours boiler plume tends to drift down there.

iii.) There is often a nice breeze travelling along the road which isn't so apparent at the back of the house.


2.) From my research, I think this would be the most appropriate HRV unit as it is only a small house : Nuaire MRXBOX95-loft. It would be attached to the ceiling joists to minimise noise transmission from the unit through the bedroom ceilings.

3.) From the HRV unit, fresh warmed air would travel along rigid duct and out of fresh air vents in the bedrooms (see video at 01:09 and 01:30, diagrams 3 &4).

4.) Stale warm air would return into the system from the ceiling of the stairwell (see video at 00:32, diagram 5)

5.) Exhaust air from the HRV unit would exit out into the loft space. The condensate drain from the unit would exit on the garden facing side of the house through the eve (see video at 03:30, diagram 6)


My questions are :

a.) Would the HRV unit be appropriate ?

b.) Is it ok to dump the exhaust air directly into the loft in terms of the wood beams and other structures up there ? I have assumed so because the system would not be extracting from the kitchen or bathroom areas.

c.) What kind of duct, grilles, u-bend unit and connectors would I use to connect everything, and where could I buy them from ? How would the metal u-bend pipe connect to the airbrick and ceiling in the bedroom in an attractive way ? Would these two components let much noise in from outside ?

d.) If I wanted to expand the system at some point in the future to include air outlets downstairs in the two lounges, could I do so without replacing any of the existing equipment ?

e.) Would an electrician need to wire the unit in ? There is a power socket in the loft.

f.) Are there any other problems with this plan that I have not considered ?

 

[dhutch]

I would have thought it was much more sensible and common to take the fresh air from the attic, and the exit the stale air via a vent in the wall/roof/gable. This would also mean (if you used the gable) no visible ducting.

That's what my parents whole-house mhrv system does, which was designed by villavent back in the late 80's and fitted DIY.

My grans 'attic only' install, done about 10 years ago, also DIY with villavent kit and design support has two outside vents, on the sable gable, for in an out.

Out ducting came with, or was ordered with, the unit but I am sure you can use what you like up to a point. Ours was expanding aluminum, used for the lot, with galv t-pieces and or reducers where required, with the last 1m perforated, with 20mm of glass wool covered in polythene, to act as a silencer on the outlets (not inlets) maybe a little more for bedrooms.

Where is your bathroom? Suitable extraction point?

If you want to add more, you would need to spec the unit large enough, if you do, no issue, if not, performance will be at reduced level, clearly.

Don't know, and have not looked at, the exact unit.

 

[jim574783]

Hi,

I don't like the idea of taking air from inside the loft as it is quite dusty and generally not a nice atmosphere. Again, fully aware this might be an irrational fear, but I don't want to take any risks if the alternative isn't too complex.

The air brick location i suggested above would be the optimal position for fresh air quality, that is why I chose it.

In terms of where the air exits the system, I could use the wall/roof/gable, I am just anxious about making any more holes in the house than I need to !

The bathroom is some distance from the loft downstairs at the back of the house, and already has its own extraction fan, so would be happy to leave that out of the system, for the time being at least.

Thankyou

 

[highleigh]

I am no expert but I was told not to take the inlet from the loft space due to amount of dust in there so that seems good logic to me.

Why would you want the exhaust going into the loft either? It will contain moist air and I doubt that is going to be good for any loft space.

Other points:

Make sure you insulate any pipes in unheated areas, i.e the loft. you can buy insulated flexible aluminium duct which should work fine in the loft areas and help with noise reduction too.

For the ceiling grills get adjustable ones (different for inlet and outlet) this will allow you to control the flow to each room. You will want to ensure the total inlet volume matches the exhaust volume to retain the air pressure.

No reason why you couldn't expand it providing it can shift the amount of air. Not sure how many air changes an hour you will need but I am sure a quick google will give you an idea.

My heat recovery system had a wire that I put a plug on, no need for a qualified electrician.

It is normal to extract from the dampest area, this would normally be the bathrooms/kitchen but in your case this may well be the bedroom so you may consider reversing your flow?

Good luck, I don't know how efficient your system will end up being but it must be better than having an open vent to outside.

 

[SimonH2]

If not insulated, the metal elbow will be cold from the cold outside air, and you will get condensation on it.

There are unit which are designed to draw air from the loft. As long as it's filtered I wouldn't have thought it was a big problem. As a side effect, it will positively ventilate the loft which should eliminate any damp/condensation issues up there.

Don't even think of putting your exhaust air into the loft. It will be damp and will cause condensation - rotting your timbers.

 

[dhutch]

We have had no issues in 20 years from taking the inlet air from the attic space of my parents house, as worst, its only dusty when your in there crashing about, and there are inlet filters. The attic should be well ventilated from outside, if not perhaps well enough to take the exhaust air without condensation issues.

If you do find it an issue, you can/could always add the proposed inlet and duct afterwards, but I think its unnecessary.

Daniel

 

[jim574783]

Thanks for all your help so far.

Someone suggested that I could have the air inlet and outlet in the front and back soffit/gutter line. This sounds like a good solution to me, and means I wouldn't have to have the metal elbow in the front bedroom.

Here is a picture of the front soffit/gutter line:

Do you think there is enough room there ? The dimensions are almost exactly the same at the back.

 

[SimonH2]

There's enough space on the soffit itself - you can (if you make/adapt the trunking etc) have a wide & thin terminal. However, you need to get up in the loft and see if it's physically possible to get a ducting there.

It all depends on how the roof & ceiling were constructed relative to the top of the wall. My experience is limited, but generally it's "quite tight" in that area and the only way to physically get at it might be to take the bottom couple of rows of tiles off the roof (plus battens and undersheet) so you can work from above.
Alternatively, using round flexible ducting it can be squashed (though that affects airflow) to an oval. To keep a decent duct area you might need 2 or 3 smaller ducts and connect them to the larger duct internally with a "Y" piece.

 

[dhutch]

I did a sofet vent for my bathroom extractor, using the flat ducting (forget the exact dims, but 180*60mm ish) and that worked ok. Flat ducting, to a 45deg elbow, down to the sofet vent. The vent was a bit more restrictive than I would have liked but then I was using 6inch ducting with a 4inch fan so left it like it was. Otherwise I might have removed half the slats of the vent with a knife or similar. You could always put several in.

But I personally really would be happy with a loft intake, and would have used an end gable for my bathroom extractor if I had one, but thats the down side (is there an upside?) of a hipped roof.

Daniel

 

[jim574783]

The system has been installed with supply and extract vents in each bedroom and in the upstairs hallway. Unit used is the Nuaire MR BOX95-WM2.

Big problem is that it is too noisy for me to sleep with it on due to fan noise travelling through the ductwork. Very dissapointed as I was expecting something silent like you would find in a Hotel Ibis.

Does anyone have any suggestions for tweaks I could make to solve this problem? I was thinking silencers fitted on intake and extract, but not sure how effective this would be.

 

[SimonH2]

Does it use rigid or flexible ducting ? If it's rigid, replace it with flexible and the noise level will drop significantly - rigid ducting acts as a sound tube (think about the speaking tubes you see on ships in old films) while flexible ducting will allow the noise to leak out through the flexible walls. Downside is that flexible ducting is slightly more restrictive - particularly if you don't extend it fully to get the walls as smooth as practical.

 

[ban-all-sheds]

Don't even think of putting your exhaust air into the loft. It will be damp and will cause condensation - rotting your timbers.

Surely if the heat has been recovered from it it won't hold much moisture any more?

 

[SimonH2]

Why so ?
Take warm moist air and cool it - the amount of moisture will not change. That is the case until the degree of cooling means the cooled air reaches the dewpoint for the amount of water in the air. At that point, moisture will condense (giving up latent heat) - and the air coming out will be fully saturated.

So when it's cold outside, you stand a good chance of putting fully saturated air into the loft. Now that may be OK if everything in the loft is warmer than the air. But that won't be the case - there will be times when parts of the roof are colder than the outside air and you will get condensation.

Even if you don't get condensation, the high humidity levels will promote mould growth.
The only way to avoid this would be to overcool the exhaust air to condense moisture out, and then reheat it. In effect, you'd have to dehumidify the exhaust air and/or heat the loft (wasting energy) to avoid the damp/condensation/mould growth.


As an alternative to exhausting air into the loft, some units are specifically designed to take the intake air from the loft. As long as it's filtered (as air from outside needs to be) then there's no problem - and it will positively ventilate the loft which will reduce any damp problems there might be up there.

 

[dhutch]

Does anyone have any suggestions for tweaks I could make to solve this problem? I was thinking silencers fitted on intake and extract, but not sure how effective this would be.

On the systems I have fitted the last 6-8ft of each outlet has been perforated tube (expanded aluminum tube, with holes punched in it, in this case) which is then covered in an inch of glass wool, covered in polythene, taped the the tube joints at each end.

Effectively you are then creating a silencer, noise leaves via the holes, and is absorbed by the glass wool, with the polythene stopping the air getting out.

Commercially available from 'villavent' if not others.


Daniel