MVHR Design for review and discussion

[TerryE]

On 05/01/2017 at 19:17, ProDave said:

It's not necessarily the case that all mvhr pipework is "within the warm environment"

 

I have recently wired a new house, where the mvhr unit, and a lot of the distribution. pipework is up in the cold, ventilated loft space. Without lagging that would be a condensation nightmare (it might still be so)

 

Dave, I absolutely agree with this point general advice, but in Ian's particular case he his house is almost a 1½ storey configuration with all of his MVHR ducting running in the floor void between the ground an upper floors.

[le-cerveau]

@readiescards  you may not need to anti bacterially clean the ductwork but they will still gather dust, that is an inevitable fact of life no matter what you do, so there is some cleaning element to be done.

[TerryE]

In practice, I would only expect dust build-up on extracts from particularly dusty rooms -- which is one reason by BPC and others strongly recommend that you don't commission the MVHR until all of the building and decorating is done.

 

IMO, one advantage of the manifold-type systems is that all runs are the same diameter; they run direct from the plenum vent to the plenum manifold; and bends are usually limited by the tightest radius that the semi-rigid pipe can take.  So the you should only expect material build up on specific runs where there is a dusty environment.  And in this case you will often be able to flush the pipe using a sponge-ball  & draw-cord using a vacuum hose in plenum chamber.

 

As I said this is only my opinion so I would welcome other views.

[Bitpipe]

Also remember to switch off your MVHR if doing dust generating work post move-in. We've been reinforcing the stairwell and have been cutting out the old PB and vacuum sanding back the new work. Also, if you have ground level intake / extract vents (like us) its a good idea to switch off if doing dusty external work, like cutting slabs etc.

Edited January 7, 2017 by Bitpipe

[ToughButterCup]

Knowing me, I'll remember just that little bit too late....

But thanks for the nudge.

[dogman]

And Bonfires

[joe90]

I would be tempted to seal over the plumums till everything completed, can't forget then!

[SteamyTea]

I decided to look at my temperature and RH data for my kitchen (25 m³) as this is the room I use the most, and is probably the one with the highest long term humidity (bathroom is short and sweet and is soon sucked out by the fan).

By using the maximum RH and AH figures I can calculate what size extraction system I would need.

In my place I don't think I have much of a problem.

 

[Stones]

On 1/3/2017 at 14:02, PeterW said:

 Makes interesting reading from the installation manual..

 

• When using the product where it is exposed to high temperatures and humidity (40˚C or higher, RH 80% or higher), or where fog occurs frequently, moisture is likely to condense in the core, and may result in condensation build up in the unit. The product should not be used under such conditions.
• Outdoor air may enter the Lossnay owing to the pressure difference between indoor and outdoor or external winds even when the product is not operated. It is recommended to install an Electrically operated damper to block the outdoor air.
• In a cold weather area, an area with strong external winds or where fog occurs frequently, cold outdoor air, external winds or fog may be introduced into the product when its operation is stopped. It is recommended to install an Electrically operated damper. l When using the product in an environment where there is a window, or opening near the outdoor louvre , where insects are likely to gather around the interior or exterior light , take note that small insects may intrude into the product.
• In a cold weather area, or others, dewing or freezing could occur on the main unit, where the duct is connected, or other sections, depending on the conditions of outdoor air and indoor temperature and moisture, even if they are within the range of operating conditions. Make sure to confirm the operating conditions and other precautions, and do not use the product if dewing or freezing is anticipated. *Example of dewing condition - Outdoor air: -5˚C or lower, dew-point temperature at installation place: 10˚C or higher (When the indoor temperature is 22˚C or higher with the relative humidity higher than 50%, or other)

Not quite sure how to read that last statement !

 

I remember discussing these units when several were on sale on ebay.  I found a paper published in New Zealand suggesting these units were not ideally suited to climates with high RH. I wasn't convinced (despite the attractive price) that it was the right way to go because of that.

On 1/5/2017 at 19:03, TerryE said:

Yes, the heat exchanger cools the out-going air and this can drop it below dew point causing humidity to condense out.  The drain takes this away.

 

That's certainly been our experience (3 previous MVHR systems), at certain times / temps condensation would form and condense out.  Maybe the paper cores of these particular units mean this isn't an issue?

 

On 1/5/2017 at 19:17, ProDave said:

Terry

 

It's not necessarily the case that all mvhr pipework is "within the warm environment"

 

I have recently wired a new house, where the mvhr unit, and a lot of the distribution. pipework is up in the cold, ventilated loft space. Without lagging that would be a condensation nightmare (it might still be so)

 

If I could give just ONE recommendation on modern house design, it would be DO NOT have an old fashioned, ventilated cold roof space where you rely on insulation at ceiling level. Even if you are not doing "room in roof" it is SO much better to have a warm roof design where the insulation is at eaves level. SO much easier to detail to get an air tight house etc etc.

 

 

All of our MVHR ductwork, in both previous and current build has been installed in the cold roof space, but crucially, underneath a nice blanket of insulation, or where ducting was open to the cold roof space, it was lagged.  The only issue we had was in our last house but that was specific to our EAHP which exhausted air at -7C.  I quickly noticed the exhaust pipe mineral wool insulation sleeves becoming saturated with condensation, so swapped it out for rigid EPS duct insulation which solved the problem.  No issues at all with the other MVHR ductwork in a cold roof.

 

A warm roof would have been great, but there was an additional cost (for our build) and ultimately, we had to draw the line somewhere.