MVHR - locate in warm or cold loft space?

[Stones]

Turning my attention to the MVHR service area I have, I need to decide whether to insulate the area of loft space which will house the MVHR unit and distribution boxes as a warm space (i.e. line the attic trusses with PU boards) or leave it as a cold space (i.e. earthwool insulation between ceiling joists, with MVHR and distribution boxes on a raised platform above.  

 

Insulating over the top of the ductwork and distribution boxes is easy enough in the cold roof scenario, so it's really a question of whether there are significant disadvantages to having the MVHR unit (Vent Axia Sentinal Kinetic Plus) in a cold space.  

 

Ducts to and from the MVHR unit will be insulated and would in the case of the air intake and exhaust have to pass into cold roof space at some point. Looking inside the MVHR unit itself, it's mostly an insulated box.  

 

A cold space would certainly be easier and cheaper (as I would need 6 x 100mm PU boards to insulate the space as a warm space), but that has to be set against the increased heat losses from the MVHR unit if it is in a cold rather than warm space.  Looking through the manual, it suggests insulating the unit for optimum performance if installed in a cold space, but doesn't quantify what the losses may be / how the performance would differ.

Anyone care to comment / have a stab at what the likely losses would be in a cold space?

 

 

 

 

 

[jack]

Personally, I'd keep the MVHR within the heated area to avoid potential condensation issues.  I can live with a little bit of condensation in the external ductwork.  I'm less happy with the prospect of condensation within the MVHR unit itself, mainly for the longevity of the electronics and fan bearings etc.

 

The actual difference in heat loss between the two scenarios will be difficult to quantify.  The performance of an MVHR unit is hit surprisingly hard by the length of uninsulated external ductwork coming into the heated area of the house (for an internal MVHR - vice versa if it's outside).  The unit itself will also contribute some losses, but I suspect you'd need to know the exact makeup of the interior MVHR insulation relative to the duct paths to understand whether internal or external mounting will give higher losses.  This is because the one side of the MVHR is at the external temp and the other is at the internal.  When it's very cold outside, there's a big temperature difference between these two, and it's hard to model how the unit as a whole will act in terms of heat transfer.  I suppose you could negate this issue by heavily insulating the unit itself.

[Stones]

I've previously located units both within and outwith the insulated envelope.  The only condensation issue I've ever had was with the exhaust ducting from the EAHP in my last house, but then that was pumping out air at -8C.  I resolved that issue by fitting rigid ductwork and rigid duct insulation to the exhaust duct run as opposed to the wrap around glass wool and foil type that had previously been there. 

 

As you suggest, one option could be to insulate the unit itself, either lagging it directly, or building a PU box around it.

[AliG]

Resurrecting topic.

 

My MVHR was in a warm space but I decided to change from PIR to rockwool on flat areas of ceiling so the ducting is now in a cold space.

 

The MVHR supplier has quoted £1540 for 50mm insulation and 20 rolls of duct tape to cover 150m of MVHR ducting. looking online it seems that maybe only 25mm is necessary. Can anyone point me to insulation that they have used?

[gregcope]

re insulation I used some of this off ebay over rigid tubing - go a size larger than your rigid pipe work as otherwise it will not fit.

 

Aluminium, Combi, Insulated Ducting Ventilation Flexible 4” 5” 6” 8” 10” 12"

https://www.ebay.co.uk/itm/282832906316

[gregcope]

Sorry about the double post... [Mod note: duplicate post deleted]

In terms of "where".  I would locate it somewhere easy to maintain.   You might need to access the front/rear so ensure its relatively easy to remove the ducting and condensate drain.  I think inside / heated envelope would be better to gain HE efficiency, although you need to be able to mount the condensate drain.    I would avoid really long runs to the distribution plenums.

Mine is in an unheated loft space.  Which sometimes goes below 0C as the loft insulation is rather effective!

[Marvin]

An MVHR, in my opinion, should be within the thermal envelope to help keep the return temp the same as the temp inside the thermal envelope.

[beddows_s]

12 hours ago, gregcope said:

Sorry about the double post... [Mod note: duplicate post deleted]

In terms of "where".  I would locate it somewhere easy to maintain.   You might need to access the front/rear so ensure its relatively easy to remove the ducting and condensate drain.  I think inside / heated envelope would be better to gain HE efficiency, although you need to be able to mount the condensate drain.    I would avoid really long runs to the distribution plenums.

Mine is in an unheated loft space.  Which sometimes goes below 0C as the loft insulation is rather effective!

Hi Greg 

 

What would you class as a "really long run"?  I'm currently in the stage of designing our system for a retrofit and looking at options and duct runs is something i'm looking at; both from unit to manifolds and then manifolds to plenums.

[gregcope]

Something around 10M?

 

This will add drag and hence you might need to run the fans higher to achieve the required room supply/extract rates.

 

Ideally you want the unit near the plenums that are in a “mid point” so that runs are minimised.   Reality might be different.

 

For context my unit input / extract are 2M.   My house feed is 3M and extract 1M.   My unit is in the “middle” of the loft.

 

i would suggest mounting the unit somewhere easy to maintain it.

 

its all a balance of differing priorities.