MHRV Ducting in Attic and insulation

tails_naf

I have an MHRV system installed, and most of the ducting is run through the unheated attic. It uses the solid white PVC ducts. The supplier provided it with a roll of 1-inch foil-backed insulation, to wrap the ducts in, which has been done.

The unit reports the temperature of the air, and it is a good few degrees down from the room temp (i.e. when the room temp is 20, the air temp reported from the unit is 15C).

So the basic question is - how much insulation of the duct-work is required typically?
If I took some standard attic insulation and wrapped the ducts in that (say to 4-inch thickness) would that be suitable?


Any tips greatly appreciated!

BryanF

tails_naf wrote: »

I have an MHRV system installed, and most of the ducting is run through the unheated attic. It uses the solid white PVC ducts. The supplier provided it with a roll of 1-inch foil-backed insulation, to wrap the ducts in, which has been done.

The unit reports the temperature of the air, and it is a good few degrees down from the room temp (i.e. when the room temp is 20, the air temp reported from the unit is 15C).

So the basic question is - how much insulation of the duct-work is required typically?
If I took some standard attic insulation and wrapped the ducts in that (say to 4-inch thickness) would that be suitable?


Any tips greatly appreciated!

Put the insulation over pipes and keep a close eye on temps as you've described above. If this helps great, but I wonder do you need the supplier to come in and check temp sensors and service

AstraMonti

I am not sure how your unit works, but you probably only need to insulate only the two of the pipes, the one that brings the air from the house to the unit and the one that brings the air from the unit to the house. The others will either bring cold air in from outside or bring the "bad" air outside and I can't see a reason why you need to insulate them.

I am no expert at all with this, I was just very closing watching my HRV installation.

Andrew_Doran

AstraMonti wrote: »

I am not sure how your unit works, but you probably only need to insulate only the two of the pipes, the one that brings the air from the house to the unit and the one that brings the air from the unit to the house. The others will either bring cold air in from outside or bring the "bad" air outside and I can't see a reason why you need to insulate them.

The pipes to and from the atmosphere are usually recommended by MVHR manufacturers to be covered with vapour-impermeable insulation, with all joints in the insulation taped.

The thinking is that attics can get hot in the sunshine, and these pipes could be cold in such a situation which could in turn cause condensation to occur on the pipes.

Another question might be whether or not that is a valid concern in the Irish climate -- I don't have an answer.

creedp

Andrew_Doran wrote: »

The pipes to and from the atmosphere are usually recommended by MVHR manufacturers to be covered with vapour-impermeable insulation, with all joints in the insulation taped.

The thinking is that attics can get hot in the sunshine, and these pipes could be cold in such a situation which could in turn cause condensation to occur on the pipes.

Another question might be whether or not that is a valid concern in the Irish climate -- I don't have an answer.

I have recently discovered that the duct taking air from unit to outside has been leaking into the attic for some time - only noticed when damp patch appeared in ceiling. When I went up into attic I found the duct uncovered and the joints unsealed with water dripping from each joint. I also noticed water droplets coming from the joint where the duct meets the vent cover in the slate. I didn't think that the air leaving the unit would contain so much moisture but when I put containers under the drips they collect just under a litre a week. I then covered the duct with wool insulation and found that the amount of water reduced massively with the most coming from the joint where duct meets the air vent in slate.

I presume if I now seal the joints and keep the duct covered that the amount of moisture generated won't affect the runing of the system? Im thinking that if it gathers in pipe it will eventually drain back into unit and drain out of bottom of unit. However Im concerned now that some/all of the joints in ducting may not have been sealed and its possible the ducting could be leaking elsewhere where it is much more difficult to access.

Its not the only problem I've had as originally the installer didn't fit the drain pipe to the unit and it drained water directly into attic making a right mess of the place. Then I found out he installed the drain on the incoming air side of unit and not the waste side and because of this it wasn't able to drain properly allowing water to build up in unit causing the motors to short. Subsequently, he put in 2 drains and teed them together with a 1/2'' tee, however the drain pipes were 3/4'' and because of this he pushed 1/2'' pipe into 3/4'' pipe but didn't put clamps on the pipes and loo and behold they leaked all over the place again - about 1/2 litre a week into the attic for a number of weeks before I noticed it again. So at this point Im truly sick of dealing with water leaking from the unit.

Apologies for the meandering nature of this story but I would advise whoever people are getting to install HRV units make sure they know what they are doing because if not installed properly they can make a right mess of your house and obviously be inefficient.

Anyway, anybody with any advise re moisture in duct from unit to roof .. is it a problem or is it OK to seal and insulate?

Andrew_Doran

Disclaimer: I am not a professional/tradesman in this area.

creedp wrote: »

Apologies for the meandering nature of this story but I would advise whoever people are getting to install HRV units make sure they know what they are doing because if not installed properly they can make a right mess of your house and obviously be inefficient.

Concur. By all reports it would seem there are some installers learning as they go. I would be looking for someone with trade experience of air conditioning systems or similar if I was paying for an install.

This report (interesting reading) suggests that in one survey of MVHR installs, 50% had the ductwork connected to the wrong terminals on the unit. :eek:

http://www.zerocarbonhub.org/resourcefiles/viaqreport_web.pdf

creedp wrote: »

I presume if I now seal the joints and keep the duct covered that the amount of moisture generated won't affect the runing of the system? Im thinking that if it gathers in pipe it will eventually drain back into unit and drain out of bottom of unit.

I do not know about your system but the extract side of my HRV is designed to run wet. I am however surprised that you are getting significant condensation in the exhaust ductwork after the exchanger, as the extracted air should have been substantially cooled before it enters the exhaust ductwork, meaning that there should not be much condensation left to occur.

creedp wrote: »

Its not the only problem I've had as originally the installer didn't fit the drain pipe to the unit and it drained water directly into attic making a right mess of the place. Then I found out he installed the drain on the incoming air side of unit and not the waste side and because of this it wasn't able to drain properly allowing water to build up in unit causing the motors to short. Subsequently, he put in 2 drains and teed them together with a 1/2'' tee, however the drain pipes were 3/4'' and because of this he pushed 1/2'' pipe into 3/4'' pipe but didn't put clamps on the pipes and loo and behold they leaked all over the place again - about 1/2 litre a week into the attic for a number of weeks before I noticed it again. So at this point Im truly sick of dealing with water leaking from the unit.

In case you have not already considered it, the extract side of an MVHR system is an environment conducive to the growth of bacterial and fungal nasties. The Y in the piping may mean that the two airspaces are bridged (supply and exhaust).

Some food for thought:

• Duct joints can be sealed with a suitable duct mastic and aluminium foil tape (not duct tape, despite the name it is unsuitable). Available from ventilation / air conditioning suppliers.
• Do all terminals on the unit have the correct ducts connected to them? (seriously -- see above)
• If I were in your position I would remove the Y from the condensate drain and cap off connection to the wrong side of the exchanger.

creedp

Andrew_Doran wrote: »

I do not know about your system but the extract side of my HRV is designed to run wet. I am however surprised that you are getting significant condensation in the exhaust ductwork after the exchanger, as the extracted air should have been substantially cooled before it enters the exhaust ductwork, meaning that there should not be much condensation left to occur.

I was surprised myself and this is why I initially though it was the roof leaking and got builder back to look at it. As I said when I insulated this duct the amount of water leaking from it decreased substantially except where it meet the vent cover in the roof where almost a half a litre of water leaked during last week alone. I will seal these duct joints and see what happens.

In case you have not already considered it, the extract side of an MVHR system is an environment conducive to the growth of bacterial and fungal nasties. The Y in the piping may mean that the two airspaces are bridged (supply and exhaust).

Some food for thought:

• Duct joints can be sealed with a suitable duct mastic and aluminium foil tape (not duct tape, despite the name it is unsuitable). Available from ventilation / air conditioning suppliers.
• Do all terminals on the unit have the correct ducts connected to them? (seriously -- see above)
• If I were in your position I would remove the Y from the condensate drain and cap off connection to the wrong side of the exchanger.

To be honest I hadn't considered this issue. Im not familiar with the construction of the units but it would appear from what your saying that one side of unit is sealed from the other and by teeing the condensate drain from each side of unit I could be linking them and creating a possibility of moisture getting into the supply side of the unit.

In my unit, it would appear that unit is split top from bottom with the exhaust supply entering into/exiting from the bottom half and the supply side entering/exiting from the top side. The condensate drains then seem to be draining the unit where the exhaust air enters the unit and where it leaves it after passing through the heat exchanger. I think the latter drain may mainly be draining the moisture falling back into the unit from the duct taking the cooled exhaust air to the roof. If that actually is the case then I don't think there is a danger that the tee could contaminate the supply side. However, Im not confident about this and I think I will contact the manufacturer and get clarity on this matter. I've tried to discuss this with the installer a number of times but am not happy with the responses I get.

In the meantime I'll have a look at unit and see can I identify if the ducts are connected correctly. I have to say the unit has been installed in one of most ridiculous places it could be in the attic. You would think there was no space in the attic for it so it was located in a little alcove where you can barely access to examine it. It makes it very difficult to do any maintenance on it such as changing filters etc.

Another issue I have is that the boost setting for showers etc does not stay on long enough .. five min max. I need to increase this to 20 min but as I said to access the unit to do this is extremely difficult. Such a pain!!

Anyway thank you very much for the advice and I will certainly follow up on the condensate drain issue asap.