MVHR - Poor Install

[OneSmallThing]

Hi,

I hired a company to supply and install an MVHR system in my bungalow, and to say I have significant concerns about their performance is an understatement! Some of it I know enough about and am picking up with them (e.g. the 'fixes' to holes they've made), but I'm concerned about the ducting that they've put in. It appears to follow an illogical path for me as it unnecessarily goes up and down as shown in the photos, which I supsect may add noise? Also does the ducting going to the valves and the manifolds themselves not need to be insulated, given that they travel through a cold pitch roof?

Thanks in advance for any help

 

[stem]

A slight duct level variation won't increase noticeable noise but it might restrict the airflow when compared to a dead straight run. However, a slight rise and fall in the duct will only cause a minimal additional resistance and nowhere near the same effect that a 90 degree bend would have. The fans will also have a performance specification and the duct resistance could be well within their duty anyway.

The trick with MVHR is to get the 'in' and 'out' airflow balanced, so that they are are as close as possible, this is so that as much as possible of the air entering and leaving the building goes through the heat exchanger and does not leak via other entry / exit points. This balance might be affected for example, if the incoming duct was short, and the outgoing duct was much longer by comparison, or vice versa. So, the duct lengths may have been extended on purpose to keep them similar lengths............maybe

When it comes to insulation, duct leaving the heat exchanger is waste air that has transferred its available heat anyway and is being exhausted outside, so insulation would be pointless. Unless the air being expelled is very humid and could condense inside a cold duct. In which case insulation may be advised. However, this often is only minimal and other arrangements can mitigate it such as sloping the duct down towards the outside to expel any water there, or the installation of a Condensation Trap.

Likewise the outside air coming in will be ambient and still to collect any recovered heat, so unless you are planning to run the system in the summer, and most users don't want heat transfer in the summer, (unless they have aircon when it operates in reverse) so they don't use it then, or run it in extract mode only. So, again insulating it wouldn't be necessary.

Ducts carrying air from the 'rooms to the heat exchanger' should definitely be insulated, as should the ducts carrying air from the 'heat exchanger into the rooms' as they both contain the warmth you want to keep.

 

[OneSmallThing]

Thanks for your reply stem. I do intend to run the system all year (we’ve done away with things like trickle vents and have air con running in kids bedrooms in summer).

And so the black ducting in all of the photos are the ducts carrying air from and to rooms to the manifold, which in then has the ducting to the unit (which is within the utility space). So if I’m understanding correctly what you are saying is that these should all be insulated? At present the installer has left them and said that they are finished.

 

[stem]

And so the black ducting in all of the photos are the ducts carrying air from and to rooms to the manifold, which in then has the ducting to the unit (which is within the utility space). So if I’m understanding correctly what you are saying is that these should all be insulated? At present the installer has left them and said that they are finished.

Yes, because you want the air being removed from the rooms to keep as much heat in it as possible so that it's available for recovery when it reaches the heat exchanger. So the duct should definitely be insulated where it runs through a cold loft or any areas where the temperature could be lower than the house temperature. This may not apply to the 'utility space' if it's at the same temperature as the rest of the house.

Likewise, once the incoming air from outside has picked up the heat at the heat exchanger you want to keep it as warm as possible for transfer to
the rooms

The picture below shows an industrial unit, but the principle is the same. Brown duct to outside uninsulated, duct with silver cladding and insulation is the warm air to and from the rooms.

 

[OneSmallThing]

Thanks Stem, really appreciate it. Confirms my suspicions that the install is missing a very significant element! Job for me it looks like! Assuming I can take the puppies down from where they have been tacked up, is simply running rockwool insulation over them likely to be sufficient (and for the manifold)?

Thank you again

 

[stem]

You could. Sometimes rerouting long straight sections under the roof insulation and applying separate insulation were it exits it can be sufficient. There are also proprietary products available. There's an example here.

I don't know what your arrangement with the installer was, if you asked them to install the parts you gave them, then you've got what you asked for. If they were contracted to do the complete install, I would argue, that unless it's specifically listed as an exclusion that the insulation should have been done by them.

There have been changes to the building regulations over the last few years and Parts F (Ventilation) and L (Conservation of fuel and power) may stipulate what should be done, if you want to take the time to investigate further.

 

[OneSmallThing]

It was for them to design, supply and install, so I think legally I’m in a good place, just sadly struggling for them to resolve it in the timescales needed.

Thanks for the notes on Part F and L, I’ve looked into those and both are explicit that all ducting should be insulated whether in a cold or a warm roof, so I think I’ve got a big fight on my hands to get this done correctly.

 

[endecotp]

The MVHR unit’s installation instructions may also have something to say about insulating the ducts. Saying “you have not installed it according to the manufacturer’s instructions” might be effective.