Opinions on this MVHR/AC combo

[yuumei]

There have been one or two threads on how to combine MVHR and AC but I don't think there is consensus on it I want to retrofit AC and was thinking to do MVHR at the same time. I think having the indoor unit in the loft with ductwork going to the rooms is the easiest. What do you think of this layout for reusing the AC ducts for MVHR too? (If I didn't have the "zoning" part, I could probably do away with the one-way valves)

[JohnMo]

You would really need do some calculations, but I have the following comments

 

Flow from AC will exceed that of the MVHR by a considerable margin. Could the flow from the MVHR actually break in to the flow of the AC? To understand this you need to understand the pressure drop over the AC system and where on the curve your MVHR fans are when AC is running and when it's off. Once set up to work with AC your MVHR would most likely be completely out of calibration if the AC fan wasn't running or any speed changes occur.

 

Nice idea, but think it would be difficult (impossible) to set up to run correctly all the time. Run them as separate systems.

[yuumei]

Thanks for the comments John! Yes I think the issue is that when both AC and MVHR are set could be balanced but then when anything is turned up or down it will unbalance it. I think MVHR isn't too much of a problem since it will be on low 99% of the time and boost only for a small time. I am still worried that when the MVHR Is on low and the AC is turned up, it might get past the non return valves and exit that way which would be a bit pants.

[JohnMo]

Some MVHR control schemes will give out as set flow rate, once commissioned and automatically adjust fan speed to compensate for blocked filters etc.

 

I would just disengage the 2 systems - KISS is always best

[joth]

10 hours ago, yuumei said:

I don't think there is consensus on it

I think there is a consensus: that it's not easy to get it right.

 

What I've done in one room is share the AC outlet plnum with the MVHR pipe, so in essence they're separate systems right up the an inch from the grill face, but from the room side all you see is one shared linear slot grill. That's as integrated as I was comfortable with. (and only did this because the most convenient retrofit location for the AC linear grill was where the MVHR outlet already was in that room. Other rooms I kept them separate)

 

 

[kevinm]

I considered the same idea but decided against it.

 

Air has such a low heat capacity, you need to move a lot of air (noisy) to get a decent cooling effect. Also any cooled ducts will need to be insulated to stop condensation forming in the void spaces (potential for mould of drips staining ceilings.

 

I'm still waiting for the phpp calcs to be done, but I'll most likely be going independant MHRV and a few hidden splits in ceilings where needed.

[Mike]

On 05/10/2023 at 11:47, yuumei said:

I want to retrofit AC and was thinking to do MVHR at the same time

Take a look at the Zehnder ComfoCool, which can be bolted on to the MVHR unit.

[JohnMo]

1 minute ago, Mike said:

Take a look at the Zehnder ComfoCool, which can be bolted on to the MVHR unit.

If you do, really read the spec, especially cooling effect v flow rate.