Green Building Forum - MVHR cooling vs Fan Coil Units

MVHR cooling vs Fan Coil Units

maxsm1 — Thu, 26 Sep 2024 09:36:48 +0100

Hi all, just hoping for some advice regarding cooling. We are about to start an eco retrofit aiming for 34kWh pa and <1.5ach @50pa. We would like to build in some form of cooling as several low energy builds we have visited have suffered from overheating issues. We are already planning external shutters on all South-ish facing roof lights and have the option of a brise soleil on the ground floor.

We were looking at adding a Comfopost CW8 unit to our Zehnder MVHR but I have doubts about how effective it will be.

Another option would be FCUs in the three rooms which are most likely to be affected by overheating. There is also a useful void space which attaches to all three rooms, which would make plumbing in the FCUs (or installing one larger concealed unit?) relatively straightforward.

I'm interested in advice on the relative merits of whole-house Comfopost vs localised FCUs... the cost should be quite similar.

Thanks in advance for any help, L

MVHR cooling vs Fan Coil Units

djh — Thu, 26 Sep 2024 12:05:55 +0100

Why are you looking at these units instead of normal domestic heat pumps?

MVHR cooling vs Fan Coil Units

maxsm1 — Thu, 26 Sep 2024 12:43:03 +0100

Hi DJH, sorry I should have made it clear - we will have a reversible ASHP so I'm hoping to use the cool feed on that to cool the house in some way on overheating days..

MVHR cooling vs Fan Coil Units

Doubting_Thomas — Tue, 01 Oct 2024 16:41:21 +0100

Not much to add, other than to say that localised Fan Coils will have localised condensation plumbing associated with them, which you need to factor in a way of plumbing into your waste system. I'm working on a commercial project where the MEP strategy changed to FCU's and we now have lots of extra vertical pipe drops which weren't in the original design.

There's also a question of noise - MVHR's tend to be a centralised system in a utility room or loft space, moving air at fairly low fan speeds. The actual terminals are usually very quiet indeed (if the system has been designed properly). By contrast, I would guess that localised FCU's would be smaller and slightly noisier, but I might be wrong.

All that being said, there's some good info here about cooling with air and the limitations of this approach. It is always better to stop the heat from entering in the first place:
https://www.weare21degrees.co.uk/guides/maximizing-comfort-and-efficiency:-our-guide-to-mvhr-systems-and-cooling/

"The reason cooling isn’t very effective through the MVHR system is not down to the technology; it is down to the nature of air and heat/coolth. Air does not have a great heat/coolth carrying capacity, so if a space is to be cooled by air, an awful lot of air needs to be supplied to that space..."

MVHR cooling vs Fan Coil Units

tony — Thu, 03 Oct 2024 07:28:20 +0100

There can be problems with condensation on the outside of the cooled air ducts.

Best plan is to design out the need for cooling by using a building physics model to inform small design changes and shading

MVHR cooling vs Fan Coil Units

djh — Fri, 04 Oct 2024 20:36:09 +0100

Posted By: tonyThere can be problems with condensation on the outside of the cooled air ducts.

Good point. Thanks!

MVHR cooling vs Fan Coil Units

maxsm1 — Fri, 18 Oct 2024 12:14:51 +0100

thankyou all, very useful