MVHR options and layout for small flat

[Bryn P]

Hi all, I've come across several posts on this forum while trying to decipher differing quotes and designs from suppliers (BPC, Paul Heat Recovery, the Airshop, ...). I'm still unsure on a few points and hope I can get some help here!

We're renovating and slightly extending our 2-bed flat, taking the opportunity to make it airtight and install MVHR, among other things. It's going to be about 71m2 with an average ceiling height of about 2.5m. That's approx 89m3/hr for 0.5 ACH, 53m3/hr for 0.3 ACH (PHR say commission on 95m3/hr). We've been specced a Vent-Axia Kinetic Sentinal BH or a Zehnder CA 180. The questions I still have are:
 

1. several people have mentioned setting their units to effectively their lowest speed, and even wishing they could go lower. They seem to always be talking about larger spaces than ours and I worry that we'll never be able to reach the low speeds we might eventually desire. The two units specced reach down to about 50m3/hr, are we going to feel like that's still too high?

 

2. linked to (1); the more premium units I find on most websites (e.g., filtering on passivhaus certified) seem always for larger spaces (and would _definitely_ not allow us to reach lower flow rates). Only the Zehnder CA 160 is shown on BPC when filtering up to 200m3/hr + passivehaus certified. What are some of the units that people recommend for <100m2 spaces (<125m3/hr incl. boost)? 

3. We've had a few different recommendations on vent layout, with BPC suggesting two outlet and two inlet vents in our proposed open plan kitchen/diner/living area (~34m2). This seems excessive to me, especially since PHR suggest only one double plenum inlet and outlet for the same space, but they won't budge on that layout when pushed. I've attached what I think makes most sense but am now unsure after BPC's strong recommendation for more vents!

4. The only place we can feasibly fit our hot water tank and MVHR unit is in a space carved out of the main bedroom wardrobe area, with access also only realistically possible from that direction (i.e., we can't create a complete sound-insulating seal as there'll be an access hatch). Are we likely to be able to hear it in operation from the bed?

5. Finally, given that each room is relatively compact, we're finding it hard to find good spots for our vents to avoid "cold spots" where people might feel the flow over them as they sit/sleep v. close to a vent. Particularly, in the living/dining area since exact positioning of furniture is TBC and we want to avoid a vent ultimately dictating where we can put things. Where could it go so that it's able to do its job properly but also won't cause any discomfort? Do we just need to think about a pre-heater for that vent?

Thanks all!

[Mike]

My apartment isn't so different in volume, so overall no worries in principle.

 

1 hour ago, Bryn P said:

1. several people have mentioned setting their units to effectively their lowest speed, and even wishing they could go lower.

Ultimately it's about how much fresh air you need per person in a room. For example, at 'normal' fan speed a double bedroom requires from 15 m³ / person / hr; in a room measuring 3 x 4 x 2.5 that's 1 ach (air change / hour), but double the size of the room and that drops to 0.5 ach. Low fan speed is typically 40% below normal speed, so those figures become 0.6 & 0.3 ach. In short, in a smaller home the minimum acceptable ach will be higher than in a larger home.

 

2 hours ago, Bryn P said:

The two units specced reach down to about 50m3/hr, are we going to feel like that's still too high?

Probably not. I don't intend to be running mine lower than 0.33 ach; in your case that would translate to a little over 50m³/hr. The exception might be during an extended absence, but at least some controllers (including my Zehnder) allow a schedule to be set so that the average daily airflow could be cut well back by having it run for only certain hours per day; that's not recommended except for longer term absence.

 

2 hours ago, Bryn P said:

What are some of the units that people recommend for <100m2 spaces (<125m3/hr incl. boost)?

I have the Zehnder CA225 (similar to the CA220 that's sold in the UK) - 60 to 150 m³/hr according to the PH certificate & 50 to 200 according to Zehnder. That's partly because I also wanted to sling it from the ceiling where I have some more space.

 

2 hours ago, Bryn P said:

We've had a few different recommendations on vent layout, with BPC suggesting two outlet and two inlet vents in our proposed open plan kitchen/diner/living area (~34m2)

That doesn't seem unreasonable if you plan to use semi-flexible ducting, to keep the airspeed (and hence the noise risk) low. You could experiment using my MVHR calculator.

 

2 hours ago, Bryn P said:

The only place we can feasibly fit our hot water tank and MVHR unit is in a space carved out of the main bedroom wardrobe area

It wouldn't be my first choice; I'd probably make that shower tray shorter to create a cupboard off the corridor. But in an enclosure with good sound insulation & anti-vibration mounts it should be OK in principle...

 

2 hours ago, Bryn P said:

Finally, given that each room is relatively compact, we're finding it hard to find good spots for our vents to avoid "cold spots" where people might feel the flow over them as they sit/sleep v. close to a vent.

Choose ceiling mounted Coanda Effect vents - Zehnder Luna, for example - and there should be no problem; keep them away from the walls by at least 350mm (750mm better, centre of room ideal subject to light fittings taking precedence).

 

[JohnMo]

9 hours ago, Bryn P said:

several people have mentioned setting their units to effectively their lowest speed

Wouldn't get bogged down, small footprint flat will not suffer from being too dry in winter. Big house only a couple of people in it, does. Set to building regs and move on.

 

The layout as you have shown looks fine. Mount MVHR on a well damped surface. I used 2x 18mm ply full wall height and it works well by bedrooms. Or buy avm mounts

 

Coanda effect nozzles are good - flow follows ceiling, so gets to room temp before coming down. Zero draft.