Sizing aircon in passive house

[Ultima357]

Now nearly three years in our new house and very happy with it's performance. However, we get plenty of solar gain and are considering putting in some aircon to three main rooms including the master bedroom. I've had one quote in but not convinced that they appreciate the passive side of things and to cool three rooms are recommending 10kw main unit with three indoor units. Given we heat the entire house with just a 12kw ashp to 22 to 23 deg and in three years have consumed only 7500kwh, a 10kw aircon seems excessive. Only info of passive house cooling I've found to date comes from Perth, Australia where they recommend circa 20w/m2 giving 6kw for a 200m2 property. This compared to 15 to 18kw for a standard build. 

 

Anyone had any experience or recommendation on this?

 

NB the three rooms are 35m2, 34m2 and 24m2, the two largest facing south. 

[crispy_wafer]

Does seem a bit oversized in my non professional opinion.  Any plans in the future to increase the number of indoor units?  The 10kW unit might possibly be good for up to 5 connected indoor units, giving you a bit of future proofing. 

 

In terms of energy usage I'd find out what the condenser will modulate down to, then compare that to a 6/7kW unit see if there is any appreciable difference.  There might not be too much!  If so then the only differences are a small extra initial purchase cost, physical size, and maybe noise when at full chat, and the advantage that you can add to it in future - maybe... 

[Nickfromwales]

Was the overheat recognised in the PHPP analysis? If so, what % did they give this, and was it a general analysis or room by room?

I’m managing a 5% “risk” with air handling units on the MVHR for a current (certified) PH clients build. The idea being that prevention neutralises the need for the cure. This one doesn’t have a huge amount of glazing though to be fair. Yours therefore may be north of 7%which would likely mandate A/C, but as PH gives with one hand, it takes with the other.

As always, there’s nothing for nothing and a compromise often needs to be struck. 
Was your MVHR ductwork insulated for the fresh air supplies to rooms?

 

If you do go A/C, I doubt 10kW is massively oversized tbh, and it may be beneficial to have that headroom for cooling, with the added benefit that these will give a smudge of heating too. Most rooms / spaces which need cooling “could do with a little bit of auxiliary heat” in the depths of winter.

PH is a bit of a merry dance, with the perfect balance often just out of reach.

[Simon R]

8 hours ago, Ultima357 said:

we get plenty of solar gain and are considering putting in some aircon to three main rooms including the master bedroom. I've had one quote in but not convinced that they appreciate the passive side of things

We ended up with more solar gain than our model forecast and installed two A2A units.  My experience is that you need far less than expected. Our open plan living area is approximately 120m2 and has significant east facing glazing.  The house is south facing, but the windows are shaded and don't get direct sun May-October. The east facing glazing caused the house to get uncomfortably warm. A 3.5kW A2A unit running from 7am to 7pm keeps the living area at a very comfortable 22c with no difficulty, indeed it's way below capacity, consuming just around 3kWh yesterday which was a very warm day indeed (all of which is from solar). We also have a second 2kWh unit for the master bedroom which has not been required for cooling but is lovely for a blast of warmth on a winter morning.

Just as an aside, once you have an A2A unit disable your MVHR summer bypass feature as the cool air from the house will cool the incoming air. 

[joth]

39 minutes ago, Simon R said:

Just as an aside, once you have an A2A unit disable your MVHR summer bypass feature as the cool air from the house will cool the incoming air. 

Summer bypass should be automatic based on relative indoor/outdoor temperatures (at least it is on mine) so it's still useful even with Aircon as it will only engage if the indoor temperature is above target and the outdoor temperature is cooler than that. 

[Simon R]

53 minutes ago, joth said:

Summer bypass should be automatic based on relative indoor/outdoor temperatures (at least it is on mine) so it's still useful even with Aircon as it will only engage if the indoor temperature is above target and the outdoor temperature is cooler than that. 

That's the way I expected it to work, but on our Blauberg it appeared to be kicking in when the outside temperature is above target, so I disabled it. I'll have to check if it's still operating that way with the current software.

[SteamyTea]

@Ultima357

Have you plotted, at the hourly interval, solar power, OAT and IAT to see what is really happening.

You may find that a bit of solar film cures 99% of the issue.

[Ultima357]

On 09/06/2023 at 07:36, Simon R said:

We ended up with more solar gain than our model forecast and installed two A2A units.  My experience is that you need far less than expected. Our open plan living area is approximately 120m2 and has significant east facing glazing.  The house is south facing, but the windows are shaded and don't get direct sun May-October. The east facing glazing caused the house to get uncomfortably warm. A 3.5kW A2A unit running from 7am to 7pm keeps the living area at a very comfortable 22c with no difficulty, indeed it's way below capacity, consuming just around 3kWh yesterday which was a very warm day indeed (all of which is from solar). We also have a second 2kWh unit for the master bedroom which has not been required for cooling but is lovely for a blast of warmth on a winter morning.

Just as an aside, once you have an A2A unit disable your MVHR summer bypass feature as the cool air from the house will cool the incoming air. 

Thanks for that. Your experience indicates my thoughts are along the right lines. Good tip re mvhr too! 

[Ultima357]

On 09/06/2023 at 11:06, SteamyTea said:

@Ultima357

Have you plotted, at the hourly interval, solar power, OAT and IAT to see what is really happening.

You may find that a bit of solar film cures 99% of the issue.

No I've not plotted those but have all the data. I didn't do the full passive certification when we built as didn't think it was worth it. To be honest, the solar gain was expected and probably needs a bit more exterior shading but awnings don't fit easily to a Cedral clad exterior on a timber frame. But there's also not that many days when you can't cool down in the evening simply by opening a window or door, it's just adds to the household dust load though! 

 

Solar film is possible but then reduces the advantages in the winter sun when we can get nicely warmed without any other input. 

Edited June 13, 2023 by Ultima357
More thoughts

[RodW]

I've been involved with a few projects using A2AHPs for heating and cooling PH buildings.  Some have worked well with only a couple of (2-3kW) units, one on the bedroom corridor and the second in an open planned GF.  In winter, bedrooms with the doors open will be at the corridor temp whereas closed rooms will be 2-3 cooler.  Bedroom cooling from the corridor isn't as reliable.  A bulkhead unit with supply ducts to multiple bedrooms with the return duct with a filter to the loft works better but approximately doubles the installation cost 2 -> 4K for that fancoil.  A single unit upstairs is also fairly effective at cooling the GF provided that most of the GF windows are shaded by structure or awnings/blinds. 

 

2-3 kW units will rarely need to run above their lowest level, which produces a similar sound level as the MVHR valves.  The current PHPP assumes fancoil unis will be running at their lowest level when using mechanical cooling. 

 

Running ASHP heating or chilled water through a duct heater/cooling of a MVHR unit can work well but if the heating and cooling loads vary significantly between rooms it will be tricky/expensive to direct additional air volumes to particular rooms.  It is also less effective and more costly than a single fancoil. 

 

 

 

 

[markocosic]

On 09/06/2023 at 09:36, Simon R said:

My experience is that you need far less than expected

 

Ditto for conventional builds.

 

 

3.5 kW for an apartment of 75 m2 and 3m ceilings with a bucketload of South and West glazing is more than you need.

 

 

Oversized units have two failings.

 

1) Efficiency at minimum turndown often isn't great

 

But the main one is actually 

 

2) Dehumidification capability is compromised 

 

 

Much of the comfort benefit comes from drying the air. You do that by cooling a piece of it, condensing/dropping out the water, and mixing it back in with the rest of the air in the room.

 

The runtime needs to be long enough for droplets to form and drip off the coils and down the drain, and for the amount that evaporates back off the coil when the compressor stops to be trivial.

 

Oversized units don't run long enough to dehumidify.

 

 

We find the 3.5 dehumidifies better if you overcool (to get it to run hard enough long enough) then allow the place to warm up again afterwards before the next cycles.

 

A 2.5 kW unit would have been a better choice.

 

I'll be fitting a 2.5 kW unit in the cabin build (not passive but not nasty either) in light of this learning by mistakes method!

[markocosic]

On 13/06/2023 at 15:17, RodW said:

I've been involved with a few projects using A2AHPs for heating and cooling PH buildings.  Some have worked well with only a couple of (2-3kW) units, one on the bedroom corridor and the second in an open planned GF.  In winter, bedrooms with the doors open will be at the corridor temp whereas closed rooms will be 2-3 cooler.  Bedroom cooling from the corridor isn't as reliable.  A bulkhead unit with supply ducts to multiple bedrooms with the return duct with a filter to the loft works better but approximately doubles the installation cost 2 -> 4K for that fancoil.  A single unit upstairs is also fairly effective at cooling the GF provided that most of the GF windows are shaded by structure or awnings/blinds.

 

Any details on these that you can share?

 

What prevents bedroom cooling from the corridor in your experience? Closed doors in the daytime when unit is running or something more subtle?

 

Did any have US style transfer grilles for airflow in the doors / above the doors?