How much cooling does a passive house need?

[WisteriaMews]

I realise this is a slightly "how long is a piece of string" question....

 

We are finalising the technical design / specification for our passive house and are keen to install some cooling, to counter the over heating days and effects of global warming (as best we can).

 

PHPP will tell us our heating demand (e.g. 10W/m² heating load), but I'm not aware of it giving a cooling load. We are considering a Zhender MVHR with Comfoclime 36 add-on that can provide 1.7 kW cooling capacity and 2.2 kW heating capacity. Has anyone got any advice on whether that cooling capacity will be enough?

 

We're currently also finalising the external shading, but intend to have shading to stop the house getting too hot in the first place.

[JohnMo]

3 minutes ago, WisteriaMews said:

We are considering a Zhender MVHR with Comfoclime 36 add-on that can provide 1.7 kW cooling capacity

Have you actually worked out the figures of Comfoclime for your flow rates? Because to get 1.7kW cooling you need flow rates of over 400m3/h, so passivhaus flow rates for MVHR that a 500m² house give or take.

 

Cooling and MVHR is a bit of waste of money as you are only changing the air every 2 to 3 hrs. It's not good enough.

 

We are not passive, but get overheated with big windows and the sun coming in under the canopy roof. Over heat days overall is not much, but the hours it is hot - without intervention it's very hot.

 

Think, solar glazing, blinds external or less effective (but still good) internal blinds Real Aircon or ASHP with fan coils or UFH with UFH (cooling).

[Duncan62]

11 hours ago, WisteriaMews said:

But I'm not aware of it giving a cooling load.

 

Passivhaus is as much for warm climate as it is cold. Cooling demand is in PHPP.

 

As above. Recirculation of cooling air required to make any difference. Unlikely to work using the MVHR input with low flow rates - Unless you have a very well designed (i.e. certified) PH.

[SteamyTea]

Why not put in a larger MVHR system that can cope with moving greater amounts of air about?

There seems to be a fixation that MVHR can never be used for effective cooling.

It may not be the most cost effective method, and probably harder to install, but buildings all over the world have used forced air heating a cooling, except in the UK, where we like to think we are World Class and know best.

 

[WisteriaMews]

12 hours ago, JohnMo said:

Have you actually worked out the figures of Comfoclime for your flow rates? Because to get 1.7kW cooling you need flow rates of over 400m3/h, so passivhaus flow rates for MVHR that a 500m² house give or take.

The MVHR design / modelling we've had done shows 229 m3/h or 286 m3/h on boost. The Zhender data sheet (thanks for the suggestion to double check all this) shows 1.1 kW of cooling at 315 m3/h, so we'd probably get ~1kW on boost. Possibly not enough.....

 

What types of intervention do you use on hot days?

[WisteriaMews]

1 hour ago, Duncan62 said:

Passivhaus is as much for warm climate as it is cold. Cooling demand is in PHPP.

 

As above. Recirculation of cooling air required to make any difference. Unlikely to work using the MVHR input with low flow rates - Unless you have a very well designed (i.e. certified) PH.

Our architects are doing the PHPP modelling so I'll ask them for the cooling load. 

 

We are currently aiming to be certified.

[JohnMo]

21 minutes ago, WisteriaMews said:

What types of intervention do you use on hot days?

We are NE Scotland so hot days are less extreme, but we get quite a bit of solar gain especially in spring and autumn. Have had up to 27 inside the lounge when about 10 outside.

 

When hot outside

Close all windows and doors.

Close blinds if sun coming in

If a warm period have the UFH in cooling mode. UF cooling will temper the extreme, but makes the house feel cooler than the air temperature would suggest. Your body heat is moving to the floor (radiating).

Once inside is warmer than outside open windows and doors. But leave cooling mode on.

[MikeSharp01]

We did a good deal of work with the PHPP / design to get our are overheating days / % down. My wife was / is worried about it so we also installed some passive cooling, in the form of, what my wife describes,  as the rat window at the front of the house - opens at ground level with some rooflights high up at the back of the house with no doors in between thus creating a good draft even on still days and are intending to cool our slab with the ASHP and have a small heat / cool battery in the Upstairs, only two bedrooms, supply side system so we can do a little to cool them if needs be.

[Alan Ambrose]

>>> PHPP will tell us our heating demand (e.g. 10W/m² heating load), but I'm not aware of it giving a cooling load.

 

Here are the tabs in the PHPP spreadsheet (below) - a bunch for heating / shading / ventilation, a bunch for cooling, and a bunch for overall energy use and HP sizing. Also look at https://passipedia.org/

 

Your architects should be on top of both heating and cooling.

 

Personally I don't see any environmental problem with running A/C or HP in cooling mode if it is driven by PV.

 

By and large, the heating / cooling balance is governed by the amount of glazing - particularly S, W, E glazing. Little glazing roughly means easy to heat and cool, much glazing means potential heating and cooling problem. You can intervene with shading, external binds, low g glass etc but the amount of glazing is key. I'm wrangling this balance too atm. I'm coming round to the idea that you can somewhat balance winter solar gains (desired) with summer solar gains (somewhat undesired). FYI PHPP has an analysis of positive winter heat gains by window vs. undesirable summer heat gains (bottom image) which I'm trying to fully understand.

 

 

 

 

 

 

 

 

 

[JohnMo]

59 minutes ago, Alan Ambrose said:

I'm wrangling this balance too atm. I'm coming round to the idea that you can somewhat balance winter solar gains (desired) with summer solar gains (somewhat undesired)

We wrangled and then the wife said we are building there for the views, big windows or I veto the build - decision made, triple glazing added to the plan. Later after build ASHP for active cooling.

[Alan Ambrose]

>>> I veto the build

 

Ah, a reminder that there are some things which trump the laws of physics 

 

It would be most thermally efficient if there were no windows at all - but we could show a real time picture of the garden on the TV 😒

[SteamyTea]

38 minutes ago, Alan Ambrose said:

some things which trump the laws of physics

Donald Trump does, he sees value in everything that suits himself.

[Nick Laslett]

On 12/07/2024 at 21:17, WisteriaMews said:

I realise this is a slightly "how long is a piece of string" question....

 

We are finalising the technical design / specification for our passive house and are keen to install some cooling, to counter the over heating days and effects of global warming (as best we can).

 

PHPP will tell us our heating demand (e.g. 10W/m² heating load), but I'm not aware of it giving a cooling load. We are considering a Zhender MVHR with Comfoclime 36 add-on that can provide 1.7 kW cooling capacity and 2.2 kW heating capacity. Has anyone got any advice on whether that cooling capacity will be enough?

 

We're currently also finalising the external shading, but intend to have shading to stop the house getting too hot in the first place.

I was interested in the ComfoClime, but found I did not have enough space for the unit. I had a short discussion with Paul Heat Recovery and they felt the ComfoPost unit was better for cooling anyway. My MVHR is not commissioned, so not sure if the ComfoPost was worth the effort. Most discussion on here point to fan coils being a better cooling solution. 
 

The overheating issue, was a topic many Buildhub members discussed when I first joined the forum back in 2019. If you can, you need to design out as much overheating as possible with shading and smart window choices. We fitted the 70/30 suncool glass, which we are very happy with, there was no discernible degradation in the interior light quality. 
 

We also lent on the cooling capabilities of ASHP. We were already having a poured concrete 1st floor, so it made sense to fit UFH upstairs for cooling purposes. There are a few recent threads on this approach. Unfortunately this year, the weather has been so poor, it has been difficult to ascertain how effect the ASHP cooling will be. 
 

We nearly put in a roof window in the flat roof at the top of our house, where the ridge has been made flat to accommodate a planning height requirement, but ultimately I could not face the extra stress and complexity of getting it fitted. The objective with this would be to utilise the heat column/stack effect for some additional cooling. 
 

One small issue which @Jeremy Harris had, was his hot water solution leaking too much heat into the house. He went with a Sunamp in the end to address this issue. Oso do a very well insulated unvented cylinder. I guess the point I trying to make is that you can easily overlook the heat loss from the hot water system and it’s contribution to overheating a house. 

[Russdl]

On 13/07/2024 at 09:54, WisteriaMews said:

What types of intervention do you use on hot days?

External blinds here, works very well but we have three east facing windows that we didn’t fix external blinds to and that was a mistake. If I were to do this again I’d have external blinds on all East, South and West facing glazing. 
 

So far any overheating risk has been easily managed with a very active regime of window opening and closing, the closing is the important bit. If we get another ‘summer of 76’ it may well be more difficult to control. 

[WisteriaMews]

5 hours ago, Nick Laslett said:

Oso do a very well insulated unvented cylinder. I guess the point I trying to make is that you can easily overlook the heat loss from the hot water system and it’s contribution to overheating a house. 

Thanks for all the info. We are looking at an Oso cylinder having seen it mentioned on the forum.

[WisteriaMews]

1 hour ago, Russdl said:

External blinds here, works very well but we have three east facing windows that we didn’t fix external blinds to and that was a mistake. If I were to do this again I’d have external blinds on all East, South and West facing glazing. 
 

So far any overheating risk has been easily managed with a very active regime of window opening and closing, the closing is the important bit. If we get another ‘summer of 76’ it may well be more difficult to control. 

If you are happy with them, could you share who supplied your external blinds? I think we'll be installing some as well.

[Russdl]

2 hours ago, WisteriaMews said:

could you share who supplied your external blinds?

We used Phantom Screens but there are various other types, styles and suppliers around. 

[Thorfun]

just install AC. We did and love it. 

We have external blinds which shade nicely but have big windows that we like to look through so sometimes want the blinds open when the sun shines. 
other areas have large windows that we can’t shade so the heat comes in. 
we just turn on the AC which is powered from the solar so we just use it and enjoy our house. 
we also leave it on overnight if required to maintain a nice temperature in our bedroom. It uses very little electricity to maintain a temperature. 
love it. So glad we installed it. 
 

Edited July 14 by Thorfun

[Gus Potter]

On 13/07/2024 at 10:44, MikeSharp01 said:

We did a good deal of work with the PHPP / design to get our are overheating days / % down. My wife was / is worried about it so we also installed some passive cooling, in the form of, what my wife describes,  as the rat window at the front of the house - opens at ground level with some rooflights high up at the back of the house with no doors in between thus creating a good draft even on still days and are intending to cool our slab with the ASHP and have a small heat / cool battery in the Upstairs, only two bedrooms, supply side system so we can do a little to cool them if needs be.

I like this approach.. it's like passive stack ventilation simple and effective. No maintenance.

 

In principle I'm all for PH type houses.. what I'm not in favour of is designing in elements that will require costly maintenance in the future and possible early replacement.. When I see folk doing this I think vanity has taken over from sanity.

 

Off now to support England at the footy.

 

 

[joth]

On 13/07/2024 at 09:20, SteamyTea said:

Why not put in a larger MVHR system that can cope with moving greater amounts of air about?

There seems to be a fixation that MVHR can never be used for effective cooling.

It may not be the most cost effective method, and probably harder to install, but buildings all over the world have used forced air heating a cooling, except in the UK, where we like to think we are World Class and know best.

 

Forced air heating/ cooling as used in every other country is recirculating. Mvhr is not. 

Recirculation you can increase air speed without decreasing efficiency, so you can blow a gale and nothing is lost outside. But with Mvhr the amount of wasted energy (thrown out the building) increases linearly to the air speed 

 

Edited July 14 by joth

[joth]

On 13/07/2024 at 08:28, Duncan62 said:

Passivhaus is as much for warm climate as it is cold. Cooling demand is in PHPP.

 

Phpp gives a total building cooling load, but no critical room analysis for cooling. This was a problem for us as the building as a whole does not overheat but a few specific rooms seriously do.

Passive stack venting is great for the building as a whole but no good for individual rooms if they're closed off a d not open to the stack venting flow.

 

I'd suggest good analysis of shading (passive and active), stack venting, and either install or plan for easy retrofit of a/c in specific critical rooms (bedrooms and living areas)