Keeping a passive house cool

[Russdl]

We learnt how to keep our house cool this summer and thought we'd share what we've learnt in case its of use to others. We're in a timber frame building built to the Passivhaus standard but not certified.

 

We don't have an ASHP, so no slab cooling for us but without a doubt our (not so secret) weapon are the external blinds. All our east and south facing windows (with the exception of 3 east facing windows - more on them later) have external blinds and as predicted by the PHPP these are the game changer.

 

Our secret weapon is the little personal weather station (PWS) in the garden and with it's help this is how we've kept the house cool:

 

In the evening when the outside temperature according to the PWS is below our target internal temperature of 21 degrees all doors and windows are opened and the MVHR ramped up. Overnight all the inward tilting windows are left open with the MVHR staying at a higher level and it stays like this until the PWS shows that the external temperature is rising towards our target 21 degrees - that happened at 07:50 this morning.

 

As soon as the exterior temperature reaches our target then all doors and windows are closed and the MVHR is reduced to its lowest setting, we monitor the air quality with a couple of CO2 monitors as recommended by @Adrian Walker and increase the MVHR speed as required to keep good air quality. All sun facing windows have the blinds down, we have the mesh style blinds that don't block off the light but stop lots of UV.

 

With this technique our hallway temperature has peaked in the early evening at around 21-22 degrees and the kitchen/dining room which has lots of east and south facing glazing has stayed below 24 degrees. As soon as we open up we get rid of most of that unwanted heat. The hallway starts in the mornings around 19-20 degrees.

 

The reason I think the PWS is a secret weapon is as far as I'm concerned you need to accurately know the exterior temperature and when to let it in and when to keep it out. On a windy day, 26 degrees can feel cooler than 21 degrees but if you let that stuff in the house you're gonna regret it. If you don't have a PWS you'll probably be able to find one situated close to you on the Weather Underground website.

 

Oh, and the 3 problem east facing windows without external blinds: 

 

1 is the utility room. That has an internal blind and as it's the utility room the internal door to the rest of the house is kept closed. That room gets warm 25-27 degrees but the heat stays in that room which of course has an MVHR extract so the heat is slowly being pumped out of the house until it's door/window opening time then we throw the heat away with vigour. 

 

2 is the ensuite which we deal with in exactly the same way as the utility.

 

3 is the main bedroom which has a great big window and curtains. The curtains remain closed while the sun is coming in that window but the doors are open so as that room warms up its heat is not being contained in that room. As the sun moves round and that window is no longer in direct sun the curtains are opened and a big old chunk of hot air falls out to be slowly extracted until window opening time.

 

To me, its like the house taking a long overnight gulp of cold air and then holds its breath throughout the day until the temperature is 'just right' again.

 

[Adrian Walker]

I’m sure that lots of people will find that useful and adaptable to their own house. Thank you for sharing. 

[ProDave]

Our secret weapon is a row of trees on the south of the plot.  Conveniently they grow leaves in summer and shade most of the garden and a lot of the windows.  In winter they fall off and let the sun into the house.

[jayc89]

What's your TF insulated with? I know a lot has been said about decrement delay with the likes of PIR, which I know can be common in TF.

Pretty jealous about those temps. Our upstairs peaked at 27c yesterday evening  

[Thorfun]

our house isn't finished yet and we have no ventilation apart from where I've yet to membrane and tape! but we're already seeing the benefits of our external blinds. earlier this week with all the blinds open it got to 31° while working on the FF. I went around and closed the blinds and angled them to allow light in but not sun left windows open over night and the next day we closed the windows and, even though some openings in the house were open as we were working and going in and out, that same room only got to 27°. once we can completely airtight the house and get the MVHR working I've no doubt that the external blinds will do a 'blinding' job of keeping temperatures down.

[Russdl]

3 hours ago, jayc89 said:

What's your TF insulated with?

 

It's Warmcel

 

[Russdl]

And in case anyone is interested in the blinds, it's these. 

[Russdl]

2 hours ago, Thorfun said:

external blinds will do a 'blinding' job of keeping temperatures down.

 

I think you'll find you're right 👍

[Russdl]

4 hours ago, ProDave said:

Our secret weapon is a row of trees on the south of the plot.  Conveniently they grow leaves in summer and shade most of the garden and a lot of the windows.  In winter they fall off and let the sun into the house.

 

Clearly the best and easiest solution if you have the space to do it, and you can do it without killing the PV at the same time.

[Thorfun]

11 minutes ago, Russdl said:

And in case anyone is interested in the blinds, it's these. 

cor...they're lovely! we went for standard Venetians so we can use them for privacy as well. negates the need for internal curtains/blinds.

[Big Jimbo]

Russdl. Is that your gaff they used on the website. Must be nice to have such a cool house in this weather fella.

[Russdl]

58 minutes ago, Big Jimbo said:

Russdl. Is that your gaff they used on the website.

😃 I wish!! 
 

But they do work well on smaller hoses as well. 

[Russdl]

1 hour ago, Thorfun said:

we went for standard Venetians so we can use them for privacy as well.

Good point. Ours provide plenty of privacy by day - you can’t see in at all - but by night (when they’re usually up anyway) they may as well not be there. A bit like the net curtains of old. 

[Thorfun]

17 minutes ago, Russdl said:

Good point. Ours provide plenty of privacy by day - you can’t see in at all - but by night (when they’re usually up anyway) they may as well not be there. A bit like the net curtains of old. 

I really like the way our house looks with the blinds down. but I am a bit biased!

 

[Russdl]

15 minutes ago, Thorfun said:

I really like the way our house looks with the blinds down.

We clearly have similar tastes 👍

[Russdl]

6 hours ago, Russdl said:

The reason I think the PWS is a secret weapon is as far as I'm concerned you need to accurately know the exterior temperature and when to let it in and when to keep it out.

To highlight that point, our neighbours have just returned from a weekend away, probably to a hot house, and they have now opened all the windows.
 

It’s lovely and breezy out there, but it’s also 30 degrees out there so they will be filling they’re house with 30 degree air which probably isn’t for the best. 
 

I could go and tell them but as they don’t speak to us since the slab was power floated, I won’t bother. 😁

[Ferdinand]

Russ

 

Is this automated, or manual? And if automated, which system?

 

(Asking for an acquaintance I sent a couple of your paras to.)

 

I get the feeling it is manual from you account.

 

F

[Russdl]

@Ferdinand it’s very manual (so not very passive) and relies completely on my unhealthy obsession with the outside air temperature. 
 

The blinds and MVHR could be automated I suspect but not the door/window opening. 

[Ferdinand]

Thanks.

[Russdl]

We have something else to add to the mix now. Our battery is in a small room in the warm roof alongside the MVHR and it warms that room up as it charges/discharges, quite significantly, what used to be the coldest room in the house is now the warmest and it seems to be working as a pre heater for the MVHR, currently it’s 20.3 degrees outside but the MVHR supply temperature is 21.8 degrees, the summer bypass is open. 
 

We haven’t had a long enough hot spell this year for the extra unwanted heat to cause any issue's and the previously described procedures keep everything hunky dory. 

 

I wonder what the effect will be in winter, a 1.5 degree increase in supply temperature just because the battery is doing its thing may be very welcome. 

[Nick Laslett]

On 14/08/2022 at 11:57, Russdl said:

We don't have an ASHP, …

 

Our secret weapon is the little personal weather station (PWS) in the garden and with it's help this is how we've kept the house cool:

…

 

Just to add to this as others might not know this, most ASHP will have a thermometer for the weather compensation, so you don’t necessarily need to have an additional weather station outside. 

[Russdl]

22 minutes ago, Nick Laslett said:

most ASHP will have a thermometer for the weather compensation,

Good point, the MVHR measures external temperature as well, so as you say the weather station is not required…

 

…unless you’re a bit obsessed like me!

[Alan Ambrose]

>>> Our secret weapon is a row of trees on the south of the plot

 

Yes, in our existing single storey, we let the olive trees and other shrubs outside the windows grow in the summer and they make for a lovely view out of the window … and they love the sun. In the autumn, I give them a good short back and sides and we get a bit of solar gain for the winter. Very effective. This mitigation is not acknowledged in simple part O - I’m not sure for TM59 and PHPP.

 

 

Edited August 12 by Alan Ambrose

[JohnMo]

Just to add further.

We have had our cooling on 24/7 for the past 6 weeks. It just cools when it needs to no thermostat, no timers.

 

Made some adjustments to get the system self modulating. Settings are such that the when the return temp to the heat pump gets to 19.4 degs the heat pump starts. If rooms are hotter from solar gain etc, the heat pump runs longer and more frequently. Overnight the heat pump may start a couple of times.

 

Below are the flow temps over 24 hrs (yesterday), the two big spikes are DHW heating.

 

As you can see the heat pump doesn't actually run that much. 11 starts in 24 hrs for cooling. Power consumed around 1.5kWh, about 8-10kWh of cooling delivered.

 

Below is house temperature, it's pretty stable. House temps do spike a little when we get solar gain but they settle back down quite quickly.

 

 

 

[Alan Ambrose]

FYI:

 

Part O - "Foliage from shrubs or trees is not a valid method of shading for calculation."

 

An unfortunate choice by the algorithm designers, as that's by far the greenest solution.

 

PHPP has a busy system of shading calcs that includes window reveals, adjacent buildings, eave overhangs, brise soleil, external blinds, low-g glazing etc. It also allows external software calcs. It has a section for trees but nothing for shrubs as far as I can see.

 

TM59 (I don't have direct experience yet) seems to have a similar feature set to PHPP.

 

The relevant PHPP page below:

 

FYI:

 

 

 

 

Edited August 12 by Alan Ambrose